Quartz Cementation in Sedimentary Basins

Influence of hydrothermal silicification on the physical properties of a basin-boundary fault affecting arkosic porous sandstones, Rio do Peixe Basin, Brazil

Article

Dec 2022
MAR PETROL GEOL

Hydrothermal silicification and deformation bands influence the physical properties of porous siliciclastic rocks. However, the impacts generated by the coexistence of these two processes on fault zone flow properties, such as porosity and permeability reductions, are still debatable. We integrated structural, geomechanical, and petro-physical data to investigate the influence of hydrothermal silicification on the physical properties of a fault zone. The fault affects Precambrian crystalline rocks and porous sandstones in the Rio do Peixe Basin, northeastern Brazil. The results indicate that quartz cementation is confined to a halo along the fault. Silicification decreases away from the fault zone toward the basin, which we subdivided into three main zones: (1) nonsilicified sandstone Zone I, (2) moderately silicified sandstone-Zone II, and (3) intensely silicified sandstone-Zone III. The elongated geometry, the thickness of the silicified body, the proximity to the fault, and the high silica cement concentration indicate an external silica source. Nevertheless, we also propose internal sources related to feldspar dissolution. The primary porosity is rarely preserved in Zone III, while the secondary porosity is filled with silica cement. The primary sandstone porosity is well-preserved in Zone II, while many dissolved feldspar grains are not filled with cement. The petrographic and petrophysical analysis (cataclastic matrix amount) indicates that cement precipitation also occurred inside the deformation bands. Silicification increased the uniaxial compressive strength (UCS) by one order of magnitude in the damage zone relative to the protolith. On the other hand, the porosity and permeability in the silicified fault zone exhibit a reduction of two and four orders of magnitude , respectively, relative to the undeformed and nonsilicified protolith. This study indicates that silicification along a fault zone has crucial implications for the reservoir properties of siliciclastic rocks, strongly reducing the fluid flow properties and increasing the UCS values.

Identification and quantitative evaluation of pores and throats of a tight sandstone reservoir (Upper Triassic Xujiahe Formation, Sichuan Basin, China)

Article

Mar 2022
MAR PETROL GEOL

Pores and throats are the key parameters for determining the pore structure and fluid flow behavior in tight sandstone reservoirs. In this paper, pores and throats of the tight sandstone reservoir of the Upper Triassic Xujiahe Formation in the northern Sichuan Basin were investigated using classical petrography, scanning electron microscopy, and confocal laser scanning microscopy. In addition, a method of pore as well as throat identification and quantitatification using high-pressure mercury injection (HPMI) measurements was worked out. The results show that clay mineral-related pores are well developed in lithic sandstone and feldspathic lithic sandstone in addition to pores produced by dissolution of calcite cement and clastic grains. Intergranular pores are connected by lamellar, curved lamellar, and narrow pore throats. The pores and throats associated with intragranular dissolution and clay minerals are mostly tube shaped. The rapid-increasing and slow-increasing profiles of the mercury injection curves represent the dominance of pores and throats, respectively, allowing to differentiate pores and throats. Calculations show, in addition to fractured lithic sandstone, that has micron-sized pore–throat systems, the other lithic sandstones possess dominatly nano-sized pore–throat systems. In case of feldspathic lithic sandstones, micron-sized and nano-sized pore throat systems are equally important. In micron-sized pore throat dominated reservoirs, the pore volume is slightly larger than the throat volume, whereas for the nano-sized pore throat system, the pore volume is considerably larger than the throat volume. The key factors that are favorable for the formation of lithic sandstone pore throat systems are early calcite dissolution, lithic fragments dissolution, and early chlorite cementation. In contrast the key factors that are favorable for the formation of feldspathic lithic sandstone pore throat systems are dissolution of feldspars and lithic fragments.

The genesis of authigenic minerals and the porosity evolution of various lithologies and their implications for identifying high-quality reservoirs in the fourth member of Xujiahe Formation (Northeast-Sichuan Basin, China)

Article

May 2022
J PETROL SCI ENG

With petroleum exploration extending to deeper reservoirs, it becomes more and more difficult to find relatively high-quality reservoirs in heterogeneous sandstones. This research integrates fluid inclusion analysis, petrographic, mineralogical, and geochemical data to determine the origin of authigenic minerals, establish the porosity evolution model and discuss diagenetic effects on reservoir quality of the fourth member of the Upper Triassic Xujiahe Formation. The main rock types of studied sandstones are identified by thin-section, SEM and XRD analysis-feldspathic litharenites, litharenites, calcareous sandstone and chlorite-rich sandstone. Of the four types of sandstone, reservoir porosity of the chlorite-rich sandstone is generally the highest. Target sandstones have experienced complicated diagenetic histories including mechanical compaction, three stages of carbonate cementation, the dissolution of K-feldspar or rock fragments, three kinds of authigenic chlorite precipitation, and two stages of quartz precipitation. Two reaction pathways between K-feldspar and kaolinite were determined and limited dissolution was generated in the closed diagenetic system. Diagenetic sequence and porosity evolution suggest that the critical factors in reducing reservoir quality can be attributed to early intense compaction and widespread calcite cementation. The genesis of high-quality reservoirs among the various lithologies is different. In feldspathic litharenites and litharenites, the key factors for the development of relatively high-quality reservoirs is the acidic dissolution of K-feldspar and less cement being present in the rock. Because chlorite helps retain primary pores by inhibiting quartz overgrowths, almost all chlorite-rich sandstone are relatively high-quality reservoirs. In general, the reservoir quality of calcareous sandstone is poor unless fractures exist that can improve permeability. Determining the porosity evolution of various lithologies is an effective method to predict relatively high-quality reservoirs, which can provide a useful reference for deep petroleum exploration in other areas.

Source of quartz cement in tight gas sandstone: Evidence from the Upper Triassic Xujiahe Formation in the western Sichuan Basin, SW China

Article

May 2022
J PETROL SCI ENG

Quartz cement is a significant authigenic mineral in the tight gas sandstones of the Upper Triassic Xujiahe Formation, fourth member (Xu4 sandstones) in the western Sichuan Basin. Here we report mineralogical, petrological and geochemical data from the Xu4 sandstones to constrain the silica sources of their quartz cements. The quartz cements mainly occur as quartz grain overgrowths and pore-filling quartz cement, with major associated authigenic minerals including calcite, dolomite, illite, kaolinite, and chlorite. The homogenization temperatures of quartz cement fluid inclusions range from 49.2 °C to 143.4 °C and show a multistage continuous precipitation process. Raman spectrometry results show the presence of methane (CH4) and CO2 in quartz cement fluid inclusions. The dissolution of feldspar, the illitization of kaolinite and smectite, as well as the pressure dissolution of quartz grains provided silica sources for quartz cement. The dissolution of feldspar by organic acid (carboxylic acid) is the main silica source for quartz cement in the 80–120 °C interval; the illitization of smectite and kaolinite have different initial temperatures, and both consume potassium. The pressure dissolution of quartz grains results in stylolite formation. The low-temperature quartz cement (<70 °C) is mainly derived from the dissolution of feldspar by meteoric water. At the Late Triassic/Jurassic transition, the uplift of the western basin led to leaching by meteoric water in the upper part of the Xu4 sandstone, forming a weakly acidic fluid with a low K⁺/H⁺ ratio and giving this type of sandstone low potassium and sodium concentrations.

The influence of sedimentary facies, mineralogy, and diagenesis on reservoir properties of the coal-bearing Upper Carboniferous of NW Germany

Article

Jan 2024

Former coal mines hosted in Upper Carboniferous silt- and sandstones in the Ruhr Basin, NW Germany, are currently examined for post-mining applications (e.g., geothermal energy) and are also important tight-gas reservoir analogs. Core material from well Pelkum-1, comprising Westphalian A (Bashkirian) delta deposits, was studied. The sandstones and siltstones are generally tight (mean porosity 5.5 %; mean permeability 0.26 mD). Poor reservoir properties primarily result from pronounced mechanical compaction (mean COPL 38.8 %) due to deep burial and high contents of ductile rock fragments. Better reservoir properties in sandstones (> 8 %; > 0.01 mD) are due to (1) lower volumes of ductile grains (< 38 %) that deform during mechanical compaction and (2) higher volumes in feldspar and unstable rock fragments. During burial these form secondary porosity (> 1.5 %) resulting from acidic pore water from organic matter maturation. Still, sandstones with enhanced porosities only show a small increase in permeability since authigenic clays (i.e., kaolinite and illite) or late diagenetic carbonates (i.e., siderite and ferroan dolomite/ankerite) clog secondary porosity. Quartz cementation has a minor impact on reservoir properties. Evaluating the Si/Al ratio can be a suitable proxy to assess grain sizes and may be a convenient tool for further exploration. Supplementary material: https://doi.org/10.6084/m9.figshare.c.7003156

Alteration of chlorite coats and sandstone porosity reduction: Insights from reactive transport modeling

Article

Dec 2023
MAR PETROL GEOL

Chlorite coats are widely recognized as a key element in preserving sandstone porosity because it can inhibit the growth of quartz cements. However, the alteration of chlorite coats and its potential influences on sandstone porosity are rarely discussed. Therefore, this work used reactive transport models under different petrographic and geochemical conditions to investigate the influence of chlorite coats on sandstone porosity in a major dissolution window (100 °C). The HCO3-rich (CO2-charged) and HCO3-depleted (organic acids-charged) waters were injected to induce mineral dissolution and precipitation. The results indicate that the alteration of chlorite coats may result in sandstone porosity reduction. The HCO3-rich water leads to a porosity decrease mainly through the precipitation of magnesite and siderite resulting from chlorite dissolution. In contrast, the HCO3-depleted water causes a porosity decrease mainly through the redistribution of kaolinite and quartz cements. Factors, including pCO2, organic acid concentration, coat coverage, coat thickness, and grain size, have secondary influences on net porosity change. In comparison, factors, including chlorite mineralogy, detrital lithology, and the reduction of K-feldspar dissolution rate caused by chlorite coat, have negligible influences. The alteration of chlorite coats may introduce significant mis-interpretation to the analysis of the relationship between chlorite coats and sedimentary facies. Moreover, the actual impact of pore-filling chlorite on porosity reduction may be either underestimated or overestimated. Therefore, the alteration of chlorite coats should be taken into consideration in future studies.

PETROGRAPHY OF PYROBITUMENS IN MIDDLE – LATE JURASSIC SANDSTONES FROM THERMALLY DEGRADED HYDROCARBON ACCUMULATIONS, EAST GREENLAND

Article

Full-text available

Sep 2023

A number of exhumed hydrocarbon traps have been described from the Traill Ø region of East Greenland. This study focuses on the Bjørnedal area where the distribution of bitumen has been mapped out. Bitumen staining clearly has a cross‐cutting relationship to stratigraphic units and can be shown to form distinct palaeo‐accumulations. Detailed petrographic studies show that bitumen occurs as late diagenetic phases in Middle to Late Jurassic sandstones, and is present both as both grain‐coating and pore‐filling phases. Geochemical analyses confirm that the bitumen is organic in composition and is composed largely of carbon and hydrogen. Both H/C ratios and bonds identified by FTIR behave as expected with increasing maturity measured using bitumen reflectance. Together, these results provide strong evidence that the material is pyrobitumen derived from the in situ thermal degradation of a liquid hydrocarbon precursor. On the basis of textures in transmitted and reflected light and quantitative bitumen reflectance distributions, two populations of pyrobitumen may be recognised in some samples. Two phases of Paleogene magmatism occurred in the Traill Ø region. The first late Paleocene – early Eocene phase was related to the opening of the northern North Atlantic in the earliest Eocene, and was experienced throughout East Greenland and the northwest European margin. The later magmatic phase was related to the gradual separation of the Jan Mayen microcontinent from East Greenland through the late Eocene – early Oligocene. A single pyrobitumen phase is recognised in accumulations only affected by the early magmatism, and a second phase is only observed in areas affected by both the early and later magmatism. This relationship is interpreted as evidence for a direct relationship between magmatic phases and bitumen generation. The presence of bitumen formed by the thermal degradation of liquid hydrocarbons during the later magmatic event suggests that a viable petroleum system remained active following the early magmatic event.

Diagenesis, facies and palaeocurrent analysis of Upper Rewa Sandstone around Sagar, Central India

Article

Full-text available

Jul 2023

The stratigraphic surface represented by a major contact between the Archean BundelkhandGranite and the extensive Proterozoic Vindhyan sediments is a regional basement cover unconformity. Thiscrystallineesedimentary interface reflects an intense weathering of continental crust during marine trans-gression. Three time-transgressive sand deposition events viz. Kaimur, Rewa and Bhander are mainly witnessedduring the entire Vindhyan sedimentation. Stratigraphically, the Upper Rewa Sandstone comprises one of theseevents in the Vindhyan Basin. Considerable progress has been made in interpreting these sandstones as afunction of entirely marine process to a combination of tidalefluvioeeolian activities. All the results have sofar been attained on the basis of sedimentary facies, provenance, palaeocurrent analysis, and some ofpetrography. A distinct differentiation between marine and fluvial components of the Upper Rewa Sandstonestill remains uncertain. Here, we use diagenesis as a parameter for the first time along with facies andpalaeocurrent analysis to acquire a clear comprehension of marine- and fluvial-dominated processes. Thepresent study spans 27 square kilometer area covering 15 stratigraphic sections with a collection of 571directional data from the facies specific sedimentary structures, and 28 samples obtained for the thin sectionanalysis.The lower unit of the Upper Rewa Sandstone shows facies association typical of tidal environment, alongwith polymodal palaeocurrent. These rocks are well sorted and dominantly lithified by authigenic cement.Profuse development of a complex of syntaxial, passive pore fills, and grain replacive cements reflect normallypressured nature of the lower unit sandstones. The upper unit of the Upper Rewa Sandstone however, reveals afacies pattern resembling fluvial processes and unimodal palaeocurrent plot. They exhibit poor sorting, sili-ceous, and ferruginous matrix, and development of intense stylolites. Since, pressure solution is a dominantmode of lithification; the upper unit rocks therefore, are overpressured. Sand deposition event pertaining tothe Upper Rewa Sandstone is found to comprise both marine and fluvial processes. Such a diagenesis basedapproach can be applied for marking a possible facies based unconformity between genetically differentdepositional units. (PDF) Diagenesis, facies and palaeocurrent analysis of Upper Rewa Sandstone around Sagar, Central India. Available from: https://www.researchgate.net/publication/372728855_Diagenesis_facies_and_palaeocurrent_analysis_of_Upper_Rewa_Sandstone_around_Sagar_Central_India [accessed May 07 2024].

Effects of overpressure on deep sandstone reservoir quality: A case study of the Medium and Lower Jurassic formation in the Shawan Sag, central Junggar Basin, western China

Article

Jul 2023

Diagenetic Evolution Control on Reservoir Quality of the Oligocene Weizhou Formation in Weixinan Sag of Beibuwan Basin, Northern South China Sea

Article

Full-text available

Jul 2023

This study focuses on the diagenetic evolution of the reservoir in the third member of the Oligocene Weizhou Formation (E3w3) in Weinan Sag of Beibuwan Basin, northern South China Sea. Based on the contents and occurrence states of chlorite, illite, kaolinite and quartz cement were described by scanning electron microscopy (SEM), thin section and X-ray diffraction. In the E3w3 reservoir, three different occurrences of chlorite are recognized. These include chlorite coating, leaf-shaped chlorite and tongue-shaped chlorite. Furthermore, three major types of diagenetic kaolinite are observed. These include kaolin crystallization promoted by fluids of meteoric origin, kaolin formed by organic acids and blocky kaolin. In addition, feldspar, by organic acid dissolution, can mainly form kaolinite but also form illite or chlorite in the E3w3 reservoir. Different clay minerals have different effects on reservoir quality, which are controlled by occurrence and diagenetic environment. Compared to the other minerals, chlorite coating has a greater effect on quartz overgrowth. However, If minerals are packed in intergranular pores, they have a negative effect on reservoir quality.

Effects of Quartz Precipitation on the Abundance and Preservation of Organic Matter Pores in Cambrian Marine Shale in South China

Article

Full-text available

Jun 2023

To evaluate the effects of quartz precipitation on the abundance and preservation of organic matter pores in marine shale reservoirs, the type of authigenic quartz and the source of silica, as well as the corresponding relation of the Lower Cambrian Shuijingtuo Formation shale in South China were investigated. Quartz in the Shuijingtuo shale occurs as four different types: detrital quartz, replacement of biosiliceous debris, euhedral quartz filled in interparticle pores, and microquartz dispersed in a clay matrix. Euhedral quartz (1–5 μm) and matrix-dispersed microquartz (100–400 nm) are found to be the dominant forms of authigenic quartz. The euhedral quartz accumulates along the interparticle pores, and the porous organic matter fills the interior of the space. Microquartz is mainly wrapped in porous organic matter. Two silica sources were revealed: biogenic silica and clay-derived silica. Biogenic Si is most likely the major source for authigenic quartz in the organic-rich (total organic carbon (TOC) > 2.55 wt.%) samples, which accounts for 23–57 wt.% (average 35 wt.%) of the total Si. Based on petrographic observations, we posit that the precipitation of large-sized euhedral quartz in the interparticle pores most likely originated from biogenic silica in the early stage of diagenesis and that the silica for the clay matrix-dispersed microquartz is provided by biogenic silica and clay-derived silica. The observation of SEM images indicates that the precipitation of early diagenetic euhedral quartz in the interparticle pores enhances rock stiffness, and the buttressing effect can protect the organic matter pores from compaction during the late-stage burial diagenesis. In contrast, the precipitation of late diagenetic microquartz in the clay matrix can lead to a reduction in the capacity of the accommodation space to host retained petroleum, consequently leading to a reduction in the development of organic matter pores and the generation of shale gas.

Impact of coal evolution on formation of tight sandstone reservoirs: A case study in the Daniudi gas field, Ordos Basin, China

Article

Full-text available

Sep 2023
ENERG EXPLOR EXPLOIT

Tight sandstone gas in coal-bearing strata has become an important unconventional gas worldwide. This research aims at understanding the impacts of coal evolution on diagenesis and reservoir quality of sandstones by means of thin section, cathode luminescence (CL), scanning electron microscope (SEM), stable isotope analysis, and fluid inclusions. The results show that carbonate cement was the main type of cement developed in tight sandstone reservoirs, of which the carbonate ions were jointly provided by coal layers and reaction in adjacent mudstones. For sandstones close to coal layers, their carbonate ions were mainly controlled by coal evolution and there was various carbonate cement developed due to cations supplied by seawater and the intense dissolution of volcanic fragments and feldspars. Combined with coal thermal simulation and burial history, it implies that at different maturity stages of coal evolution, its impact on sandstone diagenesis was different. In the early stage, the main product was CO 2 , which caused the dissolution of early calcite and promoted intense compaction. In the mature stage, CO 2 and other hydrocarbon gases were released, resulting in more dissolution and silica cementation. In the over-mature stage, the main product was methane, and CO 2 that did not spill out preserved by forming carbonate cement. The coal evolution had a strong impact on the reservoir quality of sandstone with large grain sizes, but little effect on medium- and fine-grained sandstones. High porosity and low permeability reservoirs were usually formed in small conglomerates and coarse-grained sandstones away from coal layers. While low porosity and high permeability reservoirs were distributed close to coal layers. This study aids in understanding the formation mechanism of tight sandstone reservoirs in coal measures and provides theoretic support for further exploration.

Article

May 2023

Recently, the realm of hydrocarbon exploration in China has been expanded to deep tight oil and gas from the Lower Jurassic sandstone reservoirs in the central Junggar Basin. These deeply-buried sandstones (>4.5 km) have been generally overpressured due to hydrocarbon saturation, but have not been intensively investigated in terms of diagenesis and predicting reservoir quality. The Lower Jurassic sandstones are feldspathic litharenites (average Q 28 F 29 R 43 ) and were deposited in the lacustrine-deltaic environments of the high-paleolatitude continental basin under warm, humid climatic conditions. Progressive burial with only slight uplift of the strata led to a relatively stable burial-thermal regime, resulting in minor changes in diagenetic modifications consisting of compaction, cementation by carbonates, quartz and clay minerals, and partial dissolution of feldspars and rock fragments. Despite generally similar diagenetic histories, the studied sandstones reveal considerable variations in the development of pore systems along with values of porosity and permeability. The best reservoirs are represented by medium- to coarse-grained depositional facies, having relatively higher porosity (average 8.43%) and permeability (average 0.59 millidarcys). All the evidence suggests that integrated considerations of the roles of depositional settings and diagenesis when linked with burial-thermal history and overpressure evolution conditions, provide a helpful means of predicting reservoir quality. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6617980

Integration of charging time, migration pathways and sealing analysis to understand hydrocarbon accumulation in complex fault blocks, the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin

Article

Mar 2023
MAR PETROL GEOL

A large number of oil and gas fields have been successfully discovered in complex fault blocks worldwide. However, the role played by the fault-rock seal in the hanging wall traps is not fully understood due to the paucity of datasets. The trap examples of the Pinghu Slope Belt are used to analyze the migration and accumulation of hydrocarbons in the complex fault blocks, as well as the fault-sealing mechanism of the hanging wall traps. The burial history in typical wells, combined with the homogenization temperature of fluid inclusions, is used to determine the hydrocarbon charging time. Based on detailed 3D seismic interpretation, the paleo-structural features at critical moments and their control on hydrocarbon migration were discussed. Hydrocarbon accumulation characteristics were illustrated based on triangle juxtaposition diagrams. The result shows that there are two hydrocarbon charging episodes in the study area, respectively, 10.6–8.1 Ma and 2.8 Ma-present, with the latter being predominant. This phenomenon that the homogenization temperatures of inclusions are generally higher than the measured formation temperatures at the same depth is attributed to late fluid accumulation and not reaching thermal equilibrium with the reservoir fluids. The analysis of hydrocarbon generation history, trap fixation history and fluid inclusions indicate that the critical moment of the generation-migration-accumulation of most hydrocarbons ranges from 2.8 Ma to the present. Due to the difficulty in effectively recognizing the reflector (the top of the Santan Fm) and the nearly horizontal sediments above T20, the reflector T20 (5 Ma) near the top of the Santan Fm was chosen as an approximate proxy for this critical time (2.8 Ma). The paleo-structures of the Pinghu Slope Belt at 5 Ma are a gentle southeast-dipping slope. Based on the numerical simulation of three different fault scenarios (no faults, closed faults and open faults), wells A-H are located on the preferential migration pathways. Fault seal analysis shows that multiple juxtaposition types on the fault surface and moderate-potential to effective SGR values at the sandstone reservoirs in the middle and lower Pinghu Formation. Lateral and vertical reservoir compartmentalizations are presented in clean sand-sand juxtapositions with low-potential sealing (SGR<20%) in well A. The SGR-buoyancy pressure relationship from hanging wall traps and the global calibration plots from footwall traps have a consistent trend. The fault sealing mechanism of hanging wall traps is similar to that of the footwall traps. Therefore， those global calibration plots can be simultaneously applied to the fault seal evaluation of both hanging wall traps and footwall traps.

The importance of facies, grain size, and clay content in controlling fluvial reservoir quality – An example from the Triassic Skagerrak Formation, Central North Sea, UK

Article

Full-text available

Feb 2023

Clay-coated grains play an important role in preserving reservoir quality in high-pressure high-temperature (HPHT) sandstone reservoirs. Previous studies have shown that the completeness of coverage of clay coats effectively inhibits quartz cementation. However, the main factors controlling the extent of coverage remain controversial. This research sheds light on the influence of different depositional processes and hydrodynamics on clay coat coverage and reservoir quality evolution. Detailed petrographic analysis of core samples from the Triassic fluvial Skagerrak Formation, Central North Sea, identified that channel facies offer the best reservoir quality; however, this varies as a function of depositional energy, grain size and clay content. Due to their coarser grain size and lower clay content, high energy channel sandstones have higher permeabilities (100-1150 mD) than low energy channel sandstones (<100 mD). Porosity is preserved due to grain-coating clays, with clay coat coverage correlating with grain size, clay coat volume and quartz cement. Higher coverage (70-98%) occurs in finer-grained, low energy channel sandstones. In contrast, lower coverage (<50%) occurs in coarser-grained, high energy channel sandstones. Quartz cement modelling showed a clear correlation between available quartz surface area and quartz cement volume. Although high energy channel sandstones have better reservoir quality, they present moderate quartz overgrowths due to lesser coat coverage, thus prone to allowing further quartz cementation and porosity loss in ultra-deep HPHT settings. Conversely, low energy channel sandstones containing moderate amounts of clay occurring as clay coats are more likely to preserve porosity in ultra-deep HPHT settings and form viable reservoirs for exploration. Supplementary material: of data and technique used in this study are available at https://doi.org/10.6084/m9.figshare.c.6438450.v1

TRANSGRESSIVE-REGRESSIVE CYCLES IN PERMIAN BARREN MEASURES FORMATION, PRANHITA- GODAVARI VALLEY, INDIA – IMPLICATIONS ON PALAEOGEOGRAPHY

Thesis

Dec 2022

Abhirup Saha

Gondwana basins in peninsular India are riftogenic, half-graben basins preserving thick succession of syn-rift and post-rift sedimentary deposits. The rocks of the Gondwana Supergroup (Late Carboniferous – Early Cretaceous) are scattered across several isolated basins in palaeo-rift valleys in Peninsular India. One such rift valley is the Pranhita-Godavari Valley, where a thick repository of the entire Lower and Upper Gondwana succession is preserved. The Middle Permian Barren Measures Formation of the Lower Gondwana Group is relatively poorly understood from a sedimentological point of view. An integrated approach using facies analysis, geochemical analysis and sequence stratigraphic analysis of the Barren Measures Formation can potentially provide significant insights into the palaeoenvironment, palaeogeography, and palaeoclimate existing during Middle Permian time. The Barren Measures Formation is traditionally interpreted as fluvio-lacustrine deposit at its type area, Jharia coalfield, and other Gondwana basins in peninsular India. The present research focuses on outcrop based sedimentological and sequence stratigraphic analysis supplemented by laboratory-based investigations of diagenesis. The in-depth facies analysis from selected outcrops, recorded eight distinct facies types, categorized into three facies associations, namely, (i) conglomerate-sandstone facies association (BM-CS); (ii) sandstone-mudstone facies association (BM-SM); and (iii) mudstone facies association (BM-M). The BM-CS facies association comprises pebbly, coarse-grained trough cross-stratified sandstone facies (BM-CS1), planar tabular cross-stratified sandstone facies (BM-CS2), pebbly coarse-grained plane-bedded sandstone facies (BM-CS3), red, ferruginous pebbly cross-stratified sandstone facies (BM-CS4) and lensoidal conglomerate facies (BM-CS5). The sandstones of the BM-CS facies association show lenticular geometry, concave-up basal contacts, large scale cross-stratifications with pebbles concentrated near the base and that are vertically stacked in nature. The BM-SM facies association comprises fine-grained yellow sandstone facies (BM-SM1) and fine-grained cross-stratified heterolithic facies (BM-SM2). Each facies preserves feature such as: (i) tidal bundles characterized by laterally accreting foresets draped with mud and separated by reactivation surfaces; (ii) foresets with laterally variable thickness occurring within a single cross-strata set; (iii) mutually opposite foreset bundles in vertically adjacent beds; (iv) sigmoidal strata bundles; and (v) abundant flaser beddings. The BM-M facies association is composed of ferruginous siltstone/mudstone facies (BM-M1) and calcareous mudstone facies (BM-M2). The facies types bear signatures of pedogenic overprinting in the form of rhizocretions, root tubules and colour mottling effects. Petrographically, the sandstones and mudstones are dominantly composed of quartz and orthoclase, remnant plagioclase, heavy minerals, biotite, muscovite, clay minerals, matrix, ferruginous, silica and calcite cement. The entire succession of the Barren Measures Formation is characterized by multiple vertically stacked, fining-upward cycles. Each fining-up cycle is defined by a coarse-grained pebbly trough cross-stratified sandstone near the base and fine-grained, heterolithic sandstone-mudstone or a mudstone near the top. The overall facies architecture reveals deposition under high energy unidirectional currents. The vertically stacked, multi-storeyed nature of the cross-stratified sandstones of the BM-CS facies association in the lower part of the successions with paucity of fine-grained sediments of the BM-M facies association points to a high-energy river deposit. The occurrence of tidal bundles, reactivation surfaces, spring-neap-spring tidal cyclicity, and systematic changeover of different types of foresets, oppositely directed cross-strata sets, and flaser bedding signify tidally influenced fluvial channels and overbanks. The mudstone facies association (BM-M) indicates deposition in a low energy calm environment, which underwent post-depositional pedogenic modifications resulting in the formation of hardpan calcrete horizons. A nearby low-grade metamorphic source rock with a short transport path is inferred from the petrographic study of the sandstones. The pebbly coarse-grained sandstone beds of the BM-CS present near the base of most of the fining-upward cycles record various soft-sediment deformation structures (SSDS). SSDS such as: (i) complexly deformed layers; (ii) pseudonodules; (iii) load and flame structures; (iv) various water escape structures including sedimentary dikes; and (v) syn-sedimentary faults are generally sandwiched between two undeformed beds. The deformed beds are present in the close vicinity of several intrabasinal faults. The SSDS are unique because of their (i) extremely large size, (ii) complex pattern with normal grading and (iii) predominance in coarse-grained sandstone. In the absence of triggering agents like storms/pounding waves, rapid dumping (massive beds), slumps, or sediment gravity flows in the study area, frequent palaeo-earthquake shocks are possibly the only trigger responsible for liquefaction of the sediments. The extensive liquefaction along the sediment-water interface is evidenced by the complex character and large size of the deformation structures. The coarse-grained sandstone beds bearing large SSDS, characteristically present at the basal part of each fining-up cycle, are thus identified as seismites, indicating possible liquefaction by frequent palaeo-seismic events. The facies architecture reveals the control of autogenic and allogenic processes on the cyclical sedimentation pattern and the development of different stratal stacking patterns within the basin. A stratal stacking pattern with a high channel to overbank ratio resulted in a High Amalgamation Systems Tracts (HAST), and a stratal stacking pattern with a low channel to overbank ratio, resulted in the development of Low Amalgamation Systems Tracts (LAST). There are two High Amalgamation Systems Tracts (HAST-1 and 2), each represented by coarsening-up succession of the vertically stacked, multi-storeyed channel-fill deposits of BM-CS facies association, with rare overbank deposits (BM-M). There are also two Low Amalgamation Systems Tracts (marked as LAST-1 and LAST-2, respectively) characterized by fining-up succession of single-storeyed sandstone with large SSDS and tidal signatures, embedded in thick overbank deposits. The deposition of the HAST sediments took place under autogenic upstream-controlled low-accommodation condition. The deposition of LAST sediments indicate low rates of channel amalgamation under higher accommodation space caused by an interplay of varying rates of tectonic subsidence within a rift basin along with flooding by the encroaching tidal currents. Such encroachment of tidal currents in a syn-rift basin may be related to basin tectonism, evident from the frequent occurrences of seismite beds. The varied stratal stacking patterns observed from the rock record in the study area manifest modifications of the river equilibrium profile (base level), with changing accommodation, as a result of interplay of the different autogenic and allogenic controls. The landward encroachment of the tidal limit, causes frequent flooding and modification of the river equilibrium profile, leading to a changing stratal stacking pattern. The allogenic processes such as encroaching tidal currents and syn-depositional tectonism, influenced the sequence stratigraphic architecture in the upstream controlled settings apart from the common autogenic processes. Thus, the prevalent sequence architecture is attributed to the alternate phases of autogenic mechanisms followed by the allogenic mechanisms including the syn-rift tectonic subsidence and the encroaching tidal currents, in the upstream controlled fluvial depositional systems. The facies and sequence stratigraphic architecture reveal a river deposit, with minor tidal-influences in the upper part, indicating a fluvial-tidal depositional system, during Middle Permian sedimentation. Post-depositional early diagenetic changes in the sandstones and mudstones include: (i) partial dissolution of detrital grains; (ii) biotite kaolinization; (iii) kaolinite formation and clay infiltration; (iv) localized extensive ferruginous cementation; (v) vadose and phreatic zone calcite cementation; and (vi) formation of glauconite pellets, which are observed at different stratigraphic levels. Geochemical signatures reveal low-Mg calcite as the common cement composition of the sandstone and mudstone in the study area. The micrite, pendant and microspar coating morphology of the calcite cement in the mudstones point to vadose zone cementation. The poikilotopic, blocky, bladed and prismatic morphologies of the calcite cement in the sandstones point to phreatic zone cementation. The presence of silica cementation in the form of quartz overgrowth and development of secondary porosity in the sandstones and mudstones indicate a post-burial mesogenetic change. The abundances of the diagenetic features show a definite link with the grain size and sorting of the sandstones in the study area. During the early diagenesis stage, the action of meteoric water resulted in the development of biotite kaolinization, kaolinite, alteration of the feldspars and phreatic zone calcite cementation within the sandstones and mudstones. With the onset of pedogenic (calcretization) processes vadose zone cementation took place within the fine-grained overbank mudstones. The calcite cement chemistry changed under the influence of marine (tidal) water, interacting with the fluvial sediments under a semi-arid climate. Increasing marine influences, recorded in the form of abundant authigenic glauconites, suggest a sustained marine invasion during deposition of the Barren Measures sediments. These modifications occurring due to early diagenetic (near-surface) changes appear to be predictive in nature within a sequence stratigraphic framework. The changes taking place during early diagenesis significantly impacted the development of silica cementation and secondary porosity during the burial diagenetic (mesogenetic) stage. Integrating all observations, a predictive model is established to understand the effects of syn- to post-depositional changes on the development of heterogeneity within the Barren Measures Formation of the Pranhita-Godavari Valley.

Diagenesis and the effects of cataclastic deformation on the Permo-Triassic New Red Sandstone, Isle of Arran, Scotland

Article

Feb 2023

Diagenesis in the Permo-Triassic New Red Sandstone, Isle of Arran is characterized by early cementation of hematite, clay, and calcite minerals, followed by burial compaction, quartz, feldspar, and pyrite cementation. Cataclasis post-dated the quartz and feldspar cementation and reduced the grain and pore aperture size in deformed samples. Samples with cataclastic bands typically have 18% porosity and 8.81 mD permeability on average. Whereas, undeformed samples have an average porosity of 22% and an average permeability of 381 mD. Cataclasis was not as important as diagenesis in controlling sandstone porosity and permeability. However, cataclasis resulted in lower porosity and very poor to medium permeability in deformed samples. Cataclastic bands compartmentalize reservoir sands and cause a high heterogeneity in undeformed porous sandstones. Poikilotopic and blocky calcite cement postdates early clay and hematite cement. In addition, burial quartz and feldspar overgrowths also postdate the early clay and hematite. However, the poikilotopic calcite fills in framework grains that have larger void volumes than the grain/grain contacts where quartz overgrowths are present. Cataclasis resulted in fracturing of quartz and feldspar overgrowths. Therefore, the cataclasis occurred after the development of quartz and feldspar cementation. Dissolution postdated the formation of authigenic feldspar and pyrite formation resulted from hematite reduction. The distributions of grain and pore sizes against cumulative mercury volumes in studied samples shows a high level of reduction of grain and pore aperture sizes for deformed samples from single-cataclastic and multi-cataclastic bands. The distribution of apex volumes illustrates that the effective mercury porosity of the multi-cataclastic band sample may be reduced up to > 2 times in comparison to undeformed samples. However, the sample of a thin single cataclastic band has only a slightly lower apex volume in comparison to the host sample.

Reservoir characteristics and diagenesis of the Buntsandstein sandstones in the Campine Basin (NE Belgium)

Article

Full-text available

Dec 2022

The red beds of the Buntsandstein (Early Triassic) in the Campine Basin (NE Belgium) display porosities between 5.3–20.2% (average 13.7%) and permeabilities varying between 0.02–296.4 mD (average 38.7 mD). Knowledge of their reservoir controlling properties, which today are missing, is important in view of potential geological storage of CO2 or natural gas and geothermal reservoir potential within these sandstones. Therefore the effects of diagenesis were assessed based on petrography, stable isotope analyses, fluid inclusion microthermometry, X-ray diffraction, electron microprobe and porosity-permeability core analyses. These sandstones were deposited by a dryland river system, in a warm, mostly arid climate with episodic rainfall and high evaporation rates. During wetter periods especially feldspars were dissolved. Strong evaporation during dry periods led to reprecipitation of the dissolved species as K-feldspar and quartz overgrowths, smectite and calcite/dolomite. Sediment reworking resulted in framework grains becoming clay coated. The clay coats are better developed in finer than in coarser grained sediments. The original smectite composing the rims converted to illite during burial. The tangential orientation of the clay platelets in the rims led to illite-mica-induced dissolution of quartz during burial/compaction, which is manifested as bedding parallel dissolution seams that are filled with clays and micas, especially in the fine-grained sandstone/siltstone/claystone. These constitute important barriers to the vertical flow within the reservoir. The released silica did not really affect the red sandstones but was exported (often on mm to cm scale) to nearby bleached horizons, where nucleation inhibiting clay rims are less well developed. The red colour of the sandstones arises from the presence of small amounts of Fe-oxides in the inherited clay rims. Migration of fluids enriched in organic acids, expelled from underlying Carboniferous coal-bearing strata, resulted in local bleaching of coarser grained horizons. In the finer grained sediments, the red colour was mostly preserved, which suggests that the reductive capacity of the fluid was limited.

Seal, and overburden rock characterization of potential CO2 storage sites in the northern North Sea

Thesis

Dec 2022

Md Jamilur Rahman

Anthropogenic atmospheric carbon dioxide (CO2) is one of the major greenhouse gases that are responsible for human-induced global warming. To avoid further damage, we need to cut half of the CO2 emission by 2030 and net-zero emissions by 2050. Subsurface geological storage of CO2 is one of the viable solutions that cut emissions with the lowest possible cost. The injected CO2 will be captured from the point sources, transported, and permanently stored in suitable subsurface geological formations. However, injecting CO2 into the subsurface has several failure risks. Therefore, a detailed integrated analysis is needed to build confidence for safe and permanent subsurface geological CO2 storage. This doctoral thesis study evaluates the impermeable layers called seals/caprocks, which prevent the less dense CO2 plume from migrating back to the atmosphere. A workflow is developed to characterize target caprock fracture risks by integrating different analytical techniques. Characterizing the fracture risks helps build our knowledge and confidence in safe and permanent subsurface CO2 storage. Without permanently removing greenhouse gases, we might trigger irreversible environmental tipping points. Crossing the critical threshold might be devastating for the human race. Therefore, this study partially contributes to the fight against human-induced global warming.

Dissolution and Reprecipitation of Amorphous Silica in Silica Rich Shales Induces Non-Monotonic Evolution of Porosity in Acidic Reactive Environments

Article

Dec 2022
FUEL

Advances in sustainable subsurface energy technologies are crucial for meeting our energy and resource needs for a climate-resilient future. Novel strategies to harness subsurface shale reservoirs for recovering valuable metals and for enabling CO2 storage are influenced by the morphological and mineralogical heterogeneities of these materials. In this context, delineating the interactions of highly acidic solutions such as wet supercritical CO2 on shales with varying mineralogy is crucial to inform the stability of caprock seal for CO2 storage and enhancements in permeability for fluid transport, reactivity, and storage. The feedback chemical effects associated with the interactions of acidic solutions on the morphologies and mineralogies of shales have not been extensively investigated. These insights are crucial for assessing temporal changes in the reactivity and the fate of the fluids in subsurface environments. In this study, we investigate the effect of 1 M HCl solution on the chemistry and morphology of three different shale samples with varying carbonate, clay and silica contents. An increase in the amorphous content, from 37 % to 41.3%, of silica-rich and carbonate/clay lean shale is noted due to reactions with an acidic solution which is attributed to the dissolution of Si-bearing phases such as clays, accompanied by SiO2 precipitation. In shales bearing high content of clays and carbonates, significant increases in the pore volumes and surface areas are noted. Non-monotonic changes in the micron-scale porosity of silica rich – carbonate/clay lean (e.g., Mowry shale) are noted using in-situ X-ray microtomography experiments. Due to the initial mobilization of silica and dissolution of carbonate/clay phases, the total porosity slightly increases from 6.7% to 10.7% followed by a decrease to ~4% caused by SiO2 reprecipitation. These findings suggest that even though silica is less reactive in acidic environments, the changes in the amorphous and crystalline content due to dissolution and reprecipitation alter the porosity and fluid flow paths.

Diagenesis Study of Jatiluhur Formation at Cipamingkis River, Bogor Regency, West Java, Indonesia

Article

Full-text available

Dec 2021

Diagenesis studies in the Jatiluhur Formation are still relatively new, especially in the Cipamingkis River. This research can provide information in the form of components and characteristics of sandstone in the Jatiluhur Formation which can be used as a basis for further research or useful information in the oil and gas industry. Knowledge of diagenetic could be one of the factors that affect in raservoir quality, espesially in sandstone. In this study, data collection was carried out through surface mapping, which is 55 rock samples were obtained from stratigraphic measuring section with a path length of ±2 Km in the Cipamingkis River. The data is in the form of information on sedimentarry structure, textures and composition. There were 23 sandstone and 2 limestone samples which were then subjected to petrographic analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results of study are several features of diagenesis were found including compaction that works in the form of point contact, long contact, concavo-convex contact and suture contact and dominated by mechanical compaction, while in limestone there is a brittle fracture feature in bioclasts. The cement found in the form of calcite cement, quartz and clay minerals that the form of kaolinite, smectite and illite, while the limestone is in the form of blocky and fibrous to bleded which is filled with calcite minerals. Dissolution occurs in the minerals of quartz, feldspar, and mica. The mineral replacement that is commonly found occurs in quartz and feldspar minerals. In limestone, there is an intergranular micritization. The dominant type of porosity found was interparticle with an average of 10.4% found between 3 – 23%. The history of diagenesis that occurs in rocks in the Jatiluhur Formation begins with the initial deposition of eogenesis, followed by burial mesogenesis and ends with telogenesis which reveals rocks on the surface.

Impacts of Sedimentation and Diagenesis on Deeply Buried Reservoir Quality of a Rift Basin: A Case Study of Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, China

Article

Full-text available

Aug 2022
GEOFLUIDS

The deeply buried reservoirs of Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, display strong heterogeneity, and the major controls for the development of high-quality reservoirs remain unclear. To address these issues, we conducted a series of experiment analyses, including petrographic microscope, scanning electron microscopy, and X-ray diffraction, and analyzed the impacts of sedimentation and diagenesis on the quality of deeply buried reservoirs. The results demonstrate that the sandbodies of subaqueous distributary channel and mouth bar deposited in lowstand systems tract (LST) and highstand systems tract (HST), as compared to the beach-bar and subaqueous fan sandstones deposited in transgressive systems tract (TST), have coarser grain size, higher quartz content, and lower muddy matrix content, which induced stronger anti-compaction capability, higher preservation of intergranular pore spaces, and thus better reservoir qualities. The reservoir types developed in subaqueous distributary channel and mouth bar are mainly types I, II, and III with medium-low porosity and low-ultra low permeability, while beach-bar and subaqueous fan mainly developed type III reservoir with low-porosity and ultra-low permeability. The reservoirs developed in E2w of the study area have undergone strong compaction, intense dissolution, but weak cementation. The subaqueous distributary channel and mouth bar reservoirs in LST are adjacent to Ew4 source rock in spatial distribution, resulting in strong organic acid dissolution, and developed numerous dissolved pores. The charging of hydrocarbons before deep burial further inhibited the later compaction and cementation and protects the preservation of residual primary intergranular pores and secondary dissolved pores. The combination of these factors leads to the development of the abnormally high porosity and high-quality reservoirs in LST. The results of this study reveal the genetic mechanism of deep, high-quality reservoirs in the rift basin and guide the selection of high-quality reservoirs in the later stage.

Diagenesis and reservoir quality evolution of the Lower Cretaceous turbidite sandstones of the Agat Formation (Norwegian North Sea): Impact of clay grain coating and carbonate cement

Article

May 2022
MAR PETROL GEOL

The turbidite sandstone of the Agat Formation is one of the most important hydrocarbon reservoirs in the Lower Cretaceous succession of the Norwegian North Sea. The diagenetic history of these sandstones has greatly impacted the reservoir quality, resulting in a high heterogeneity in porosity and permeability. To investigate this impact, sixty thin sections and rock samples were investigated through petrographic observations, geochemical microanalysis, and X-ray diffraction. Three main diagenetic events occurred during burial and influenced the reservoir properties: (1) mutual growth of chlorite, siderite, and apatite, (2) calcite cementation, and (3) dissolution of K-feldspar and calcite. Early diagenesis occurs under strongly reducing conditions, leading to the formation of Fe-chlorite/berthierine, siderite, pyrite, and apatite forming coatings around the detrital grains. Chlorite coatings have a key role in controlling the reservoir quality in the Agat Formation. In the lower part of the reservoir, chlorite forms thick coats (15–25 μm) that strongly reduce permeability. Conversely, in the upper part of the reservoir, chlorite coats are thinner (4–10 μm), favoring porosity preservation by inhibiting quartz overgrowth. X-ray diffraction and electron microscope observations indicate Fe-rich chlorite of Ib polytype, suggesting solid-state transformation from a berthierine precursor. The kinetics of berthierine/chlorite growth are governed by the dissolution of Fe-rich grains and fine fraction materials found as matrix and early coats. These grains are abundant in the lower part of the reservoir favoring the formation of thick chlorite coats. Chlorite was followed by pervasive calcite cementation in deep burial conditions creating low permeability barriers in the reservoir. The source of this calcite is internal and probably related to the dissolution of an early diagenetic calcite cement forming at shallow depth, with a minor contribution from marine carbonates. During late diagenesis, K-feldspar grains and calcite cements underwent intense dissolution creating secondary porosity and consequently increasing permeability. The input of acidic fluids associated with the maturation of the source rocks could have facilitated these dissolution reactions. Our results highlight the importance of inherited mineralogy in controlling the distribution of chlorite, carbonates, and secondary porosity in the Agat Formation. This study provides useful indicators to help predict diagenetic reactions that can occur in deep marine siliciclastic reservoirs.

Diagenesis and Controls on Reservoir Quality of Lower Triassic Sandstones (Buntsandstein) from a Marginal Basin Facies

Conference Paper

Jan 2022

Controls on near-surface and burial diagenesis of a syn-rift siliciclastic rock succession: A study from Permian Barren Measures Formation, Southern India

Article

May 2022
SEDIMENT GEOL

Distinct processes during the syn-depositional sedimentation and the post-depositional diagenesis determine the fabric of a clastic sedimentary rock. Temporal and spatial variations in any of these processes cause development of specialized characters in the sedimentary rocks. The Permian Barren Measures Formation in the Pranhita-Godavari Valley in southern peninsular India, is characterized by signatures of early to late diagenetic cementations within coarse-grained sandstones and mudstones. An integrated petrographic-geochemical study is carried out to understand the post-depositional near-surface and burial processes. Facies architecture manifests sedimentation in fluvial (channel, bar and overbank) to tide-influenced fluvio-tidal mixed depositional condition. Post-depositional near-surface early diagenesis changes include – (i) partial dissolution of detrital grains, (ii) biotite kaolinization, (iii) localized extensive ferruginous cementation, (iii) calcite cementation, and (iv) development of rhizocretions observed at different stratigraphic levels. These point to the role of meteoric water during the pre-burial stage that changed the pore water chemistry within the fluvial channel and bar sediments, along with onset of pedogenic (calcretization) processes in the vadose zone within the fine-grained overbank deposits. Marine (tidal) water influx in fluvial regime under a semi-arid climate modified the pore water chemistry. Shallow burial diagenetic changes, under elevated temperature and pressure include – (i) extensive calcite cementation in intergranular pore spaces and fractures, and (ii) late phase partial dissolution producing secondary porosity. A predictive model is derived to decipher the role of post-depositional changes in developing potential reservoir heterogeneity in a fluvio-tidal mixed depositional system.

Diagenesis and controls on reservoir quality of Lower Triassic red bed sandstones (Buntsandstein) from a marginal basin facies, southwest Germany

Article

May 2022
MAR PETROL GEOL

After decades of hydrocarbon exploration and production in the Lower Triassic Buntsandstein from the Central European Basin center, the marginal southern German Buntsandstein has gained interest as a potential lithology for geothermal applications and lithium brine extractions. For this purpose and in order to better understand reservoir quality, porosity and permeability in combination with point-counted petrographic data of sandstones from SW Germany covering the Lower, Middle, and Upper Buntsandstein are studied. Textural, mineralogical, and petrophysical data show that the eolian and fluvial sandstones represent a heterogeneous reservoir system. Permeabilities vary over six orders of magnitude from 0.001 to 515 mD, while porosities are in the range of 7.9–21.5%. Different controls on reservoir quality exist. Large grain sizes ≥0.31 mm together with total quartz contents are the major controls on reservoir quality. Additional controls on reservoir quality are exerted by authigenic phases. Since porosity loss is dominated by compaction, the influence of authigenic clay phases on reservoir quality is minor. However, illite coatings on detrital quartz efficiently inhibited syntaxial quartz overgrowth. Where illite coatings lack, syntaxial quartz overgrowths locally stabilized the granular framework and, thus, preserved primary porosity. These authigenic phases, however, represent a local control, which cannot be applied on a regional scale. Similarly, enhanced secondary porosity due to feldspar dissolution is locally restricted and minor. The comparison with other studies on the Buntsandstein shows that its reservoir system is highly heterogeneous over small and large scales and within the same stratigraphic levels. In this study, quartz-rich and authigenic clay-poor sandstones with relative large grain sizes interpreted as eolian deposits reveal the best reservoir qualities.

The significance of outcrop analog data for reservoir quality assessment: A comparative case study of Lower Triassic Buntsandstein sandstones in the Upper Rhine Graben

Article

Apr 2022
MAR PETROL GEOL

Subsurface reservoir quality assessments are among the most important workflows in reservoir geology. The application has been standard practice in the hydrocarbon industry and is also adopted for geothermal reservoir systems hosted in clastic rocks. With the emergence of fluid mining for metals, such as lithium for the energy transition, yet another field may adopt the standard practice. However, as drill cores and hard geological data from the subsurface are expensive commodities they may be substituted by outcrop analog studies to fill in data gaps in the understanding of the deep subsurface. However, reservoir quality controls, such as pore-filling cements or natural fractures may be altered or overprinted during uplift to the surface and exposure to undersaturated meteoric fluids and associated stress release. A complete understanding of geochemical and geomechanical processes affecting the rocks since their deposition is needed to account for that, otherwise data may not be representative of the subsurface. In this case study using petrographic (point-counting) and petrophysical (porosity and permeability) analyses we highlight the possibilities and limitations of outcrop analog data used for the evaluation of subsurface reservoir quality. The focus is on the Triassic Buntsandstein within the Upper Rhine Graben in southwestern Germany and eastern France and exposed outcrop analogs along the western margin of the graben. Here, we show that some data (detrital composition, quartz cement, K-feldspar cement, and clay mineral cement volumes) may directly be applicable to subsurface reservoir quality assessment. However, other data (porosity, permeability, IGV, carbonate cement, and iron hydroxide cement contents) cannot be used analogously to subsurface samples and we outline possible approaches to mitigate some of the mismatches in the frame of a reservoir quality study. In particular, the correction for uplift diagenetic dissolution of carbonate cemented nodules and goethite precipitation will be the key to gain more accurate results in the presented area.

Experimental Simulation of Burial Diagenesis and Subsequent 2D-3D Characterization of Sandstone Reservoir Quality

Article

Full-text available

Feb 2022

Characterization of deeply buried sandstones and their reservoir quality is of paramount importance for exploring, developing, and subsurface storage of energy resources. High reservoir quality in deeply buried sandstones is commonly correlated with the occurrence of grain coatings that inhibit quartz cementation. The development of reliable models that can predict reservoir quality relies on incorporating quantitative understanding of these diagenetic processes. Hydrothermal experiments simulating burial diagenesis were integrated with multi-scale X-ray tomography to quantify the 3-dimensional evolution of grain coating volume and porosity with increasing temperature; while microscopic and automated quantitative mineralogy analysis were used to track the associated mineralogical alterations. To simulate reservoir evolution, sandstone samples from the Lower Jurassic Cook Formation (Oseberg Field, 30/6-17R, Norway) were exposed to a silica supersaturated Na2CO3 (0.1 M) solution for up to 360 h at temperatures of 100–250°C. The experimental results show the main porosity and permeability reduction window is associated with pore-filling kaolinite, and lies between 150 and 200°C, above which little change occurs. Volumetric increases in grain coating start to occur at ∼150°C through precipitation of authigenic chlorite, and continue to 250°C, irrespective of the experimental duration. Together with preexisting siderite coatings, the newly precipitated chlorite prevents the loss of reservoir quality by inhibiting quartz overgrowth development. Pore flow simulations based on the observed temperature-dependent 3-dimensional pore networks allow us to characterize pore-throat and permeability evolution and gain quantitative understanding of the impact of diagenetic overprinting on deeply buried sandstone reservoirs.

Origin of Quartz Cement in the Paleogene Sandstone Reservoir of Shahejie Formation, Bohai Bay Basin, China

Article

Full-text available

Nov 2021

The precipitation of authigenic quartz plays a significant role to reduce the reservoir characteristics and enhance the stiffness of the rock. The Es1 sandstone of Shahejie Formation is acting as a significant hydrocarbon producing rock in the Nanpu Sag. Various methods like thin section Petrography, cathodoluminescence (CL), scanning electron microscope (SEM, with EDS), and electron microprobe analysis has been used to reveal the origin of quartz cement as well as to evaluate the effect of quartz cement on reservoir quality. The studied sandstone is classified as immature to mature feldspathic litharenite and lithic arkose and consists of quartz, feldspar, rock fragments and micas. Petrographic studies and SEM analysis shows that the authigenic quartz is acting a significant cement that reduces the reservoir quality. Whereas clay minerals (Kaolinite and mixed layer illite to smectite) are dominant in the Es1 sandstone, that can reduce the reservoir quality. SEM, CL and thin section analysis reveal that there are two stages of quartz cement in the studied samples; that are pore filling authigenic cement and quartz overgrowth cement. Fluid inclusion homogenization temperatures shows that stages of quartz cement were developed with continuous process from 70ºC to 130ºC. Quartz cements were generally originated from I/S reaction, feldspar dissolution, conversion of rock fragments and pressure solution. Feldspar dissolution (K‐feldspar) and kaolinite to illite reaction is an insignificant silica source for the silica cement which is internally precipitated in a close system with diffusion transporting mechanism. Overall, quartz cement significantly enhance the rock strengthen and brittleness effectively as well as it reduce the overall reservoir quality.

Petrofacies investigations of the Cretaceous Pab Formation Rakhi Gorge Eastern Sulaiman Range Pakistan - Implication for reservoir potential

Article

Full-text available

Nov 2021

Petrologic investigations of the sixty representative thin sections from the Pab Formation within Rakhi Gorge section Eastern Sulaiman Range Pakistan have been used to characterize different diagenetic patterns, identification of primary composition and reservoir characters. Sublithic, lithic and quartzose sandstones have been the principal constituents of the formation. The processes responsible for the late-stage alteration and diagenetic processes identified during the petrographic study include compaction of lithic fragments and mud clasts, formation of quartz overgrowth structures, feldspar replacement and alteration, cementation, and replacement of grains by clay minerals other ferrous elements and diagenetic minerals. The study shows that the initial porosity has been primary intergranular, but the secondary porosity has been originated in the rocks as a result of the alteration of the primary and secondary constituents, as well as fracturing. These facies characters replicate the reservoir quality including, good, medium, low-quality, and non-reservoir. Samples displaying average total porosity greater than 10 % have been declared as good quality. While rocks samples having 7 % or greater porosity are grouped as a medium reservoir. Those samples consisting of porosity values between 1 and 4% are considered as a low quality reservoir while those samples having porosity ranges low than 1% have been called non-reservoir. Based on the investigated parameters the studied samples from the Pab Formation displayed the characters of a medium reservoir that may hold significant hydrocarbon. This different quality including good and low quality zonation can be attributed to the facies and diagenetic change in the formation.

Diagenetic Controls on the Quality of Shallow Marine Sandstones: An Example from the Cambro-Ordovician Saq Formation, Central Saudi Arabia

Article

May 2024
J AFR EARTH SCI

Modelling diagenetic reactions and secondary porosity generation in sandstones controlled by the advection of low‐molecular‐weight organic acids

Article

Mar 2024

Higher secondary porosity was observed in the centre of a sandstone unit in the Eocene Shahejie Formation fan delta front sandstones from the Bozhong Depression, Bohai Bay Basin. This differs from past studies showing secondary porosity mainly in the marginal parts of sandstones adjacent to shales. This study utilized reactive transport models involving low‐molecular‐weight organic acids (LMWOA) to discuss potential processes resulting in the contrary distribution of secondary porosity. An interface model simulating LMWOA diffusion from adjacent shales to the sandstone resulted in secondary porosity in sandstones adjacent to shales. In contrast, an advection model simulating advective transport of LMWOA parallel to the sandstone bedding successfully generated higher secondary porosity in the central part. The central part of the sandstone exhibited better grain sorting (greater depositional porosity) and significantly less early carbonate cements compared to the marginal sandstone parts. Consequently, the central part had greater porosity prior to the dissolution through LMWOA. The initially higher porosity in the central part allowed for a higher advective flux of LMWOA‐rich water and associated lower pH, resulting in decreased oligoclase saturation, higher oligoclase dissolution rates and ultimately higher secondary porosity. This study indicates that grain sorting during sediment deposition, early carbonate cementation, LMWOA production in adjacent shales, and advection processes collectively control the diagenetic reactions and the distribution of secondary porosity in sandstones.

CHAPTER 5: APPLICATIONS OF CATHODOLUMINESCENCE IMAGING TO SEDIMENTARY ROCKS

Chapter

Jan 2014

Redbed diagenesis links district-scale mineralising fluids to Cu source rock in the Polaris Zn-Pb(-Cu) district, Arctic Canada

Article

Full-text available

Feb 2024
ORE GEOL REV

Diagenesis as a control on the tight sandstone reservoir quality of the upper Carboniferous strata in the northeastern Ordos basin, China

Article

Oct 2023
MAR PETROL GEOL

Diagenesis, paleoenvironments, and petroleum geology of sediments lining the West and Central African Rift System, Koum Basin, North Cameroon

Article

Oct 2023

The West and Central African Rift System (WCARS) hosts all the sedimentary basins of West and Central Africa, and constitutes a priority target for sedimentological studies in the area. Numerous basins in this area, notably the Koum Basin, remain poorly known greenfields, owing to a dearth of studies associated with poor accessibility, security limitations, and accrued exploration focus on hydrocarbon-producing oceanic basins in the area. The present study seeks to reduce the knowledge gap by providing new data on the diagenesis, petroleum system, and paleoenvironments of the Koum Basin. NNW–SSE traverse mapping across the basin aided in the identification of conglomerates, sandstones, and shales/mudstones, hosting dinosaur tracks, plant matter and are intercalated by siltstones and bivalve-rich carbonates. Lithofacies analyses, petrography, scanning electron microscopy, and X-ray diffractometry were applied. The studied facies form three facies associations reflecting fluvio-lacustrine paleoenvironments. Petrography exposed concavo-convex to sutured mineral margins, presenting silicate overgrowths plus calcite, silica, hematite, and clay cements. Albite, quartz, calcite, and microcline dominate bulk mineralogy, with analcime, goethite, and hematite as minors, while chlorite-smectite, illite, and smectite dominate clay mineralogy, all pointing to the pre-eminence of mesodiagenesis within the Koum Basin. Studied shales and siltstones present traces of oil/oil migration paths in organic pores, whereas sandstones and carbonates are devoid of mature organic matter and interconnected porosity. Consequently, a source-rock petroleum system with shales and siltstones playing the dual roles of hydrocarbon source and reservoir is inferred for this basin that can be considered a potential unconventional hydrocarbon reserve.

Digital Rock Advances from an Material Point Method Approach for Simulation of Frame Moduli and a Sedimentary Petrology Inspired Method for Creation of Synthetic Samples Through Simulation of Deposition and Diagenesis

Article

Sep 2023

We compare hydromechanical simulation results that use two alternative sources of 3D digital rock input: microCT analysis and synthetic rocks created using a newly developed process simulation methodology that more rigorously reflects knowledge from sedimentary petrology compared to previous efforts. We evaluate performance for these alternative representations using St. Peter Sandstone samples where dry static bulk modulus (K) and shear modulus (G) are simulated using a new extension of the Material Point Method that resolves contacts using high-resolution surface meshes and that considers three alternative contact modeling approaches: purely frictional, fully bonded, and cohesive zones. We evaluate model performance on two samples from the dataset with multiple static moduli measurements (sample 1_2: porosity 24.6 vol%: K 10.2 14.7 GPa, G 11.6 14.0 GP; sample 11_2: porosity 12.4 vol%: K 13.5 24.6 GPa, G 12.8 17.9 GPa). Purely frictional results underpredict measured modulus values whereas fully bonded results overpredict them. Measured values are most closely approximated by results with cohesive zones that consider sets of discrete spring-like features at contacts. Shear modulus results from FEM simulations on structured grids, by contrast, tend to be significantly greater than measured values, particularly for samples with lt; 18 vol% porosity. Permeability values from digital rock physics simulations for the studied samples are within factors of 2 5 of conventional core analysis measurements (2860 and 58 md for sample 1_2 and 11_2, respectively). We demonstrate that the process modeling approach (1) accurately reproduces measured rock microstructure parameters from thin section analysis, (2) leads to simulation results for dry static moduli and permeability of comparable accuracy to simulations that employ microCT samples, and (3) provides a rigorous basis for predicting diagenetically induced variations in hydromechanical properties over the range from unconsolidated sand to indurated rock.

Response of clastic reservoir to magmatic intrusion: Advances and prospects

Article

May 2023

Análise de proveniência por composição detrítica e sua importância na exploração de hidrocarbonetos

Article

Full-text available

Apr 2023

Introdução. A análise de proveniência sedimentar abrange a restauração dos elos existentes entre a rocha sedimentar e suas respectivas áreas fonte de sedimento. Objetivo e Metodologia. A unidade fundamental é a composição sedimentar detrítica. Entretanto, a composição detrítica não é reflexo unicamente da composição das rochas das áreas fontes, mas também da sobreposição de diversos agentes e processos, interdependentes, modificadores da assinatura de proveniências, atuantes desde a formação e o intemperismo da rocha fonte até processos diagenéticos durante o soterramento. Resultados. Por critérios didáticos, estes fatores e agentes podem ser sumariados em: tectônica e reciclagem sedimentar, natureza e composição das áreas-fonte, clima, relevo, transporte sedimentar e diagênese. Conclusão. Não se deve pressupor que a composição detrítica reflita, unicamente, composição da rocha fonte, mas ela é o produto desse conjunto complexo de fatores interdependentes. A análise de proveniência constitui uma ferramenta muito útil na exploração de hidrocarbonetos, principalmente em termos de caracterização preditiva das qualidades petrofísicas de reservatórios.

The origin of two distinct diagenetic depth zones in hydrocarbon-bearing sediments of Huanghekou Sag, offshore Bohai Bay Basin, China

Article

Feb 2023
MAR PETROL GEOL

Mineral diagenesis in a carbonate-rich mudstone: The Lower Carboniferous Hodder Shale, UK

Article

Nov 2022

Mineral authigenesis in mudstones respond differently across the compositional spectrum of fine-grained rocks resulting in a complex array of possible rock fabrics that affect the mechanical capacity of mudstones. Within carbonate-rich mudstones, geochemical controls on silicate and carbonate mineral reactions are variable and poorly understood. Here we analyse the diagenetic minerals of the carbonate-rich Hodder Mudstone from the Carboniferous Bowland Basin, UK, using SEM petrography combined with X-ray-based inorganic geochemical data. Our findings show that up to 90% of quartz cement in the samples are diagenetic in origin, originating mainly from biogenic silica dissolution, and clay mineral reactions. It is also evident that due to the varying diagenetic silica-yielding reactions, authigenic silica overgrowths and isolated authigenic quartz crystals are localised in argillaceous samples while calcareous samples are characterized by silica replacement textures and quartz/calcite intergrowths. Moreover, euhedral dolomite crystals are concentrated within argillaceous units in comparison with calcareous units which are characterised by anhedral dolomite precipitation and replacement textures. These finding presents a case for facies-selective cementation as both early and burial diagenetic alterations were observed to be controlled by primary depositional components of biogenic debris and extrabasinal silicate detritus that resulted in complex and variable precipitation of authigenic minerals. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6269030

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.

Rock Physics-Based Pore-Filling and Seismic Amplitude Modeling of Barail Sandstone from Assam Geologic Province, India

Article

Jan 2022

This study attempted a rock physics template (RPT) based approach to model the impact of mechanical compaction and chemical diagenesis on overall porosity reduction of the BarailSandstone across the Assam Shelf and Schuppen Belt area. Then, a sensitivity analysis was carried out based on the probabilistic amplitude variation with angle (AVA) approach to study the impact of diagenesis, rock frame constituents, and pore fluids on seismic amplitude response. The RPT depth trend suggests a significant upliftment (2500 m) within the supra-thrust region, while the Assam Shelf and sub-thrust part of the Schuppen Belt were normally compacted throughout the geologic time. The geologically constrained RPT was used to (i) identify the primary process for chemical diagenesis as feldspar dissolution and(ii) estimate total cement, non-contact cement (NCC), and contact cement (CC) values. Laboratory-based analysis of rock samples validated the modeled cement volumes. The RPT-based CC (mode value) estimates were observed as 3%, 5%, and 7%, whereas the NCC varied as 6%, 8.5%, and 11% sequentially from extensional through supra-thrust to sub-thrust regions. The extensional and supra-thrust regions dominantly exhibited class IVAVA for oil and gas saturated rocks mainly due to the presence of coal and the relatively under compacted nature of the Barail Sandstones. A change in AVA class was observed to class II and IIP within the sub-thrust region due to enhanced cementation. The forward modeling revealed that a small amount (2%) of coal presence could potentially change the AVA class from II to IV, and an increase in CC from 5 to 7% could cause the AVA class to change from IV to II. This study efficiently modeled the digenetic effects and seismic amplitude in this tectonically complex region.

Unconformity-controlled bleaching of Jurassic-Triassic sandstones in the Ordos Basin, China

Article

Jan 2022
J PETROL SCI ENG

Bleaching of red beds, a type of hydrocarbon-induced alteration, is generally attributed to redox reactions between ferric iron minerals and hydrocarbon-bearing solutions. Herein, we report sandstone bleaching occurs interbedded with the coal- and dark mudstone-bearing strata at shallow depths below two unconformity surfaces separating sandstone formations of Triassic-Jurassic age in the Ordos Basin, China. Field observations, petrography, and geochemistry suggest that uplift events controlled the formation of red beds via supergene alteration and bleaching via hydrocarbon circulation. The color of sandstones below the unconformities grade from red, yellow, and white colors at shallow depths (few meters to tens of meters) to dark yellow, gray-green and gray colors at deeper depths. Organic matter (carbonaceous plant debris) and pyrite in the unaltered sandstone gave rise to the gray color. The red/yellow sandstones are characterized by the presence of extensive iron oxide/hydroxide grain coatings, exhibit intense dissolution and extensive kaolinization of detrital feldspar and biotite and lithics and are mainly composed of detrital quartz. The white, bleached sandstone presents similar petrographic characteristics as the unbleached sandstone except for a lack of iron oxide/hydroxide cements. δ18OVSMOW (9.8‰ to 15.8‰) and δDVSMOW (−103‰ to −119‰) values of kaolinite, and chemical indices of alteration of sandstones indicate a weathering origin for the kaolinite and the dissolution of labile minerals in the red and yellow sandstones. The original color of the bleached sandstone was gray during very early diagenesis and shifted to red/yellow due to the oxidation of pyrite and ferromagnesian silicate minerals (e.g., biotite) into hematite or goethite cements by the meteoric water circulation during regional uplift following the deposition of each formation. Supergene alteration associated with unconformities also created significant secondary porosity, and allowed later hydrocarbons to flow along the unconformities. The lithological properties of the weathered rocks below unconformities are highly heterogeneous both vertically and laterally and have a significant influence on fluid flow. This study provides direct evidence for hydrocarbon migration along unconformities and improves understanding of fluid-rock interaction in subsurface reservoirs.

The porosity preserving effect of basin wide illitic coating in deeply buried sandstone intervals of the lower Jurassic Stø Formation, Barents Sea

Article

Dec 2021
MAR PETROL GEOL

The Stø Formation contains the main target reservoirs for petroleum exploration in the Norwegian Barents Sea Area, however, the clean sandstones of the Stø Formation can be heavily quartz cemented in areas that have undergone extensive post depositional burial. The Stø Formation in well 7219/8-2 has previously been identified as having porosities well above the expected porosity depth trend due to the porosity preserving effects of illitic grain coating. In this study the Stø Formation in 14 wells with maximum burial depths at top of formation ranging from 2672 to 3623 m have been investigated and several new intervals of the formation show abnormally high porosities relative to the expected burial trend. Thin sections from 4 wells were available and in all of these remnants of grain coating illitic clay was observed in varying amounts, with the effectiveness in reducing quartz cement overgrowth being determined by the continuity of grain coats. This indicates that the degree of grain coating coverage exerts the main control on the porosity of the clean deeply buried parts of the Stø Formation. With increasing burial depths, the porosity difference between intervals with continuous grain coats and intervals with less continuous grain coats becomes ever larger. Intervals observed to have extensive grain coating coverage compact with a significantly reduced rate with increasing burial compared to poorer coated intervals in the chemical compaction domain. The noticeable and consistent difference in the chemical compaction trend between coated- and negligible coated intervals within the Stø Formation could allow for establishment of powerful predictive models without the need for expensive petrographic- or core plug data.

Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria

Article

Dec 2021

This article presents the results of cementation characteristics and their effect on sandstone reservoir quality of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations in selected fields in the Euphrates Graben area, Syria. This study emphasises the role of cementation in the evaluation of the diagenetic history of the sediments, developing effective porosity, as well as evaluation of reservoirs stimulation procedures and potential for formation damage of the sandstone reservoirs. Quartz cement is present as well developed tabular or pyramidal syntaxial overgrowths. Kaolinite cement is present as vermicular aggregates which are most abundant within sandstones of the Mulussa F Formation. Carbonate cements include siderite and dolomite. Four lithofacies were identified within the studied formations; lithofacies-1 and 2 correspond to fluvial depositional environments, lithofacies-3 and 4 correspond to fluvial to estuarine channel environments. The Post Judea Sandstone and the Lower Rutbah reservoir units are typically lithofacies-3 sequences in which quartz overgrowths are the dominant cement. Because the total cement is more extensive in the Post Judea Sandstone Formation than in the Lower Rutbah Formation, resulting in high porosity (up to 26%) and permeability (6 000 mD), the reservoir quality is predicted to be best in the Post Judea Sandstone Formation. The reservoir units in the Mulussa F Formation contain the highest cement volumes comprised of early siderite and kaolinite, which, with the development of compaction-resisting quartz overgrowths and resultant compactional pore volume loss, has resulted in typically lower porosity being preserved than in the Lower Rutbah and Post Judea Sandstone formations.

References

Article

Dec 2021

Craig M. Bethke

An indispensable primer and reference textbook, the third edition of Geochemical and Biogeochemical Reaction Modeling carries the reader from the field's origins and theoretical underpinnings through to a collection of fully worked examples. A clear exposition of the underlying equations and calculation techniques is balanced by real-world example calculations. The book depicts geochemical reaction modeling as a vibrant field of study applicable to a wide spectrum of issues of scientific, practical, and societal concern. The new edition offers a thorough description of surface complexation modeling, including two- and three-layer methods; broader treatment of kinetic rate laws; the effect of stagnant zones on transport; and techniques for determining gas partial pressures. This handbook demystifies and makes broadly accessible an elegant technique for portraying chemical processes in the geosphere. It will again prove to be invaluable for geochemists, environmental scientists and engineers, aqueous and surface chemists, microbiologists, university teachers, and government regulators.

The impact of diagenesis on the reservoir quality of the early Cretaceous Lower Goru sandstones in the Lower Indus Basin, Pakistan

Article

Full-text available

Dec 2021

The sandstone units of the Early Cretaceous Lower Goru Formation are significant reservoir for gas, oil, and condensates in the Lower Indus Basin of Pakistan. Even though these sandstones are significant reservoir rocks for hydrocarbon exploration , the diagenetic controls on the reservoir properties of the sandstones are poorly documented. For effective exploration, production, and appraisal of a promising reservoir, the diagenesis and reservoir properties must be comprehensively analyzed first. For this study, core samples from depths of more than 3100 m from the KD-01 well within the central division of the basin have been studied. These sandstones were analyzed using petrographic, X-ray diffraction, and scanning electron microscopic analyses to unravel diagenetic impacts on reservoir properties of the sandstone. Medium to coarse-grained and well-sorted sandstone have been identified during petrographic study. The sandstone are categorized as arkose and lithic arkose. Principal diagenetic events which have resulted in changing the primary characters of the sandstones are compaction, cementation, dissolution, and mineral replacement. The observed diagenetic processes can be grouped into early, burial, and late diagenesis. Chlorite is the dominant diagenetic constituent that occurs as rims, coatings, and replacing grains. The early phase of coating of authigenic chlorite has preserved the primary porosity. The recrystallization of chlorite into chamosite has massively reduced the original pore space because of its bridging structure. The current study reveals that diagenetic processes have altered the original rock properties and reservoir characteristics of the Lower Goru sandstone. These preliminary outcomes of this study have great potential to improve the understanding of diagenetic process and their impact on reservoir properties of the Lower Goru sandstone in the Lower Indus Basin and adjoining areas.

Reservoir quality of the Middle-Late Triassic Yanchang Formation (Ordos Basin) as controlled by sedimentology and diagenesis

Chapter

Jan 2022

Deep-lacustrine sediments deposited by gravity flows are important, though low-permeability constituents of tight oil reservoirs in the Late Triassic Yanchang Formation (southern Ordos Basin, China). Petrographic, mineralogical, and geochemical data were investigated in order to better constrain the controls on reservoir quality. In particular, sandy debrites and turbidites have been addressed. They were significantly altered by diagenesis, due to compaction, authigenic clay formation, and carbonate and quartz cementation and dissolution. Volcanic ashes provided silica for quartz cementation and easily transformed to authigenic smectite. The negative δ¹³C and δ¹⁸O values of carbonate cement in the sandstones reflect decarboxylation of organic matter as an important carbonate source. Reservoir quality is influenced by depositional (e.g., grain size, grain sorting) and diagenetic processes (e.g., compaction, resulting in loss of primary porosity, cementation, and dissolution). Quartz cement stabilizes the framework and preserves the open porosity, but is of minor importance. All these factors have great implications for tight oil exploration and production.

Occurrence and origin of chlorite and associated impact on tight sandstone reservoir quality: A case study of the Xujiahe Formation (NE Sichuan Basin, China)

Article

Nov 2021
J PETROL SCI ENG

Chlorite is beneficial to the preservation of primary pores, and its formation mechanism in marine sandstone has been widely reported. However, few studies have focused on the genetic mechanism of various types of chlorite in lacustrine deltaic sandstone. Therefore, the occurrence, diagenetic time and origin of chlorite in Xujiahe tight sandstone were investigated using microscopic images and mineralogical chemical analysis, and three stages of chlorite were identified, namely, grain-coating (Ch1), pore-lining (Ch2), and pore-filling (Ch3) chlorite, which belong to the Fe chlorite, Al chlorite and transitional types, respectively. The variations in morphology and chemical composition reflect differences in the sources and formation mechanisms. Chlorite sandstones are mainly distributed in the underwater distributary channel, which has strong hydrodynamic conditions. The formation of Ch1 is controlled by the sedimentary environment and flocculation. Salinity differences resulted in a larger amount of flocculation deposits in the estuaries during the syngenetic period. In the shallow burial stage, the clay coating underwent in situ transformation under thermal impacts and formed the Ch1. Xujiahe sandstones experienced complex diagenetic processes, which include early compaction, three stages of chlorite cementation, two stages of quartz precipitation, dissolution and two stages of carbonate cementation. Organic acid dissolution of volcanic rock fragments, phyllite, and biotite likely supplied sufficient Fe²⁺ for the growth of Ch2 in a relatively closed diagenetic system. As the pore water became weakly alkaline, diagenetic fluid migration originating from the clay mineral transformation in mudstone mainly provided Fe²⁺ and Mg²⁺ for Ch3 growth in sandstones. Chlorite is considered to have “composite” impacts on deep tight sandstones, with grain-coating and pore-lining chlorites protecting the primary pores by restricting the silica supply and occupying potential quartz nucleation sites; however, grain-coating chlorite caused throat obstruction or even closure under strong mechanical compaction, which reduced the permeability of the reservoir.

Quartz Cementation in Sedimentary Basins

Abstract

No full-text available

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