Article

The fate of feldspar in Brent Group reservoirs, North Sea: A regional synthesis of diagenesis in shallow, intermediate, and deep burial environments

June 1992
Geological Society London Special Publications 61(1):329-350

June 1992
61(1):329-350

DOI:10.1144/GSL.SP.1992.061.01.17

Authors:

Feldspar dissolution shows a strong relationship to reservoir temperature and pore water chemistry. In low temperature, brackish, shallow to moderately buried reservoirs (2400-3700 m, reservoir temperature less than 100-120°C) feldspar is generally a major detrital component whose abundance is affected by local variations in facies, extent of early meteoric leaching, and kaolinitization. In deeply buried Brent reservoirs (3700-4700 m) where present burial temperatures exceed 130-140°C detrital K-feldspar is absent and sandstones are characterized by major illitization of early formed kaolinite, precipitation of fibrous illite, extensive quartz cementation, and variable albitization. Although some feldspar is dissolved during early burial history by low temperature circulation of meteoric water, only deeply buried Brent reservoirs are characterized by the complete diagenetic removal of feldspar; however, the secondary porosity generated by late dissolution is of little benefit to reservoir quality because accompanying cementation results in major loss of permeability. -from Author

典型含油气盆地深层富长石碎屑岩储层长石溶蚀接力成孔认识及其油气地质意义

Article

Mar 2022

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

A review of feldspar alteration and its geological significance in sedimentary basins: From shallow aquifers to deep hydrocarbon reservoirs

Article

Full-text available

Feb 2019
EARTH-SCI REV

The feldspar group is one of the most common types of minerals in the earth's crust. Feldspar alteration (including the whole processes of feldspar dissolution, transfer of released solutes, and secondary mineral precipitation) is ubiquitous and important in fields including resources and environmental sciences. This paper provides a critical review of feldspar alteration and its geological significance in shallow aquifers to deep hydrocarbon reservoirs, as assessed from peer-reviewed paper in the literature. A variety of mechanisms such as the surface reaction-controlled dissolution mechanism, the preferential leaching-diffusion controlled mechanism, the diffusion-precipitation controlled dissolution mechanism and the interfacial dissolution-reprecipitation mechanism have been proposed to be responsible for the dissolution of feldspars. Feldspar dissolution rates can be affected by the crystal structure, Al/Si ordering, temperature, pH, surface area, organic acids, chemical affinity, and precipitation of secondary minerals. Five main dissolution rate laws have been used to describe feldspar dissolution rates, including the linear transition state theory (L-TST) rate law, non-linear TST rate law, parallel rate law, stepwave model rate law, and partial equilibrium law. The rate inconsistency between laboratory experiments and field observations is interpreted with hypotheses that include the armoring effects of the coating secondary minerals on feldspar surfaces, the possible effects of leached layers, the approach to saturation with respect to feldspars, the inhibition by absorbed Al ³⁺ on the feldspar surface, and the inhibition by simultaneous slow clay precipitation rates. The inorganic-original (meteoric water and deep hot water) and organic-original (kerogen and hydrocarbon degradation) hydrogen ion (H ⁺ ) in a fluid can probably act as a significant catalyzer of fast dissolution of feldspars in shallow aquifers and deep hydrocarbon reservoirs. Various mineral assemblages including extensively leached feldspars with a wide range of associated amounts of clay minerals and quartz cements can be identified in subsurface reservoirs under different geological conditions. Feldspar dissolution can generate enhanced secondary porosities and rock permeability in open geochemical systems at shallow depth or at a moderate-deep depth where faults develop widely. While in closed geochemical systems at moderate-deep depth, feldspar dissolution is likely to generate redistributional secondary porosities and to decrease rock permeability. Authigenic clay minerals formed following feldspar dissolution alter rock wettability and affect the charging and entrapment of hydrocarbons in reservoir. Feldspar alteration may promote hydrocarbon degradation by promoting bioactivity or by consuming low molecular weight organic acids and CO 2 produced via oil degradation. Further work should be conducted to study hydrocarbon-water-feldspar interactions in deeply buried hydrocarbon reservoirs. Feldspar alteration may promote CO 2 sequestration by consumption of H ⁺ , generation of HCO 3⁻ , and pH buffering of formation water. K-feldspar alteration may also promote illitization in interbedded mudstones by supplying K ⁺ .

Petrography, fluid-inclusion, isotope, and trace-element constraints on the origin of quartz cementation and feldspar dissolution and the associated fluid evolution in arkosic sandstones

Article

Full-text available

May 2018
AAPG BULL

Feldspar dissolution, quartz cementation, and clay cementation are significant diagenetic processes in deep-water fan feldspar-rich sandstones in the Shengtuo area, Dongying sag (East China). The timing and origin of these water-rock interactions as well as the paleofluids in which relevant chemical reactions occurred were deduced using data from microscopy, fluid-inclusion microthermometry, oxygen-isotope microanalysis, and trace-element microanalysis. Three distinct, separate episodes of quartz overgrowths (referred to as Q1, Q2, and Q3) were distinguished by cathodoluminescence microscopy. The Q1 quartz, identified in all porous sandstones from 2500 to 3600 m (8200 to 11,800 ft), was formed at approximately 100°C-115°C (212°F-239°F) before oil filled the reservoirs. The Q2 quartz was mainly precipitated at 115°C-130°C (239°F-275°F), accompanying or slightly postdating the main phase of oil filling, and was identified in samples from 2800 to 3600 m (9200 to 11,800 ft). The Q3 quartz was only identified in sandstones buried deeper than 3500 m (11,400 ft) and was likely precipitated in the Quaternary when temperature exceeded 130°C-135°C (266°F-275°F). Secondary ion mass spectrometry oxygen-isotope microanalyses yielded δ¹⁸OVSMOW (Vienna standard mean ocean water) values ranging from 21.42‰ to 24.35‰ for Q1 quartz, from 22.03‰ to 24.99‰ for Q2 quartz, and from 21.72‰ to 22.91‰ for Q3 quartz. A mass-balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones. Positive δ¹⁸O(water) values (+0.5‰ to +4.5‰ VSMOW) of quartz-forming waters indicate that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water. The variations of δ¹⁸O(cements) and trace-element compositions from Q1 quartz to Q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly; further, δ¹⁸O(water) values of the pore water increased by approximately 2‰ VSMOW after oil filling. Meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution. The detected CO2 and hydrocarbons in fluid inclusions in the quartz cements, the existence of CO2 in hydrocarbon-rich natural gas, and the pyrobitumen in the feldspar-hosted pores suggest that acids derived from organic matter (kerogen in source rocks and hydrocarbons in reservoirs) probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones. The ongoing development of Q2 and Q3 quartz suggests that quartz precipitation did not cease after oil filling. Further, diagenetic reactions likely have proceeded from water-rock interactions to hydrocarbon-water-rock interactions. Copyright © 2018. The American Association of Petroleum Geologists. All rights reserved.

Reactive transport modeling of coupled feldspar dissolution and secondary mineral precipitation and its implication for diagenetic interaction in sandstones

Article

Mar 2017
GEOCHIM COSMOCHIM AC

Guanghui Yuan

Dissolution of feldspars and precipitation of secondary minerals (kaolinite, illite and quartz) are significant diagenetic processes in arkosic sandstones. We examined moderately buried sandstones in the Eocene Shahejie Formation from two sags in the Bohai Bay Basin, East China. Three different types of mineral assemblages (MA) were identified: extensively leached feldspars with a large amount of authigenic kaolinite and quartz cement (MA-1), extensively leached feldspars with a large amount of authigenic kaolinite and minor quartz cement (MA-2), and extensively leached feldspars with a small amount of both authigenic kaolinite and quartz cement (MA-3).

Models of burial diagenesis for deep exploration plays in Jurassic fault traps of the Central and Northern North Sea

Chapter

Full-text available

Jan 1993

Stuart D Burley

With increasing burial depth, intergranular porosity declines in a predictable manner according to theoretical and empirically established mathematical relationships. In basins with simple structural and thermal histories, average porosity can often be represented as an exponential function of depth. There are, however, common significant deviations from such a simple model of porosity evolution. Four mechanisms are recognized by which enhanced porosity may be generated or maintained during burial: development of overpressures due to restricted pore fluid escape; inhibition of mechanical compaction by selective cementation; inhibition of cementation as a result of hydrocarbon accumulation; or generation of secondary porosity due to either grain or cement dissolution together with simultaneous removal of reaction products. Secondary porosity may be generated either near the surface by the ingress of dilute meteoric-derived waters or at depth by the generation of ‘aggressive’ pore fluids. Deep burial diagnesis may encompass both closed and open system porosity generating reactions. In closed systems, increase in temperature concomitant with burial overcomes kinetic barriers enabling dissolution of metastable detrital minerals. By contrast, in more open systems, movement of fluids from either mudstones or evaporites to either overlying or adjacent sandstones introduces chemical disequilibrium that may cause mineral dissolution. Rapid, intermittent movement of fluids along active faults may additionally introduce temperature disequilibrium to further facilitate mineral dissolution in the proximity of faults. Over geological periods of time, however, faults are barriers to fluid flow and provide a mechanism for generating overpressure. In the Central and Northern North Sea basins, an upper diagenetic regime is separated from a lower diagenetic regime by a regional cover of Cretaceous mudstones and chalks. The Cretaceous sediments provide a regional seal which often coincides with the development of overpressure above deep graben centres reflecting restricted pore water escape. Fluid and solute movement between these two regimes is only possible where faults or gas chimneys penetrate the Cretaceous seal. In the upper regime, porosity loss in Tertiary sediments broadly conforms to the predicted decline of porosity with depth. In the lower regime, Jurassic sandstones on structural highs are characterized by extensive secondary porosity resulting from feldspar grain and carbonate cement dissolution, best preserved where oil accumulation rapidly follows porosity generation. Enhanced porosity can be expected either where sandstones interdigitate with mudstones or in graben centre plays adjacent to faults. This contrasts with graben margin plays where faults are the locus of pronounced cementation and complex diagenetic sequences are developed. In the absence of either oil accumulation or overpressure, secondary porosity either compacts or is occluded by a characteristic deep burial authigenic mineral assemblage. The present heat flow in the North Sea can be interpreted in terms of a residual thermobaric fluid flow from basin centres to basin margins which was probably initiated during the early Tertiary, coincident with the onset of rapid thermal relaxation subsidence.

Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group, Murchison Field, North Viking Graben (northern North Sea)—reply

Article

Oct 1994
SEDIMENT GEOL

Diagenesis of the Hild Brent Sandstones, Northern North Sea: Isotopic Evidence for the Prevailing Influence of Deep Basinal Water

Article

Nov 2002

The deeply buried (3.8-4.3 km) Brent reservoir sandstones of the Hild Field, Northern North Sea, have undergone intense diagenetic alteration. Fluid inclusion, O-C-H-Sr isotope and K-Ar age data combined with petrography and burial history constrain the conditions and timing of the main diagenetic transformations as well as the nature and origin of diagenetic fluids. Non-ferroan calcite cement formed at shallow depth (< 2 km) and low temperature (20-70°C) from predominantly marine-brackish connate water. Subsequently, pore-filling vermicular kaolinite and porelining illite, developed at shallow-intermediate burial depth and temperatures (< 100°C) during the Cretaceous, from water that could not be precisely constrained. The late diagenetic events include precipitation of quartz overgrowths, pore-filling ferroan calcite, pore-filling dickite, and pore-bridging, fibrous illite. They occurred at depths of 2.5 to 4.3 km and temperatures of 95 to 155°C from the Early Tertiary to the present. The late, deep-burial cements formed from evolved basinal waters, isotopically similar to present-day formation water, that originated from deeply buried mudstones in the nearby Viking graben and, to a lesser extent, in the East Shetland Basin. Introduction of these compactional waters in the reservoir began in the Late Cretaceous-Early Tertiary and both preceded and accompanied hydrocarbon migration. The reconstructed diagenesis of the Hild Brent reservoirs displays notable differences with diagenetic models developed in many fields of the northern North Sea. In particular, diagenesis in Hild is characterized by the prevailing influence of deep basinal water, a feature related to the immediate proximity of the Viking Graben. This study further illustrates that diagenetic processes related to compaction-derived fluids, commonly reported in sediments located within rift basins, should also be expected to occur in neighboring sub-basins along rift margins.

Reservoir Quality of Upper Jurassic Corallian Sandstones, Weald Basin, UK

Article

Full-text available

Oct 2021

The Upper Jurassic, shallow marine Corallian sandstones of the Weald Basin, UK, are significant onshore reservoirs due to their future potential for carbon capture and storage (CCS) and hydrogen storage. These reservoir rocks, buried to no deeper than 1700 m before uplift to 850 to 900 m at the present time, also provide an opportunity to study the pivotal role of shallow marine sandstone eodiagenesis. With little evidence of compaction, these rocks show low to moderate porosity for their relatively shallow burial depths. Their porosity ranges from 0.8 to 30% with an average of 12.6% and permeability range from 0.01 to 887 mD with an average of 31 mD. The Corallian sandstones of the Weald Basin are relatively poorly studied; consequently, there is a paucity of data on their reservoir quality which limits any ability to predict porosity and permeability away from wells. This study presents a potential first in the examination of diagenetic controls of reservoir quality of the Corallian sandstones, of the Weald Basin’s Palmers Wood and Bletchingley oil fields, using a combination of core analysis, sedimentary core logs, petrography, wireline analysis, SEM-EDS analysis and geochemical analysis to understand the extent of diagenetic evolution of the sandstones and its effects on reservoir quality. The analyses show a dominant quartz arenite lithology with minor feldspars, bioclasts, Fe-ooids and extra-basinal lithic grains. We conclude that little compactional porosity-loss occurred with cementation being the main process that caused porosity-loss. Early calcite cement, from neomorphism of contemporaneously deposited bioclasts, represents the majority of the early cement, which subsequently prevented mechanical compaction. Calcite cement is also interpreted to have formed during burial from decarboxylation-derived CO2 during source rock maturation. Other cements include the Fe-clay berthierine, apatite, pyrite, dolomite, siderite, quartz, illite and kaolinite. Reservoir quality in the Corallian sandstones show no significant depositional textural controls; it was reduced by dominant calcite cementation, locally preserved by berthierine grain coats that inhibited quartz cement and enhanced by detrital grain dissolution as well as cement dissolution. Reservoir quality in the Corallian sandstones can therefore be predicted by considering abundance of calcite cement from bioclasts, organically derived CO2 and Fe-clay coats.

The dating of petroleum fluid residence time in subsurface reservoirs. Part 1: A radiolysis-based geochemical toolbox

Article

Jul 2019
GEOCHIM COSMOCHIM AC

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Chemo-mechanical Alteration of Silicate-Rich Shale Rock after Exposure to CO2-Rich Brine at High Temperature and Pressure

Article

Full-text available

Dec 2023
ROCK MECH ROCK ENG

Owing to their application in rock weathering concerning geostructural stability, enhanced geothermal systems, carbon sequestration, and enhanced oil recovery, the effect of rock–brine–CO2 interactions on the microstructural and mechanical properties of rocks has become a prevalent topic. Understanding the interplay among chemical, microstructural, and mechanical processes is essential to comprehend how they affect rock mechanical alteration. In this study, we examined the effects of chemo-mechanical loading on the microstructural features and mechanical alterations of individual components within the rock. Experiments involved exposing the Permian rock samples to either CO2 or N2-rich brine (a control condition) at a temperature and pressure of 100 °C and 1800 Psi, respectively, for varying duration (14 and 28 days). The ionic strength of the solution was adjusted to 1 M using NaCl. Micro-CT image analysis showed the dissolution of clay- and quartz-rich phases followed by their precipitation. After 14 days, the depth of the outer reacted zone reached roughly 1100 µm, and after 28 days, the depth increased to 1500 µm. Microscale mechanical analysis showed decreased indentation modulus of the clay- and quartz-rich phases after reacting with CO2-rich brine. This decrease in indentation modulus was more than 50% for quartz-rich phases for 28 days of reaction and was lower adjacent to the reacted surface. The decrease in mechanical properties was more pronounced at a distance of 400–600 µm from the reacted surface after 14 days of reaction with CO2-rich brine due to the pore-size controlled solubility phenomenon. Experiments conducted at a greater distance from the reacted surface (approximately 5 mm) revealed a weaker clay–quartz interface, possibly due to the formation of microcracks induced by the swelling of clay particles. Results for the N2 condition show a superficial mechanical alteration of the rock constituents limited to a depth of 200 µm from the reacted surface.

In Situ Stress Distribution in Cretaceous Ultra-Deep Gas Field From 1D Mechanical Earth Model and 3D Heterogeneous Geomechanical Model, Kuqa Depression, Tarim Basin, NW China

Article

Full-text available

Jul 2022

The Kuqa Depression boasts rich cretaceous ultra-deep hydrocarbon resources. However, it is in complex geological conditions. At present, sufficient understandings on the in situ stress distribution and influencing factors are lacking, which restricts the process of hydrocarbon exploitation. Therefore, in this study, the Bozi gas field is selected as an example, and a 1D mechanical earth model (1D MEM) is established with the drilling data and logging data through the geomechanical method to clarify the in situ stress distribution of the wellbore. A 3D heterogeneous geomechanical model (3D HGM) is established with the constraint of 1D HEM to clarify the distribution characteristics of the 3D in situ stress field in the Bozi gas field and discuss its influencing factors. The results show that: 1) the Bozi gas field is in an extremely strong in situ stress condition with high stress values. The minimum horizontal principal stress (Sh) of the cretaceous system is 153∼180 MPa, and the maximum horizontal principal stress (SH) is nearly 200 MPa; 2) the in situ stress in the Bozi gas field has obvious vertical stratification characteristics, which can be divided into three stress sequences of “low–high–low”, with great differences in interlayer stress; 3) the in situ stress distribution of the Bozi gas field is greatly affected by the types of faulted anticline. Different types indicate different stress distribution; 4) within the influence range of overthrusts, the in situ stress in the footwall is lower than that of the hanging wall. The greater the fault offset, the greater the in situ stress difference between the hanging wall and footwall. Moreover, the lower the stress in the footwall, the higher is the degree of overthrust, and the larger is the range of footwall stress area; and 5) the means of highly deviated wells is more helpful to the Bozi gas field for hydrocarbon exploitation.

Illite and chlorite cementation of siliciclastic sandstones influenced by clay grain cutans

Article

Jul 2021
MAR PETROL GEOL

The distribution and amount of clay rim cements are different for Permian Rotliegend and Lower Triassic Bunter sandstones in the Southern Permian and German Triassic Basins, respectively. Both are similar fluvial-aeolian deposits in hot-arid endorheic basins. In both Permian and Triassic sandstones clay grain cutans can be present, but clay rim cement is often lacking or rare in Bunter sandstones. At first sight it would appear that the presence of cutans is thus not relevant for the development of clay cementation. However, at closer inspection, it appears that authigenic clay is indeed present in both cases. In Rotliegend sandstones, the authigenic clay mainly developed as rims around the clay grain cutans, which are often thin and well microlaminated with few intra-micropores and composed of platy particles. The clay crystals in the rim decrease the pore interconnectivity and lower permeability significantly. In Bunter sandstones, most of the authigenic clay developed within the cutans. These cutans are less well microlaminated, have ample micropores, and are composed of more equant-shaped particles. The latter looser structure facilitated clay authigenesis within the cutans and their micropores. This often led to exfoliation of the cutan laminae and expansion of the entire cutan. These expanded cutans also lower pore connectivity. The presence and thickness of clay rim cement on top of cutans and grains is thus not correlated with the thickness of the cutans but with the texture of the cutans. The latter determined where authigenic clays precipitated and the thickness of cutans is partly the result of clay authigenesis within the cutans. This demonstrates that the composition and the texture of sedimentary components constrained and controlled burial diagenesis.

Influence of detrital composition on the diagenetic history of tight sandstones with implications for reservoir quality: Examples from the Permian Xiashihezi Formation and Carboniferous Taiyuan Formation, Daniudi gas field, Ordos Basin, China

Article

May 2017

How Can Technical Aspects Help Improving K-Ar Isotopic Data of Illite-Rich Clay Materials into Meaningful Ages? The Case of the Dominique Peter Uranium Deposit (Saskatchewan, Canada)

Article

Full-text available

Jul 2020

Norbert Clauer

Previously published K-Ar dating results of <2 μm illite separates from uranium-hosting and associated barren rocks from Dominique Peter district of the Carswell circular structure in the Athabasca Basin (Canada) were considered to trace four distinct tectonic-thermal events at 1447 ± 45, 1282 ± 40 and 1184 ± 15 (all errors at ± 2 Ma, with a younger, less constrained episode at ~900 Ma. Recent analyses of K-Ar ages of additional <0.2 μm illite fractions from a few initial samples demonstrate that the intermediate ages at ~1280 and ~1185 Ma result, in fact, from mixing of two generations of illite that precipitated at ~1450 and ~900 Ma. They have, therefore, no stratigraphic value, while the two tectonic-thermal episodes at 1448 ± 48 and 937 ± 39 Ma appear to be historically sound. In fact, the analytical procedure of isotopic dating clay materials is of more importance than is often stated. For instance, a safe way to evaluate and constrain best numerical isotopic data of clay separates into ages is combining data of two size splits from several samples. If such age data, especially from size fractions of indurated host rocks, are scattered and point towards the higher data of the coarser fractions, they are potentially enriched in earlier crystallized K-rich components, and should consequently be discarded. The occurrence of detrital or early-crystallized components in clay-rich separates becomes a serious concern when comparing ages generated by various isotopic methods on mineral separates of various whole rocks. It is especially verified in very old, metal-rich deposits such as the uranium-rich deposits of the Saskatchewan Basin. These deposits and their host rocks were studied extensively by a large spectrum of isotopic methods on many types of rocks in a widely dispersed area, and for which the numerical statistics became, sometimes, more central in the interpretation than the specificity of the successive events in the host rocks.

Linking the influence of diagenetic properties and clay texture on reservoir quality in sandstones from NW Borneo

Article

Jun 2020
MAR PETROL GEOL

Linking the influence of diagenetic properties and clay texture on reservoir quality in sandstones from NW Borneo

Article

Jun 2020
MAR PETROL GEOL

Reservoir studies in the Sandakan sub-basin NW Borneo are challenging and inexplicable due to complex diagenetic effects such as compaction, cementation, dissolution and depositional means including grain size, sorting and packing with reference to clay texture. These diagenetic effects together impart significant amounts of uncertainty on reservoir quality assessment. In this regard, we documented the mineralogical, diagenetic and reservoir properties of sandstones from the Sandakan Formation, NW Borneo through thin sections, SEM, XRD, XRF, He-porosimeter and poroperm. In order to analyze the influential parameters, we used matrix, grain rearrangement and clay coating around detrital grains for the reservoir quality. Our results show that the sub-litharenites to quartz arenite that dominate in the sandstones are drawn from recycled orogenic sand. Furthermore, reservoir properties of the sandstones were ascertained to be controlled by diagenesis, micro quartz with clay coatings and pressure dissolution of feldspar with the alteration of lithic fragments. The result shows that the physical compaction, rearrangements and ductility of grains were the reasons that reduced porosity and permeability. The main reason for porosity reduction in the present study is pressure dissolution that resulted in the form of chemical compaction induced by micas with grain contacts. Nonetheless, through SEM and petrography, clay coatings around the grains and dissolution of feldspar were found to optimally contribute to the preservation of reservoir properties. Dissolution of feldspar and kaolinization resulted in the formation of secondary porosity that allows meteoric water to percolate through the open and detachment spaces of quartz or other grains. This percolation is also responsible in enhancing the pore throat radii in the reservoir media of the analyzed sandstones. These traits are obliging to diagenetic effects on porosity and permeability, which are linked with the physical compaction and clays coating to reduce and preserve the reservoir properties respectively of same type of reservoirs globally.

A renewed look at calcite cement in marine-deltaic sandstones: the Brent Reservoir, Heather Field, Northern North Sea, UK

Article

Full-text available

Feb 2019

Wireline and seismic acoustic impedance imaging show that the marine part of the clastic Brent Group reservoir in the Heather Field, northern North Sea, contains much calcite cement in the flank parts of the structure. The non-marine Ness Formation and crest parts of the structure contain negligible calcite cement. This localized calcite cement has led to relatively poor reservoir performance since first oil in 1978, although a new suite of wells has boosted production with plans to keep the field active until 2030. Understanding the origin and distribution of calcite cement would help the development of more realistic reservoir models and boost production rates through optimum well location. We have thus used a suite of techniques, including standard point counting, SEM-EDS mineralogy, BSE microscopy, fluid inclusion thermometry and stable isotope analysis, to develop new and improved models of calcite distribution. Calcite seems to have attributes of both early and late diagenetic cement. A 30–40% intergranular volume in calcite cemented beds seems to support pre-compactional growth but high-temperature fluid inclusions and the presence of primary oil inclusions suggest late growth. Much calcite may have developed early but it seems to have recrystallized, and possibly undergone redistribution, at close to maximum burial or had a late growth event. Calcite cement probably originated as marine-derived micrite, bioclasts or early marine cement but adopted the isotopic characteristics of high-temperature growth as it recrystallized. Quartz grains have corroded outlines in calcite-cemented areas with one sample, with 79% calcite cement, displaying signs of nearly total replacement of quartz grains by calcite. The flank localization of calcite cement remains to be explained, although it could be due to primary depositional factors, early diagenetic loss of calcite from crestal regions or late diagenetic loss of calcite from crestal regions. Controversially, the growth of calcite seems to be associated with quartz dissolution, although the geochemical and petrophysical cause of this remains obscure. Diagenetic loss of quartz from sandstones cannot easily be explained by conventional modelling approaches and yet seems to be an important phenomenon in Heather sandstones.

Clay Cutans and the Origin of Illite Rim Cement: An Example from the Siliciclastic Rotliegend Sandstone in the Dutch Southern Permian Basin

Article

Full-text available

May 2018

In Permian Rotliegend sandstones in the Dutch part of the Southern Permian Basin illite can be a practical annoyance because it strongly influences reservoir properties. Despite its practical importance, the origin and in particular the prediction of the distribution of authigenic illite in siliciclastic sandstone is still under discussion. Three main types of authigenic illite exist, each of which result from different diagenetic processes at different stages during burial. The earliest illite is tangential illite, parallel to the grain surfaces. It has formed by infiltration of suspended clay-size particles into loose surficial sands and can be interpreted as cutanic clay. The clay cutans were occasionally formed in situ but are mostly inherited with grains having been reworked from interdune and sabkha areas by fluvial and aeolian action. The second illite type is authigenic and occurs on top of the clay cutans. Commonly two or three illite generations with different crystal habit are present, ranging from platy to hairy. Clay cutans were nucleation sites and generated the mass for the illite rim cement. Different amounts and habits of illite rim cement are linked to differences in the thickness and completeness of the clay cutans. The more iron oxide in the cutans, the less authigenic illite formed. The illite rim cement precipitated after mechanical compaction, and is probably related to illitization of smectitic clay in the cutans. Illite rim cement is thus related to the local paleogeography of the basin, including aridisol development and erosion of such soils. The third type of authigenic illite occurs in secondary pores after dissolution of detrital K-feldspar or rock fragments containing K-feldspar. The latter illite is not the product of kaolinite illitization but it precipitated as a primary mineral without a kaolinite precursor. The illite in secondary pores after feldspar dissolution thus depends on the detrital composition. Only the illite rim cement reduces intergranular porosity to microporosity, thereby increasing the specific surface area. This results in a high irreducible water content. In consequence the permeability for a given porosity can be reduced by up to two orders of magnitude. The other two types of illite have no significant influence on reservoir properties. © 2018 SEPM Society for Sedimentary Geology. All rights reserved.

Article

Sep 2017
MAR PETROL GEOL

Diagenetic alterations related to falling stage and lowstand systems tracts of shelf, slope and basin floor sandstones (Eocene Central Basin, Spitsbergen)

Article

Full-text available

Jan 2012

Diagenetic alterations and detrital composition of shelf slope and basin floor litharenitic to sublitharenitic sandstones from the Eocene Central Basin of Spitsbergen display fairly systematic variations among the falling stage (FSST) and lowstand systems tracts (LST). The diagenetic processes in both systems tracts include mechanical and chemical (pressure dissolution of quartz grains) compaction, kaolinitization of detrital silicates such as mica and feldspar, cementation by carbonates and quartz overgrowths and illitization of kaolinite. Overall, compaction is more important in porosity destruction than cementation due to the abundance of ductile grains. Kaolinitization of detrital silicates is more common in the FSST than in the LST sandstones due to greater meteoric-water flux during sea-level fall. However, the mechanisms of meteoric water flux into the FSST slope deposits and basin-floor fan sandstones is enigmatic but could have been as a consequence of hydraulic head creation along the basin margin during a major fall in the relative sea-level. Cementation by carbonates such as calcite and dolomite occurred in both FSST and LST sandstones, being most extensive in sandstones immediately below marine and maximum flooding surfaces, which is partly attributed to the presence of detrital carbonate grains below these surfaces. The Eocene Central Basin of Spitsbergen is a potential analogue for the study of other deep-sea reservoirs in similar basinal settings in which reservoir quality assessment is of considerable importance. This study shows that constructing a conceptual model for the distribution of diagenetic alterations and thus reservoirquality evolution in deep-marine turbiditic sandstones is possible by integrating diagenesis and sequence stratigraphy.

CO2 interaction with aquifer and seal on geological timescales: the Miller oilfield, UK North Sea

Thesis

Jan 2008

Jiemin Lu

Carbon Capture and Storage (CCS) has been identified as a feasible technology to reduce CO2 emissions whilst permitting the continued use of fossil fuels. Injected CO2 must remain efficiently isolated from the atmosphere on a timescale of the order of 10000 years and greater. Natural CO2-rich sites can be investigated to understand the behaviour of CO2 in geological formations on such a timescale. This thesis examines the reservoir and seal on one such oilfield. Several hydrocarbon fields in the South Viking Graben of the North Sea naturally contain CO2, which is thought to have charged from depth along the western boundary fault of the graben. The Miller oil field which contains ~ 28 mol% CO2, of isotopic composition δ13C = -8.2‰. The Upper Jurassic Brae Formation reservoir sandstones and the Kimmeridge Clay Formation (KCF) seal have been exposed to the CO2 accumulation since its emplacement. Rock samples from the reservoir sandstone and bottom of the seal mudrock were examined using multiple techniques, including XRD, SEM, fluid inclusion and carbonate stable isotope analyses. The sandstones show no features directly attributable to abundant CO2 charge. SEM analyses reveal significant heterogeneities in diagenesis within the KCF. The silt/sand lithologies of the KCF have undergone a diagenetic history similar to that of the Brae Formation sandstones. In contrast, the KCF shales display a distinctly different diagenesis of dominant dissolution of quartz and feldspar with little evidence of mineral precipitation. In both the Brae Formation and the KCF, pore-filling kaolinite, illite and carbonates are relatively late diagenetic events which can be associated with CO2-induced feldspar dissolution. Mudrock X-ray diffraction mineralogical data reveal abrupt vertical mineralogical variations across the reservoir crest in the Miller Field, while such variations are absent in a low-CO2 control well in the same geological settings. This suggests that reactions induced by abundant CO2 dissolved feldspar and produced kaolinite, carbonates and quartz in the seal, while oil emplacement inhibited the reactions in the oil leg. However, petrographic evidence and comparison between different sections argue against CO2 reactions as the sole cause for such large mineralogical variations, especially for quartz. The vertical mineralogical variations to a certain extend represent original sedimentary heterogeneity. Linear variations of carbonate δ13C with depth were discovered in both shale and silt/sand lithologies of the KCF in a 12m zone immediately above the reservoir. These features are absent in the low-CO2 control well. These trends are interpreted as dissolution of original carbonates by CO2 slowly ascending from the reservoir. New carbonates precipitated from a carbon source with upwards decreasing δ13C due to mixing between three carbon sources with different C isotopes at systematically varying ratios. The isotopes in the reservoir and the bottom of the seal suggests initial CO2 charge at about 70-80 Ma. CO2 infiltration rate is estimated at about 9.8×10-7g·cm-2·y-1. Geochemical modelling was applied to reconstruct the reservoir fluid evolution by calibrating it to mineralogy, fluid chemistry, diagenesis and fluid inclusion data. The modelling suggests that CO2 migrated into the reservoir together with a saline basinal fluid derived from the underlying evaporites at ~ 70 Ma. The CO2 and basinal water charge imposed an important influence on the mineral reactions and fluid chemistry. This study suggests that the KCF has formed an excellent CO2 seal, with no substantial breach since its charge at 70-80 Ma.

Albitite from the West Driefontein Mine, South Africa

Article

Full-text available

Mar 1998

Andesitic magma with alkalic affinity intruded the Upper Jeppestown Subgroup of the Witwatersrand Supergroup in the West Driefontein Mine as a network of dykes. In the southwestern part of the mine a composite dyke, belonging to the network and consisting of amphibole stilpnomelane albitite, is exposed in the 23-43 CL Cross Cut, 60 m below the Carbon Leader Reef. The dyke comprises three separate intrusions that are probably related through fractional crystallization at deeper levels. Albitization of feldspar and uralitization of magmatic mafic minerals, as well as the neoformation of actinolitic amphibole and stilpnomelane transformed the original dioritic rock into amphibole stilpnomelane albitite. Trace-element discrimination diagrams show similarities to Goedgenoegd and Rietgat Formations of the Platberg Group, Ventersdorp Supergroup. Rb-Sr data indicate a date of 1933 ± 54 Ma, which is regarded as the age of hydrothermal alteration during a prolonged metamorphic event related to the intrusion of the Bushveld Complex.

Diagenetic Alterations Related to Falling Stage and Lowstand Systems Tracts of Shelf, Slope and Basin Floor Sandstones (Eocene Central Basin, Spitsbergen)

Chapter

May 2013

This chapter shows that constructing a conceptual model for the distribution of diagenetic alterations and thus reservoir quality evolution in deep-marine turbiditic sandstones is possible by integrating diagenesis and sequence stratigraphy. It provides data highlighting that the detrital composition and distribution of some diagenetic alterations, including kaolinitization of the detrital silicates and cementation by carbonate displays fairly systematic variation within the sequence stratigraphic context of the falling stage and lowstand shelf, slope and basin floor sandstones of the Eocene Central Basin, Spitsbergen. Wave reworking across marine flooding and maximum flooding surfaces resulted in incorporation of detrital carbonate grains into the sand deposits, which acted as potential sources and nucleation sites for carbonate cementation.

Open versus closed mesogenetic systems in Cretaceous fluvial and tidal sandstones, Sirt Basin, Libya

Article

Full-text available

Apr 2014
GEOARABIA

This study constrains factors controlling the distribution of diagenetic alteration and their impact on reservoir quality of the Cretaceous sandstones from the Al- Bayda Platform, located in the southern Sirt Basin (Libya). These factors include the presence of early cements as well as the influx of hot basinal brines. The studied samples come from two blocks in the Khalifa Field, which are dislocated by a major normal fault. The deep-burial (mesogenetic) alteration includes the partial to pervasive replacement of early (eogenetic) dolomite and calcite cements by ferroan-dolomite, ankerite and siderite, precipitation of grain-coating chlorite, and cementation by quartz overgrowths, barite and anhydrite, particularly in the downthrown block. The association of quartz overgrowths with barite suggests that deep burial was influenced by the influx of hot basinal brines through faults. Conversely, deep-burial alteration in braided fluvial deposits of the Nubian sandstones of the upthrown block include: illitization of eogenetic smectite, quartz cementation and formation of chlorite. This study shows that deep burial of the studied sandstones did not occur in a closed system, but was affected by the influx of hot basinal brines through faults, which formed during basin rifting. This interpretation is supported by the relatively high homogenization temperatures (100–110°C) of primary fluid inclusions within quartz overgrowths, which exceed the maximum burial temperatures experienced by the Cretaceous succession, and by the high salinity of these inclusions.

Sedimentary environment and diagenesis of the Lower Cretaceous Chaswood Formation, southeastern Canada: The origin of kaolin-rich mudstones

Article

Jul 2005
SEDIMENT GEOL

The Lower Cretaceous fluvial sandstone–mudstone succession of the Chaswood Formation is the proximal equivalent of offshore deltaic rocks of the Scotian Basin that are reservoirs for producing gas fields. This study interprets the mineralogical consequences of Cretaceous weathering and early diagenesis in a 130-m core from the Chaswood Formation in order to better understand detrital and diagenetic minerals in equivalent rocks offshore. Mineralogy was determined by X-ray diffraction, electron microprobe analysis and scanning electron microscopy. The rocks can be divided into five facies associations: light gray mudstone, dark gray mudstone, silty mudstone and muddy sandstone, sorted sandstone and conglomerate, and paleosols. Facies transitions in coarser facies are related to deposition in and near fluvial channels. In the mudstones, they indicate an evolutionary progression from the dark gray mudstone facies association (swamps and floodplain soils) to mottled paleosols (well-drained oxisols and ultisols following syntectonic uplift). Facies transitions and regional distribution indicate that the light gray mudstone facies association formed from early diagenetic oxidation and alteration of the dark gray mudstone facies association, probably by meteoric water. Principal minerals in mudstones are illite/muscovite, kaolinite, vermiculite and quartz. Illite/muscovite is of detrital origin, but variations in abundance show that it has altered to kaolinite in the light gray mudstone facies association and in oxisols. Vermiculite developed from the weathering of biotite and is present in ultisols. The earliest phase of sandstone cementation in reducing conditions in swamps and ponds produced siderite nodules and framboidal pyrite, which were corroded and oxidized during subsequent development of paleosols. Kaolinite is an early cement, coating quartz grains and as well-crystallized, pore-filling booklets that was probably synchronous with the formation of the light gray mudstone facies association. Later illite and barite cement indicate a source of abundant K and Ba from formation water. This late diagenesis of sandstone took place when the Chaswood Formation was in continuity with the main Scotian Basin, prior to Oligocene uplift of the eastern Scotian Shelf. Findings of this study are applicable to other mid-latitude Cretaceous weathering and early diagenetic environments.

Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group, Murchison Field, North Viking Graben (northern North Sea)

Article

Nov 1993
SEDIMENT GEOL

Up to four calcite-cemented horizons (doggers) form impermeable barriers to fluid flow within the Middle Jurassic Rannoch Formation and are correlatable across the Murchison Field. Calcite precipitated during early diagenesis, within high porosity/permeability sandstones at the top of coarsening (shoaling) upward shoreface cycles. Calcite δ¹³C and δ¹⁸O compositions range from -4.1 to -13.4‰ PDB, and -6.6 to -16.7‰ PDB, respectively. Sr concentrations of up to 1334 ppm are consistent with marine carbonate sources (probably shell fragments), but no viable intraformational carbonate source has been identified in the Murchison Field area. Initial ⁸⁷Sr/⁸⁶Sr compositions (0.71109–0.71266) are higher than Middle Jurassic seawater (0.7073), and consistent with precipitation from modified porewaters containing significant proportions of continentally derived “meteoric” fluids enriched in ⁸⁷Sr as a result of basement weathering, or percolation through hinterland soils/unconsolidated detritus. An internal source of ⁸⁷Sr is not considered viable in view of the high proportion (up to 25‰ clastic constituents) of unaltered detrital alkali feldspar and mica within the Rannoch Formation.

Evidence for resetting of fluid inclusions from quartz cements in oil fields - discussion

Article

Jan 1995

Clay Diagenesis in the Sandstone Reservoir of the Ellon Field (Alwyn, North Sea)

Article

Full-text available

Jun 1999

The nature, composition, and relative abundance of clay minerals in the sandstones of the Brent Group reservoir were studied between 3200–3300 m in a well of the Ellon Field (Alwyn area, North Sea). The sandstones have a heterogeneous calcite cement which occurred during early-diagenesis. Clay diagenesis of the cemented and uncemented sandstones was investigated using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction analyses (XRD), and infrared spectroscopy (IR). The influence of cementation on clay neoformation is demonstrated in this study. Detrital illite and authigenic kaolinite are present in both the calcite-cemented and uncemented sandstones suggesting that kaolinite precipitated before calcite cementation. In the uncemented sandstones, blocky dickite replaces vermiform kaolinite with increasing depth. At 3205 m, authigenic illite begins to replace kaolinite and shows progressive morphological changes (fibrous to lath-shape transition). At 3260 m, all sandstones are not cemented by calcite. Illite is the only clay mineral and shows a platelet morphology. In the cemented samples, vermiform kaolinite is preserved at all depths, suggesting that dickite transformation was inhibited by the presence of the calcite cement. This observation suggests that calcite cement would prevent fluid circulation and dissolution-precipitation reactions.

Genetic Model and Exploration Guidelines for Kaolin Beneath Unconformities in the Lower Cretaceous Fluvial Chaswood Formation, Nova Scotia

Article

Jan 2006
Explor Min Geol

Potentially commercial deposits of kaolin are found in the fluvial Lower Cretaceous Chaswood Formation, the sedimentologically proximal equivalent of deltaic sediments of the offshore Scotian basin. The geological setting of the kaolin deposits has been interpreted from high-resolution seismic-reflection profiles and boreholes, and mineralogical studies on one reference borehole. The kaolin is a product of early diagenesis involving meteoric water below the water table. The distribution of kaolin is controlled by the presence of intraformational unconformities that were sites of ground water recharge, and are marked by the development of oxisols above the water table. Such diagenesis was facilitated by interbedded, loosely consolidated, permeable sandstone. The highest-grade kaolin deposits are found in areas close to river belts, where overbank muds were not sufficiently drained to develop paleosols, but where later uplift created intraformational unconformities. In the sandstone, meteoric water flow altered feldspars to kaolin, thus yielding commercial silica sand deposits. It also altered ilmenite to rutile, which is concentrated as secondary placers in modern rivers that have eroded the Chaswood Formation and tills derived therefrom.

Closed-System Burial Diagenesis in Reservoir Sandstones: Examples from the Garn Formation at Haltenbanken Area, Offshore Mid-Norway

Article

Full-text available

Jan 2001

The Middle Jurassic Garn Formation of the Haltenbanken area has been studied using mineralogical and geochemical data from 21 wells, ranging in burial depths from 2.0 to 4.1 km relative to seafloor (RSF). K-feldspar and plagioclase contents show variations on a re- gional scale both laterally and as a function of burial depth. The con- tent of pore-filling authigenic illite increases sharply, and the content of K-feldspar and kaolinite decreases in Garn sandstones presently at depths greater than 3.6-3.7 km RSF (120-130 8C). The depletion in K- feldspar below 3.7 km RSF is not accompanied by lower potassium values in the bulk chemical composition (wt % K 2O). This suggests that the potassium released during K-feldspar dissolution is retained in the sandstones and is precipitated as illite. The variations in bulk contents of potassium and sodium are therefore considered to be re- lated principally to primary variations in sandstone mineralogy. The shallower sandstones ( , 3.7 km RSF) with average wt % K 2O greater than 0.95 (K/Al molar ratio . 1/3) have a K-feldspar:kaolinite ratio greater than one. The deeply buried (. 3.7 km RSF) sandstones with similar potassium contents contain excess K-feldspar and most of the kaolinite is illitized. However, deeply buried sandstones containing an average of 0.38 wt % K 2O (K/Al molar ratio , 1/4) contain a significant amount of kaolinite but negligible K-feldspar. This suggests that the K-feldspar:kaolinite ratio before the onset of illitization was less than one, and hence that the kaolinite-illite reaction has been re- stricted by an insufficient supply of potassium (absence of K-feldspar). This illustrates how illitization of kaolinite depends upon K-feldspar as a local source of potassium. Prediction of illitization in sandstones, therefore, must be based on integration of models for provenance, fa- cies, and early diagenesis in addition to burial and thermal history. The formation of pore-filling authigenic illite in these sandstones is an important influence on the total reservoir quality.

How does the burial rate control the diagenesis of sandstone? Insights from a diagenetic physical simulation experiment

Article

Apr 2024
CHEM GEOL

Brine reflux as a possible first order control on the alkali geochemistry of baseline sedimentary rocks in the Proterozoic North Australian Zinc Belt

Article

Jun 2023
PRECAMBRIAN RES

The use of a Multi-Sensor Core Scanner Workflow as the Backbone of a Digital Core Repository

Article

Mar 2023

James William Shreeve

A core repository is a physical record of a country's or commercial organization's subsurface wealth. Some of the largest core repositories hold 1000s of km of core material and it is a challenge to turn this physical archive into an accessible digital resource for all. Non-destructive multi-sensor core logger (MSCL), hyperspectral and X-ray imaging techniques offer a unique chance to rescue valuable data trapped within core samples, improving the way that a core repository delivers data to academic or industrial end users. Here we present a case study of an archived petroleum core acquired in 1985 at the Osprey Field, United Kingdom Continental Shelf (UKCS). Data from the UK National Data Repository are augmented by a MSCL, Hyperspectral and X-ray dataset that is uploaded into a cloud-based digital repository. The data were analysed using a multi-variant analysis to reclassify the original lithological interpretations uncovering a greater proportion of clay and cemented horizons than was previously interpreted. A workflow is established to optimize the use of legacy cores and exploit the abundance of data trapped within the core repository using continuous multi-sensor core scanning and imaging data which are stored within the virtual environment for visualization and access to all.

Differential development characteristics of secondary pores and effects on pore structure and movable fluid distribution in tight gas sandstones in the lower Permian, northeastern Ordos Basin, China

Article

Feb 2023

An integrated thermal and isotopic study of the diagenesis of the Brent Group, Alwyn South, UK North Sea

Article

Full-text available

May 1995
APPL GEOCHEM

The petrography, fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore-fluid history of the Middle Jurassic Brent Group reservoir sandstones in the Alwyn South area of the U.K. North Sea. The study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels. The cement assemblage of kaolinite, quartz and illite has resulted in severe deterioration of otherwise good reservoir quality. Early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation. Temperature estimates for kaolinite precipitation of 80°C andδ18O of ≈ + 15‰ (±3‰) suggest co-existing fluids ofδ18O ≈ −3‰. Quartz cementation overlapped both kaolinite and illite formation. Fluid inclusion data indicate that quartz cementation took place at temperatures of 109±7°C. Pore fluid salinities were ≈4 wt% NaCl with a H2OO isotopic composition of ≈ -1 %o ± 0.5‰ SMOW. The fluids which precipitated coexisting illite were compositionally homogeneous with equilibriumδ18O water compositions of +0.5‰ SMOW. Illite SD values range from −33 to −50‰ SMOW. These fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform, marine signature. This is consistent with the predominantly marine to paralic depositional context of the Brent Group in Alywn South. Illite precipitation was followed by hydrocarbon emplacement between the Middle Eocene and Lower Oligocene.

The Value of Core in Mature Field Development – Examples from the UK North Sea

Article

Aug 2022

Core-based studies have had material impacts on the understanding of a number of late-life, mature North Sea Brent Group hydrocarbon reservoirs. These studies have included sedimentological, diagenetic and reservoir quality focused evaluations of core. The primary objective of the studies has been to improve conceptual and qualitative models that can be utilized in reservoir modelling and also for infill drilling and well workover evaluations. Most of these studies have been undertaken on old core samples collected in the 1980s and 1990s. Two case studies are described here that provide examples of the utility of core in mature fields. (1) Heather Field calcite: to quantitatively assess the distribution of calcite cements and their impact on hydrocarbon volumes and reservoir quality distribution in Brent reservoirs. (2) Thistle Field Etive Formation barriers and baffles: to characterize and describe the origin and distribution of low-permeability intervals within the Etive Formation reservoir. These two studies used a wide variety of core-based techniques including core logging and description, optical microscopy and petrographical studies, isotope analyses, X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) (FEI Company analysis tool and software, QEMSCAN)-based mineralogy, portable-X-ray fluorescence (XRF), NDTr and Thermo Scientific Inc. NITON TM operational software (NDT) geochemical analysis, as well as image analysis of grain size and texture. These data were then integrated with other subsurface datasets, such as well log, seismic data and well performance data, in order to address the specific reservoir challenge. These new and focused reappraisals of core demonstrate the dual value of core-based studies, which can: improve the understanding of producing hydrocarbon reservoirs, leading to improved productivity and recovery. Core is a full asset life-cycle resource and provides critical insight at all stages of field maturity as production behaviour changes and alternative development strategies are considered; further our general knowledge and understanding of clastic sedimentology and diagenesis using rich and diverse core-based datasets backed up by substantial well log and seismic datasets.

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

Article

Jul 2022

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

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.

Diagnosing Nuclear Power Plant Pipe Wall Thinning due to Flow Accelerated Corrosion using a Passive, Thermal Non-destructive Evaluation Method: Feasibility Assessment via Numerical Experiments

Article

Oct 2021

Organic-inorganic interactions between hydrocarbons and most minerals in deeply buried reservoirs remain unclear. In this study, gold capsules and fused silica capillary capsules (FSCCs) with different combinations of nC16H34, water (distilled water, CaCl2 water) and minerals (quartz, feldspar, calcite, kaolinite, smectite, and illite) were heated at 340 °C for 3–10 d, to investigate the evolution and reaction pathways of the organic-inorganic interactions in different hot systems. After heating, minerals exhibited little alteration in the anhydrous systems. Mineral alterations, however, occurred obviously in the hydrous systems. Different inorganic components affected nC16H34 degradation differently. Overall, water promoted the free-radical thermal-cracking reaction and step oxidation reaction but suppressed the free-radical cross-linking reaction. The impact of CaCl2 water on the nC16H34 degradation was weaker than the distilled water as high Ca²⁺ concentration suppressed the formation of free radicals. The presence of different waters also affects the impact of different minerals on nC16H34 degradation, via its impact on mineral alterations. In the anhydrous nC16H34-mineral systems, calcite and clays catalyzed generation of low-molecular-weight (LMW) alkanes, particularly the clays. Quartz, feldspar, and calcite catalyzed generation of high-molecular-weight (HMW) alkanes and PAHs, whereas clays catalyzed the generation of LMW alkanes and mono-bicyclic aromatic hydrocarbons (M-BAHs). In the hydrous nC16H34-distilled water-mineral systems, all minerals but quartz promoted nC16H34 degradation to generate more LMW alkanes, less HMW alkanes and PAHs. In the nC16H34-CaCl2 water-mineral systems, the promotion impact of minerals was weaker than that in the systems with distilled water. This study demonstrated the generation of different hydrocarbons with different fluorescence colors in the different nC16H34-water-mineral systems after heating for the same time, implying that fluorescence colors need to be interpreted carefully in investigation of hydrocarbon charging histories and oil origins in deeply buried reservoirs. Besides, the organic-inorganic interactions in different nC16H34-water-mineral systems proceeded in different pathways at different rates, which likely led to preservation of liquid hydrocarbons at different depth (temperature). Thus, quantitative investigations of the reaction kinetics in different hydrocarbon-water-rock systems are required to improve the prediction of hydrocarbon evolution in deeply buried hydrocarbon reservoirs.

The process of rubefication and its relationship with gold mineralization of Sizhuang gold deposit, northwestern Jiaodong Peninsula, eastern China

Article

Jan 2020

Coupled mineral alteration and oil degradation in thermal oil-water-feldspar systems and implications for organic-inorganic interactions in hydrocarbon reservoirs

Article

Full-text available

Mar 2019
GEOCHIM COSMOCHIM AC

Organic-inorganic interactions after oil charging are critical for determining the ongoing evolution of hydrocarbons and rock quality in water-wet siliciclastic reservoirs. It is the conceptual approach of this study to simulate and decipher these interactions by using quantitative analyses of the interrelated changes of minerals, water, hydrocarbons, gases, and organic acids in heated oil-water-rock systems. The experimental results show that organic-inorganic interactions occur between the organic oil and inorganic feldspar in the presence of water. Water promotes the oil degradation by an extra supply of H+ and OH- ions. In the oil-water-rock systems, mutual exchanges of H+ and OH- ions among minerals, water, and hydrocarbons probably result in the mutual interactions between oil degradation and mineral alteration, with water serving as a matrix for the ion exchange. In the oil-water-feldspar system, feldspar alteration does not cease in the oil zone in the presence of some water and, inversely, oil degradation can even accelerate the alterations of the aluminosilicate minerals. The mineral alterations from feldspar to boehmite, illite, and muscovite promote the oil degradation in the oil-deionized water-feldspar systems with the mutual ion exchange. Due to a possible competition between the Brønsted acid sites in the minerals and the halide anions or the direct replacement of K-feldspar by albite, the mineral alterations retard the oil degradation in the oil-NaCl water-feldspar systems. The experimental results also indicate that oil degradation via free radical cross-linking and free radical thermal cracking is extensive in anhydrous oil systems and may lead to blocking of pores and throats by the generated pyrobitumen in the oil zone of a reservoir. In contrast, in the oil-water transition zone of a reservoir, oil degradation via thermal cracking and oxidative decomposition may dominate the oil degradation and may lead to the generation of secondary pores with leaching of minerals by the generated CO2 and organic acids. This study clearly demonstrates that organic-inorganic interactions are very complex and proceed in diverse pathways in different hydrogeochemical systems. Thus, further quantitative investigations of the reaction pathways and reaction kinetics of coupled mineral alteration and oil degradation in oil-water-rock systems are merited. Meanwhile, the incorporation of such organic-inorganic reactions into geochemical models will improve the prediction of the evolution of organic and inorganic species in petroleum reservoirs.

Diagenesis and reservoir properties of Middle Jurassic sandstones, Traill Ø, East Greenland: The influence of magmatism and faulting

Article

Dec 2016
MAR PETROL GEOL

Jens Therkelsen

The c. 500 m thick Middle Jurassic sandstones of the fluvial Bristol Elv and marine Pelion Formations of the East Greenland Basin are evaluated here in order to improve the understanding of the processes that influenced the diagenetic evolution. The study may help to predict the reservoir properties of sandstones affected by magmatism and faulting, both in general and specifically in undrilled areas on- and offshore East Greenland and, in the Vøring Basin on the Mid-Norwegian shelf. The study shows a variety of authigenic mineral phases dominated by quartz cement, carbonate cement, illite and iron-oxide. One of the clear differences between the two formations is the presence of early carbonate-cemented horizons in the marine sandstones; these horizons are inferred to reflect a primary concentration of biogenic clasts and fossil shells. Intense quartz cementation occurs primarily in the fluvial sandstones but the marine sandstones are also highly quartz-cemented. Two episodes of burial and uplift are recorded in the diagenetic sequence and widespread grain-crushing in coarse-grained intervals is believed to result from overpressure and subsequent compaction due to sudden pressure release along major faults. Maximum burial depths may only have been around 2000–2500 m. Cathodoluminescence analyses show that grain crushing was followed by intense quartz cementation. The quartz cement is to a great deal believed to have formed due to increased surface area from crushing of detrital quartz grains, creating fresh nucleation sites for the quartz. Cathodoluminescence investigations also show that only minor pressure dissolution has taken place between detrital quartz grains and that the ubiquitous quartz cementation displays several growth zones, and was thus in part the result of the introduction of silica-rich extra-formational fluids related to the flow of hot fluids along reactivated faults and increased heat flow and temperature due to magmatism. This interpretation is supported by fluid inclusion homogenization temperatures between 117 and 158 °C in quartz cements. In one of the two study areas, the development of macroscopic stylolites has significantly enhanced quartz cementation, probably in connection with thermal convection flow. As a result of the magmatic and fault-related quartz cementation and illitization, the reservoir quality of the sandstone formations deteriorated and changed drastically.

Post-depositional history of the Miocene Gorgoglione Formation (southern Apennines, Italy): Inferences from mineralogical and structural analyses

Article

Full-text available

Jul 2015

The Gorgoglione Formation represents the infill of a thrust-top basin, which records the tectonic evolution of the southern Apennines (Italy) since Upper Miocene times. The Upper Miocene basin was divided into two main sub-basins, showing both about NNW-SSW elongation. During ongoing contractional deformation, the Gorgoglione basin was incorporated into the allochthonous units of the Apennine fold-and-thrust belt, as outlined by the emplacement of thrust sheets of internal provenance (Sicilide Unit) and by the development of two main generations of tight to open folds. In this work, the modalities of deformation associated to the incorporation of the Gorgoglione Formation into the fold-and-thrust belt has been documented by means of mineralogical and structural analyses. Mineralogical data show that, in the northeastern sub-basin of the Gorgoglione Formation, the illite content and the order of illite/smectite mixed layer increase near the contact with the overlaying Sicilide Unit. These data are hence used to estimate the relative tectonic load produced by thrusting of the aforementioned tectonic unit. Structural data are consistent with tight, NNW-trending, meso-scale folds particularly frequent in the upper pelitic/arenaceous portion of the Gorgoglione Formation. Within these folded pelitic levels, a well-developed axial-plane foliation, and an ordered I/S (R1 and R3) with higher illite content, is documented. Instead, other folds developed far away from the Sicilide klippen show an incipient axial-plane foliation and a random I/S (R0) with lower illite amount is found in pelites. The data suggest that the eastern sub-basin of the Gorgoglione Formation underwent variable tectonic load, increasing from E to W, as well as deformation produced by thrusting of Sicilide sheet. The mineralogical data also indicate that illitization process is favoured for the high availability of potassium due to the dissolution of k-feldspar. High potassium availability affects the samples subjected to the highest diagenetic degree, characterized by kaolinite illitization process.

Clay mineral stratigraphy of Mesozoic and Paleozoic red beds, Northern North Sea

Chapter

Jan 1993

Differences in the clay mineralogy and stable isotope geochemistry of Triassic, Permian and Devonian red beds in UK Quadrant 9 aid in distinguishing samples of different age and provide a basis for delineating stratigraphic position during drilling of these unfossiliferous units. Triassic red beds are characterized by the presence of expandable clays (smectite and illite/smectite), kaolinite, illite and chlorite. Smectitic and kaolinitic samples are isotopically heavy (δ¹⁸O = 18.7–20.7‰) and probably represent clays formed near the earth’s surface from circulating meteoric water. The illite, mixed-layer illite/smectite (I/S), chlorite clay assemblage is isotopically lighter (δ¹⁸O = 16.7‰) and probably formed during burial diagenesis at elevated temperatures. The clay mineral assemblage of Permian Rotliegendes red beds is characterized by the presence of illite, ordered interstratified illite/smectite, kaolinite (shallow samples) and chlorite. Devonian red beds in Quadrant 9 are characterized by an isotopically light (δ¹⁸O = 14.5‰) illite-chlorite clay mineral assemblage, in conjunction with accessory albite and calcite. Clay geothermometry suggests that the mineralogy of Devonian red beds developed by metamorphic alteration and was either inherited from metamorphic provenance, or developed diagenetically during the Caledonian orogenesis. Clay crystal morphology provides additional criteria for distinguishing different-aged red beds: Triassic clays contain extremely small plates and laths of smectite or small plates of illite and chlorite; Permian red beds contain abundant lath-shaped and platy illite; Devonian red beds are dominated by coarse recrystallized mica and chlorite and lack lath-shaped or small dispersed crystallites.

Variation in kaolinite morphology with growth temperature in isotopically mixed pore-fluids, Brent Group, UK North Sea

Article

Jan 1994

Fluid flow across the entire basin was driven by a hydrostatic head on the East Shetland Platform palaeo-landmass to the west. The development of the two kaolinite morphologies is possibly related to the degree of supersaturation at the time of precipitation. At low degrees of supersaturation, vermiform kaolinite precipitated slowly upon detrital mica surfaces. -from Authors

Episodic and simultaneous illitization in oil-bearing Brent Group and Fulmar Formation sandstones from the northern and southern North Sea based on illite K-Ar dating

Article

Dec 2013
AAPG BULL

Size fractions (<0.4 and 0.4-1.0 μm) of Brent Group sandstones from the northern North Sea contain mostly illitesmectite mixed layers with kaolinite, whereas the same size fractions of Fulmar Formation sandstones from the southcentral North Sea consist of illite-smectite mixed layers with minor chlorite. Transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking. The K-Ar data of the fractions less than 0.4 pm of Brent Group samples plot on two arrays in a 40Ar/36Ar vs. 40K/36Ar diagram that have isochron characteristics with ages of 76.5 ± 4.2 and 40.0 ±1.5 Ma, and initial 40Ar/36Ar ratios of 253 ±16 and 301 ± 18, respectively. For the Fulmar Formation sampies, the data points of the fractions less than 0.2 and less than 0.4 pm also fit two isochrons with ages of 76.6 ±1.4 and 47.9 ± 0.5 Ma and initial 40Ar/36Ar ratios of 359 ± 52 and 304 ± 2, respectively. Some of the coarser 0.4-1.0-μm fractions also plot on the two isochrons, but most plot above indicating the presence of detrital components more than 0.4 μm. The almost identical ages obtained from illite-type crystals of sandstones with different deposition ages that are located about 600 km (373 mi) apart record two simultaneous illitization episodes. These events were not induced by local burial conditions, but are related to episodic pressure and/or temperature increases in the studied reservoirs, probably induced by hydrocarbon injection. This interpretation is indirectly supported by notably different K-Ar illite ages from cores of a nearby reservoir at hydrostatic pressure. Illite is not as well crystallized as expected for potential crystallization temperatures above 160°C measured by fluidinclusion determinations. In both the northern and south-central North Sea, the two illite generations remain unaffected after crystallization despite continued burial, suggesting notably higher crystallization temperatures than those estimated from geothermal gradients. No recent illite crystallization or alteration is recorded in the K-Ar ages, despite a dramatic regional acceleration of the subsidence in the southern North Sea. Copyright © 2013. The American Association of Petroleum Geologists. All rights reserved.

The K-Ar and 40Ar/39Ar methods revisited for dating fine-grained K-bearing clay minerals

Article

Sep 2013
CHEM GEOL

Norbert Clauer

The strengths and weaknesses of the two Ar isotopic methods (K–Ar and 40Ar/39Ar) were evaluated on the basis of respective recent applications mainly on low-temperature K-bearing illite-type clay minerals. This review includes a presentation of basic, analytical and technical aspects for both methods, as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization. Whenever possible, the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates. The comparative review examines stratigraphic dating of glauconites, indirect dating of low-temperature ore deposits, dating of burial-related illitization, and dating of polyphased tectono-thermal activity, more specifically of fault gouges. Some pending questions such as the necessary encapsulation due to 39Ar recoil and its restoration into step-heating patterns are also raised, together with the new potential of Ar-dating of nanometric illite crystals.

Laser 40Ar/39Ar microprobe analyses of fine-grained illite

Article

Sep 1997
GEOCHIM COSMOCHIM AC

Fine-grained (<0.02 μm) to coarse-grained (2.0-0.2 μm) illite separates and finely powdered muscovite standards were analyzed with a microencapsulation technique and an laser microprobe. The integrated ages of the illite agreed within error with conventional analyses, even though the sample sizes, 1–100 micrograms, were at least a 10,000-fold less. Incremental laser heating of an artificial mixture of illite and muscovite of two different ages yielded a stair step profile, where the youngest and oldest incremental ages approximately coincided with their ages. The thermally activated argon release rate from illite was distinct from that of the muscovite and may result from differences in grain thickness, lower K concentration, and the presence of cis vs. trans-sited vacancies. Incremental heating, therefore, may prove capable of delineating detrital from authigenic components in illite extracted from shale and sandstone. Microencapsulation and laser analyses, when combined with sophisticated techniques for separating clays, will permit dating of samples where clay is a minor constituent, such as sandstones and meteorites, and will enhance identification of endmember ages in naturally occurring clay.

Discussion of ‘Evidence for resetting of fluid inclusion temperatures from quartz cements in oilfields’ by Osborne and Haszeldine (1993)

Article

Dec 1995

Effects of Diagenesis on Neural-Network Grain-Size Prediction

Article

Mar 2000

The use of gamma-ray log shapes to determine grain-size trends is commonplace despite obvious limitations (Rider 1990, Hurst1990). In a recent development, this implied qualitative relationship between the gamma-ray response and grain-size was exploited in developing a quantitative means of predicting grain-size from well logs (Oyeneyin and Faga, 1999) using neural networks. However, for this in situ prediction methodology to succeed the relationship between gamma-ray log values and clay content and between clay content and grain-size needs to be consistent within a range that will not negate the generalisation capabilities of a Backpropagation-Neural-Network. One of the main factors affecting the natural variability that exists in these relationships and causes reduction in porosity and permeability is diagenesis. The others include compositional and textural factors (Rider 1990). This paper presents the results and recommendations of a study carried out to determine the effects of diagenesis on the accuracy of nonlinear grain-size modelling in sandstones. The results of the study indicate that grain-size modelling in diagenetically modified formations is feasible. It characterises the effects of cemented zones. The study further concludes that cross-depositional environment prediction of grain-size in the case of the Brent and Statfjord formations generates representative grain-size trends and values.

Diagenetic Albitization of Detrital K-feldspar in Jurassic, Lower Cretaceous, and Tertiary Clastic Reservoir Rocks from Offshore Norway, I. Textures and Origin

Article

Full-text available

Jan 1988

Albitization proceeds by a dissolution-precipitation mechanism. At shallower depth, dissolution of the parent K-feldspar is more rapid than the precipitation of albite, and rates of albite precipitation increase with depth, favoring pseudomorphic replacement of K-feldspars, Other factors that influence albitization processes are a Na+/a K+, permeability reduction, and surface-dissolution kinetics of the parent grain. -from Authors

Numerical Assessment of Dissolution Versus Replacement in the Subsurface Destruction of Detrital Feldspars, Oligocene Frio Formation, South Texas

Article

Full-text available

Jan 1989
J Sediment Petrol

Volumetric loss of detrital feldspar on the observed scale over such a short time span has major implications for provenance interpretations and for understanding the basin-wide mass balance of several major diagenetic components. The wide variation of feldspar content and composition at any given depth (temperature) attests to the premier significance of fluid composition and flux in driving the reactions that bring about dissolution of feldspars in these rocks. -from Authors

Albitization of Detrital Plagioclase in Triassic Reservoir Sandstones from the Snorre Field, Norwegian North Sea

Article

Full-text available

Jan 1990

Sadoon Morad

Detrital plagioclase (An12-An28) in Triassic reservoir sandstones of the Lunde Formation in the Snorre Field, offshore Norway has been partially to completely albitized during burial diagenesis (depths of about 2,500-3,000 m; 75-100°C present temperature). Calcite (CaCO3 > 99 mole %) and kaolinite were formed as by-products of plagioclase albitization. Sodium-rich detrital plagioclase (An2-An11) are usually unalbitized. Detrital K-feldspar is fresh or, rarely, partly albitized. Differences in albitization behavior of detrital plagioclase and K-feldspar are shown to be related to the chemistry of formation waters, reactivity of kaolinite, and temperature. The diagenetically formed albite pseudomorphs are very pure (Ab > 9 ), well-ordered low-albite, dark-luminescent, vacuolized, mostly untwinned, and comprised of numerous euhedral albite crystals. Some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase. The amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable Ca-rich plagioclase. The microporosity formed during albitization is proportional to the anorthite component, which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite. The albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of Na+ from pore solutions.

Diagenetic albitization of detrital K-feldspars in Jurassic, Lower Cretaceous and Tertiary clastic reservoir rocks from offshore Norway; II, Formation water chemistry and kinetic considerations

Article

Full-text available

Jul 1990

Formation waters from clastic reservoirs offshore Norway are saturated or supersaturated with respect to albite deeper than 2.5 km. Thermodynamic and mass constraints show that the albitization process is more sensitive to potassium removal than sodium supply. Kinetic considerations indicate that at temperatures below 145°C the dissolution of K-feldspar is always faster than albite growth. Combined with replacement constraints, the kinetic data also explain the observed coarsening of growing albite crystals with increasing burial temperature. -from Authors

Illite-Smectite Diagenesis and Palaeotemperatures in Northern North Sea Quaternary to Mesozoic Shale Sequences

Article

Full-text available

Jun 1988

Michael J. Pearson

Clay mineral abundances and illite-smectite (I/S) compositions have been determined by X-ray diffraction (XRD) in shales of Permo-Triassic to Quaternary age from seven wells in the Viking Graben and Moray Firth. Chemical analyses of size fractions provide evidence that diagenetic illitization of smectite has occurred during burial by uptake of Al and K, and release of Si. K-feldspar was probably the main source of K for illitization. The depth at which random I/S disappears occurs at similar temperatures (mean 93°C) in each well for which reliable measurements are available. Vitrinite reflectance measurements at this depth are also similar (mean 0·64% R 0 ) and correspond to early oil generation. I/S diagenetic levels may have been imprinted by a Tertiary heating event.

Diagenesis of a Deeply Buried Sandstone Reservoir: Hild Field, Northern North Sea

Article

Full-text available

Oct 1986

Arve Lønøy

Calcite cementation and extensive dissolution of feldspar with formation of authigenic kaolinite, quartz cement and secondary porosity are the main diagenetic processes in the deeply buried Hild Field. Mineralogical and isotopic analyses, reservoir pressure and depositional environment suggest that these diagenetic processes occurred prior to burial at a depth of 1·5–2 km. The timing of the diagenetic sequence suggests that feldspar dissolution is related to meteoric water flow. Calcite occurs as an early diagenetic iron-poor cement, and as two types of later diagenetic (<120°C) ferroan calcite cements. The ferroan calcites are mainly an in situ dissolution-reprecipitation product of the early diagenetic phase. Extensive local dissolution of calcite was important for forming secondary porosity which is closely associated with a prominent ‘gas chimney’ in the area studied. A high CO 2 content in the natural gases of the reservoir suggests that the solvent was carbonic acid formed from CO 2 generated during thermal maturation of organic matter. Calcite dissolution probably occurred between 70° and 100°C.

Frio Sandstone Diagenesis, Texas Gulf CoastA Regional Isotopic Study

Chapter

Jan 1984

Lynton S. Land

Reservoir Diagenesis and Convective Fluid Flow

Chapter

Jan 1984

Formation of Secondary PorosityHow Important Is It?

Chapter

Jan 1984

Knut Bjørlykke

Chapter 7. ISOTOPIC STUDIES OF PHYLLOSILICATES: (Exclusive of Micas)

Chapter

Dec 1988

Temporal reconstruction of sandstone diagenetic histories

Article

Jan 1989

P.D. Lundegard

Stable isotopes as tracers in clastic diagenesis

Article

Jan 1989

Fred John Longstaffe

Diagenesis and Hydrocarbon Accumulation, Brent Sandstone (Jurassic), Bergen High Area, North Sea

Article

Jan 1989
AAPG BULL

R. A. Clark J. R. Glasmann

Timing and Conditions of Permian Rotliegende Sandstone Diagenesis, Southern North Sea: K/Ar and Oxygen Isotopic Data

Article

Jan 1989
AAPG BULL

James L. Aronson Mingchou Lee

Oseberg area-integrated basin modelling.

Article

Jan 1987

An improved deterministic, dynamic basin model has been developed and applied to the study of hydrocarbon generation and migration in the Oseberg region of the Norwegian Continental Shelf. The results suggested that significant oil generation occurred in the drainage ares of the Oseberg Alpha and Beta South structures from early Maastrichtian time. Modelling studies also showed that a significant additional heat input occurred within the region during Eocene/Miocene time.-from Authors

(Study of fluid inclusions in the silica overgrowths of the North Sea reservoir sandstones: a possible new diagenetic history of the Brent of the Alwyn area).

Article

Jan 1986

The study of fluid inclusions found in silica overgrowths indicates that silicification is contemporaneous with the first migration stages of hydrocarbons in the reservoir sandstones. Temperature data do not fit with the depth of burial at this time. In contrast, they are likely to indicate an introducation in the reservoirs of hot fluids generated from source rocks situated in deeper parts of the basin. -English summary

Diagenesis of a deeply buried sandstone reservoir: Hild Field, Northern North Sea.

Article

Jan 1986

Calcite cementation and extensive dissolution of feldspar with formation of authigenic kaolinite, quartz cement and secondary porosity are the main diagenetic processes in the deeply buried Hild Field. Mineralogical and isotopic analyses, reservoir pressure and depositional environment suggest that these diagenetic processes occurred prior to burial at a depth of 1.5-2 km. The timing of the diagenetic sequence suggests that feldspar dissolution is related to meteoric water flow. Calcite occurs as an early diagenetic iron-poor cement, and as two types of later diagenetic (<120oC) ferroan calcite cements. The ferroan calcites are mainly an in situ dissolution-reprecipitation product of the early diagenetic phase. Extensive local dissolution of calcite was important for forming secondary porosity which is closely associated with a prominent 'gas chimney' in the area studied. A high CO2 content in the natural gases of the reservoir suggests that the solvent was carbonic acid formed from CO2 generated during thermal maturation of organic matter. Calcite dissolution probably occurred between 70o and 100oC. (Authors' abstract)-D.J.M.

Procedures in sedimentary petrology, chapter 22

Article

Jan 1971

G.M. Friedman

Organic facies and hydrocarbon distributions in the Norwegian North Sea

Chapter

Jan 1985

Significant hydrocarbon accumulations have been found in North Sea reservoirs ranging in age from Devonian (Buchan Field) to Eocene (Frigg area). In Norwegian waters, the oldest reservoirs are at present Rhaeto—Liassic Statfjord Formation sands. Source rock development in the northern North Sea is restricted almost entirely to the Jurassic. Rich oil-prone source rocks occur mainly in the Humber Group, and particularly within the Kimmeridge Clay Equivalent (Upper Jurassic ‘Hot Shale’). In basinal settings within the Viking and Central Grabens, Kimmeridge source rock sections may be over 1000 m thick. Net source rock thicknesses exceeding 400 m are documented in several wells.

Origin, Recognition, and Importance of Erosional Unconformities in Sedimentary Basins

Chapter

Jan 1988

G. Shanmugam

Erosional unconformities of different scales (local to global) are an ubiquitous element of all sedimentary basins. Erosional unconformities of subaerial origin are believed to have been caused by tectonic uplifts and by eustatic sea-level fall. Erosional unconformities of submarine origin may be related to transgression, mass movements, turbidity currents, thermohaline currents, carbonate dissolution, storms, and clastic influx on carbonate shelves. Important criteria for recognizing subaerial unconformities include discordance of dip, karst facies, basal conglomerate, and a major gap in the fossil record. Paleosol horizons, duricrust, and continental deposits, indicative of subaerial exposure, can also be used to define surfaces of potential subaerial unconformities. Submarine unconformities may be recognized by mass-movement deposits, glauconitic minerals, and manganese nodules. Recognition of unconformities is useful for subdividing stratigraphic units, determining the timing of tectonic activity, interpreting lateral facies relationships, constructing burial and uplift curves, correlating certain stratigraphic boundaries, interpreting sea-level changes, and for reconstructing paleogeography. Erosional unconformities may be important to exploration because they can be used to predict deep-sea turbidite reservoir facies; they can mark upper boundaries of zones of increased porosity (e.g., Statfjord Field, North Sea); they can provide an ideal juxtaposition of reservoir and source rocks (e.g., Prudhoe Bay Field, Alaska); they can act as avenues of hydrocarbon migration (e.g., Maracaibo Basin, Venezuela); they can generate hydrocarbon traps (e.g., Messla Field, Libya); and they can be favorable sites for mineralization (e.g., uranium, aluminum, phosphates, and gold).

Formation of secondary porosity: How important is it?

Article

Jan 1984

Knut Bjørlykke

Reservoir diagenesis and convective fluid flow

Article

Jan 1984

Organic geochemistry in exploration of the Northern North Sea

Article

Jan 1985

J.D. Field

A subsurface geochemical study has been conducted in the Northern North Sea with the following objectives: 1. to identify potential hydrocarbon-source horizons; 2. to determine their regional maturity, and hence the most likely type of hydrocarbons expelled into carrier beds and 3. to identify secondary migration routes and to evaluate the probable gravity of accumulated hydrocarbons.

Carbon dioxide and organic acids: their role in porosity enhancement and cementation, Paleogene of the Texas Gulf Coast

Article

Jan 1986

Considerations of the solubility of carbon dioxide and the distribution of organic acids with respect to temperature suggest that these species will be of great importance in porosity enhancement only if generated locally. Other potential sources of acid for dissolution reactions are reverse weathering reactions in shales and hydrous pyrolysis reactions between organic carbon and oxygen in H2O (including synthesis of organic acids from kerogen). -from Authors

Timing and conditions of Permian Rotliegende sandstone diagenesis, Southern North Sea: K/Ar and oxygen isotopic data

Article

Jan 1989

Illitic clay, ranging from pure illite to highly illitic illite/smectite (I/S), is the most abundant diagenetic phase in the eolian and sabkha facies of the Rotliegende Formation of the southern North Sea and northeastern Netherlands. Most K/Ar ages of diagenetic illite and I/S in the eolian sandstones are between 100 and 175 Ma. Illite formation is related to two major phases of tectonic activity, the Jurassic Kimmerian orogenic movements and Late Cretaceous-early Tertiary inversion movements. Calculated δ18O values of most illite-forming fluids lie between 0 and 4‰ (SMOW), indicative of meteoric or marine waters modified by interaction with rock. However, 18O-depleted fluids (as low as -5‰), indicative of a major meteoric component, are inferred to have been involved in the formation of illite in the Groningen gas field at a time when nearby sections of the Rotliegende were eroded and exposed at the surface. -from Authors

Diagenesis of Frio sandstones, Texas Gulf Coast: A regional isotopic study

Article

Jan 1984

L.S. Land

Generally quartz is the first major cement and constitutes 2.5% by volume of the sandstone. The delta 18O of quartz is +31per mille + or - 1.5per mille (SMOW). Calcite constitutes 5% of the Frio sandstone. Most of the calcite has delta 18O of -7.2 + or - 2per mille (PDB). The delta 13C values are approx -4 + or - 2per mille (PDB).-K.A.R.

The application of stable isotopes to studies of the origin of dolomite and to problems of diagenesis of clastic sediments

Article

Jan 1983

L.S. Land

Demonstrates the application of stable isotope geochemistry to the origin of sedimentary dolomite and diagenesis in clastic systems - clay mineral authigenesis, quartz, carbonate, and zeolite cementation. Case studies on diagenesis illustrate the mode of application of stable isotopic analyses. -from Editor

Geochemical Evidence for the History of Diagenesis and Fluid Migration: Brent Sandstone, Heather Field, North Sea

Article

Jan 1989

J. R. Glasmann

Preparing doubly polished sections of temperature sensitive sedimentary rocks

Article

Jun 1984

Heating soft and/or cleavable minerals can cause expansion of inclusion cavities and/or leakage of fluid from inclusion cavities, resulting in an increased temperature of homogenization. Describes a set of grinding, polishing, and mounting techniques for preparing doubly polished rock sections with a minimum of heating above room temperature.-from Authors

Diagenesis and hydrocarbon accumulation, Brent Sandtone (Jurassic), Bergen Area, North Sea

Article

Nov 1989
AAPG BULL

Calcite and kaolinite are low-temperature diagenetic precipitates that formed as meteoric pore water entered the Brent at the end of Brent deposition and later, during late Cimmerian erosion. A major period of feldspar dissolution associated with quartz and illite cementation occurred at Huldra field. Precipitation of diagenetic illite in Brent sandstones was closely linked with hydrocarbon maturation in the adjacent Viking graben and accumulation of generated fluids in traps at structural highs. Pore-water character during illite diagenesis, interpreted from oxygen isotope and quartz overgrowth fluid inclusion data, was warmer than predicted by burial history analysis and one and a half to two times seawater salinity. The incursion of saline, isotopically evolved fluids at Huldra suggests replacement of paleometeoric water by basin compaction fluid. -from Authors

Procedures in Sedimentary Petrology [book review]

Article

Sep 1972

A. M. Gutstadt

Shale Diagenesis in the Bergen High Area, North Sea

Article

Jan 1989

J. R. Glasmann

Illite diagenesis in Tertiary and Mesozoic shales in the Bergen High area, northern North Sea, was studied using mineralogic, isotopic, and computerized thermal modeling techniques. The Tertiary shales are dominated by smectite, with lesser amounts of illite, kaolinite, and chlorite. At present burial temperatures of >70°C smectite is absent, and the shales contain abundant lath-shaped illite which yields a mixed-layer illite/smectite (I/S) X-ray powder diffraction (XRD) pattern. Transmission electron microscopy (TEM) indicates that the illite laths increase in abundance and thickness with increasing depth; XRD patterns indicate a progressive increase in the illite component of the I/S. The deepest samples were found to contain long-range ordered (R=3) I/S, which showed platy particle morphology with the TEM. K-Ar ages of most of the <0.1- μ m-size illite separates imply that illitization was a relatively brief event affecting a thick sequence of sediments during late Cretaceous to early Paleocene time (65–87 Ma); however, measured ages were affected by trace levels of detrital Ar contamination and do not represent the true age of diagenesis. Several methods of quantifying Ar contamination were used to correct measured ages to obtain a reasonable estimate of the true age of diagenesis. The corrected ages are imprecise due to uncertainties in quantifying the levels of sample contamination, but generally suggest a Paleogene (38–66 Ma) period of illitization. In contrast, simple kinetic models of smectite-illitization suggest much younger ages of diagenesis (0–40 Ma at the Veslefrikk field; 0–60 Ma at the Huldra field). The timing of the diagenesis and the morphologic aspects of the authigenic illite suggest that illite precipitated before late Tertiary compaction and resulted in a decrease in fluid permeability. Low trapping efficiency of early Tertiary sediments, vertical escape of warm fluid from the Brent sandstone, and high heat flow may have promoted illite diagenesis in the shales prior to deep late-Tertiary burial.

Mechanism of burial metamorphism of argillaceous sediment: 2. Radiogenic argon evidence

Article

May 1976
GEOL SOC AM BULL, GSA Bulletin

Variations in the 40*Ar of the whole rock and the <0.1-μm fraction were used to monitor the involvement of K2O in the inferred burial metamorphic reactions of argillaceous sediment from Texas Gulf Coast well CWRU (Case Western Reserve University) 6. These data strongly support the occurrence of the reactions involving the illite/smectite mixed-layer clay inferred from depth-dependent mineralogical and chemical changes reported in a companion paper by Hower and others (this issue). The 40Ar released from all samples was generally greater than 40 percent radiogenic (40*Ar), allowing precise measurement of differences of 40*Ar with depth. The whole-rock K-Ar apparent age shows a decrease from about 150 m.y. to 75 m.y. over just the same depth interval (1,850 to 3,700 m) that the illite/smectite mixed-layer clay progressively changes from 20 percent illite layers to 80 percent. This decrease in apparent age of the total shale is due to loss of 40*Ar from the rock. The 40*Ar loss is not caused by outgassing from increasing temperature, because the finest (<0.1-μm) fraction, which is nearly pure illite/smectite, actually gains 40*Ar with depth. The coarser fraction, from which the 40*Ar loss is occurring, concentrates the older 40*Ar-rich phases of the rock such as K-feldspar and mica. These results strongly imply that the K2O for the new illite layers of the illite/smectite is derived by chemical decomposition of the K-feldspar and mica. The gain in 40*Ar of the finest fraction also just corresponds with depth to the change in mineralogy of the illite/smectite and a large gain of K2O in this fraction from 2 to 5 percent K2O. The 40*Ar gain indicates that the mean time of K2O gain, which is the mean time of burial metamorphism, was about 18 m.y. ago.

Soil Chemical Analysis — An Advanced Course

Book

Jan 1974

M. L. G. Jackson

Steiger, R. H. & Jaeger, E. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359-362

Article

Oct 1977
EARTH PLANET SC LETT

K/Ar dating of time of gas emplacement in Rotliegendes Sandstone, Netherlands

Article

Sep 1985

The time of emplacement of gas into each of two reservoirs in the Rotliegendes Sandstone, one in the Groningen field and one in the Broad Fourteens basin, was estimated from potassium/argon (K/Ar) ages of diagenetic illitic clays and careful petrographic observations of the rocks in which the clays occurred. In each reservoir, the intensity of diagenetic alteration increased downward within a relatively thin interval, apparently reflecting a longer period of alteration for deeper samples. In each case reported here, the age of the deeper sample is younger than that of the shallower. We interpret the age of the finest illite fraction in the gas zone of each reservoir to indicate the time at which illite formation ceased as the result of gas emplacement and pore fluid displacement in the rock. Our results indicate that gas emplacement in the Groningen field commenced prior to 150 Ma, and that the growth of diagenetic illite continued in the present-day water zone from that time until at least 120 Ma.

Reply [to “Comment on ‘A numerical model of compaction-driven groundwater flow and heat transfer and its application to the paleohydrology of intracratonic sedimentary basins’ by Craig M. Bethke”]

Article

Jan 1985

Craig M. Bethke

A new numerical method allows calculation of compaction-driven groundwater flow and associated heat transfer in evolving sedimentary basins. The model is formulated in Lagrangian coordinates and considers two-dimensional flow in heterogeneous, anisotropic, and accreting domains. Both the continuity of the deforming medium and aquathermal pressuring are explicitly taken into account. A calculation of compaction-driven flow during evolution of an idealized intracratonic sedimentary basin including a basal aquifer predicts slow groundwater movement over long time periods. Fluids in shallow sediments tend to move upward toward the sedimentation surface, and deeper fluids move laterally. The hydraulic potential gradient with depth reverses itself near the basal aquifer, and fluids in this area have a tendency to migrate obliquely into stratigraphically lower sediments. Only small excess pressures develop, suggesting that intracratonic basins are not subject to overpressuring during their evolutions. Owing to the small fluid velocities, heat transfer is conduction-dominated, and the geothermal gradient is not disturbed. Variational studies show that excess hydraulic potentials, but not fluid velocities, depend on assumptions of permeability and that both excess potentials and velocities scale with sedimentation rate. Aquathermal pressuring is found to account for

Hydrogen and oxygen isotope exchange reactions between clay minerals and water

Article

Feb 1976
GEOCHIM COSMOCHIM AC

The extent of hydrogen and oxygen isotope exchange between clay minerals and water has been measured in the temperature range 100-350° for bomb runs of up to almost 2 years. Hydrogen isotope exchange between water and the clays was demonstrable at 100°. Exchange rates were 3-5 times greater for montmorillonite than for kaolinite or illite and this is attributed to the presence of interlayer water in the montmorillonite structure. Negligible oxygen isotope exchange occurred at these low temperatures. The great disparity in D and O 18 exchange rates observed in every experiment demonstrates that hydrogen isotope exchange occurred by a mechanism of proton exchange independent of the slower process of O 18 exchange. At 350° kaolinite reacted to form pyrophyllite and diaspore. This was accompanied by essentially complete D exchange but minor O 18 exchange and implies that intact structural units in the pyrophyllite were inherited from the kaolinite precursor.

Illite Crystal Morphology as Indicator of Diagenetic Environment: ABSTRACT

Article

Jan 1989
AAPG BULL

J. Reed Glasmann

Diagenetic Sequences and K/Ar Dating in Jurassic Sandstones, Central Viking Graben: Effects on Reservoir Properties

Article

Oct 1986

M. Thomas

Differences in the porosity-permeability values and the clay mineralogy of the Brent and Statfjord reservoirs of two closely related wells from the Central Viking Graben appear to be due to diagenesis. The diagenetic phases have been arranged into three distinct sequences, each induced by distinct hydrodynamic systems. These systems are related to the compaction-lithification phase, the Kimmerian to Lower Cretaceous emersive phase, and the hydrological loading of the deep parts of the basins and/or fault panels. The late diagenetic sequence is expressed by quartz overgrowths, blocky kaolinite and illite authigenesis. With a probable 'catalytic' effect of T, this sequence is the most intensely developed and so greatly damages the porosity and permeability. In the oil-bearing reservoirs of Well 25/4-1, the stage of evolution of this third diagenetic sequence is not well advanced: illitization was rapidly stopped by the hydrocarbon charge so that permeabilities remained satisfactory (up to 500 mD). In the water-zone of Well 25/4-5, illitization is extensive and permeabilities considerably reduced (lower than 5 mD). K/Ar dating of authigenic illites provides evidence of a late Eocene phase (40-44 m.y.) as the maximum age for the hydrocarbon entrapment in Well 25/4-1, whereas the last illite authigenesis from the water zone of Well 25/4-5 gives an Oligocene phase (28 m.y. in the Statfjord formation). These data indicate that fluid circulation was effective until this time in the down-dip faulted panel of Well 25/4-5, and has therefore been responsible for the drastic permeability reduction. (Author's abstract)-C.N.

The Effect of Fluid/Rock Ratio on Feldspar Dissolution and Illite Formation under Reservoir Conditions

Article

Oct 1986

W. L. Huang

Experiments on the conversion of feldspar to illite reveal that variation of the fluid/rock ratio (flow rate) has a significant effect on the kinetics of feldspar dissolution and illite formation. Dissolution of albite in near-neutral KCl solution at 200°C and 500 bar shows that the Si and Na release rate per unit surface area of albite is faster with higher fluid/rock ratio. These results have application to the interpretation of secondary porosity formation. The experiments also reveal a special set of conditions under which rapid illite formation can occur. In contrast to the slow process of illite formation in neutral solution, there can be mass nucleation and growth of illite platelets on albite surfaces in an initially acidic solution with low fluid/rock ratio. In similar experiments with higher fluid/rock ratio, kaolinite forms instead of illite because there is insufficient solid to titrate the large volume of solution. This study suggests that illitization of feldspar or kaolinite may be triggered by a decreased rate of acidic fluid-influx during burial diagenesis.

The Nature, Origin and Distribution of Authigenic Clay Minerals from Middle Jurassic Ravenscar and Brent Group Sandstones

Article

Jun 1984

J. D. Kantorowicz

Middle Jurassic sandstones contain a variety of authigenic clay materials, the origin and distribution of which are related to the influence of several factors, principally depositional pore-fluid chemistry, sandbody geometry and the migration of aggressive fluids. Pore-lining illite, pore-lining chlorite and pore-filling vermiform kaolinite in the Ravenscar group mutually exclude each other because of depositional pore-water chemistry: sea-water for illite, anoxic fresh-water for chlorite and oxygenated fresh-water for kaolinite. Blocky pore-filling dickite occurs ubiquitously within the large connected sandbodies. Its origin may be related to the migration of aggressive fluids and its distribution to depositional sandbody geometry. Mixed-layer chlorite-vermiculite also occurs, and is believed to have formed from chlorite during Recent weathering. Similar patterns occur in Ninian Field (N North Sea). Brent group sandstones. Here, pore-lining illite occurs in marine sandstones and pore-filling vermiform kandites in non-marine sandstones. However, vermiform kandites also occur in the marine sandstones, perhaps due to fresh-watertable development following progradation. The more blocky kandites occur in large connected sandbodies. Finally, a second phase of illitization occurs, postdating blocky kandites, perhaps caused by alkaline formation waters. The occurrence of chlorite in the Broom formation is anomalous and its possible origin is discussed.-D.J.M.

Electron Microscope and X-ray Diffraction Studies of Filamentous Illitic Clay from Sandstones of the Magnus Field

Article

Mar 1982

W.J. McHardy

Scanning electron microscopy revealed that the organization of the interstitial micaceous mineral in the title sandstones depended on the manner of drying of the specimen. After air- and freeze-drying, the lath-like mineral tended to occur as mats densely packed against the pore walls, whereas after critical-point drying an open tangled web of very long, thin ribbons virtually filling the pore space was observed (this arrangement is the one most likely to occur in situ ). The ends of these filamentous ribbons often appear to be bedded in authigenic quartz overgrowths on sand grains, thus anchoring them firmly in the pores. From transmission electron microscope observations, the individual laths were frequently only 2–3 nm thick and, from electron diffraction patterns, elongated along the a-axis: the stacking modification is 1 M . X-ray diffraction patterns are those that would be expected from a fully-ordered interstratified mica-smectite containing ∼20% smectite layers. It is difficult to reconcile this with the thickness as observed under the transmission electron microscope, but a possible explanation is that the ‘smectite’ interlayers in reality represent spaces between individual crystals which formed when they sedimented into an oriented aggregate.

K-Ar dating of illites in Brent Group reservoirs: A regional perspective

Article

Jun 1992

This paper reviews existing K-Ar dates from diagenetic illites from the Brent Group and presents new age data from 11 wells from across the East Shetland Basin. K-Ar ages from diagenetic illites record illite growth in various parts of the Brent Group of the East Shetland Basin from about 60 Ma until 17 Ma. Diagenetic illites from the Cormorant field yield ages, which when combined with burial history simulation, suggest substantial illite growth from 75°C. This temperature is believed to be close to the temperature for the start of illite growth in the Brent Group. Other illite dates, when combined with burial history simulations, demonstrate continued illite growth to temperatures in excess of 110°C. Etive Formation samples of wells in which the overlying Kimmeridge Clay Formation source rocks are mature for oil generation, show a strong correlation between the K-Ar age of the finest size fraction of diagenetic illite and the calculated time at which the overlying source rocks reached a vitrinite reflectance of 0.62%. Data from other formations do not follow this trend and frequently give ages which are considerably older than those from the Etive Formation. It is considered likely that many of these older dates may arise as a result of contamination by detrital illites which are likely to be less common in the highly reworked sands of the Etive Formation barrier complex. No recent (< 10 Ma) K-Ar dates have been obtained from diagenetic illites of the water zone of any wells so far studied in the Brent province. It is also apparent from many wells where diagenetic illites from both the hydrocarbon and water zones have been dated, that illite growth apparently stops in the water zone relatively soon after hydrocarbon emplacement. Furthermore, the time span for illitization, suggested by dating different illite size fractions of the same sample, is much shorter than that suggested by comparison of the range of temperatures illite apparently grew at and burial histories of Brent Group wells. Consideration of these points has led the authors to conclude that illite begins to form at a temperature of about 75°C probably at quite a low reaction rate. The process of oil migration at somewhat higher temperature perturbs the static aqueous pore fluid medium and increases effective water-to-rock ratio by fluid transport. This results in increased rates of illite growth. Subsequent to the peak of oil generation, fluid flow rates decrease, with a concomitant decrease in illite-forming reaction rates. Any attempt to isolate illite formed at lower or higher temperatures will inevitably also sample illite from the most rapid period of growth and so yield a false and reduced time span of illite formation and apparently old ages for most recently formed illite in the water zone.

The reservoir properties and diagenesis of the Brent Group: A regional perspective

Article

Jun 1992

A regional study of Brent Group diagenesis and reservoir properties has been undertaken in order to determine the main controls on porosity and permeability in the sandstone reservoirs. Data from 44 wells from block 211/7 in the north to block 3/8 in the south and spanning current depths from 6700 to 13 400 ft include 9000 porosity, permeability and grain density determinations, quantitative petrographic information from 850 thin sections, and stable isotopic and K/Ar analyses of authigenic phases. The diagenesis of the sediments is similar across the study area and most of the diagenetic phases occur in all formations. The sequence of precipitates and dissolution events reflects early porewater evolution in shallow burial environments and later reactions which were essentially isochemical and controlled largely by increasing burial depth and therefore temperature. Early diagenetic products include siderite, calcite, chlorite and vermicular kaolinite. Only where the calcite cements form concretions or cemented horizons have they a significant effect on reservoir properties. Local dissolution of feldspars and carbonate cements took place on the crest of some fault blocks inferred to have been emergent during the Jurassic and this has caused local enhancement of porosity in some crestal wells. In general, the porosity of each of the reservoir facies decreases systematically with depth but permeability only starts to decrease significantly at depths greater than 10 200 ft (3109.0 m). The general decrease in porosity can be attributed to compaction, together with burial cementation by quartz and iron-rich carbonates. Secondary porosity resulting from feldspar dissolution is increasingly common at depth but there is no net increase in porosity as much of the dissolution was evidently accompanied by the precipitation of authigenic quartz initially with kaolinite but at greater depths with illite. The systematic changes in porosity and the decrease in permeability are compatible with thermally driven dissolution of feldspar and the local reprecipitation of the authigenic silicates; the decrease in permeability corresponds to the presence of increased quantities of illite at depth.

Geochemistry of Porosity Enhancement and Reduction in Clastic Sediments

Article

Jan 1983

C. D. Curtis

The diagenetic history of many reservoir sandstones appears to be divisible into three stages. The first includes destruction of primary porosity, the second porosity enhancement by chemical leaching, and the third a return to loss by cementation and recrystallisation. In an attempt to account for the solution transport necessarily involved, the several types of water-rock interaction that take place during burial of clastic sediment sequences are reviewed. It is concluded that kinetic factors play an important role: some reactions are spontaneous and fast, others are dependent on ‘initiating’ reactions and are slower. ‘Initiating’ reactions modify pore water composition drastically and are thought to be mainly redox or acid-generating reactions. They take place mostly in mudrocks which contain much more unstable material than sandstones. Comparisons between mudrock and sandstone diagenetic patterns lend support to previous suggestions that much diagenetic alteration in sandstones is affected by pore waters (and solutes) originating in mudrocks. Aluminium is thought to be mobile under these conditions. A brief consideration of mineral/water reactions in soils leads to the view that pore waters derived directly from meteoric water are unlikely to be effective agents for leaching at depth.

Hydrocarbon Generation and Migration from Jurassic Source Rocks in the East Shetland Basin and Viking Graben of the Northern North Sea

Article

Jun 1983

J. C. Goff

In the E Shetland Basin oil generation began 65 Ma ago; peak oil generation maturity occurs today at 3250 m (0.7% R 0 ) and was first reached 40–50 Ma ago; the oil generation threshold is at 2500 m. Highest oil saturations in the Kimmeridge Clay occur at 0.8% R 0 ; oil expulsion efficiencies are >20–30%. Oil phase migration has probably occurred through oil wet kerogen laminae, and through interconnected large pores aided by low oil/water interfacial tensions. Oil migrated along strong lateral fluid pressure gradients, from overpressured source rocks in half grabens to Jurassic reservoirs in tilted fault blocks. In the Viking Graben the Kimmeridge Clay is at oil floor maturity below 4500 m; oil and peak oil generation began 7C–80 and 55–65 Ma ago respectively; 40 Ma ago the Kimmeridge Clay passed through peak generation, and gas generation by cracking of oil had begun. Peak dry gas generation from Brent coals occurs today below 5000 m, and began 40 Ma ago. The Frigg Field gas, probably generated from late Jurassic source rocks, migrated through microfractures in overpressured mudstones below 3500 m; above 3500 m methane probably migrated in aqueous solution and was exsolved in the early Tertiary aquifer.

Diagenesis of Jurassic Sandstones in the Viking Graben

Article

Feb 1978

F. Sommer

The development of reservoir qualities during the diagenetic evolution of sand bodies of the Brent Sand Formation in the Viking Graben is described. There are 3 main diagenetic phases: early silicification; strong leaching by circulating fresh waters resulting in kaolinization; and late illitization contemporaneous with the migration of hydrocarbons and associated water. The illitization processes are shown to be more complicated than previously indicated; a source rock outside the Brent Sand Formation is postulated. Analysis of the diagenetic minerals and the pore waters is used to elucidate migration behaviour in reservoirs. Migration is dated as probably occurring during Lower Eocene times.

Mechanism of burial metamorphism of argillaceous sediments: 3. O-isotope evidence

Article

Jan 1977

O-isotope analysis of shales sampled from wells drilled through sedimentary deposits in the Gulf of Mexico region indicates that the sediments and rocks are not isotopically equilibrated systems - even those that have been buried to depths where temperatures are as high as 170 °C. In comparison with the coarser fractions, the finer fractions of both clay minerals and quartz are almost always richer in O18. O-isotope disequilibrium among the clay fractions becomes less marked as burial temperature increases. O-isotope exchange between clay and pore water become more extensive at higher temperatures; this corresponds to more extensive diagenetic alteration of mixed-layer illite-smectite. There is no evidence for O-isotope exchange between detrital quartz and pore water. However, quartz that forms diagenetically as an accompaniment to the conversion of smectite to illite layers in the mixed-layer clay forms in equilibrium with the pore water. The usefulness of O-isotope geothermometry for determination of the maximum temperatures to which shales have been heated during burial was investigated. Temperatures were calculated from the O-isotope fractionations between coexisting fine-grained quartz and clay from three wells; these calculated temperatures progressively approached the measured well (logged) temperatures as depth of burial and temperature increased. In one well, good agreement between calculated and measured temperatures was obtained for measured temperatures between 100 and 180 °C. In two other wells, satisfactory agreement was approached but not obtained at measured temperatures as high as 120 °C. Temperatures calculated from the O-isotope fractionations of quartz and calcite or calcite and clay were not reasonable. This probably reflects isotope exchange between calcite and pore water after the silicates attained their measured isotope ratios. Consequently, calcite is not a suitable mineral for use in isotope geothermometry of diagenetically altered shales.

Sandstone diagenesis in the Pattani Basin (Gulf of Thailand): history of water-rock interaction and comparison with the Gulf of Mexico

Article

Sep 1990
APPL GEOCHEM

In the Pattani Basin, a failed-rift basin, extensive water-rock interaction has occurred between subquartzose alluvial sandstones of Miocene age and their pore fluids. Diagenetic rates and pathways have been strongly influenced by high geothermal gradients, high CO2 fugacities, and low pore water salinities. Depositional pore water was fresh to brackish, depending on the depositional environment of the sediments. Chloride concentrations in modern formation water are believed primarily to reflect the proportions of river and sea water in the depositional environment. However, the concentration of other important solutes and the isotopic composition of the formation waters can not be explained by roportional mixing of these two end-member waters. Dissolution of detrital plagioclase (An = 3) and K- feldspar are reactions of major significance that are reflected chemically in the Na/Cl and K/Cl ratios of the formation water. Despite the high temperature of the sandstones (120–200°C), diagenetic albite does not occur. Geochemical calculations indicate the formation water is undersaturated with respect to both orthoclase and albite. This style of feldspar diagenesis differs significantly from that of sandstones of similar composition in other basins, and has probably influenced other aspects of silicate diagenesis.

The fate of feldspar in Brent Group reservoirs, North Sea: A regional synthesis of diagenesis in shallow, intermediate, and deep burial environments

Abstract

No full-text available

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