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Clay Diagenesis in the Kimmeridge Clay Formation, Onshore UK, and Its Relation to Organic Maturation
Abstract and Figures
Conversion of randomly ordered illite-smectite to ordered illite-smectite in the Upper Jurassic Kimmeridge Clay formation from the North Sea has been recorded within the 'oil window' and has been suggested as an indicator of oil source rock maturity. Studies of authigenic clay minerals in the fine fraction (>0.5 mu m) of the Kimmeridge Clay mudstones from 14 locations along the outcrop between Dorset and North Yorkshire, England, show that they are mainly ordered illite-smectites. The onshore Kimmeridge Clay section is organically immature, suggesting that illite-smectite ordering cannot be extrapolated between basins as an indicator of maturity levels. The results also have important implications in models of source-rock hydrocarbon expulsion and migration which involve shale dewatering as a flushing agent. However, dewatering of shales may aid migration as it could cause fracturing of the shale bands separating the organic-rich layers within the source rock, prior to hydrocarbon generation.-R.A.H.
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... The conversion from smectite to illite (clay diagenesis) with increasing depth occurs in all shales (Scotchman, 1987) and can be described by the widely accepted model proposed by Pytte and Reynolds (1989) based on a fifth-order kinetic reaction of the Arrhenius type. The result of the conversion is that the stiffness of the minerals composing the shale increases with depth. ...
... The conversion smectite/illite occurs in all shales with a general release of bound water into the pore space (Scotchman 1987). Smectite dehydration implies a stiffer matrix due to the presence of more illite, and therefore higher velocities. ...
We simulate the effects of diagenesis, cementation and compaction on the elastic properties of shales and sandstones with four different petro-elastic theories and a basin-evolution model, based on constant heating and sedimentation rates. We consider shales composed of clay minerals, mainly smectite and illite, depending on the burial depth, and the pore space is assumed to be saturated with water at hydrostatic conditions. Diagenesis in shale (smectite/illite transformation here) as a function of depth is described by a fifth-order kinetic equation, based on an Arrhenius reaction rate. On the other hand, quartz cementation in sandstones is based on a model that estimates the volume of precipitated quartz cement and the resulting porosity loss from the temperature history, using an equation relating the precipitation rate to temperature. Effective pressure effects (additional compaction) are accounted for using the Athy equation and the Hertz–Mindlin model. The petro-elastic models yield similar seismic velocities, despite the different levels of complexity and physics approaches, with increasing density and seismic velocities as a function of depth. The methodology provides a simple procedure to obtain the velocity of shales and sandstones versus temperature and pressure due to the diagenesis-cementation-compaction process.
... We consider two dissimilar theories of the diagenesis-cementation process, namely smectite/illite conversion based on a kinetic reaction given by the Arrhenius equation (Pytte and Reynolds, 1989), and sandstone compaction and cementation (Walderhaug, 1996;Avseth and Lehocki, 2016). The conversion from smectite to illite (clay diagenesis) with increasing depth occurs in all shales (Scotchman, 1987) and can be described by the widely accepted model proposed by Pytte and Reynolds (1989) based on a 5th-order kinetic reaction of the Arrhenius type. The result of the conversion is that the stiffnesses of the minerals composing the shale increase with depth. ...
... The conversion smectite/illite occurs in all shales with a general release of bound water into the pore space (Scotchman, 1987). Smectite dehydration implies a stiffer matrix due to the presence of more illite and therefore higher velocities. ...
We simulate the effects of diagenesis, cementation and compaction on the elastic properties of shales and sandstones with four different petro-elastical theories and a basin-evolution model, based on constant heating and sedimentation rates. We consider shales composed of clay minerals, mainly smectite and illite, depending on the burial depth, and the pore space is assumed to be saturated with water at hydrostatic conditions. Diagenesis in shale (smectite/illite transformation here) as a function of depth is described by a 5th-order kinetic equation, based on an Arrhenius reaction rate. On the other hand, quartz cementation in sandstones is based on a model that estimates the volume of precipitated quartz cement and the resulting porosity loss from the temperature history, using an equation relating the precipitation rate to temperature. Effective pressure effects (additional compaction) are accounted for by using Athy equation and the Hertz-Mindlin model. The petro-elastic models yield similar seismic velocities, despite the different level of complexity and physics approaches, with increasing density and seismic velocities as a function of depth. The methodology provides a simple procedure to obtain the velocity of shales and sandstones versus temperature and pressure due to the diagenesis-cementation-compaction process.
... The higher smectite content in the clay fraction of KWC compared to KBC is possibly due to smectite to illite transformation, which is typically active at a depth shallower than 2 -3 km, while illite precipitation occurs at higher temperatures at depths of more than 2 -3 km (Dypvik, 1983;Weaver, 1989). Scotschman (1987) has also reported that the ordered illite -smectite is of authigenic origin for Kimmeridge clay. The difference in kaolinite content is assumed to be due to initial differences in kaolinite composition (Scotschman, 1987). ...
... Scotschman (1987) has also reported that the ordered illite -smectite is of authigenic origin for Kimmeridge clay. The difference in kaolinite content is assumed to be due to initial differences in kaolinite composition (Scotschman, 1987). In KBC, dissolution of carbonate fossils has occurred to a great extent (Scotschman, 1989). ...
The paper presents the results of laboratory compaction tests on samples of Kimmeridge clay taken from two different locations in the UK. The main objective of the laboratory tests is to determine how diagenesis affects the hydro-mechanical properties and compaction behavior of argillaceous sediments. The two Kimmeridge clays used in the tests are essentially from the same parent material but have undergone different degrees of mechanical and chemical diagenesis. The laboratory tests are designed to determine the effects of mechanical loading and chemical processes on changes in porosity, permeability, compressibility, strength and effective horizontal stress in argillaceous materials. Another objective of the study is to investigate the adequacy of soil mechanical constitutive models in predicting the response of argillaceous materials to conditions corresponding to burial depths in sedimentary basins. The tests on Kimmeridge clay are carried out to as high as 150 MPa effective vertical stress corresponding to about 9 km in burial. The results indicate that mechanical compaction and burial depth cannot explain the differences in the hydro-mechanical behavior of the tested materials. The differences in hydro-mechanical response are attributed to chemical diagenesis by precipitation of minerals in the pores of the sediment. Other observed effects of chemical diagenesis on hydro-mechanical behavior of lithified sediments are: (1) increase in apparent pre-consolidation stress; (2) decrease in compressibility; (3) decrease in permeability; and (4) decrease in effective horizontal stress. Implications of the results to fluid flow and overpressure development in sedimentary basins are discussed.
... The Kimmeridge Clay Formation (KCF) sample was collected from the Blackstone Band exposed at Clavell's Hard (coordinates: 50 • 36 ′ 12 ′′ N, 2 • 7 ′ 26 ′′ W), 1 km east of Kimmeridge Bay in Dorset, UK ( Fig. 1a and b). The Blackstone Band is a Jurassic, relatively homogeneous dark brown, exceptionally organic-rich, fissile mudstone comprising shelly materials and FeS 2 nodules with intermittent hard, thin carbonate-rich layers (Scotchman, 1987(Scotchman, , 1989Shaw and Primmer, 1991;Macquaker and Gawthorpe, 1993;Morgans-Bell et al., 2001). The Blackstone Band is the most organic-rich layer in the KCF and is characterised by the presence of other important diagenetic materials such as Fe, detrital silica and carbonate, and kaolinite. ...
Incorporation of reduced inorganic sulfur (S) into organic matter (OM) in euxinic environments is considered a vital route through which OM is preserved for millions of years. However, precipitation of iron sulfides is believed to compete with OM preservation via sulfurisation. High resolution geochemical, petrographic, and electron microprobe (EMPA) analyses were employed to better understand S controls on OM preservation with particular focus on iron sulfides precipitation in organic S-rich mudstones from the Kimmeridge Clay (KCF), Monterey (MF), and Whitby Mudstone [Grey Shale (GS) and Jet Rock (JR) Members] Formations. Analyses indicated that S in the KCF and MF is mostly bound to OM as shown by the relatively high abundance of organic sulfur compounds (OSCs) in their pyrolysates. The relatively low amounts of iron (Fe) in these mudstones permitted extensive reaction of S and OM to occur efficiently, promoting preservation of high amounts OM through sulfurisation. State-of-the-art EMPA imaging of total S and Fe indicated that the S was mainly bound to organics, supporting the geochemical analyses. In contrast, S in the GS and JR was predominantly sequestered as pyrite (FeS2) rather than organically bound, as indicated by the relatively low quantity/absence of OSCs in their pyrolysates, hence, preservation through sulfurisation was less significant in these settings. EMPA imaging of S in these mudstones shows discrete shapes ranging in geometry and size and in most cases correlated with Fe in the corresponding Fe EMPA images, confirmed that the S exists in discrete authigenic FeS2. Overall, in line with previous research, this study indicates that preservation of OM through sulfurisation is an important pathway for sequestration of OM. However, an abundance of reactive Fe could hinder this process by preferentially reacting with reduced S, to form iron sulfides (mainly FeS2), thus impeding reactions between OM and S to occur efficiently.
... Ramseyer and Boles, 1986;Pytte and Reynolds, 1989;Velde and Vasseur, 1992), (ii) chemistry of pore solutions contained in the rock (e.g. Eberl and Hower, 1976;Scotchman, 1987;Š ucha et al., 1993) or (iii) compositional heterogeneity of the source smectite (e.g. Wei et al., 1996). ...
Results are presented on the relationships between the vitrinite reflectance VRo, the Tmax temperatures determined
with Rock-Eval pyrolysis, and the smectite (S) indices representing the percentage of smectite in the
mixed-layer illite-smectite (I/S) from claystones. Samples locations are recovered from the Polish part of the
Outer Carpathian fold-and-thrust belt. Organic indices show thermal maturities of most outcrop samples in the
range of 0.47 to 1.49% Ro and 421 to 485 °C Tmax. Inorganic S indices values range from 8 to 60%. Lateral variability
of the maturity measured with both organic and inorganic indices reflects mainly differences of maximum
tectonic burial in the accretionary prism and subsequent exhumation and denudation. For the Western Outer
Carpathians the calculated formulae for Tmax to VRo and S to VRo conversion are VRo = 0.0152 * Tmax - 5.938
(r2 = 0.87) and VRo = 1.1432–0.01295 * S (r2 = 0.65), respectively. Maximum paleotemperatures were calculated
for individual samples from S indices by applying the novel formula T = ln (1.1432–0.01295 * S) + 1.68/
0.0124. The proposed new conversion formulae might be useful in other regions with a similar geological setting,
i.e. for claystone formations dominated by kerogen type III in fold–and-thrust belts, characterized by rapid
tectonic burial accompanied by a low geothermal gradient, as well as maturities of the sub-bituminous to the
medium volatile bituminous rank (0.5 to 1.50% Ro).
... Clay minerals and organic matter usually coexist in clastic rocks and are used as a tool to identify potential hydrocarbon generation and expulsion due to their high susceptibility to temperature changes that control related mineral conversions and organic maturity [1][2][3][4][5]. Clay minerals and organic matter also have significant influences on the origin, preservation, and production of shale gas due to the substantial role of their nanoscale pores in the generation, storage, and seepage of shale gas [6]. ...
The present work is conducted on the Paleozoic (Ordovician) Khabour and the (Silurian) Akkas shales in the Akkas-1 well of western Iraq. The study is aiming to determine the implications of clay mineral transformation, organic mineral distribution and maturity of hydrocarbon generation, using X-ray diffraction (XRD), scanning electron microscopy (SEM) in addition to organic matter concentrations. In the shale of the Khabour Formation, amorphous organic matter is common and includes various Tasmanite-type organic matter, vitrinite, inertinite, and bituminite. The main clay minerals observed include illite, chlorite, kaolinite, in addition to mixed-layer illite-smectite and rare smectite. In Silurian shale, high content of organic matter is recorded in addition to abundant vitrinite and low content of grainy organic matter (Tasmanites) and pyrite. Illite and kaolinite are commonly found in addition to chlorite and illite-smectite clay minerals. Conversion of smectite to mixed-layer illite-smectite (I-S) and an increase in vitrinite reflectance are commonly observed below 2500 m depth in the studied formations, which coincides with oil and gas generation. These results could be used as an indication of higher maturity and hydrocarbon generation in the deeply buried shale of the Khabour and Akkas formations in western Iraq.
... Clay minerals and organic matter usually coexist in clastic rocks and are used as a tool to identify potential hydrocarbon generation and expulsion due to their high susceptibility to temperature changes that control related mineral conversions and organic maturity [1][2][3][4][5]. Clay minerals and organic matter also have significant influences on the origin, preservation, and production of shale gas due to their substantial role of nanoscale pores in the generation, storage, and seepage of shale gas [6]. ...
The present work is conducted on the Paleozoic (Ordovician) Khabour and the (Silurian) Akkas shales in the Akkas-1 well of western Iraq aiming to determine the implications of clay mineral transformation and organic mineral distribution and maturity for hydrocarbon generation using X-ray diffraction (XRD), scanning electron microscopy (SEM) in addition to organic matter concentrates. In the shale of the Khabour Formation, amorphous organic matter is common and includes various Tasmanite-type organic matter, vitrinite, inertinite and bituminite. The main clay minerals observed include; illite, chlorite, kaolinite, in addition to mixed-layer illite-smectite and rare smectite. In Silurian shale, a lot of organic matter is recorded in addition to abundant vitrinite, some grainy organic matter (Tasmanites) and pyrite with common illite, kaolinite in addition to chlorite and illite-smectite clay minerals. Conversion of smectite to mixed-layer illite-smectite (I-S) and increase in vitrinite reflectance are commonly observed below 2500 m depth in the studied formations and this coincides with oil and gas generation. These results could be used as an indication of higher maturity and hydrocarbon generation in the deeply buried shale of the Khabour and Akkas formations in western Iraq.
... the presence and significance of methanogenesis (McHargue and Price, 1982;Scotchman, 1987). Any dolomites that formed in the previous stage of diagenesis will likely show a ferroan rim surrounding the nonferroan core, precipitated as a result of BSR. ...
Marine carbonate δ¹³C and δ¹⁸O values are commonly utilized as indicators of past climate and are thought to record primary seawater conditions. However, these minerals can be altered following deposition and burial, overprinting the primary signal to reflect subsurface conditions. Whereas the influence of post-depositional alteration is widely understood in carbonate-dominated sediments (Swart, 2015), the influence has not been as widely studied in mud-rich settings. This study analyzed the carbonate δ¹³C and δ¹⁸O values from carbonate intervals in the Late Pennsylvanian Wolfcamp D shale unit in the Midland Basin to evaluate the impact of post-depositional alteration on the interpretation of stable isotope compositions of carbonates in a mud-rich depositional environment. Here we show the importance of using multiple indicators to fully evaluate the impact of post-depositional alteration on the interpretation of the marine carbonate δ¹³C and δ¹⁸O values in mud-rich environments. The extent of diagenesis in the Wolfcamp D was primarily controlled by fluctuations in benthic redox chemistry, which controlled sulfate reduction and the preservation of organic matter. These processes influenced the extent of carbonate cementation in the bacterial sulfate reduction zone and during deep-burial, making the carbonates unreliable for intra-basinal correlations. The results of this study were incorporated into a conceptual model that can likely be applied as a framework for other studies in mud-rich depositional environments.
... Firstly, it has been suggested that the black Cretaceous shales as well as the underlying Kimmeridge Clay Formation are too immature to generate oils (Fig. 14) (Scotchman, 1987(Scotchman, , 1989Underhill & Stoneley, 1998). Oil inclusions were not found within the calcite veins, so there is no direct evidence that overpressure could have developed as a result of hydrocarbon generation. ...
Bedding-parallel fibrous calcite veins in black shales (Cretaceous, southern UK) were investigated using a combined field, stable isotopic geochemistry, petrographic and crystallographic method to examine their formation mechanism. Calcite veins occur in all shale beds and are most abundant in the bituminous shales of the Chief Beef Beds. The calcite fibres in these veins exhibit either an antitaxial fibre growth with curvy stylolites as the median zone, or a predominantly syntaxial, upwards growth. The calcite veins range from –0.49 to 1.78‰ of δ ¹³ C values, and –6.53 to –0.03‰ of δ ¹⁸ O values, which are both similar to those of their host shales. Our petrographic observations demonstrate that subhorizontal and interconnecting microstylolite networks commonly occur within the calcite veins. Equant calcite grains in the median zones exhibit indenting, truncating and also interpenetrating grain contacts. It is interpreted that the fibrous calcite veins were sourced by neomorphic calcite from their host shales, with evidence from the δ ¹³ C signatures, pressure-solution features (stylolites, microstylolites and grain contact styles) and embedded fossil ghosts within the veins. The diagenetic fluids, from which calcite was precipitated, were a mixing of the original seawaters and ¹⁸ O-depleted meteoric waters. Development of bedding-parallel calcite veins is considered to have been enhanced by pressure solution as a positive feedback mechanism, which was facilitated by the overburden pressure as the maximum principal stress. Calcite fibres, with a predominant subvertical c-axis orientation, exhibit a displacive growth in porous shales and a replacive growth at vein-limestone contacts. This study highlights the critical role of pressure solution in the formation of bedding-parallel calcite veins during burial and diagenesis of immature black shales.
Geochemical parameters, e.g. maturity, and total organic carbon (TOC) content, play a crucial role in the prediction of sweet spots and the exploration of oil and gas in organic-rich shales. Thermal maturity significantly affects the conversion of solid organic matter (OM) into hydrocarbons and the evolution of microstructures, thereby altering the overall elastic properties of shales. To clarify how the maturity affects shale property, we propose a novel Rock Physics Model (RPM) of organic-rich shale, in which we consider the continuous process of thermal maturity.#xD;Firstly, we present how to estimate maturity level, TOC content, and organic porosity using logging data. Secondly, different from only considering the discrete-stage maturity, we establish a novel RPM, in which a continuous kerogen maturation process serves as a key control condition. Furthermore, we propose how to calibrate the volumetric proportion of each porosity type as a function of maturation. Finally, we apply the RPM to investigate how sweet spot parameters (thermal maturity, TOC content, and brittle mineral content), overpressure and diagenesis affect the overall elastic properties and anisotropy of shale. Results demonstrate that using the proposed RPM we may predict acoustic velocity of shale formations reliably, and kerogen evolution has a noticeable impact on the elastic properties of shale rocks, particularly during the wet gas window stage of mid-to-high maturation. We conclude that thermal maturity emerges as a crucial sweet spot parameter in the case of exploration of oil and gas in organic-rich shales.
A detailed study has been made of changes in interlayered clay minerals, sediment mineralogy, vitrinite reflectance, kerogen composition, and chemistry of the soluble organic extract through Well 16/22-2 in the South Viking Graben. The well sampled sediments range in age from Oligocene to, at total depth, Middle Jurassic (Dogger).
Lithologies are typical of the South Viking Graben/Central Graben area with Upper Jurassic black pyritic shales, a thick Cretaceous chalk succession and prominent top Palaeocene tuff. Sandstones are common only in the Upper Jurassic Palaeocene and Eocene. Tuffs are sporadically present in the Middle Jurassic. Randomly interlayered illitesmectite is the dominant clay mineral through the Tertiary and Cretaceous and the change to an ordered structure which occurs just above the Jurassic/Cretaceous unconformity is interpreted as a diagenetic reaction. Estimates of illite-smectite composition indicate a reversal to smectite rich clays in the Lower Eocene-Top Palaeocene which is attributed to an insufficient supply of K ⁺ from detrital feldspar and mica due to the dominance of basic volcanic weathering products.
The vitrinite reflectance profile for this well indicates progressive maturation of kerogen to marginal maturity in the Upper Jurassic. A large jump of 0.2% R o occurs at the Upper/Middle Jurassic boundary below which kerogen maturity is at the peak oil-generating stage. This maturation break is supported by an increase in spore colour index. Chemical parameters are strongly influenced by a change in kerogen type from mixed algal and humic to predominantly humic at this stratigraphic level. Very high heat flow due to localized intrusive igneous activity during the late Middle Jurassic is thought to have caused the increased maturity of the Middle Jurassic sediments.
Vitrinite reflectance studies in non-coal-bearing rocks have been conducted over the past several years; the techniques for extraction, analysis, and interpretation of small vitrinite particles vary slightly from standard coal petrographic procedures. Two areas in California afford a comparison of thermal history utilizing authigenic minerals and vitrinite reflectance: the Great Valley sequence of late Mesozoic age near Cache Creek west of the Sacramento Valley, and Miocene to Eocene sediments in the Tejon area of the San Joaquin Valley. In the Tejon area, laumontite is a postcompaction authigenic mineral and first occurs at 9,500 ft and is abundant below 10,500 ft. Vitrinite reflectance studies of Tejon area wells demonstrate an equivalent coal rank of high-volatile C bituminous to subbituminous A at 10,500 ft. \ t this depth, the temperature is 218°F (104°C), and the hydrostatic pressure is 4,600 psi (317 bars). Electrical log temperatures, as corrected by comparison with accurate subsurface temperature profiles, were used to reconstruct paleothermal conditions. At Cache Creek, laumontite is found as an alteration product in sandstones formerly buried at depths greater than 32,500 ft; the calculated paleopressure at this level is 27,000 psi (1,860 bars). Vitrinite reflectance measurements within the laumontite-bearing interval indicate a thermal history equivalent to high-volatile B bituminous coal. Based on the temperature- reflectance relation at Tejon, the temperature at the top of the laumontite zone at Cache Creek is estimated to be 265°F (130°C), and the paleogeothermal gradient in the Great Valley sequence is around 0.62°F per 100 ft (1.1°C per 100 m). Because the formation of zeolites is governed by a complex interaction of physical and chemical factors, we judge that vitrinite reflectance, which is largely temperature dependent, is the more promising tool for the measurement of thermal history.