Article

Possible links between sandstone diagenesis and depth-related geochemical reactions occurring in enclosing mudstones

February 1978
Journal of the Geological Society 135(1):107-117

February 1978
135(1):107-117

DOI:10.1144/gsjgs.135.1.0107

Authors:

Within compacting mudstone sequences several depth zones can be recognised in which distinctive diagenetic reactions take place. Most striking are those involving carbonate precipitation or replacement. The major carbon source is organic matter which is degraded by various microbiological and inorganic reactions to bicarbonate as one product. Other reactions involving silicates and sesquioxides introduce cations to the porewater system and carbonates are precipitated. Their chemical and isotopic composition can be linked with temperature and depth of formation. Analysis of some of the literature of sandstone diagenesis suggests that carbonate cements in sandstones often originate in mudstones, and that precipitation may take place far from the site of solute generation. Vast volumes of porewater excluded from mudstones during compaction are channelled into and travel great distances within sand units. Early diagenetic reactions drastically modify the permeability of both potential source and reservoir rocks. The component of horizontal flow is greatly increased and this must affect later migration of liquid hydrocarbons.

Fluid evolution and related fluid–rock interactions of the Oligocene Zhuhai sandstones in the Baiyun Sag, northern margin of the South China Sea

Article

Full-text available

Aug 2023

Pore fluids control the diagenetic processes and storage spaces of deep clastic rock reservoirs and have become a major area of interest within the fields of sedimentology and petroleum geology. This paper aims to relate the diagenetic processes of the Oligocene Zhuhai sandstones in the Baiyun Sag to pore fluids varying with burial depth. The types and distribution patterns of authigenic minerals are investigated through analysis of petrographic, mineralogical, and geochemical features to illustrate the origin and flow patterns of pore fluids and their influences on reservoir diagenesis. Strong cementation of eogenetic carbonate cement near the sandstone–mudstone interface was a consequence of material migration from adjacent mudstones on a large scale. The pore fluids were mainly affected by microbial methanogenesis and carbonate mineral dissolution in adjacent mudstones during eogenesis. The pore fluids were diffusively transported in a relatively open geochemical system within a local range. Support for this model is provided by the heavier stable isotopic values present in eogenetic calcite and dolomite. Feldspar dissolution during early mesogenesis was spatially accompanied by the precipitation of authigenic quartz and ferroan carbonate cement. Pore fluids in this period were rich in organic acids and CO2, and their migration mechanism was diffusive transport. The obviously lighter carbon and oxygen isotopic compositions of the ferroan calcite support this inference. During late mesogenesis, the input of deep hydrothermal fluid might have been partly responsible for the precipitation of ankerite, barite and authigenic albite. Oil charging may have inhibited carbonate cementation and compaction, accordingly preserving porosity, and together with authigenic kaolinite, might have promoted the transition of the reservoir from water wet to oil wet to the benefit of oil entrapment. The findings reported here shed new light on the evaluation and prediction of sandstone reservoirs that have experienced multiple periods of fluid flow.

Fluid evolution and related fluid–rock interactions of the Oligocene Zhuhai Sandstones in the Baiyun Sag, North Margin of the South China Sea

Preprint

Full-text available

May 2023

The pore fluids control the diagenetic processes and storage spaces of the deep clastic rock reservoirs and have become a major area of interest within the fields of sedimentology and petroleum geology. This paper aims to relate diagenetic processes of the Oligocene Zhuhai sandstones in the Baiyun Sag, to pore fluids varying with burial depth. The types and distribution patterns of authigenic minerals are investigated by analyzing petrographic, mineralogical, and geochemical features, to illustrate the originand flow patterns of pore fluids and their influences on reservoir diagenesis. Strong cementation of eogenetic carbonate cement near the sandstone–mudstone interface was a consequence of material migration from adjacent mudstones on a large scale. The pore fluids were mainly affected by microbial methanogenesis and carbonate mineral dissolution in adjacent mudstones during eogenesis. It was diffusively transported in a relatively open geochemical system within a local range. Support for this model is provided by heavier stable isotopic values in eogenetic calcite and dolmite. Feldspar dissolution during early mesogenesis is spatially accompanied by the precipitation of authigenic quartz and ferroan carbonate cement. Pore fluids in this period were rich in organic acid and CO2, and its migration mechanism was diffusive transport. The obviously lighter carbon and oxygen isotopic compositions of ferroan calcite supports this deduction. During late mesogenesis, the input of deep hydrothermal fluid might be partly responsible for the precipitation of ankerite, barite and authigenic albite. Oil charge may inhibit the carbonate cementation and compaction, accordingly preserving porosity, and together with the authigenic kaolinite, might promote the reservoir from water wet to oil wet, to the benefit of oil entrapment. The findings reported here shed new light on the evaluation and prediction of sandstone reservoirs that have experienced multiple periods of fluid flow.

Sedimentology - 2022 - Cunningham - Organic matter deposition and preservation in ancient deep‐sea levee sediments

Preprint

Nov 2022

Organic matter deposition and preservation in ancient deep‐sea levée sediments: Implications for Neoproterozoic trends in carbon burial

Article

Oct 2022

Deep‐marine levées are areally extensive features that experience high rates of sedimentation, making them ideal sites for significant carbon burial and preservation. Although modern deep‐marine levées have been shown to sequester a large proportion of the world's total buried organic carbon, few studies have attempted to assess carbon deposition and preservation in ancient deep‐marine levée deposits. Observations of well‐exposed levée deposits of the Neoproterozoic Windermere Supergroup in British Columbia, Canada, have shown that depositional processes in levées can result in the concentration and enrichment of sedimentary marine organic matter. In contrast to many previous studies where organic‐rich strata occur as black shales formed in anoxic conditions, organic matter in this study occurs mostly in banded, mud‐rich sandstones deposited under oxic conditions. Scanning electron microscopy shows that this organic carbon occurs primarily as nano‐scale coatings on clay particles, but also as uncommon sand‐sized organomineralic aggregates and discrete sand‐sized amorphous grains. As flows overspilled the channel margins the rapid collapse of the turbulent suspension resulted in elevated rates of sediment fallout that promoted aggregation of organic matter and clay particles, increasing cohesive forces in the flow, and ultimately en masse deposition. Rapid burial plus association with clay mineral surfaces prevented organic matter degradation, thereby effectively sequestering significant amounts of carbon in the sediment and therefore a net sink for atmospheric CO2. However, the mechanisms and efficiency by which organic matter is buried and preserved on geological timescales is influenced by many factors, including glacial, sea level and tectonic cycles. This work elucidates the fundamental physical and chemical processes that control organic matter accumulation and preservation in deep‐marine levées, and how these processes have evolved throughout geological time.

Formation of zoned ankerite in gravity-flow sandstones in the Linnan Sag, Bohai Bay Basin, eastern China: Evidence of episodic fluid flow revealed from in-situ trace elemental analysis

Article

Nov 2019
MAR PETROL GEOL

It has been shown that ankerite in clastic reservoirs can preserve an extended record of diagenesis and thus the fluid flow history in sedimentary basins. Ankerite cements with distinct zoning textures are well developed in gravity-flow sandstones associated with magmatic intrusions in the Linnan Sag, Eastern China. This example offering a rare opportunity to investigate the history of complex diagenetic fluid flow events in a lacustrine rift basin. An integrated study including thin section petrography, fluid inclusion micro-thermometry, carbon and oxygen isotope, SEM-EDS, XRD, CL, BSE-EPMA, LA-ICP-MS analyses was undertaken. The aim is to resolve the mineralogical, geochemical and isotopic composition of the sandstone and constitutional minerals. The sandstones are mainly of lithic arkoses (Q43F34L23), with an average carbonate cements of approximately 8.2 wt %. The carbonate cements are dominated by zoned ankerite appearing as rhombohedral and poikilotopic blocky crystals. Calcite, ferroan calcite, dolomite and siderite cements are also present. Non-carbonate cements including barite, authigenic clay minerals, and quartz overgrowth are common in the sandstone. The δ¹³ C values of ferroan calcite range from −4.2‰ to +1.2‰ PDB. Ferroan calcite was re-precipitated at a temperature of 93.1 °C at the expense of the dissolution of eodiagenesis carbonate cement in the sandstone. Ankerite was precipitated at temperature ranges from 107.8 to 140.9 °C, accompanied with the formation of pore-filling barite. The δ¹³C values of ankerite range from −1.8‰ to +1.1‰, and δ¹⁸O values range from −14.8‰ to −10.5‰, indicating an involvement of hydrothermal fluids in the formation of the zoned ankerite. The hydrothermal fluids were probably related to the mafic magmatic activities and migrated into the sandstones along faults. These fluids are rich in iron and magnesium, and also contains abundant inorganic carbon dioxide and sulphate, episodic charging into the reservoir. The abundances of most of the elements, including magnesium, iron, REEs and trace elements in the zoned ankerite consistently decrease away from the magma intrusion centres. We therefore conclude that multiple magmatic activities were primarily responsible for the zoned ankerite precipitation in the research area. The intimate relationship between magmatic activity and carbonate cementation in sandstone reservoirs documented in this paper may have significant implications for studying fluid flow in lacustrine rift basins.

Diagenetic Evolution and Petrophysical Characteristics of Paleogene Sandstone Reservoirs in the Southwest Baiyun Sag, Northern South China Sea

Article

Full-text available

Sep 2023

In addressing the critical need to understand the geological and diagenetic factors affecting Paleogene sandstone reservoirs in the Baiyun Sag—a region vital for oil and gas exploration—this study delves into the Paleogene Zhuhai and Enping formations. Advanced methodologies, including petrographic thin-section analysis, scanning electron microscopy, wavelet analysis, and carbon and oxygen isotope analysis, were employed to evaluate their petrological characteristics, pore structures, physical properties, and key diagenetic processes. The results indicate that the Zhuhai Formation is primarily composed of feldspathic quartz sandstones with prevalent intergranular dissolution pores, while the Enping Formation consists mainly of feldspathic sandstones with intragranular solution pores. The diagenetic temperatures across both formations ranged from 42.6 to 116.3 °C. The studied reservoirs have experienced porosity alterations due to mechanical compaction (23.07% reduction), carbonate cementation (9.02% reduction), and dissolution (5.09% enhancement). Notably, feldspar dissolution emerged as a significant contributor to high-quality reservoirs, particularly in the upper Enping Formation. These findings offer invaluable insights into the diagenetic evolution of sandstone reservoirs in the Baiyun Sag and hold significant implications for guiding future oil and gas exploration efforts in the region.

Advances in Geology of Unconventional Hydrocarbon Resources

Book

Full-text available

May 2022

For deepening our understanding on geological theory of unconventional oil and gas, we initiated this research topic and have gathered contributions from scientists working in diverse geoscience disciplines who have common interests in unconventional resources. In total, 16 papers have been included. The main topics include the following aspects: 1) shale gas in the Wufeng and Longmaxi formations, in the Marcellus formation, and shale gas exploration potential in southwestern Shandong Province of China; 2) shale oil in the Ordos Basin, in the Bohai Bay Basin, and in the Junggar Basin; 3) tight sandstone reservoirs and hydrocarbon reservoirs in volcanic rocks.

Mineralogi og geokemi af det Øvre Miocæne Gram ler

Article

Aug 1989

Det øvre miocæne Gram ler ved Gram Teglværk er undersøgt. Et 13 m højt profil er beskrevet og 27 prøver analyseret for mineralogi, kornstørrelse samt organisk kulstof og svovl. Endvidere er seks prøver analyseret for Ca, Mg, Mn, K, Fe, Na og Al. Sedimentet består af glimmerholdigt siltet generel stigende kor-ns tørrelse opad i ler med profilet en og øverst erkendes to storm lag af lamineret f insand. Nederst i profilet findes fem horisonter af siderit- og calcitkonkretioner. Gram leret er af lejret pi shelfen i det tertiære Nordsøbassin. En væsentlig del af materialet er aflejret under relativt høje energiforhold i forbindelse med stormsituationer og det mest finkornede materiale er aflejret fra suspension. Den nederste del af den undersøgte sekvens· er aflejret fjernest fra kysten med dominans af finkornede materialer aflejret fra suspension og enkelte distale stormlag. Sedimentet fremtræder i dag homogent p.g.a. bioturbation. De to stormlag øverst i profilet er aflejret på dybder over stormbølgebasis. Hele sekvensen afspejler en regressiv udvikling. Den diagenetiske udvikling er karakteriseret ved ophold i tre zoner, der er relateret til nedbrydningsprocesser af det organiske materiale. I den oxiske zone er der dannet mindre mængder glaukonit i fækale pellets. Anoxiske sulfidiske forhold er opnået relativt hurtigt p.g.a. høj aflejringsrate og højt indhold af organisk materiale. I denne zone dannedes pyri t og Fe-fattige karbonater. På større indsynkningsdybde etableredes anoxiske metaniske forhold, hvor udfældning af Fe-rige karbonater er foregået.

The Effect of Diagenetic Evolution on Shale Gas Exploration and Development of the Longmaxi Formation Shale, Sichuan Basin, China

Article

Full-text available

Apr 2021

Diagenetic evolution is an important controlling factor of shale gas reservoirs. In this study, based on field outcrop and drilling core data, analytical techniques including X-ray diffraction (XRD), field emission scanning electron microscope combined with a focused ion beam (FIB-FESEM), and energy-dispersive spectroscopy (EDS) analyses were performed to determine the diagenetic evolution of the Longmaxi Formation shale and reveal the effect of diagenetic evolution on the shale gas exploration and development in the Sichuan Basin, Southwest China. The eodiagenesis phase was subdivided into two evolution stages, and the mesodiagenesis phase was subdivided into three evolution stages in the basin margin and center. Absorbed capacity and artificial fracturing effect of the Longmaxi Formation shale gas were related to mineral composition, which was influenced by sedimentary characteristics and diagenetic evolution. The diagenetic system in the basin margin was more open than that in the basin center due to a different burial history. The more open diagenetic system, with more micro-fractures and soluble constitute (e.g., feldspar), was in favor for the formation and preservation of secondary dissolved pores and organic pores in the basin margin. The relatively closed diagenetic system with stronger compaction resulted in deformation of pore space in the central basin.

Diagenetic evolution and chemical changes of deep-water mudstones of Shahejie Formation in the Dongying Sag, Jiyang Depression, Eastern China

Article

Feb 2018
MAR PETROL GEOL

The diagenetic evolution and chemical changes of mudstones have a significant influence on the reservoir quality of the mudstones and their interbedded sandstones. To investigate, a variety of methods were applied to the mudstones in the middle of the third member of the Shahejie Formation (Es3z), which were formed in a lacustrine basin during 43.7–38.2 Ma. Optical microscopy and XRD analysis show that the mudstones are dominated by clay minerals (56.8%) followed by carbonate (18.2%), quartz (14.9%), feldspar (8.4%), pyrite (0.9%), anhydrite (0.8%), and total organic carbon (TOC: 2.4%). Cementation and replacement are the main diagenetic events in the mudstone of Es3z. Carbonates and clay minerals are the most common cements, but occasionally authigenic pyrite, albite, and quartz are also present. At the eogenetic stage, the diagenetic events comprised precipitation of pyrite, siderite, dolomite, and calcite besides transformation of K-feldspar into kaolinite. During the mesogenetic stage, the main diagenetic events included precipitation of fracture-filling calcite, ferroan calcite, ankerite, microcrystalline quartz, quartz overgrowth, and transformation of K-feldspar into illite and albite. The limited variation of the Nb/Ta and Zr/Hf ratios and the almost identical distribution pattern of the REE and trace elements indicate that the detrital material in the mudstones was sourced by a common provenance of felsic igneous rocks. The CIA values of Es3z mudstones vary from 50.59 to 65.05 and the PIA values vary from 50.86 to 75.58, which confirm a low-intensity weathering of the source area. Thus, the variation in the bulk-rock geochemical composition of the mudstones may not be primary, but instead a result of mass transfer with the interbedded sandstones during diagenesis. Changes ratios between aim elements (Al, Si, Ca and K) and immobile elements with burial depth in combination with mass balance calculation indicate that aluminum and silicon were retained by the precipitation of authigenic minerals in the mudstone. The mudstones have received progressively more potassium with increasing depths, whereas calcium has been expelled. The reservoir quality of the Es3z sandstones has mainly been affected by the diagenetically induced decrease in the carbonate content in the mudstones causing carbonate cementation in the sandstones.

Genesis and distribution pattern of carbonate cements in lacustrine deep-water gravity-flow sandstone reservoirs in the third member of the Shahejie Formation in the Dongying Sag, Jiyang Depression, Eastern China

Article

Nov 2017
MAR PETROL GEOL

The lacustrine deep-water gravity-flow sandstone reservoirs in the third member of the Shahejie Formation are the main exploration target for hydrocarbons in the Dongying Sag, Eastern China. Carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep-water gravity-flow sandstone reservoirs. The sandstones are mainly lithic arkose with an average framework composition of Q43F33L24. The carbonate cements are dominated by calcite, ferroan calcite, ankerite and a small amount of dolomite. The calcite and ferroan calcite are mainly poikilotopic blocky crystals, while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores. The relatively positive δ¹³C values (−2‰ to +3.9‰) of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon. From 32 Ma to 25 Ma, the pore water was rich in bicarbonate and Ca²⁺ due to carbonate dissolution in mudstone, and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium-to-thick sandstone beds. From 12 Ma to present, abundant Ca²⁺, Fe³⁺, Fe²⁺, Mg²⁺ and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium-to-thick bedded sandstones. Ankerite is mainly distributed in the reservoirs associated with oil migration or charge, because change of Fe³⁺ to Fe²⁺ from oil charge may supply sufficient Fe²⁺ for ankerite precipitation. The center of sandstone beds (>0.6 m) is with potential of high-quality reservoirs in the research area. Carbonate cementation appears to be an important factor that controls the accumulation of oil in deep-water gravity-flow sandstone reservoirs in the study area.

Provenance and Diagenetic Features of Sandstones in the Surma -Tipam Transitional Sequence exposed in the Schuppen Belt, Naga Hills, NE India

Article

Full-text available

Dec 2023

The Schuppen Belt, a part of the Indo-Burma Ranges is basically comprised of molasses of Tertiary age. This tectono–stratigraphic belt is restricted by two major thrust faults, namely Disang and Naga. In the southern part of the Naga Hills a significant part is occupied by Surma–Tipam Transitional Sequences (STTS). This study deals with the petrography, provenance, and tectonic setting of the STTS sandstones. The sandstones are classified as arkose and arkosic wacke types. The major contribution of detritus has been observed from the recycled orogen, dissected arc, transitional continental and basement uplift. The geochemistry data reveals that the sandstones were derived from a collisional setting of an active continental margin. The low degree of chemical maturity indicates that the sandstones were deposited in an arid climatic condition. The diagenetic signatures observed in these sandstones such as, point, long and concavo-convex grain boundary, albitization, crushing and squashing of quartz grains, warping of mica around detrital grains, and bending of mica suggest early to a late stage deep burial diagenesis.

Outsized Turbidity Currents as a Primary Mechanism for Neoproterozoic Organic Carbon Delivery to the Deep Sea

Article

Full-text available

Jul 2023

Levees in modern deep-marine systems have been shown to sequester significant amounts of organic carbon due largely to their expanse and high rates of sedimentation. However, relatively few studies have examined organic carbon sequestration in ancient deep-marine leveed slope channel systems. Physical and geochemical analyses of well-exposed levee deposits in the Neoproterozoic Windermere Supergroup in B.C., Canada have shown that intervals of organic-rich (up to 4% TOC) strata correlate with conditions of elevated sea level and primary productivity on the shelf. Organic matter (OM) occurs primarily as micro- to nano-scale carbon adsorbed onto the surface of clay grains and notably occurs mostly in anomalously thick, mud-rich sandstone beds that are interspersed within successions of thin-bedded, comparatively organic-poor turbidites. The concentration of organic carbon in thick beds suggests that even when primary productivity is high it only becomes mobilized in significant quantities into the deep sea by uncommon, outsized turbidity currents. Although markedly more common in organic-rich intervals, thick, organic-rich beds occur also in organic-poor levee deposits, suggesting that the occurrence and frequency of outsized flows may be linked to primary productivity on the shelf. High rates of OM production and fallout would result in rapid accumulation of OM on the seafloor that then binds and provides mechanical strength to the accumulating sediment. Later this overthickened, organic-rich sediment pile becomes gravitationally unstable and ultimately remobilized downslope. These failure events create large, surge-like flows that are considerably thicker than the depth of the slope channels through which they travel. Accordingly, continuous overspill over the channel margins results in the deposition of an anomalously thick, sand- and organic-rich bed. These episodic events not only deplete the outer continental shelf of OM, but apparently also reduces the gradient slope of the local seabed, which then results in the more typical smaller, channel-confined organic-poor turbidity currents. Additionally, the abrupt and single-bed occurrence of OM-rich strata suggests that the buildup of organic-rich strata and seafloor stabilization was rapid but only of limited duration. Significantly, this study suggests that outsized turbidity currents that originate on the outer continental shelf are the primary mechanism for organic matter delivery to the deep sea, at least in pre-vegetation times, and that flow size and frequency, in addition to primary productivity, exerts an important control on the distribution of organic carbon in deep-sea sediments.

QUARTZITIC SANDSTONES IN THE NAMURIAN AND LOWER WESTPHALIAN SUCCESSION OF THE SOUTHERN NORTH SEA: A NEW HIGH‐PERMEABILITY CARBONIFEROUS RESERVOIR TYPE

Article

Full-text available

Oct 2022

Most of the potential sandstone reservoirs within the Namurian and lower Westphalian succession of the Southern North Sea Basin are originally feldspathic sands in which the feldspar has mainly been altered to microporous kaolinite clays. The sandstones provide a moderate porosity (typically 8‐15%, depending mainly on grain size), but permeability is severely limited owing to the microporous nature of much of the porosity. Permeability is typically 1 mD or less, rising to a few tens of millidarcies in occasional coarse‐ and very coarse‐grained sands. Predicting the presence of higher‐permeability reservoir zones is therefore a critical exploration problem in these successions. Quartzitic sands have been discovered in places, especially for example in the Trent field (block 43/24), where physical reworking of sands during the transgressions that preceded the deposition of marine bands removed much of the feldspar, so that less clay was formed during burial diagenesis. Although these sandstones display moderately elevated permeabilities, commonly several to several tens of millidarcies, they are usually fine‐grained, which limits their reservoir potential. A particular type of quartzitic sandstone reservoir has been identified quite widely within the Namurian and lower Westphalian succession of the Southern North Sea. This type is indistinguishable in terms of sedimentology and inferred detrital composition from the originally feldspathic facies which now form the widespread kaolinite‐rich, low‐permeability sandstones. However, it has a very low kaolinite content (commonly 1‐2%, compared with a more usual 5‐20%) so that these sandstones display permeabilities of the order of several hundred millidarcies. Wireline log data from nineteen wells within UK Quadrants 43 and 44 have been examined, and all relevant core logged in order to compare the depositional settings of the quartzitic and non‐quartzitic sandstones. Existing data from over 500 petrographic thin sections from the area have been reviewed and 78 new thin sections analysed to determine the petrographic controls on reservoir quality in both sandstone types. The diagenetic histories of each type have been interpreted. The dissolution of feldspars during diagenesis of the quartzitic sandstones, without a substantial residue of kaolinite or other aluminium‐rich mineral, is attributed to the mobilisation of aluminium within organic complexes. This is thought to require the presence of certain organic acids.

Diagenetic history of the Miocene Surma Group sandstones from the Eastern Fold Belt of the Bengal Basin

Article

Full-text available

Apr 2022

The Eastern Fold Belt (EFB) of the Bengal Basin remains an under-explored petroleum province in Bangladesh. Interbedded sandstones and shales of the Miocene Surma Group are thought to host hydrocarbon accumulations in EFB. The diagenetic history and their controls on sandstone reservoir quality and distribution are largely unknown in this region. Therefore, this paper aims to improve the understanding of diagenetic processes and their impacts on reservoir quality in the Surma Group sandstones. Thin section petrography, Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis have all been used to this end. The results of this study show that the sandstones are very fine- to medium- grained, moderately to moderately well-sorted, matrix- rich and dominantly sublitharenites. The principle diagenetic minerals/cements are calcite, authigenic chlorite, illite/illite-smectite, minor quartz cement, and minor kaolinite. Poikilotopic calcite cement drastically reduces the porosity and permeability down to 0%. Authigenic chlorite rims along quartz grain prevent quartz overgrowth, which acts to preserve porosity. The primary porosity (0-22%, average 9%) is by far the most dominant porosity in the Miocene Surma Group reservoirs, with burial enhanced secondary porosity caused by dissolution. Reservoir quality is mainly controlled by mechanical and ductile grain compaction and calcite cement.

Diagenetic evolution of the lower Yaojia Formation of Songliao Basin, China: Impact on reservoir quality

Article

Mar 2022
J PETROL SCI ENG

The oil-bearing layer in the first member of the Upper Cretaceous Yaojia Formation (K2y¹) in the middle of the Changling Sag of the southern Songliao Basin, northeast China, has low-permeability and is deemed to be an unconventional reservoir. Therefore, better evaluation of reservoir quality and related controlling factors are crucial for the reduction in exploration risk and sustainable development. The current study integrates diagenesis, sequence stratigraphy, and sedimentary facies to decipher their influence on reservoir quality using core and thin section observations, X-ray diffraction, scanning electron microscopy, fluid inclusion, and carbon and oxygen isotope analyses. The K2y¹ is composed of shallow delta sediments of transgressive system (TST) and lowstand system (LST) tracts. The depositional microfacies of the sandstones include TST deltaic front underwater distributary channels and sheet sands, and LST deltaic plain distributary channels, distributary channel intersections, and crevasse splays. The sandstones are dominantly fine-grained moderately sorted feldspathic litharenites and lithic arkoses with silt. Early diagenetic events included compaction, development of smectite, and alteration of feldspar and kaolinite whereas mesogenetic alteration included feldspar dissolution, precipitation of quartz and carbonate cements, and clay mineral alteration. The reservoir quality of the K2y¹ sandstones is generally poor, exhibiting variations among different deltaic depositional facies within the LST and TST. The (underwater) distributary channel (intersection) sandstones, particularly those of the LST, have relatively higher reservoir quality owing to their larger grain size, better sorting, lower carbonate cement content, mixed-layer I/S and illite, and greater occurrence of chlorite.

Grain assemblages and diagenesis in Ordovician-Silurian transition shale deposits of the Upper Yangtze Platform, South China

Article

Mar 2022
J ASIAN EARTH SCI

This paper considers possible impacts of mineralogy and organic matter contents and assemblages on the diagenetic history, pore structure, and brittleness of the Ordovician-Silurian Wufeng Formation and Longmaxi Formation (Northwestern Hunan, upper Yangtze Platform) of South China. Multiple analytical approaches, including X-ray diffraction, scanning electron microscopy, cathodoluminescence, and X-ray mapping, were utilized to identify favorable shale gas intervals. Analyzed shale samples of the Wufeng-Longmaxi succession are dominated by such extrabasinal components as quartz, K-feldspar, albite, lithic fragments, and K-rich and Mg-rich clay minerals. Subordinate components include siliceous tests and organic particles. Authigenic minerals present as grain replacements and pore-fillings include quartz, illite, pyrite, and carbonate minerals. Depositional and diagenetic organic matter, which are discriminated petrographically, are present in the studied deposits. Wufeng Formation samples are enriched in organic matter (mainly of diagenetic origin) and authigenic quartz. The dominant types of porosity of the Wufeng samples include pores hosted by organic particles and interparticle pores between authigenic quartz crystals. The precipitation of authigenic quartz appears to have been the principal means of porosity reduction and enhanced the brittleness of the shale. Analyzed samples of the Longmaxi Formation are dominated by extrabasinal grains and lesser authigenic minerals. Reduction of depositional porosity of these deposits appears to have been largely the result of physical compaction. Thus, extensive authigenic quartz cementation and thermal maturation of organic matter of the Wufeng Formation likely enhanced the reservoir capacity and fracturing ability of these deposits relative to the overlying Longmaxi Formation. This understanding is especially relevant to shale gas exploration and production considerations of organic-rich fine-grained sediments of similar sedimentological background.

New data on the palynostratigraphy and paleoenvironments of the late Miocene (Tortonian) Quifangondo Formation in the Cabo Ledo section, Kwanza Basin, Angola

Article

Feb 2022
J AFR EARTH SCI

A detailed palynostratigraphic and palynofacies analysis, associated with a lithological control, was carried out on eleven outcrop samples from the Quifangondo Formation in the Cabo Ledo (Petrofina) section, Kwanza Basin, Angola. The samples yield relatively diverse and well-preserved terrestrial and marine palynomorphs. A systematic analysis of the samples enables the identification of nine spore genera and 3 species, 20 pollen genera and 13 species, and 32 dinoflagellate cyst genera and 34 species. In addition, 10 genera of other aquatic palynomorphs, such as Chlorophyceae green algae, are identified. Dinoflagellate cysts and other aquatic palynomorphs typically dominate throughout the assemblages. In this section, a combined pollen - dinoflagellate cyst zonation is used to define two palynoassociations. The first palynoassociation is middle Tortonian in age and is characterized by the presence of the pollen grain Fenestrites spinosus and the first occurrence of the dinoflagellate cyst species Selenopemphix armageddonensis. Additionally, diverse Chlorophyceae green algae dominate most of this interval samples. The second palynoassociation is assigned to the late Tortonian and is characterized by a drastic decrease in Chlorophyceae green algae, as opposed to the gonyaulacales dinoflagellate cysts. The first occurrence of the pollen grain Fenestrites longispinosus marks the transition between the palynoassociations. Lithologically, the Cabo Ledo (Petrofina) section is dominated by a claystone with an increase in the silt and carbonate components towards the upper section. The lithological data, integrated with palynostratigraphy and palynofacies, suggests that the upper part of the Quifangondo Formation was mainly deposited in an inner to middle neritic environment characterized by dysoxic conditions punctuated by periods of terrestrial inflows. Such conditions typically result from seasonal fluctuations. The age and depositional environment of the upper Quifangondo Formation inferred from this new data allows a correlation with the other Quifangondo sequences previously studied by the authors. This multi-proxy approach is important for further stratigraphic analysis with other age-controlled lithostratigraphic units in the basin. Furthermore, the improvement of paleoenvironmental and depositional models for this unit is of great importance for cross-basin correlation and future petroleum exploration plays.

Effects of sedimentary processes and diagenesis on reservoir quality of submarine lobes of the Huangliu Formation in the Yinggehai Basin, China

Article

Jun 2020
MAR PETROL GEOL

Sedimentology of Shahejie Formation, Bohai Bay Basin: a case study of Es1 member in Nanpu Sag

Article

Full-text available

May 2020

The Es1 sandstone is a fruitful hydrocarbon zone of Eocene Shahejie Formation. Therefore, the present study comprises the subsurface facies analysis established on the description of seven wells. Methods include core observation, thin section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and X-ray diffraction to evaluate the microfacies, lithofacies, and rock characteristics. Two microfacies of lithofacies have been established as sandstone microfacies and mudstone microfacies on the basis of grain size. Among them, sandstone is further divided into four sub-microfacies on the basis of microscopic studies, rock fragments and mineral composition, which are arkose sandstone, lithic arkose, feldspathic litharenite, and litharenite. These microfacies consisting quartz, feldspar as abundant detrital grains followed by rock fragments and micas, whereas calcite, ferro-calcite, quartz overgrowth, and clay are the main cement. The Es1 sandstone consists of good primary porosity and permeability as well as good secondary porosity caused by fracturing and dissolution of unstable minerals that enhance the reservoir quality. Whereas cementation and compaction reduce the reservoir characteristics. Studied formation divided into five lithofacies including conglomerate sandstone lithofacies, medium to coarse-grain sandstone lithofacies, fine sandstone lithofacies, siltstone lithofacies, and mudstone lithofacies. Core observation, sedimentary structures, facies sequences of the well log, and sedimentary composition reveal that Es1 deposited into the lacustrine delta fan depositional environment. Rock grain size indicates that Es1 contains mixtures of grains including pebbles, sand-size as well as minor silt and clay size. Grain size varies from conglomerate to clay size. The Es1 sandstone was deposited in braided river (conglomerates sandstone), channel bar, lacustrine, fluvial channel (massive bedding, cross-bedding, and graded bedding sandstone), deltaic, and flood-plain (siltstone and mudstone) sedimentary environment. It is concluded that higher transport energies in channels deposited coarse-grain sediments in shallowest part of the lake setting. However, the energy decreases towards greater depth which deposited mudstone in central part of the lake.

Diagenesis and reservoir quality of lacustrine deep-water gravity-flow sandstones in the Eocene Shahejie Formation in the Dongying sag, Jiyang depression, eastern China

Article

Full-text available

May 2020
AAPG BULL

Geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities: A case study on tight oil sandstones of the Upper Triassic Yanchang Formation, southwestern Ordos Basin, China

Article

Full-text available

Oct 2019
AAPG BULL

How important is carbonate dissolution in buried sandstones: evidences from petrography, porosity, experiments, and geochemical calculations

Article

Full-text available

Jul 2019

Burial dissolution of feldspar and carbonate minerals has been proposed to generate large volumes of secondary pores in subsurface reservoirs. Secondary porosity due to feldspar dissolution is ubiquitous in buried sandstones; however, extensive burial dissolution of carbonate minerals in subsurface sandstones is still debatable. In this paper, we first present four types of typical selective dissolution assemblages of feldspars and carbonate minerals developed in different sandstones. Under the constraints of porosity data, water–rock experiments, geochemical calculations of aggressive fluids, diagenetic mass transfer, and a review of publications on mineral dissolution in sandstone reservoirs, we argue that the hypothesis for the creation of significant volumes of secondary porosity by mesodiagenetic carbonate dissolution in subsurface sandstones is in conflict with the limited volume of aggressive fluids in rocks. In addition, no transfer mechanism supports removal of the dissolution products due to the small water volume in the subsurface reservoirs and the low mass concentration gradients in the pore water. Convincing petrographic evidence supports the view that the extensive dissolution of carbonate cements in sandstone rocks is usually associated with a high flux of deep hot fluids provided via fault systems or with meteoric freshwater during the eodiagenesis and telodiagenesis stages. The presumption of extensive mesogenetic dissolution of carbonate cements producing a significant net increase in secondary porosity should be used with careful consideration of the geological background in prediction of sandstone quality.

Origin of spherulitic and cone-in-cone concretions in Cambro-Ordovician black shales, St Lawrence Estuary, Quebec, Canada

Article

Full-text available

Jun 2019
GEOL MAG

Spherulitic concretions are very rare among carbonate concretions that generally consist of micritic carbonate. The occurrence of spherulitic concretions in Cambro-Ordovician black shales of unknown stratigraphic age on a mid-channel island in the St Lawrence Estuary in Quebec is a new example in addition to only three hitherto reported occurrences of spherulitic carbonate concretions. Their origin is still poorly understood. These concretions occur in close association with, and show various transitions to, cone-in-cone structure. The spherules, measuring 0.5 to 12 mm in diameter, consist of intergrown fine fibres of ferroan calcite and quartzine, pointing to the formation of the concretions below the sulfate-reduction zone. A phenomenological theory of spherulitic crystallization relates the thickness δ of an impurity-rich layer in front of impurity-rejecting growing crystals to the impurity-diffusion coefficient D and the growth velocity G of the crystal by δ = D/G. In spherulite-forming environments, extremely small values of δ (in the order of <10 ⁻⁴ cm) in conjunction with cellulation lead to spherulitic fibre growth. The theory of spherulitic crystallization is here applied to sedimentary deposits for the first time. The intimate association of calcite and quartzine in the concretions requires a chemical change from alkaline to acidic conditions, which occurs below the carbonate-reduction zone owing to the dissolution of sponge spicules or radiolarians. The transition from spherulite to the silica-free cone-in-cone structure occurs when the silica reservoir that acted as an impurity is exhausted in the crystallization process.

Applications of Light Stable Isotopes (C, O, H) in the Study of Sandstone Diagenesis: A Review

Article

Full-text available

Feb 2019
ACTA GEOL SIN-ENGL

This article reviews the applications of light stable isotope, including carbon, oxygen and hydrogen, in the studies on origin and formation temperature of authigenic carbonate, quartz and clay minerals. Theoretical knowledge and analytical methods for major light stable isotopes are introduced in detail. Negative and positive δ¹³C values indicate significant differences on the origin of carbonate cements. The δ¹⁸O value is an effective palaeotemperature scale for authigenic minerals formation. Various fractionation equations between δ¹⁸O and temperature are proposed for carbonate cements, quartz cements and clay minerals, whose merit and demerit, applicable conditions are clarified clearly. Clumped isotope analysis can reconstruct the temperature of carbonate precipitation with no requirement on the δ¹⁸O of initial waters, which makes temperature calculation of carbonate cements formation more convenient and accurate. Hydrogen and oxygen isotopes mainly reflect the origin of diagenetic fluid for clay mineral formation, providing reliable evidence for diagenetic environment analysis. This work aims at helping researchers for better understanding the applications of light stable isotope in sandstone diagenesis.

Controls on reservoir quality in fan-deltaic conglomerates: Insight from the Lower Triassic Baikouquan Formation, Junggar Basin, China

Article

Feb 2019
MAR PETROL GEOL

Fan delta sandy conglomerates form important oil and gas reservoirs. Recently, a new giant oilfield was discovered in the Lower Triassic Baikouquan Formation (T 1 b) in the Mahu Sag of the Junggar Basin, northwestern China. However, the fan delta sandy conglomerate reservoirs are strongly heterogeneous. Most have poor permeability and host little hydrocarbons, while a subset at the delta front exhibit high porosity and permeability, forming high-quality reservoirs. To explain this disparity, the sedimentary facies, diagenesis, and reservoir quality of the sandy conglomerate strata, and their interactions, were studied. Detailed logging of 253m cores indicates that the T 1 b was deposited in a lacustrine fan delta complex, comprising ten lithofacies from coarse matrix-supported pebbly conglomerate (Gm) to fine massive mudstone (Fm) in four Facies Associations. Interpretation of optical microscope, backscattered and scanning electron microscope imaging show that these ten lithofacies underwent differential diagenesis, especially dissolution of minerals such as orthoclase. Dissolution was particularly prevalent in the subaqueous distributary channel deposits of sandstones (St), hyperpycnal-flow massive sandstones (Sm) and grain-flow deposits of sandy gravelly conglomerates (Gcs) at the delta-front, forming abundant secondary pores. These pores enhanced the reservoir performance and led to high porosity and permeability within these three lithofacies. For these reservoirs, continuous winnowing at the delta front led to relatively high primary porosities, and subsequent mineral dissolution during burial diagenesis, closely related to acidic Mn-rich hydrocarbon-bearing fluids, significantly improved the porosities further. Migration of hydrocarbon-bearing fluids was facilitated by faults cutting through the reservoir and underlying source rocks. This led to the development of belts of high-quality reservoir at the delta front near the fault conduits. This model for reservoir development and charge suggests that further reservoirs can be expected within the T 1 b of the Mahu Sag.

Diagenesis impact on a deeply buried sandstone reservoir (Es 1 Member) of the Shahejie Formation, Nanpu Sag, Bohai Bay Basin, East China

Article

Full-text available

Oct 2018

Diagenesis has a significant impact on reservoir quality in deeply buried formations. Sandstone units of the Shahejie Formation (Es1 Member) of Nanpu Sag, Bohai Bay Basin, East China is a typical deeply buried sandstone with large hydrocarbon accumulations. The methodology includes core observations and thin section studies, using fluorescence, scanning electron microscope (SEM), cathodoluminescence (CL), fluid inclusion and isotope and electron probing analysis as well as the numerical determination of reservoir characteristics. The sandstones consist of medium to coarse-grained, slight to moderate sorted lithic arkose and feldspathic litharenite. Porosity and permeability values range from 0.5 to 30% and 0.006 to 7000 mD, respectively. The diagenetic history reveals mixed episodes of diagenesis and deep burial followed by uplift. The main diagenetic events include compaction, cementation alteration, dissolution of unstable minerals and grain fracturing. Compaction resulted in densification and significantly reduced the primary porosity. Quartz, calcite and clay are the dominant pore-occluding cement and occur as euhedral to subhedral crystals. Alteration and dissolution of volcanic lithic fragments and pressure solution of feldspar grains were the key sources of quartz cement whereas carbonate cement is derived from an external source. Clay minerals resulted from the alteration of feldspar and volcanic lithic fragments. Porosity and permeability data predict a good inverse relationship with cementation whereas leaching of metastable grains, dissolution of cement and in some places formation of pore-lining chlorite enhanced the reservoir quality. The best reservoir is thicker sandstone bodies that are medium to coarse-grained, well-sorted sandstone with low primary ductile grains with a minor amount of calcite cement. The present study shows several diagenetic stages in the Es1 Member, but the overall reservoir quality is preserved.

Diagenetic variation at the lamina scale in lacustrine organic-rich shales: Implications for hydrocarbon migration and accumulation

Article

Mar 2018
GEOCHIM COSMOCHIM AC

Lacustrine carbonate-rich shales are well developed within the Mesozoic-Cenozoic strata of the Bohai Bay Basin (BBB) of eastern China and across southeast Asia. Developing an understanding of the diagenesis of these shales is essential to research on mass balance, diagenetic fluid transport and exchange, and organic-inorganic interactions in black shales. This study investigates the origin and distribution of authigenic minerals and their diagenetic characteristics, processes, and pathways at the scale of lacustrine laminae within the Es4s-Es3x shale sequence of the BBB. The research presented in this study is based on thin sections, field emission scanning electron microscope (FESEM) and SEM-catholuminescence (CL) observations of well core samples combined with the use of X-ray diffraction (XRD), energy dispersive spectroscopy, electron microprobe analysis, and carbon and oxygen isotope analyses performed using a laser microprobe mass spectrometer. The dominant lithofacies within the Es4s-Es3x sequence are a laminated calcareous shale (LCS-1) and a laminated clay shale (LCS-2). The results of this study show that calcite recrystallization¹ is the overarching diagenetic process affecting the LCS-1, related to acid generation from organic matter (OM) thermal evolution. This evolutionary transition is the key factor driving the diagenesis of this lithofacies, while the transformation of clay minerals is the main diagenetic attribute of the LCS-2. Diagenetic differences occur within different laminae and at variable locations within the same lamina level, controlled by variations in mineral composition and the properties of laminae interfaces. The diagenetic fluid migration scale is vertical and responses (dissolution and replacement) are limited to individual laminae, between zero and 100 μm in width. In contrast, the dominant migration pathway for diagenetic fluid is lateral, along the abrupt interfaces between laminae boundaries, which leads to the vertical transmission of diagenetic responses. The recrystallization boundaries between calcite laminae act as the main migration pathways for the expulsion of hydrocarbons from these carbonate-rich lacustrine shales. However, because the interaction between diagenetic fluids and the shales themselves is limited to the scale of individual lamina, this system is normally closed. The occurrence of abnormal pressure fractures can open the diagenetic system, however, and cause interactions to occur throughout laminae; in particular, the closed-open (C–O) diagenetic process at this scale is critical to this shale interval. Multi-scale C–O systems are ubiquitous and episodic ranging from the scale of laminae to the whole basin. Observations show that such small-scale systems are often superimposed onto larger ones to constitute the complex diagenetic system seen within the BBB combining fluid transport, material and energy exchange, and solid-liquid and organic-inorganic interactions.

The Effects of Metamorphism on Iron Mineralogy and the Iron Speciation Redox Proxy

Article

Dec 2017
GEOCHIM COSMOCHIM AC

As the most abundant transition metal in the Earth’s crust, iron is a key player in the planetary redox budget. Observations of iron minerals in the sedimentary record have been used to describe surface atmospheric and aqueous redox environments over the evolution of our planet; the most common method applied is iron speciation, a geochemical sequential extraction method in which proportions of different iron minerals are compared to calibrations from modern sediments to determine water-column redox state. Less is known about how this proxy records information through post-depositional processes, including diagenesis and metamorphism. To get insight into this, we examined how the iron mineral groups/pools (silicates, oxides, sulfides, etc.) and paleoredox proxy interpretations can be affected by known metamorphic processes. Well known metamorphic reactions occurring in sub-chlorite to kyanite rocks are able to move iron between different iron pools along a range of proxy vectors, potentially affecting paleoredox results. To quantify the effect strength of these reactions, we examined mineralogical and geochemical data from two classic localities where Silurian-Devonian shales, sandstones, and carbonates deposited in a marine sedimentary basin with oxygenated seawater (based on global and local biological constraints) have been regionally metamorphosed from lower-greenschist facies to granulite facies: Waits River and Gile Mountain Formations, Vermont, USA and the Waterville and Sangerville-Vassalboro Formations, Maine, USA. Plotting iron speciation ratios determined for samples from these localities revealed apparent paleoredox conditions of the depositional water column spanning the entire range from oxic to ferruginous (anoxic) to euxinic (anoxic and sulfidic). Pyrrhotite formation in samples highlighted problems within the proxy as iron pool assignment required assumptions about metamorphic reactions and pyrrhotite’s identification depended on the extraction techniques utilized. The presence of diagenetic iron carbonates in many samples severely affected the proxy even at low grade, engendering an interpretation of ferruginous conditions in all lithologies, but particularly in carbonate-bearing rocks. Increasing metamorphic grades transformed iron in carbonates into iron in silicate minerals, which when combined with a slight increase in the amount of pyrrhotite, drove the proxy toward more oxic and more euxinic conditions. Broad-classes of metamorphic reactions (e.g. decarbonation, silicate formation) occurred at distinct temperatures-pressures in carbonates versus siliciclastics, and could be either abrupt between metamorphic facies or more gradual in nature. Notably, these analyses highlighted the importance of trace iron in phases like calcite, which otherwise might not be included in iron-focused research i.e. ore-system petrogenesis, metamorphic evolution, or normative calculations of mineral abundance. The observations show that iron is mobile and reactive during diagenesis and metamorphism, and these post-depositional processes can readily overprint primary redox information held by iron speciation. However, in principle, additional mineralogical and petrographic approaches can be combined with iron speciation data to help untangle many of these post-depositional processes and arrive at more accurate estimates of paleoenvironmental redox conditions and processes, even for metamorphosed samples.

Influence of faulting on the distribution and development of cold seep-related dolomitic conduit concretions at East Cape, New Zealand

Article

Oct 2017

An intertidal shore platform in bathyal mudstones of the Late Miocene Pohutu Formation near East Cape, North Island, New Zealand, hosts many impressive dolomitic conduit concretions having predominantly doughnut morphologies up to 6 m across. Carbon isotope values (δ¹³C + 6 to + 9‰ PDB) are interpreted to show that the carbon for dolomite precipitation was derived from extensive anaerobic oxidation of thermogenic methane, while oxygen isotope values (δ¹⁸O + 2 to + 6‰ PDB) implicate the dissociation of gas hydrates at the time of carbonate precipitation. The concretions are inferred to mark the shallow sub-seafloor locations of upward migrating hydrocarbon-enriched fluids in a fossil cold seep system on a convergent margin. The distribution of concretions and faults are intimately associated. We propose that doughnut concretion growth was related to a periodically active fault-valve mechanism involving fluid pressure increase, fault rupture and fluid discharge, followed by fluid pressure decrease, mineral precipitation and fault sealing.

The Distribution and Origin of Carbonate Cements in Deep-Buried Sandstones in the Central Junggar Basin, Northwest China

Article

Full-text available

Sep 2017
GEOFLUIDS

Extremely high porosities and permeabilities are commonly discovered in the sandstones of the Xishanyao Formation in the central Junggar Basin with the burial depth greater than 5500 m, from which hydrocarbons are currently being produced. High content of carbonate cements (up to 20%) is also observed in a similar depth range. Our study aimed to improve our understanding on the origin of carbonate cements in the Xishanyao Formation, in order to provide insights into the existence of high porosity sandstones at greater depths. Integrated analyses including petrographic analysis, isotopic analysis, fluid-inclusion, and core analysis were applied to investigate the distribution and origin of carbonate cements and the influence of high fluid pressure on reservoir quality. Textural evidences demonstrate that there are two generations of carbonate cements, precipitated at the temperature of 90°C and 120°C, respectively. The carbonate cements with low δCPDB13 ranging from −19.07 to -8.95‰ dominantly occurred near the overpressure surface and especially accumulated at approximately 100 m below the surface. Our interpretation is that high content of carbonate cements is significantly influenced by early carbonate cements dissolution and migration under overpressure. Dissolution of plagioclase resulted in the development of internal pores and porosities of as much as 10% at 6500 m depth presumably.

Diagenetic evolution and reservoir quality of the Oligocene sandstones in the Baiyun Sag, Pearl River Mouth Basin, South China Sea

Article

Apr 2024

The Oligocene Zhuhai sandstones are significant reservoirs for hydrocarbons in the Baiyun Sag, South China Sea. For effective appraisal, exploration and exploitation of such a deep-water hydrocarbon sandstone, samples of five wells from depths of 850 m to 3 000 m were studied. A series of comprehensive petrographic and geochemical analyses were performed to unravel the diagenetic features and their impact on the reservoir quality. Petrographically, the sandstones are dominated by feldspathic litharenites and lithic arenites with fine to medium grain sizes and moderate to good sorting. The reservoir quality varies greatly with a range of porosity from 0.2% to 36.1% and permeability from 0.016 × 10−3 µm2 to 4 301 × 10−3 µm2, which is attributed to complex diagenetic evolution related to sedimentary facies; these include compaction, cementation of calcite, dolomite, siderite and framboidal pyrite in eogenetic stage; further compaction, feldspar dissolution, precipitation of ferrocalcite and ankerite, quartz cements, formation of kaolinite and its illitization, precipitation of albite and nodular pyrite, as well as hydrocarbon charge in mesogenetic stage. The dissolution of feldspar and illitization of kaolinite provide internal sources for the precipitation of quartz cement, while carbonate cements are derived from external sources related to interbedded mudstones and deep fluid. Compaction is the predominant factor in reducing the total porosity, followed by carbonate cementation that leads to strong heterogeneity. Feldspar dissolution and concomitant quartz and clay cementation barely changes the porosity but significantly reduces the permeability. The high-quality reservoirs can be concluded as medium-grained sandstones lying in the central parts of thick underwater distributary channel sandbodies (>2 m) with a high content of detrital quartz but low cement.

Dynamic coupling between cementation and porosity evolution of Chang 8 tight sandstone reservoir, Ordos Basin: Insights from in-situ microanalysis

Article

Apr 2024
MAR PETROL GEOL

Characteristics and origin of crystalline dolomite: A case from Paleogene lacustrine fine-grained rocks in Jiyang depression, Bohai Bay Basin, China

Article

Apr 2024
MAR PETROL GEOL

Influence of coupled dissolution-precipitation processes on the pore structure, characteristics, and evolution of tight sandstone: A case study in the upper Paleozoic reservoir of Bohai Bay Basin, eastern China

Article

Dec 2023

Petrology, Physical Properties, and Diagenetic Characteristics of Glutenite Reservoir: An Example from the Upper Urho Formation in Zhongguai Salient, Junggar Basin, China

Article

Full-text available

Oct 2022

The Upper Urho Formation on the northwest margin of the Junggar Basin is a key formation for oil and gas exploration. Based on the core observation, combined with the analysis of cast thin section, scanning electron microscope, energy dispersive spectrum, X-ray diffraction, fluid inclusion, and porosity-permeability test, the petrological characteristics, physical properties, diagenesis types, and their effects on the physical properties of the glutenite reservoir of Upper Urho Formation were studied systematically. The results show that the lithology of reservoir of the Upper Urho Formation in Zhongguai Salient is mainly conglomerate with a small amount of sandstone. The permeability of conglomerate is generally higher than that of sandstone, and the physical properties of granule conglomerate and fine pebble conglomerate are relatively better. The reservoir experienced complicated diagenesis, mainly including compaction, cementation, and dissolution, also including filler shrink seam and metasomatism. At present, the reservoir of Upper Urho Formation in Zhongguai Salient is mainly at the middle diagenetic stage A period and partly at the middle diagenetic stage B period. The original porosity lost by compaction is the largest, which is the fundamental reason for low porosity and low permeability of reservoir. The influence of cementation on reservoir physical properties has two sides. On the one hand, the development of cementation is the main reason for low porosity and low permeability of reservoir. On the other hand, the early cementation resists compaction to a certain extent and provides a material basis for the later dissolution. The reservoir of Upper Urho Formation in Zhongguai Salient is deeply buried and has experienced strong compaction and cementation, and the reservoir properties are poor, but the secondary pore space formed by dissolution improves the reservoir properties to some extent. The study concluded that the dense glutenite reservoirs of the Upper Urho Formation can develop relatively high-quality reservoirs on a local scale, which is of guiding significance for the exploration of the Upper Urho Formation.

Laminae characteristics of lacustrine shales from the Paleogene Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China: Why do laminated shales have better reservoir physical properties?

Article

Aug 2022
INT J COAL GEOL

Laminae are the most typical and representive sedimentary feature in organic-rich shales, and revealing their influences on the reservoir physical properties is vital to understand the oil enrichment mechanisms of lacustrine shale. This study investigated the laminae characteristics and their effects on the physical properties of the shales from the Second Member of the Kongdian Formation (Ek2). The research presented in this study is based on core and thin section observations, XRD analysis, low-pressure nitrogen physisorption (LNP), high-pressure mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM). The results show that the Ek2 shales developed massive and laminated sedimentary structures. The laminae can be categorized as siliceous laminae, dolomite laminae, clay-rich siliceous laminae, organic matter laminae, and calcite laminae. Analysis of SEM, LNP, MIP, and NMR show that macropores and micro-fractures contribute significantly to the total pore volume of laminated samples, while micropores and mesopores are the dominant pore types in massive samples. The pore structure and connectivity of the laminated samples are better than that of the massive samples. Comparison of laminar and massive mudstones in the Ek2 showed that the development of laminae physical properties that are more conducive for reservoir formation as a result of interlaminated fractures, optimized arrangement of minerals, and improved organic-inorganic interactions.

Successive formation of secondary pores via feldspar dissolution in deeply buried feldspar-rich clastic reservoirs in typical petroliferous basins and its petroleum geological significance

Article

Jul 2022

Clastic rock reservoirs in petroliferous basins are generally rich in feldspars. Feldspar dissolution has developed widely in clastic reservoirs, and the resulting secondary pores are crucial in deeply buried reservoirs. Based on a study of the diagenesis of clastic reservoirs in the Bohai Bay Basin, Tarim Basin, and Pearl River Mouth Basin and physical and numerical simulation experiments of fluid-rock interactions, this paper proposed a successive formation model of secondary pores via feldspar dissolution in deeply buried clastic reservoirs, considering the global research progresses in feldspar dissolution in clastic rocks. Feldspar dissolution can occur from shallow open systems to deep-ultra deep closed systems in petroliferous basins, resulting in the successive formation of secondary pores at different diagenetic stages. The successive mechanism includes three aspects. The first aspect is the succession of corrosive fluids that dissolve minerals. Meteoric freshwater dominates at the Earth’s surface and the early diagenetic A stage. Subsequently, organic acids and CO2 formed via kerogen maturation dominate at the early diagenetic B stage to the middle diagenetic stage. CO2 and organic acids formed via hydrocarbon oxidation in hydrocarbon reservoirs dominate at the middle diagenetic B stage to the late diagenetic stage. The second aspect is the successive formation processes of secondary pores via feldspar dissolution. Large-scale feldspar secondary pores identified in deep reservoirs include secondary pores formed at shallow-medium depths that are subsequently preserved into deep layers, as well as secondary pores formed at deep depths. Existing secondary pores in deeply buried reservoirs are the superposition of successively feldspar dissolution caused by different acids at different stages. The third aspect is a successive change in the feldspar alteration pathways and porosity enhancement/preservation effect. Open to semi-open diagenetic systems are developed from the Earth’s surface to the early diagenetic stage, and feldspar dissolution forms enhanced secondary pores. Nearly closed to closed diagenetic systems develop in the middle to late diagenetic stages, and feldspar dissolution forms redistributional secondary pores. The associated cementation causes compression resistance of the rock, which is favorable for the preservation of secondary pores in deep layers. These new insights extend the formation window of secondary pores in petroliferous basins from the traditional acid-oil generation window to a high-temperature gas generation window after hydrocarbon charging. The proposed model explains the genesis of deep-ultra deep high-quality reservoirs with low-permeability, medium-porosity and dominating feldspar secondary pores, which is significant for hydrocarbon exploration in deep to ultra-deep layers.

典型含油气盆地深层富长石碎屑岩储层长石溶蚀接力成孔认识及其油气地质意义

Article

Mar 2022

https://doi.org/10.1360/SSTe-2020-0157 含油气盆地碎屑岩储层中富含长石等大量铝硅酸盐矿物，长石溶蚀作用普遍发育，且长石溶蚀形成的次生孔隙是深层碎屑岩储层重要的储集空间。本文基于对渤海湾盆地、塔里木盆地和珠江口盆地等碎屑岩储层地质实例成岩作用和流体-岩石相互作用物理及数值模拟实验的研究，结合国内外碎屑岩储层长石溶蚀作用研究进展，提出了含油气盆地深层碎屑岩储层“长石溶蚀接力成孔”认识。即长石溶蚀作用从含油气盆地中浅层开放体系到深层-超深层封闭体系均可发生，表现出接力成孔效应，包含三层含义。一是溶蚀矿物的侵蚀性流体的接力：地表-早成岩A期主要是大气淡水，早成岩B期-中成岩A期主要是配套烃源岩中干酪根热演化生成有机酸和CO2等酸性流体，中成岩B期-晚成岩期主要是油气储层中烃类高温水氧化生成的CO2和有机酸等酸性流体。二是长石溶蚀成孔过程的接力：深层储层中的规模性长石次生孔隙既有中/浅层溶蚀形成后保存到深层的，又有直接在深层溶蚀形成的，是不同期次、不同来源酸性流体对长石矿物多期持续溶蚀改造的叠合。三是长石溶蚀路径和增孔-保孔效应的转换接力：地表-早成岩阶段发育开放-半开放成岩体系，长石溶蚀形成增孔型次生孔隙；中成岩-晚成岩阶段发育近封闭-封闭成岩体系，长石溶蚀形成调配型次生孔隙，伴生的胶结作用使岩石具有更强抗压性，利于次生孔隙有效保存。这一认识的提出，将含油气盆地中次生孔隙形成窗口从传统认识的生酸-生油窗拓展到成藏后高温生气窗，合理解释了深层-超深层长石溶蚀次生孔隙主导的低渗-中孔型优质储层的发育机理，对拓展深层-超深层油气勘探领域有重要意义。

Effects of precipitation and dissolution of carbonate cements on the quality of deeply buried high-temperature and overpressured clastic reservoirs: XD 10 block, Yinggehai Basin, South China Sea

Article

Feb 2022
MAR PETROL GEOL

Clarifying the precipitation and dissolution processes of carbonate cement is of great significance for reconstructing the history of reservoir diagenesis, quantitatively evaluating reservoir quality, and enhancing acidification-related oil recovery. In this study, comprehensive experiments were performed, including thin section observation, cathodoluminescence, scanning electron microscopy, electron probe, QEMSCAN, carbon and oxygen isotopes, and fluid inclusions, to investigate the carbonate cements from the lower Huangliu Formation in the XD 10 block of the Yinggehai Basin in the South China Sea. The types, distributions, paragenetic framework, and formation mechanisms of carbonate cements were studied systematically to further reveal their impacts on clastic reservoir quality. The results were as follows: (1) The average absolute content of carbonate cement was 7.5%, which accounted for 84.3% of the total authigenic minerals and showed a negative correlation with reservoir properties. The early-stage calcite and dolomite being controlled by sedimentation with the δ¹⁸OPDB values ranging from −12.28‰ to −5.28‰, filled in the intergranular pores with poikilotopic crystals, which resulted in almost 90% of the primary porosity to be lost; the late-stage ferrocalcite and ankerite that were characterized by δ¹⁸OPDB values varying from −17.0‰ to −9.02‰ filled up approximately 30% of the secondary pores caused by minerals dissolution. (2) The overpressure of deeply buried clastic reservoirs resulted from CO2-rich fluid charging, water saturation, and their distributions, which directly affected the stability of the reservoir carbonate cements. For a gas layer containing bound water, spot-like selective dissolution developed because the film-type bound water contained only a small amount of CO2. For the water layer, the carbonate cements suffered from strong dissolution owing to the long-term sufficient CO2 supply. (3) In situ dissolution experiments of carbonate cement, periodically monitoring the relationship between injection pressure and dissolution time, indicated that the intrusion pressure represented a periodic leaping change. This is attributed to the periodic leap between the “throat blocking” resulted from the precipitation of dissolved carbonate grains and the “throat opening” caused by the further dissolution of carbonate grains. (4) Analysis of 3D petrophysical properties and 2D images showed that the early-stage carbonate cements dissolution enhanced the porosity of the reservoir from 3% to 28%, which increased the permeability by 2000–4000 times. The dissolution of late-stage carbonate cement improved the porosity from 8% to 16% and resulted in a permeability increase of 5–100 times. The mean radius of 2D pores were extended from 68.4 to 266.3 μm, the mean radius of 2D throat were increased from 14.9 to 35.1 μm. Systematic analysis of the influence of carbonate cements in deep tight reservoirs with high temperature and overpressure (HTOP) on the rock quality provided some insights into the analysis of quality control factors and the later development plan of carbonate cement-rich deep tight reservoirs.

DIAGENESIS OF THE HAYMANA FORMATION SANDSTONES IN THE TUZGOLU BASIN

Thesis

Full-text available

Jun 2017

Mithat Emre Kibris

In this master thesis study, petrography and XRD, SEM-EDS, FTIR and NMR analyzes were perfermed to determine the inorganic and organic compounds that can form in the reservoir rock. Samples of sandstone-shale outcrops taken from 3 locations in Haymana Formation around Haymana were made into thin section at Nova de Lisboa University (Portugal) and XRD, FTIR, SEM-EDS analyzes were carried out at Aveiro University (Portugal). 30 surface samples collected from sandstone-shales of Haymana Formation in the Salt Lake Basin have been examined. Clay minerals were determined by XRD (X-ray analysis), SEM (scanning electron microscopy)-EDS (energy dispersive spectroscopy), FTIR (Fourier Transform infrared) spectroscopy analyzes. In addition, the chemical structures of organic and inorganic compounds in sandstone-shales and the presence of hydrocarbon have been determined by FTIR technique. Mineral content of all samples was determined by X-ray whole rock and detailed clay analysis (XRD) (Philips PW-1730). The morphological and microdissociation properties of the sample with the electron microscope (SEM) (Jeol JSM-840A), the energy dispersive spectroscopy device (EDS) (Trocor TN-5502) and the point chemical element analyzes were carried out in Aveiro University laboratories. According to the NMR results, the character constructions of the samples were determined. The smectite illite transformation zones obtained according to XRD shots are thought to coincide with hydrocarbon formation zones. Porosity and permeability analyzes were carried out in TPAO laboratories to determine reservoir properties. The laboratory analyzes revealed that the clay minerals (chlorite, illite, kaolinite and smectite) of the Haymana formation sandstone-shales and their effects on the quality of the sandstone reserves were precisely determined by the molecular chemical structure of the organic matter and the diagenetic properties of Haymana Formation sandstones. The relation between diagenesis and hydrocarbon investigations has been established. It was determined that clay minerals were found in Haymana sandstones in abundance and pore fillings and clay minerals negatively affected Haymana sandstone reservoir. June 2017, 73 pages

Applications of geochemistry and basin modeling in the diagenetic evaluation of Paleocene sandstones, Kupe Field, New Zealand

Article

Sep 2021

Paleocene sandstones in the Kupe Field of Taranaki Basin, New Zealand, are subdivided into two diagenetic zones, an upper kaolinite–siderite (K-S) zone and a lower chlorite–smectite (Ch-Sm) zone. Petrographic observations show that the K-S zone has formed from diagenetic alteration of earlier-formed Ch-Sm sandstones, whereby biotite and chlorite–smectite have been altered to form kaolinite and siderite, and plagioclase has reacted to form kaolinite and quartz. These diagenetic zones can be difficult to discriminate from downhole bulk-rock geochemistry, which is largely due to a change in element-mineral affinities without a wholesale change in element abundance. However, some elements have proven useful for delimiting the diagenetic zones, particularly Ca and Na, where much lower abundances in the K-S zone are interpreted to represent removal of labile elements during diagenesis. Multivariate analysis has also proven an effective method of distinguishing the diagenetic zones by highlighting elemental affinities that are interpreted to represent the principal diagenetic phases. These include Fe-Mg-Mn (siderite) in the K-S zone, and Ca-Mn (calcite) and Fe-Mg-Ti-Y-Sc-V (biotite and chlorite–smectite) in the Ch-Sm zone. Results from this study demonstrate that the base of the K-S zone approximately corresponds to the base of the current hydrocarbon column. An assessment with 1D basin models and published stable-isotope data show that K-S diagenesis is likely to have occurred during deep-burial diagenesis in the last 4 Myr. Modeling predicts that CO2-rich fluids were generating from thermal decarboxylation of intraformational Paleocene coals at this time, and accumulation of high partial pressures of intraformational CO2 in the hydrocarbon column is considered a viable catalyst for the diagenetic reactions. Variable CO2 concentrations and residence times are interpreted to be the reason for different levels of K-S diagenesis, which is supported by a clear relationship between the presence or absence of a well-developed K-S zone and the present-day reservoir-corrected CO2 content.

Controls of diagenesis on the quality of shallowly buried terrestrial coarse-grained clastic reservoirs: A case study of the Eocene Shahejie Formation in the Damintun Sag, Bohai Bay Basin, Eastern China

Article

Sep 2021
J ASIAN EARTH SCI

A typical shallowly buried (800–2000 m burial depth) coarse-clastic, nonmarine, fan-deltaic reservoir is well developed in the Eocene Shahejie Formation, Damintun Sag, Bohai Bay Basin, China. In this study, diagenesis and its control on the reservoir quality of fan-deltaic sandstones and conglomerates in the Shahejie Formation were investigated in detail by petrographic and geochemical analyses, including cathodoluminescence, X-ray diffraction, scanning electron microscopy, electron probe microanalyses and X-ray fluorescence. The reservoir sandstones and conglomerates mainly consist of moderately sorted feldspathic litharenite, which has undergone various diagenetic processes such as mechanical compaction, cementation, and dissolution, and are currently in the eodiagenesis stage with porosity and permeability averaging 21.56% and 435.2 mD, respectively. Mechanical compaction and cementation caused an ∼45% reduction in the original reservoir porosity, especially the former, which exerted a more significant influence. In regard to cementation, the late calcite that formed at the mudstone-sandstone interface is more destructive and enhances reservoir heterogeneity, and its source material is believed to be primarily derived from the adjacent mudstones in conjunction with oil and gas charging. Feldspar dissolution plays a limited constructive role in reservoir properties by creating ∼10% secondary porosity and changing reservoir wettability to oil wet through the formation of authigenic kaolinite. As a result, massive primary pores (∼90% of the total pores) in the shallowly buried sandstones and conglomerates in the Eocene Shahejie Formation are still preserved. This systematic study is expected to be applicable in a general way to other shallowly buried terrestrial clastic reservoir systems.

Depth of Diagenetic Processes and Lower Boundary of the Biosphere in the South Caspian Basin

Article

Dec 2020

Akper Feyzullayev

Hydrochemie, diagenetische Entwicklung, Herkunft und Verbleib der gelösten Substanzen von Porenwässern

Chapter

Jan 2021

Diagenetische Reaktionen verlangen die Anwesenheit von Wasser als Porenlösung. Das Vorhandensein von Wasser im Porenraum ist Voraussetzung für die bei der Versenkungsdiagenese auftretenden mineralchemischen Reaktionen. Wo statt Wasser andere Porenfüller wie Öl, Gas oder mineralische Zemente die Poren besetzen, wird der Ablauf advektiv oder diffusiv beherrschter diagenetischer Prozesse unterbunden oder stark gehemmt.

Genetic mechanisms of Permian Upper Shihezi sandstone reservoirs with multi-stage subsidence and uplift in the Huanghua Depression, Bohai Bay Basin, East China

Article

Oct 2020
MAR PETROL GEOL

Braided river sandstones in the Permian Upper Shihezi (US) Formation in the Huanghua Depression, Bohai Bay Basin, East China, are reservoirs with large accumulations of hydrocarbons. The Upper Paleozoic formations, including the Carboniferous and the Permian strata experienced multi-stage of subsidence and uplift in the past 320 Ma, leading to occurrence of subaerial exposure and deep burial of the Permian US sandstones. Due to complex tectonic evolution, the Upper Paleozoic formations in different tectonic zones of the Huanghua Depression experienced various burial-thermal histories, resulting in the development of long-term open (LTO), early-open to late-closed (EOLC), and long-term closed (LTC) geochemical systems in different Permian US sandstones, as well as different evolutionary patterns of diagenesis and reservoir quality. After extensive diagenetic alteration, the present sandstones are primarily quartzose, subfeldsarenite, and sublitharenite sandstones. Pores in shaly sandstones and fine-grained sandstones were destroyed extensively, only pebbly to medium-grained sandstones can be potential effective reservoirs. The Permian US sandstones with the LTO systems, which are currently overlain by the Paleogene strata and buried less than 2500 m, experienced two stages of meteoric freshwater leaching and are characterized by extensive feldspar dissolution and weak precipitation of authigenic kaolin and quartz. Intergranular and intragranular pores dominate the 10–20% porosity, and permeability ranges from 1mD to 800 mD. The Permian US sandstones with the EOLC systems, which are currently overlain by thick Jurassic and Cretaceous strata on a regional scale and buried deeper than 3000 m, experienced both meteoric freshwater leaching and deep burial leaching and are currently characterized by extensive feldspar dissolution, extensive kaolin precipitation, and weak-moderate quartz cementation. Intragranular secondary pores and kaolin intercrystal pores dominate the 5–15% porosity, and permeability ranges from 0.1 mD to 10 mD. The Permian US sandstones with the LTC systems, which are currently overlain by a thick Permian Shiqianfeng Formation on a regional scale and buried deeper than 3000 m, experienced no meteoric freshwater leaching but only burial dissolution; they are currently characterized by extensive feldspar dissolution and extensive precipitation of clays and quartz cements. Intercrystal pores in the authigenic clay and intragranular secondary pores dominate the 5–10% porosity, and permeability ranges from 0.01mD to 1mD. Since the unconformities, overlying strata, and tectonic evolution relevant to the Permian US sandstone unit can be analyzed using seismic data, the proposed genetic models can support the predrill evaluation of reservoir quality of the Permian sandstones.

Multiple organic–inorganic interactions and influences on heterogeneous carbonate‐cementation patterns: Example from Silurian deeply buried sandstones, central Tarim Basin, north‐western China

Article

Oct 2020

This study investigates the importance of multiple organic–inorganic interactions on heterogeneous carbonate cementation patterns in Silurian deeply buried sandstones, central Tarim Basin, north‐western China, to evaluate their effects on reservoir quality. Petrographic observations and mineral geochemistry identify two stages of carbonate cementation: (i) eogenetic, poikilotopic blocky calcite precipitated at 20 to 50°C (δ¹³CVPDB from −0.3‰ to +6.8‰; δ¹⁸OVPDB between −6.6‰ and −2.9‰) and dolomite precipitated at 40 to 62°C (δ¹³CVPDB from +1.9‰ to +5.3‰; δ¹⁸OVPDB between −6.2‰ and −2.1‰); and (ii) mesogenetic, isolated pore‐filling ankerite formed at 70 to 120°C (δ¹³CVPDB from −14.4‰ to −6.6‰; δ¹⁸OVPDB between −11.6‰ and −7.8‰). The eogenetic carbonates are predominantly distributed as carbonate‐cemented beds or concretions along sandstone–mudstone contacts and were derived mainly from microbial methanogenesis of organic matter and dissolution of carbonate minerals in adjacent calcareous mudstones. The mesogenetic ankerite cements occur predominantly as carbonate‐cemented beds, concretions and patches, and are more concentrated in the central sections of sandstone bodies. It is inferred that ankerite cements were sourced from dissolution of eogenetic calcite and dolomite via in situ generation of organic CO2 related to thermal decarboxylation of organic acids within sandstones. Kinetic modelling results coupled with petrographic observations illustrate that mesogenetic dissolution of eogenetic calcite and dolomite cements did not enhance total reservoir porosity due to re‐precipitation of ankerite cements on a very local scale within sandstone bodies. The development of extensively carbonate‐cemented geometries (beds, concretions and patches) created by different generations of carbonate cements derived from external sources, led to reservoir heterogeneity and significant destruction of sandstone reservoir quality especially during diagenesis. The results of this study demonstrate the importance of multi‐stage, intra‐formational mass transfer induced by concentration gradients during organic–inorganic interactions in mediating a variety of spatial patterns of carbonate cementation under different diagenetic regimes. Better insights of these organic–inorganic interactions in a coupled sandstone–mudstone system could improve predictive models of heterogeneous diagenetic alterations in deeply buried sandstones of similar origin.

Role of CO2-water-rock interactions and implications for CO2 sequestration in Eocene deeply buried sandstones in the Bonan Sag, eastern Bohai Bay Basin, China

Article

Mar 2020
CHEM GEOL

This study explores CO2-water-rock interactions in Eocene hydrocarbon sandstone reservoirs in eastern Bohai Bay Basin, China, to document mineral alterations and their implications for CO2 geological sequestration. Petrographic, geochemical and reactive transport modeling evidences reveal that various inorganic mineral alterations occur during organic CO2 intrusion from adjacent source rocks into sandstone reservoirs including: (1) pervasive dissolution of calcite and chlorite minerals that are re-precipitated as secondary, more stable dolomite and ankerite cements; (2) precipitation of low-temperature calcite cements (27–49 °C) and relatively high-temperature dolomite (88–111 °C) and ankerite (103–136 °C) cements based on their δ¹⁸OPDB values (−11.91‰ to −7.86‰ for calcite; −15.16‰ to −12.48‰ for dolomite and −17.71‰ to −14.32‰ for ankerite); (3) Carbon sources for dolomite and ankerite cements (δ¹³CPDB from −7.59‰ to −3.58‰) are a mixture of carbon derived from early-formed calcite precursors (δ¹³CPDB from–0.26‰ to +3.75‰) with significant proportions of organic carbon (δ¹³CPDB from −25‰ to −10‰); (4) plagioclase dissolution spatially associated with precipitation of quartz and kaolinite. Reactive transport modeling results illustrate that mineral dissolution is coupled temporally and spatially with precipitation of secondary mineral phases. It is inferred that advective and diffusive mass transport of dissolved species in the extremely low pore-fluids is greatly restricted and mineral kinetics exerts more influences on CO2–water–rock interactions in the deep burial environments. The overall mineral alterations in the CO2-mediated geochemical system are the consequences of complicated interconnected reactions in the simulated model. Kinetically slow dissolution reactions of aluminosilicate minerals (e.g. chlorite) are regarded as the rate-limiting step for precipitation of more stable carbonate phases and CO2 sequestration. Therefore, the occurrences, abundances and distribution patterns of the aluminosilicate minerals coupled with their kinetic reaction rates are likely to affect the long-term fate of the safe CO2 storage in the deeply buried siliciclastic reservoirs.

Sedimentological impact on reservoir quality of Es1 sandstone of Shahejie formation, Nanpu Sag, East China

Article

Aug 2019

Comprehensive sedimentological studies regarding different facies types and the factors persuading their development were carried out at Eocene (Es1) member of Shahejie formation of Nanpu Sag. To evaluate the sedimentary facies, lithofacies, and reservoir quality, wireline log analysis, a petrographic thin-section study, SEM, grain size analysis, XRD, and CL analysis were used. The Es1 consists of five sedimentary facies and seven lithofacies. Sandstone is classified on the basis of standard rock classification scheme as feldspathic litharenite and lithic arkose. Quartz grains are abundant detrital constituent and feldspars are subordinate followed by mica, chert, rock fragments, and iron oxide as a minor constituent. Primary intergranular pores, as well as secondary dissolution pores and fracture pores, are also present that enhance the reservoir quality. Grain size varies from conglomerate to clay size, with massive bedding, cross-bedding, and fine-grained ripple laminated sandstone, which predicts that formation is deposited in lacustrine and fluvial channel or channel bar environment. XRD and SEM analysis show various clay minerals that reduced the reservoir characteristics. The present study discloses the deposition and distribution of lacustrine, braided, meandering river delta, and delta front sand bodies of Es1 member as well as provides some support for reservoir quality of the correlative sedimentary system that has great significance for further exploration.

Diagenesis factor that effect the reservoir properties and gas accumulation in the 4th member of Xujiahe formation, Western Sichuan Basin, China

Article

Jan 2019

Reservoir properties of the fourth member of the Xujiahe Formation in the Western Sichuan Depression are quite poor (average porosity =5.9%, average permeability =1.41 mD). These properties were influenced by several diagenetic processes, especially compaction and carbonate cementation. Due to a combination of compaction of ductile fragments and carbonate cementation, sand/mud interfaces of the thick sandstone layer were tightly cemented, pore space also reduced sharply in positions where rich in ductile grains. Thus, space leaving for hydrocarbon is limited, except where both the detrital ductile grain content and carbonate cement content are low.

Sideritic Concretions from the Westphalian of Yorkshire: A Chemical Investigation of the Carbonate Phase

Article

Full-text available

Jun 1974

Michael J. Pearson

The carbonate phase of impure sideritic concretions from a sequence of argillaceous sediments from the Westphalian of Yorkshire has been analysed using an acid-dissolution technique and corrections allowing for the decomposition of non-carbonate minerals applied to obtain accurate compositional formulae. The siderites show considerable compositional variation, some containing Ca in excess of anticipated substitutional limits. Whilst these may be metastable it is suggested that coupled Mg-Ca substitution could provide an explanation.

Chemical Weathering in a Subtropical Igneous Terrain, Rio Ameca, Mexico

Article

Jan 1971

James I. Drever

Ferro-Magnesian Calcite Cement in Sandstones

Article

Jan 1975

Diagenesis of Gulf Coast Clayey Sediments and Its Possible Relation to Petroleum Migration

Article

Jan 1969
AAPG BULL

John F. Burst

Interstitial Water Composition and Geochemistry of Deep Gulf Coast Shales and Sandstones

Article

Jan 1973
AAPG BULL

Gene W. Schmidt

Interstitial water from shales and sandstones shows a contrast in concentration and composition. Sidewall cores of shales were taken every 500 ft between 3,000 and 14,000 ft in a well in Calcasieu Parish, Louisiana, which encountered abnormally high fluid pressures just below 10,000 ft. Significant differences between the total dissolved solids concentrations in waters from normally pressured sandstones (600-180,000 mg/l) and highly pressured sandstone (16,000-26,000 mg/l) were noted. Shale pore water has a lower salinity than the water in the adjacent normally pressured sandstones, but the concentrations are more similar in the high pressure zone. Shale water generally has a concentration order of SO4 = > HCO3- > Cl-, whe eas water in normally pressured sandstone has a reversed concentration order. Conversion from predominantly expandable to non-expandable clays accelerates near the top of the high pressure zone, which appears correlative with a major temperature gradient change, an increase in shale porosity (decrease in shale density), a lithology change to a massive shale, an increase in shale conductivity, an increase in fluid pressure, and a decrease in the salinity of the interstitial waters. The data presented suggest that the clays subjected to diagenetic change release two layers of deionized water and that this released water may be responsible for the lower salinity of the water found in the high pressure section.

New charts help estimate formation pressures

Article

Jan 1966
Oil Gas J

H.H. Ham

Gas Analyses in Sediment Samples from Legs 10, 11, 13, 14, 15, 18, and 19

Article

Jun 1973

Late-stage dehydration in deeply buried pelitic sediments

Article

Jan 1972
AAPG BULL

Early Diagenesis of Clay Minerals, Rio Ameca Basin, Mexico

Article

Jan 1971

James I. Drever

Sedimentary geochemistry: Environments and processes dominated by involvement of an aqueous phase

Article

Jan 1977

C.D. Curtis

Chapter 11 Interstitial Solutions and Diagenesis in Sediments

Chapter

Dec 1967

Wolf Von Engelhardt

In this chapter the writer explains the diagenetic processes in sandstones and, starting from these observations, draws some conclusions concerning the processes occurring in the interstitial solutions. General conclusions about the concept of diagenesis and its distinction from metamorphism follow from this.

Hydrocarbon Generation in Gulf Coast Tertiary Sediments

Article

Jan 1974
AAPG BULL

Roger E. Laplante

Organic detritus deposited in sediments is composed principally of carbon, hydrogen, oxygen, and nitrogen, and has the potential for the generation of hydrocarbons. At the time of deposition, only small amounts of hydrocarbons are present. The type and quantity of hydrocarbons eventually generated depend largely on the hydrogen content of the kerogen. With increased burial, disseminated sedimentary organic matter undergoes carbonization by processes very similar to the thermochemical reactions causing coalification. Carbonization is a thermal process marked by the generation of volatiles relatively rich in oxygen and hydrogen and by the formation of a kerogen residue increasingly enriched in carbon. The most significant oxygen-rich volatile is carbon dioxide, and the most significant hydrogen-rich volatiles are hydrocarbons. By measuring changes in the elemental composition of the kerogen as a function of depth, the principal volatile products of the carbonization reactions can be determined. Data from Oligocene and younger Cenozoic samples from the Louisiana Gulf Coast indicate that carbon dioxide is the principal volatile product of the early stage of carbonization and that hydrocarbons are not significant products until the later stages. The amounts of hydrocarbons generated during kerogen carbonization are vast compared to those formed by any other natural source or process. The data indicate that carbonization or, more specifically, hydrocarbon generation is a rate-controlled process which follows the principles of chemical kinetics. That is, the younger the sediment, the higher the temperature required to reach the level of carbonization associated with hydrocarbon generation. For example, significant hydrocarbon generation occurs in the Oligocene at a temperature of 170°F (77°C), in the lower Miocene at a temperature of 186°F (86°C), and in the upper Miocene at a temperature of 205°F (96°C). Higher temperatures are required for significant hydrocarbon generation in post-Miocene sediment. These results support the conclusions of earlier research geochemists that kerogen with relatively low hydrogen contents, e.g., similar to percentages found in coals, is restricted principally to gas generation during carbonization. Thus, the relatively low hydrogen content in kerogen from selected Louisiana wells suggests that the sections penetrated are better sources for gas than for oil.

Chapter 2 Phases of Diagenesis and Authigenesis

Chapter

Dec 1967

Rhodes W. Fairbridge

A review of marine sedimentological data leads to a classification of three phases of diagenesis; these are: (a) syndiagenesis (marked by syngenetic authigenesis in two stages, initial or oxidizing and early burial or reducing), that lasts from 1, 000 up to about 100, 000 years and may extend to depths from about 1 to 100 m; (b) anadiagenesis (marked by hypogene authigenesis, i.e., non-magmatic ascending waters and "natural chromatography"), extending from 10 3 to 10 8 years, and 1 to 10, 000 m depth; (c) epidiagenesis (marked by deep meteoric waters and epigene authigenesis) that may by-pass anadiagenesis, due to tectonism, and may extend from 10 3 to 10 9 years, and in depth about 1 to 3, 000 m. Many authigenic minerals formed during different stages of diagenesis may be experimentally duplicated, but much remains to be done.

Chapter 3 Diagenesis of Sandstones

Chapter

Dec 1967

E.C. Dapples

This chapter concerns the diagenetic modification of sands following deposition, such modifications occurring as progressive stages. Redoxomorphic (oxidation-reduction) reactions involving iron in particular characterize early burial. Locomorphic changes (cementation and mineral replacement) involving primarily silica and carbonates are typical of lithification. The phyllomorphic stage (authigenesis of micas and feldspars) is a late burial feature. Chemical reactions which occur during each of the three stages of diagenesis result in equilibrium mineral assemblages which are considered to identify the pH and Eh of the interstitial fluids. Shifts in the direction of equilibria are indicated by corresponding changes in the mineral assemblage. Although a secondary mineralogy characterizes most of the diagenetic progression there may be also removal of detrital mineral matter to produce a simple residual mineralogy, and special granular intersutured textures.

Sedimentary pyrite formation

Article

Nov 1969

RA Berner

Influence of nature and diagenesis of organic matter in formation of petroleum

Article

Jan 1974
AAPG BULL

Hydrocarbon generation in the Gulf Coast Tertiary sediments

Article

Jan 1974

R. E. La Plante

Stability of Mg-Ca carbonates

Article

Feb 1977
GEOCHIM COSMOCHIM AC

R.B. de Boer

In this article a diagram is constructed in which the stability of Mg-Ca-carbonates can be represented as a function of only two parameters. By means of this diagram the relative stability of the various carbonates can be easily compared.

Fabric and Chemistry of Pore Filling Calcite in Septarian Veins: Models for Limestone Cementation

Article

Jan 1974

R. C. Lindholm

Vein filling low magnesium calcite in septarian veins exhibits fibrous, bladed and equant fabrics, all of which occur also in limestone cements. These fabrics are associated with particular crystal habits (fibrous-combined prism and rhombohedron; bladed-steep rhombohedra; equant-flat rhombohedra) and magnesium content (fibrous and bladed approximately 2.0 mole percent MgCO3; equant generally less than 0.4 mole % MgCO3). These relationships indicate that the amount of magnesium in aqueous solution controls the crystal habit of crystallizing calcite which in turn affects the final fabric. The sequence of fibrous or bladed calcite followed by equant calcite suggests crystallization in waters where magnesium content decreased with time (changing diagenetic environmen in marine lutites with burial). The early formed fibrous calcite may have crystallized as aragonite or as high magnesium calcite (micron-sized or fibrous cement), but was later modified through neomorphism. Concurrent increase in ferrous iron content (from early to late formed calcite) indicates parallel changes in those chemical factors (iron content, Eh?) that control the incorporation of iron into the calcite lattice.

Burial Diagenesis in Gulf Coast Pelitic Sediments

Article

Ed Perry

Petroleum Generation in Gulf Coast Tertiary Sediments: ABSTRACT

Article

Jan 1972
AAPG BULL

R. E. Laplante

Organic detritus deposited in sediments is composed principally of carbon, hydrogen, oxygen, and nitrogen. At the time of deposition, only small amounts of hydrocarbons are present. However, this organic matter has the potential to generate hydrocarbons in quantities that depend largely on its hydrogen content. Studies show that organic matter disseminated in sediments, when heated, undergoes carbonization by mechanisms very similar to the thermochemical processes responsible for coalification. Carbonization is a thermal process marked by the generation of volatiles relatively rich in oxygen and hydrogen and by the formation of a kerogen residue increasingly enriched in carbon. The most significant oxygen-rich volatile is carbon dioxide and the most significant hydrogen-rich volatiles are hydrocarbons. By measuring changes in the elemental composition of the organic matter as a function of depth, the principal volatile products of the carbonization reactions can be determined. Data from the Gulf Coast Tertiary indicate carbon dioxide is the principal volatile product of early carbonization a d that hydrocarbons are not significant products until the later stages. The amounts of hydrocarbons generated during carbonization are vast compared with amounts from any other naturally occurring source or process. The data indicate that the rate of carbonization or, more specifically, hydrocarbon generation is a chemical process that follows the general rules of chemical kinetics. That is, as sediment age decreases, the temperature required to reach the level of carbonization associated with hydrocarbon generation increases. For example, significant hydrocarbon generation occurs in the Oligocene at a log temperature of 175°F and above; in lower Miocene at log temperature 186°F; and in upper Miocene at log temperature 205°F. Appreciably higher temperatures are required for significant hydrocarbon generation in post-Miocene sediment. It is suggested that kerogen with relatively low hydrogen levels (similar to levels found in coals) would generate gas rather than oil. Thus, the relatively low hydrogen level in organic matter from the rocks discussed suggests that the sections penetrated would be better sources for gas than oil. End_of_Article - Last_Page 635------------

Clay-Ironstone Concretion Preserving Fabrics Due to Progressive Compaction

Article

Jan 1972

Fabrics arising from preferred orientation of clay minerals are presented in a sequence of samples taken from the center of a diagenetic siderite concretion into the enclosing shale sediment. Compactional histories, as deduced directly from estimated porosity values before cementation with siderite, and as deduced from the rotation of mineral flakes to produce the observed fabric, confirm each other. It is concluded that the particular carbonate concretion studied started to grow within the pore space of a flocculated clay sediment soon after deposition. An early period of rapid growth is distinguished from a second, protracted phase which extended effectively until the sediment achieved its present degree of compaction.

Stages of Diagenesis in the Development of Sandstones1

Article

Aug 1962

E. C. DAPPLES

On the basis of mineral replacements and intergrowths, four oxide series are recognized among sandstones: (1) alumina, lime-magnesia, iron oxide, (2) silica, lime-magnesia, iron oxide, (3) silica, alumina, iron oxide, (4) silica, alumina, lime-magnesia. Typical reactions are: a, for series (1), hematite + calcite + Mg ⇄ siderite + ferro-dolomite + dolomite; b, for series (2), hematite + clay minerals + silica ⇄ chlorite + greenalite; c, for series (3), diaspore + silica ⇄ clay minerals, and diaspore + silica + K ⇄ glauconite + clay minerals; also, during late burial illite + Na + K + Ca → muscovite + biotite + feldspar; d, for series (4), illite or glauconite → chlorite + muscovite; and calcite + Mg + illite → micas + feldspar + dolomite; also, with deep and late burial, clay minerals → chlorite or sericite, K feldspar → sericite, plagioclase → chlorite + chert. From the type of reaction and the principal sequence of occurrence three stages of diagenesis are established: Stage (1), redoxomorphic, which dominates the episodes of deposition and early burial. Reactions principally are reduction and oxidation which form the basis for the name of the stage. Stage (2), identified as locomorphic, involves replacement of one mineral by another, the two minerals not entering into mutual reaction. Such reactions are typical of the early burial stage and are an important part of the process of lithification. Stage (3), identified as phyllomorphic, follows the principal locomorphic replacements and involves development of micas, principally from clays. Among sandstones, the progress of diagenesis may be arrested at either the locomorphic or phyllomorphic stages, in more or less parallel fashion to metamorphic grades. Also, the phyllomorphic stage overlaps the zeolite and chlorite grades of metamorphism.

Origin and Classification of Chemical Sediments in Terms of pH and Oxidation-Reduction Potentials

Article

Jan 1952

Chemical sediments of marine origin are divided into three main classes, representing deposition in normal marine open-circulation environments, restricted humid (euxinic) environments, and restricted arid (evaporite) environments. The characteristics of these environments are briefly reviewed, and the position is taken that the hydrogen-ion concentration, pH, and the oxidation-reduction potential, Eh, afford two basic controls which largely determine the kinds of chemical end-members produced by both inorganic and biochemical reactions. Within the framework of these controls, the depositions of calcium carbonate, iron minerals, manganese minerals, phosphates, evaporites, and organic matter are shown in their relation to variations in pH and Eh of the environment. Certain of the end-members depend mainly upon one or the other of the two controls, and some depend upon both. A classification of chemical sediments is proposed which shows their relation to pH and Eh and serves to indicate genetic relations among the chemical end-members. Occurrences of chemical end-members are summarized on a pH-Eh graph, which shows the range of these values which normally occurs in the environments discussed. Observed mineral associations in typical chemical sediments are listed in support of the theoretical treatment. The influence of postdepositional changes on the original mineral associations is also pointed out. The writers conclude that the environment of deposition of many ancient chemical sediments can be reconstructed in terms of its essential physicochemical characteristics from study of the mineral assemblages among the chemical end-members present in the sediment.

Stable isotopes, chemistry and petrology of carbonate concretions (Mishash Formation, Israel)

Article

Dec 1972
CHEM GEOL

Carbonate concretions of variable sizes occur in the upper member of the Senonian (Upper Cretaceous) Mishash formation in Israel. Eight concretions and their surrounding country rocks were examined in the field, in thin sections and by X-rays. The isotopic composition of carbon and oxygen in the carbonates, the amount of acid insoluble residue, as well as the concentrations of P2O5, Ca, Mg, Sr and Fe were determined. Other concretions and country rocks were analyzed for oxygen and carbon isotopes only.The concretions are composed of almost pure, microsparitic calcite, whereas the country rocks are porcellanitic-phosphoritic chalks. Compared with the surrounding rocks, the concretions are strongly enriched in 12C and are depeleted in insoluble residue, P2O5, SiO2, Fe, Sr and Mg.It is postulated that the concretions were formed by addition of CaCO3 to sites of anaerobic decomposition of organic matter, while CaCO3 was mobilized in the surrounding sediments, in which aerobic decomposition of organic matter prevailed.Several consequences of this model are considered, concerning the quantitative volume changes, as well as the distribution of SiO2 and Mg between concretions and country rocks.

The formation of sedimentary Fe-minerals

Article

May 1968

An attempt to combine laboratory and field data on recent sedimentary environments to produce mineral stability diagrams suggests that two distinct situations may be recognized. In depositional water, only ferric compounds can be truly stable. In interstitial water, beneath the sediment-water interface, ferrous compounds (pyrite, pyrrhotite, magnetite, siderite, and chamosite) are stable. Anion activity (sulfide and carbonate) appears to be the most important chemical control. The theoretical findings (Pt. 1 by C. D. Curtis) are consistent with interpretation of petrographic observations on Carboniferous and Jurassic ores of the British Isles (Pt. 2 by D. A. Spears).

Diagenesis of Gulf Coast clayed sediments and its possible relation to petroleum migration

Article

Jan 1969
AAPG BULL

J. F. Burst

Sedimentary Geochemistry: Environments and Processes Dominated by Involvement of an Aqueous Phase [and Discussion]

Article

Sep 1977
PHILOS T R SOC A

Crust and mantle processes yield mineral assemblages which are not generally stable in Earth surface environments. Uplift to the zone of weathering therefore initiates chemical reactions which produce quite different assemblages. The precise nature of the resulting minerals depends much upon the composition of the aqueous phase present. Erosion, sedimentation and diagenesis move weathering products through a succession of chemical environments. Further solid-solution reactions occur at each stage. Until recently, the least-well documented part of this cycle was burial diagenesis. New information provided by oil companies from their submarine exploration programmes has done much to rectify the situation. It is now possible to present a fairly complete account of the important chemical reactions occurring at each stage of the surface cycle. The major conclusion to be drawn from this exercise is that reactions involving an aqueous phase play a very important part in geological evolution as a whole.

The Principle of Diagenetic Facies and Some of Its Implications

Article

Jul 1960

Chemical reorganization occurs during diagenesis as a response of a rock to its physicochemical environment. The mineral assemblages produced will be dependent on the environment and the materials present. Physicochemical factors of importance in diagenesis appear to be environmental pH-cation concentration, environmental Eh, pressure, and temperature. Mineral assemblages which can be expected to appear in given rocks under given conditions can be inferred from data derived from inherent properties of minerals, such as abrasion pH, from laboratory studies on mineral stability fields and hydrothermal alteration, from theoretical studies on mineral stabilities, from hydrothermal deposits, and from diagenetic depth sequences. These data suggest the possibility of erecting a sequence of facies, analogous to metamorphic facies, starting at the surface and passing down into the low-grade metamorphic facies. Each diagenetic facies can be defined as including all rocks or sedimentary detritus which have, by a pro...

Interstitial Water Composition and Geochemistry of Gulf Coast Deep Shales and Sandstones: ABSTRACT

Article

Jan 1971
AAPG BULL

Gene W. Schmidt

Interstitial water from shales and sandstones shows a marked contrast in concentration and composition. Sidewall core shales were taken every 500 ft between 3,000 and 14,000 ft from a well in Calcasieu Parish, Louisiana, which encountered abnormally high fluid pressures just below 10,000 ft. Significant differences between the concentrations of water from normally pressured sandstones (63,000-180,000 mg/l TDS) and high pressured sandstone (16,000-26,000 mg/l TDS) were noted. Shale pore water has a lower salinity than the water in the adjacent normally pressured sandstone, but the concentrations are more similar in the high pressure zone. Shale pore water generally has a concentration order of SO4- > HCO3- > CL-, whereas, water in normally pressured sandstone has a reversed concentration order. Water in high-pressured sandstone has low salinity with the HCO3- and SO4- concentrations being intermediate between the water from normally pressured sandstone and that from shale. Conversion from predominantly expandable to nonexpendable clays accelerates near the top of the high-pressure zone which appears correlative with a major temperature gradient change, an increase in shale porosity (decrease in shale density), a lithologic change to massive shale, an increase in shale conductivity, an increase in fluid pressure, and a decrease in the salinity of the interstitial waters. These differences and correlations may have a bearing on the processes which alter subsurface waters, cause electric log responses, and could allow an understanding of the diagenesis and migration of petroleum. End_of_Article - Last_Page 363------------

Mineralogy, Chemistry, and Origin of a Concretionary Siderite Sheet (Clay-Ironstone Band) in the Westphalian of Yorkshire

Article

Dec 1975

C. D. Curtis

Concretionary siderite horizons are quite common in massive clay sequences. One such horizon, from the Westphalian of Yorkshire, has been studied in detail. Two iron-rich carbonate minerals occur together although they cannot be distinguished in thin section on account of very fine grain size. One is much richer in magnesium (pistomesite) than the other (siderite). The latter is rela-tively rich in manganese and the heavier stable carbon isotope ¹³ C whereas the former carbonate is richer in calcium and ¹² C. The most important iron source is thought to have been hydrated iron oxides originating in soils. Much of the carbonate carbon started as organic molecules. The siderite appears to have formed earlier than the pistomesite. The stratiform character of these deposits appears to reflect siltier horizons in the mudstones, which presumably channelled pore water migration during compaction. This is probably why such carbonate horizons were formerly believed to be of simple sedimentary rather than diagenetic origin.

Diagenesis and the KAr Dating of Shales and Clay Minerals

Article

Jan 1974

EDWARD A. PERRY

Gulf Coast sediments were obtained from an oil well drilled in coastal Louisiana, and their diagenesis was documented by the mineralogic changes in mixed layer illite/smectite. Samples were selected to represent varying degrees of diagenesis and separated into four size ranges for K-Ar analysis; bulk shale samples were also analyzed. The measured K-Ar ages of the mixed layer illite/smectite become progressively younger with increasing depth of burial because of the diagenetic addition of potassium caused by conversion of smectite layers to illite layers. Detrital illite and feldspar are destroyed to provide potassium for the diagenetic illite layers, and the detrital phases have lowered ages because of the greater loss of radiogenic argon relative to potassium. As a result of both processes, the measured ages of whole shale also decrease with increasing diagenesis, and radiogenic argon is lost from the system.

In situ measurement of marine sediment density by gamma radiation

Article

Oct 1968
Deep Sea Res Oceanogr Abstr

K. Preiss

Isotopic evidence for source of diagenetic carbonates formed during burial of organic-rich sediments

Article

Sep 1977

Organic matter is modified by several processes operating at different depths during burial diagenesis: (1) sulphate reduction; (2) fermentation; (3) thermally-induced decarboxylation, and so on. CO2, one common product of each, can be distinguished by its carbon isotope composition: approximately (1) −25‰, (2) +15‰, (3) −20‰ relative to PDB. These values are preserved in diagenetic carbonates of the Upper Jurassic Kimmeridge Clay. Independent corroboration of the relative dominance of each process within specific depth intervals is given by the isotopic composition of incorporated oxygen which is temperature dependent (1) 0 to −2‰, (2) −1.5 to −5‰,(3) −3.5 to −7.0‰.

Siderite- and Calcite-bearing Concretionary Nodules in the Lias of Yorkshire

Article

Jun 1967
GEOL MAG

A. Hallam

Nodules containing siderite and calcite in the Yorkshire Lias have been studied quantitatively using petrographic, X-ray and chemical methods. Ankerite is recorded from the Lias for the first time. It is shown how the relative abundance of early diagenetic siderite and calcite correlates with sandiness of the sediment and provides an index of environmental change.

Stable Carbon Isotope Ratios within Carbonate Concretions: a Clue to Place and Time of Formation

Article

Jan 1972
NATURE

NATURALLY occurring carbon shows considerable variability with respect to its stable isotope composition. The 13C/12C ratio in plant material (and in ancient residues) is generally roughly 2.5% lower than in the carbonate of marine limestones as a result of fractionation during photosynthesis. A summary of carbon isotope distribution and fractionation is given by Degens1.

The distribution of clay minerals in the ocean

Article

Jun 2006
SEDIMENTOLOGY

In order to obtain data comparable to those of Biscaye (1965) on the Atlantic Ocean, the authors carried out quantitative X-ray determinations of clay minerals in the sediments of the Indian and Pacific oceans using the Biscayemethod.

The growth of Cambrian and Liassic Concretions

Article

Dec 1971
SEDIMENTOLOGY

Robert Raiswell

Concretions from Upper Cambrian sediments in South Wales can be divided into two groups of different ages. An early group (type) are characterised by the association of septarian structures and deformed laminae; whilst later (type Ha) concretions developed both cone-in-cone structures and parallel laminae. Determinations of the cement content of individual concretions revealed centrifugal variations related to the porosity changes within the sediment during concretionary growth. Growth of type-I concretions began in sediment of approx. 70% porosity and continued until compaction had reduced the porosity to less than 40%. Later, type-IIa concretions began growth in sediment of approx. 30–40% porosity and experienced only slight compaction over their growth spans. Liassic concretions from a prolific concretionary horizon in Dorset developed parallel laminae, but grew early in the absence of compaction (type IIIb) concretions at this horizon grew within the sediment during a pause in deposition. Evidence is described for distinguishing between concretions formed in systems open or closed to seawater. It is concluded that Cambrian concretions grew in closed systems, but the early Liassic concretions may have developed in a partially open system. A model is offered to explain the closed system growth of prolific concretionary horizons. The Cambrian concretions often proliferated along 2–3 cm thick siltstone units, which it is suggested were more permeable than the surrounding mudstones and therefore became pore-water migration paths. Along these migration paths concretionary growth may have relieved the supersaturation of mobile pore waters.

Geochemistry and halmyrolysis of clay minerals, Rio Ameca, Mexico

Article

Aug 1970
GEOCHIM COSMOCHIM AC

Kenneth L. Russell

Clay minerals from the Rio Ameca, Mexico, were vised for a study of the chemical interactions between clay minerals and sea water. Montmorillonite and kaolinite/halloysite are the common clay minerals with some amorphous material and illite also present. Clay minerals from the shallow portions of the bay near the river mouth seem to be only detrital material with no visible structural diagenesis. This clay and clay filtered from the river water were analyzed for major cations. The number of equivalents of cations is the same in river clay as in marine clay. This disproves any rapid reaction between the clay and sea water to remove cation alkalinity.

Interstitial solutions and diagenesis in deeply buried marine sediments: Results from the Deep Sea Drilling Project

Article

Feb 1975
GEOCHIM COSMOCHIM AC

Through the Deep Sea Drilling Project samples of interstitial solutions of deeply buried marine sediments throughout the World Ocean have been obtained and analyzed. The studies have shown that in all but the most slowly deposited sediments pore fluids exhibit changes in composition upon burial. These changes can be grouped into a few consistent patterns that facilitate identification of the diagenetic reactions occurring in the sediments.Pelagic clays and slowly deposited (<1 cm/103yr) biogenic sediments are the only types that exhibit little evidence of reaction in the pore waters.In most biogenic sediments sea water undergoes considerable alteration. In sediments deposited at rates up to a few cm/103 yr the changes chiefly involve gains of Ca2+ and Sr2+ and losses of Mg2+ which balance the Ca2+ enrichment. The Ca-Mg substitution may often reach 30 mM/kg while Sr2+ may be enriched 15-fold over sea water. These changes reflect recrystallization of biogenic calcite and the substitution of Mg2+ for Ca2+ during this reaction. The Ca-Mg-carbonate formed is most likely a dolomitic phase. A related but more complex pattern is found in carbonate sediments deposited at somewhat greater rates. Ca2+ and Sr2+ enrichment is again characteristic, but Mg2+ losses exceed Ca2+ gains with the excess being balanced by SO4post staggered2− losses. The data indicate that the reactions are similar to those noted above, except that the Ca2+ released is not kept in solution but is precipitated by the HCO3post staggered− produced in SO4post staggered2− reduction. In both these types of pore waters Na+ is usually conservative, but K+ depletions are frequent.In several partly consolidated sediment sections approaching igneous basement contact, very marked interstitial calcium enrichment has been found (to 5.5 g/kg). These phenomena are marked by pronounced depletion in Na+, Si and CO2, and slight enhancement in Cl−. The changes are attributed to exchange of Na+ for Ca2+ in silicate minerals forming from submarine weathering of igneous rocks such as basalts. Water is also consumed in these reactions, accounting for minor increases in total interstitial salinity.Terrigenous, organic-rich sediments deposited rapidly along continental margins also exhibit significant evidences of alteration. Microbial reactions involving organic matter lead to complete removal of SO4post staggered2−, strong HCO3post staggered− enrichment, formation of NH4post staggered+, and methane synthesis from H2 and CO2 once SO4post staggered2− is eliminated. K+ and often Na+ (slightly) are depleted in the interstitial waters. Ca2+ depletion may occur owing to precipitation of CaCO3. In most cases interstitial Cl− remains relatively constant, but increases are noted over evaporitic strata, and decreases in interstitial Cl− are observed in some sediments adjacent to continents.

Changes in dissolved sulfate, calcium and carbonate from interstitial water of near-shore sediments

Article

Oct 1968
GEOCHIM COSMOCHIM AC

Dissolved trace metal concentration, sulfate, carbonate and calcium were determined in the interstitial water expressed from two marine cores taken in the borderland off Southern California. C13/C12 ratios were measured for both the total dissolved carbonate species, and for the solid carbonate in the sediment. One core consisted of oxidizing sediment from the San Pedro Basin (SP), the other came from the Santa Catalina Basin (SC) where reducing sediments are overlain by about two meters of oxidizing sediment.Total dissolved carbon dioxide increased in both cores, reaching a maximum of 34·4 mM in SP and 12·3 mM in SC. δC13 of the dissolved carbonate decreased from —1·0%. at the surface of SP to−21·2%. at 500 cm depth; in SC the decrease was from − 6·0%. at the surface to −19·3%. at 500 cm. In the SP core dissolved calcium decreased from 10·4 mM at the surface to 3·7 mM at 500 cm depth, while sulfate decreased from 6 mM to 0·1 mM. There was no marked decrease in calcium or sulfate with depth in Santa Catalina.The results indicate that the increase in dissolved carbonate with depth arises largely from metabolic activity, rather than solution of solid carbonates. Calculations based on three different approaches indicate that surface interstitial water is undersaturated in calcium carbonate compared to surface sea water, but below 2 or 3 m, the interstitial water becomes supersaturated. It is suggested that solution of calcium carbonate under low pH, below the sediment-water interface, may result in transport of calcium from the sediment column into overlying sea water.

Early diagenesis in a reducing fjord, Saanich Inlet, British Columbia—I. chemical and isotopic changes in major components of interstitial water

Article

Sep 1972
GEOCHIM COSMOCHIM AC

Water and interstitial water from the reducing fjord of Saanich Inlet, British Columbia were analyzed for their major element composition, ammonia, phosphate and silica contents, and for stable isotope composition of sulfur and carbon species. Ca was the only major element to show a significant change with depth (a 75 per cent decrease in some cases). Ammonia and phosphate are highly enriched in the interstitial water (I.W.), concentrations reaching 250 ppm and 39 ppm, respectively. Total dissolved CO2 in I.W. increases strongly with depth (20 to 30 times that in overlying sea-water) and it becomes enriched in C13 (δC13PDB ≈ + 17.8%.). Both sulfate and dissolved sulfide decrease with depth to a complete disappearance of all sulfur species from the interstitial water. The dissolved sulfide is highly enriched in S34 (δS34 ≈ + 18%.). All these changes are attributed to strong biological activity in the sediments.

Sedimentary pyrite formation: An update

Article

Apr 1984
GEOCHIM COSMOCHIM AC

Robert A. Berner

Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H2S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals.Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.

The Natural History Of Crystalline Calcium Carbonate: Effect of Magnesium Content And Salinity

Article

Jan 1974

Robert L. Folk

Morphology of calcium carbonate crystals is controlled mainly by rate of crystallization, and Mg and Na content of the precipitating waters. Together, these factors integrate to provide important indicators of environment. Magnesium selectively poisons sideward growth of calcite; thus CaCO 3 prefers to crystallize as aragonite, or as minute fibers or steep rhombs of magnesian-calcite, whose sidewise growth is generally stopped at widths of a few microns. Thus in Mg-rich environments, such as beaches or marine bottoms, micritic or fibrous aragonite and magnesian calcite cements form. As seawater is buried, Na usually remains high but Mg is selectively lost so that the Mg/Ca ratio drops from 3:1 to about 1:3. Thus, in the absence of Mg-poisoning, coarse sparry calcite cement can form in the subsurface, and crystallizes as irregular polyhedra. In meteoric waters, both Mg and Na are very low. If precipitation is very rapid, calcite micrite may form (caliche). Fresh-water calcite can also occur as euhedral rhombs in very dilute solutions. In the phreatic-meteoric zone, sparry calcite develops. Carbonate ooze initially contains much Mg. Upon lithification, it is proposed that much of the Mg is retained as a sort of "cage" around each polyhedron of calcite, preventing growth beyond a few microns. Fresh-water flushing removes this Mg-cage, and allows recrystallization to coarser microspar.

Diagenesis in sediments: De-velopments in Sedimentology Stability of Mg-Ca carbonates

Jan 1977
265-70

Diagenesis Of
Amsterdam
R B De Boer

Diagenesis of sandstones. In: LARSEN, G. & CHILINGAR, G. V. (eds) Diagenesis in sediments: De-velopments in Sedimentology 8, Elsevier, Amsterdam. DE BOER, R. B. 1977. Stability of Mg-Ca carbonates. Geochim. cosmochim. Acta 41, 265-70.

Reservoir pressures in Gulf Coast Louisiana

Jan 1953
410

G Dickinson
Dickinson G.

DICKINSON, G. 1953. Reservoir pressures in Gulf Coast Louisiana. Bull. Am. Ass. Petrol. Geol. 37, 410-32.

Chemical weathering in a subtropical igneous terrain Early diagenesis of clay minerals

Jan 1971
951-61

J I Drever

DREVER, J. I. 1971a. Chemical weathering in a subtropical igneous terrain, Rio Ameca, Mexico. J. sedim. Petrol. 41, 951-61. 1971b. Early diagenesis of clay minerals. Rio Ameca Basin, Mexico. J. sedim. Petrol. 41, 982-94.

Phases of diagenesis and au-thigenesis Diagenesis in sediments: Developments in Sedimentology 8

Jan 1967

R W Fairbridge

FAIRBRIDGE, R. W. 1967. Phases of diagenesis and au-thigenesis. In: LARSEN, G. & CHILIGAR, G. V. (eds) Diagenesis in sediments: Developments in Sedimentology 8, Elsevier, Amsterdam.

Variation in the HALLAM, A. 1967. Siderite-and calcite-bearing concretio-nary nodules in the Lias of Yorkshire

Jan 1968
222-7

E M Galimov
Yu P Girin

GALIMOV, E. M. & GIRIN, YU. P. 1968. Variation in the HALLAM, A. 1967. Siderite-and calcite-bearing concretio-nary nodules in the Lias of Yorkshire. Geol. Mag. 104, 222-7.

Isotopic evidence for several sources of carbonate and distinctive diagenetic processes in organic-rich Kimmeridgian sedi-ments

Jan 1977
209-13

H Irwin
M Coleman
C D Curtis

IRWIN, H., COLEMAN, M. t~ CURTIS, C. D. 1977. Isotopic evidence for several sources of carbonate and distinctive diagenetic processes in organic-rich Kimmeridgian sedi-ments. Nature, Lond. 269, 209-13.

Possible links between sandstone diagenesis and depth-related geochemical reactions occurring in enclosing mudstones

Abstract

No full-text available

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