Clay Mineral Diagenesis Research Articles

Mudstone diagenesis with depth and thermal maturity in the Cenomanian–Turonian Eagle Ford group. PART II: Diagenetic processes and paragenetic sequence

The Cenomanian–Turonian (C–T) Eagle Ford Group (EFG) is composed of massive, laminated, and/or burrowed limestones, marls, and lime mudstones deposited on the drowned south Texas shelf below storm-wave base. A burial diagenesis investigation using seven cored wells from shallow to deep burial (depth ranges from 7684 to 13,670 ft [2342 to 4166.6 m]; BHT from 183 to 285 °F [83.9–140.6 °C]) and from east (San Marcos Arch) to west (Maverick Basin) delineated a complex diagenetic history as a result of changes in water chemistry, redox condition, and burial history. The second part of this comprehensive study shows the variety of diagenesis and paragenesis. This study focuses especially in the organic-rich Lower Eagle Ford (LEF) member deposited largely under anoxic/euxinic conditions. Processes such as near seafloor sulfate-reduction, Fe-reduction, and methanogenesis drive various chemical reactions during lithification. Feldspar diagenesis included albitization and transformation of K-feldspar to kaolinite and albite, and from kaolinite to Mg-chlorite with depth. Clay mineral diagenesis is complex with authigenic kaolinite, mixed kaolinite/Mg-chlorite, and Mg-chlorite form in biota tests and matrix. Additionally, calcite dissolution, burial dolomitization, late Fe-chlorite replacement, and oxidation of pyrite to marcasite are also observed. Detritus-derived smectite starts to transform to mixed layered clay, illite-smectite (I/S) in the oil window; however, illitization trends were not observed in depth. This is because most smectite forms from alteration of volcanic ashes and the volcanic origin of the disorientated smectite did not follow regular illitization kinetics. The increase in Mg content from illitization transforms not only the kaolinite to chlorite, but precipitates authigenic dolomites. The clay minerals precipitated in the original interparticle pores, further reducing porosity, and impacted permeability, fluid saturation, wettability, and capillary pressure. With thermal maturation, bitumen and oil conversion increases oil-wetness of the rock and the resistivity measurement from wireline logs.

Deciphering Mineralogical Composition and Clay Diagenesis in Exposed Shales of Jaintiapur, Surma Basin: Unveiling Insights into Burial Diagenesis

Mineralogical analysis of exposed shale samples in the Jaintiapur region was conducted utilizing X-ray diffraction (XRD) techniques. The studied shales are characterized by illite, chlorite, illite/ smectite mixed layer clay, and kaolinite as the predominant clay minerals, with a relatively lower proportion of smectite. Additionally, the non-clay fraction consists mainly of quartz, feldspar, dolomite, and trace amounts of pyrite. Notably, illite and chlorite exhibit prominent abundance within the clay portion, while quartz is the prevailing non-clay mineral. Furthermore, the investigation reveals a notable enrichment of illite and chlorite with the advancement from younger to older stratigraphic formations in the studied shale samples, indicating greater intensity in clay mineral diagenesis in deeper stratigraphic formations. The transition from smectite to illite minerals can release Si, Ca, Fe, and Mg ions, potentially entering nearby sandstones and leading to cementation with quartz, chlorite, and calcite. This process aids in retaining pore water in shales. In this context, overpressure development influenced by clay diagenesis is not primarily due to cementation while undergoing smectite to illite conversion but due to illite packet formation within smectite, reducing shale permeability. During this conversion, interlayer water shifts to free water, causing volume reduction compounded by the low permeability of shale, trapping fluids in the pores. This expansion of pore water, reduced velocity, and increased pore pressure suggest that mineral transformation with burial depth might contribute to overpressure development. However, the observed decline in smectite clays in sediments transitioning from younger to older, culminating in their absence in older shale-dominated formations, explains the presence of diagenetic illite and chlorite. This study underscores the progressive diagenetic transformation in clay mineralogical composition with progressive burial across the sedimentary succession. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(2), 2023, P 69-82

Clay mineral diagenesis in alternating mudstone‐sandstone beds from Late Palaeozoic strata of the Anadarko Basin, United States

Closely interbedded mudstone and sandstone are rarely the focus of clay mineral diagenetic studies; as such, it is not understood if these various rock types follow the same or different diagenetic pathway(s) during burial. Over a depth span of ~2,300 to 4,200 m, 95 core samples of alternating Pennsylvanian mudstone/sandstone from the hydrocarbon‐rich Anadarko Basin in Oklahoma have been characterized petrographically, mineralogically, and chemically. The investigated fine‐grained sediment is argillaceous to siliceous mudstone, dominated by Illite/mica and mixed‐layer illite–smectite. The sandstone is variably classified as quartz and mudclast‐rich litharenite to sub‐litharenite, which is commonly carbonate‐cemented. This contribution provides evidence for a very similar diagenetic evolution of the clay mineral assemblages across all rock types despite the discrepancies in porosity and permeability of the host rock. We posit that the incorporation of micrometre‐scale mudclasts through bioturbation of the sandstone may explain the relatively uniform compositional characteristics of illite–smectite, chlorite, and kaolinite. Inferences are drawn from a detailed clay mineral quantification of these interbedded lithologies aid to our understanding of the clay mineral distribution and speciation in Pennsylvanian strata of the Anadarko Basin, which further helps to constrain the reservoirs' physical, chemical, and structural properties.

Clay mineral transformations of mesozoic mudstones in the central Junggar Basin, northwestern China: Implications for compaction properties and pore pressure responses

The clay mineral transformation of deeply buried mudstones is closely related to the mudstone microstructure, petrophysical properties, and pore fluid pressure (pore pressure). The influence of the burial history on the microstructure and compaction behavior of mudstones has been the focus of recent research. In this study, the mineral composition, mudstone microfabric and petrophysical properties of Mesozoic overpressured mudstones in the central Junggar Basin were analyzed to explore the influence of clay mineral diagenesis on compaction processes and overpressure responses in diagenetically altered mudstones. The clay minerals of these Mesozoic mudstones are mainly composed of mixed smectite/illite layers (I/S) and illite with small amounts of kaolinite and chlorite, but they do not contain discrete smectite. The Mesozoic mudstone underwent clay mineral diagenesis, the smectite in I/S transformed to illite at depths of 4000–4800 m (temperatures of 90–105 °C), and the I/S changed from the R0 type to the R3 type. The recrystallization and growth of clay minerals continued with increasing stress with depth, leading to the substantial realignment of clay particles observed at the microscale. The combination of the preferential alignment of clay particles and precipitation of authigenic cements caused significant decreases in the porosity of the mudstone. The results of this study support the viewpoint that the effective stress continues to work in diagenetically altered mudstones, although porosity loss is enhanced by clay fabric destabilization caused by clay mineral transformation. The top depth of smectite-to-illite transformations is consistent with the top of the overpressure, indicating that overpressure occurs simultaneously with clay diagenesis in diagenetically altered mudstones. The decrease in porosity and increase in density of mudstone caused by clay diagenesis changes the overpressure logging response to a certain extent and affects the accuracy of the pressure prediction results. The sonic transit time-density cross plot can be used to effectively distinguish the compaction stage and the cause of overpressure in diagenetically altered mudstones. Clay mineral diagenesis cannot be ignored in pore pressure analysis and pressure prediction in deeply buried mudstones. This research is of great importance for deepening the theory of mudstone compaction and has practical significance for pore pressure prediction in deeply buried layers.

New understanding of overpressure responses and pore pressure prediction: Insights from the effect of clay mineral transformations on mudstone compaction

Chemical compaction dominated by clay mineral transformations, commonly develops in deeply buried mudstones, and prominently affects the petrophysical properties of mudstones and fluid flow. This paper takes the Neogene mudstones in the Ledong Slope of the Yinggehai Basin as research object, analyzes the relationship between chemical compaction and overpressure, establishes a new multisegment normal compaction trends (NCTs) and reevaluates the overpressure mechanisms. The results show obvious clay mineral transformation with increasing depth, and the top of the clay mineral transformations is approximately equal to the depth of the overpressure. The logging responses of overpressured mudstones are affected by clay mineral diagenesis, showing a 45% increase in the sonic transit time at 3500-5000 m and a 10% decrease in the density at the same interval, and the density response exhibits a relative lag. A method comprising an inversion algorithm calibrated by measured pressure data is proposed to establish multisegment NCTs. Furthermore, new loading-unloading curves comprising a mechanical loading curve and chemical loading curve are established to reevaluate the overpressure mechanisms. The loading-unloading curves show that the pressure gradient generated by mudstone compaction maybe less than 1.8. While the pressure data with pressure gradient larger than 1.8 fall on the unloading curve, indicating elevated sandstone pressures rather than in situ mudstone pressures. The multisegment NCTs were proven to provide a satisfactory overpressure estimate in the Ledong Slope as well as the relative contribution of different overpressure mechanisms. This paper provides new insight into mudstone compaction and overpressure responses, as well as a meaningful reference for the identification of multiple-overpressure generation mechanisms and the prediction of pore pressure in deeply buried formations.

Discussion of “Quantitative evaluation of thixotropy-governed microfabric evolution in soft clays”

The diagenesis of clay minerals and thermal evolution of organic matter (OM) are consistent in space and time in most petroliferous basins. Within these processes, the synchronism of smectite illitization with peak hydrocarbon generation is one of the most important phenomena and the illitization process is believed to play an important role in catalytic effects on the conversion of OM to oil and gas. The variation in the layer charge of montmorillonite (Mt) is a key indicator of illitization. However, the effects of layer charge on OM pyrolysis have not been investigated. In this study, a series of homologous reduced-charge Mt (LiMt) samples were prepared by changing the temperature of heat treatment based on the Hofmann–Klemen effect. The 12-aminolauric acid and Mt were mechanically mixed to prepare clay–OM complexes. Pyrolysis experiments were conducted in a confined gold capsule system, and the gas components were evaluated by gas chromatography. The experimental results show charge reducing changed the solid acidity of Mt and further influence the catalytic activity of Mt on the OM pyrolysis. The solid acidity of Mt first increased and then decreased as the layer charge decreased, while the water content decreased. These sites, especially Brønsted (B) acid sites, catalyzed OM pyrolysis via the carbocation mechanism. The content of iso‐alkanes was controlled by the content of water in Mt because excess water molecules hindered the isomerization reactions occurring at the B acid sites. The CO2 content was affected by both the amount of Lewis (L) acid sites and the water content. These fundamental results suggest that changes of the layer charge in Mt which occurs during the processes of smectite illitization may play an important role in hydrocarbon generation. These findings are important for understanding the organic-inorganic interactions between clays and OM during sedimentation and diagenesis.

Effects of montmorillonite charge reduction on the high-temperature/high-pressure pyrolysis of organic matter

The diagenesis of clay minerals and thermal evolution of organic matter (OM) are consistent in space and time in most petroliferous basins. Within these processes, the synchronism of smectite illitization with peak hydrocarbon generation is one of the most important phenomena and the illitization process is believed to play an important role in catalytic effects on the conversion of OM to oil and gas. The variation in the layer charge of montmorillonite (Mt) is a key indicator of illitization. However, the effects of layer charge on OM pyrolysis have not been investigated. In this study, a series of homologous reduced-charge Mt (LiMt) samples were prepared by changing the temperature of heat treatment based on the Hofmann–Klemen effect. The 12-aminolauric acid and Mt were mechanically mixed to prepare clay–OM complexes. Pyrolysis experiments were conducted in a confined gold capsule system, and the gas components were evaluated by gas chromatography. The experimental results show charge reducing changed the solid acidity of Mt and further influence the catalytic activity of Mt on the OM pyrolysis. The solid acidity of Mt first increased and then decreased as the layer charge decreased, while the water content decreased. These sites, especially Brønsted (B) acid sites, catalyzed OM pyrolysis via the carbocation mechanism. The content of iso‐alkanes was controlled by the content of water in Mt because excess water molecules hindered the isomerization reactions occurring at the B acid sites. The CO2 content was affected by both the amount of Lewis (L) acid sites and the water content. These fundamental results suggest that changes of the layer charge in Mt which occurs during the processes of smectite illitization may play an important role in hydrocarbon generation. These findings are important for understanding the organic-inorganic interactions between clays and OM during sedimentation and diagenesis.

The acid test: An experimental microarchaeological study of guano-driven diagenesis in tropical cave sediments

Caves in tropical regions form a key resource for those reconstructing human evolution and dispersals. However, the complex sedimentary records they contain remain under-interpreted because of the poorly constrained effects of humid tropical climates upon archaeological site formation processes. Guano-driven phosphatic diagenesis impacts archaeological preservation in caves, but atypical features observed in guano layers in prominent Southeast Asian archaeological sites suggested that hot and humid conditions had promoted the formation of distinctive sedimentary environments with unknown effects on assemblage taphonomy. Few reference data exist that are relevant to geomorphic processes in tropical caves and this laboratory-based modern analogue study was designed to conclusively relate analytical observations to sedimentary palaeoenvironments and explore their effects on assemblage taphonomy. Stratigraphic models were kept under simulated tropical conditions, each containing an identical range of faunal remains, organic materials and minerals, which was buried in wet bat guano. These were excavated, one per month over 24 months, while nine further stratigraphic models were used to control for environmental variables. Sedimentary environments were investigated using sedimentological characterisations of excavated sediments, while post-depositional alterations to buried materials were investigated through optical and scanning electron microscopic analysis of thin-section samples combined with chemical characterisations of excavated samples through Fourier-transform infrared spectroscopy (FTIR). Experimental conditions prevented the sediment acidification that is typically associated with guano-driven diagenesis, but buried materials were severely, and distinctively, diagenetically altered. Organic decomposition in guano, bamboo and charcoal was associated with the activities of sulphate-reducing bacteria, but diagenetic alterations affecting charcoal remain poorly resolved due to variation in the experimental assemblage. Bone was discoloured and affected by dissolution, recrystallisation and alteration to secondary minerals, including gypsum, while carbonates reprecipitated as calcium phosphates and non-stochiometric mineral species. Diagenesis of clay minerals was observed, but related diagenetic trajectories remain poorly resolved. These alterations provide micromorphological and mineralogical indicators of sedimentary palaeoenvironments in the earliest stages of burial and this experiment provides a framework within which to understand the taphonomy of archaeological materials in tropical caves.

Diagenesis and genesis of clay minerals in the Triassic sandstones of the Panchet and Parsora formations, Damodar–Son Basin, India

The Triassic sandstones of the Panchet and Parsora formations are deposited in the Damodar and Son basins of the Peninsular India. Though both the formations have similarity in depositional and lithification processes, their diagenetic history and authigenic clay minerals formation have little differences. To improve understanding of diagenesis and associated clay minerals, the present study examined in Triassic sandstones. The Lower Triassic Panchet sandstones are poorly sorted arkose and originate from nearby granitoid terrain. The early diagenetic assemblages of the sandstones are kaolinite–iron oxide, montmorillonite, quartz and cement in the sandstones formed in oxy-acidic environment under shallow burial condition. Authigenic chlorite and recrystallised interstitial components such as orthomatrix are the main late diagenetic products formed, respectively, at oxy-acidic and reducing alkaline conditions where carbonate and allogenic iron cements were emplaced as the latest events. The Panchet and the Parsora formations are the riverine Triassic deposits and have almost identical early diagenetic assemblages. The kaolinisation in sandstone is loss of feldspar and is more pronounced in the Parsora Formation. The late diagenetic products in both the formations essentially similar and differ only in their relative chlorite content, which was formed under elevated temperature as an effect of tectonic activity. Allogenic iron and carbonate cements are post-tectonic.

Evaluation of Shale Source Rocks and Clay Mineral Diagenesis in the Permian Basin, USA: Inferences on Basin Thermal Maturity and Source Rock Potential

The use of mineral diagenetic indices and organic matter maturity is useful for reconstructing the evolution of sedimentary basins and critical assessments for potential source rocks for petroleum exploration. In this study, the relationship of clay mineral diagenesis and organic matter thermal indices (Rock-Eval Tmax) and calculated vitrinite reflectance (%Ro) were used to constrain the maximum burial depths and temperatures of three distinct intervals within the northern Permian Basin, USA. X-ray diffraction of clay fractions (<2 µm) consists of illite, chlorite, and illite-smectite intermediates. Primary clay mineral diagenetic changes progressively increase in ordering from R0 to R1 I-S between 2359.5 and 2485.9 m and the appearance of chlorite at 2338.7 m. Rock-Eval pyrolysis data show 0 to 14 wt% TOC, HI values of 40 to 520 mgHC/g TOC, and S2 values of 0 to 62 mg HC/g, with primarily type II kerogen with calculated %Ro within the early to peak oil maturation window. Evaluation of the potential for oil generation is relatively good throughout the Tonya 401 and JP Chilton wells. Organic maturation indices (Tmax, %Ro) and peak burial temperatures correlate well with clay mineral diagenesis (R0–R1 I-S), indicating that maximum burial depths and temperatures were between 2.5 and 4 km and <100 °C and 140 °C, respectively. Additionally, the use of clay mineral-derived temperatures provides insight into discrepancies between several calculated %Ro equations and thus should be further investigated for use in the Permian Basin. Accordingly, these findings show that clay mineral diagenesis, combined with other paleothermal proxies, can considerably improve the understanding of the complex burial history of the Permian Basin in the context of the evolution of the southern margin of Laurentia.

Clay mineral diagenesis and red bed colouration: A SEM study of the Gercus Formation (Middle Eocene), northern Iraq

Detailed high magnification scanning electron microscopic (SEM) study supported by petrographic, mineralogical, and geochemical (major and trace element) analysis of the Gercus Formation clastics (sandstones and mudstones) from northern Iraq shows that the main authigenic components are carbonate (dolomite and calcite), clay and haematite minerals with rare quartz, feldspar, and gypsum. The carbonates and sulphate minerals were the main early diagenetic phase in the Gercus Formation and were produced from the post‐depositional breakdown of ferromagnesian minerals. These latter are unstable in the oxidizing conditions of the interstitial waters below the desert surface. The main authigenic clay minerals are illite, illite‐smectite, and palygorskite. The presence of tangential illite coating and illite‐smectite in the sandstones can be related to their desert origin. Palygorskite is associated with dolomite, calcite, and haematite. These mineral associations may indicate early diagenesis in evaporitic and saline conditions. It also confirms the syndepositional and diagenetic origin for the clayey‐haematitic matrix and pigment of the Gercus Formation. Haematite occurs as extremely fine‐grained crystals mixed with authigenic clays and exhibits a range of shapes (e.g., fibrous to lath‐shaped aggregates, small spherical grains, specularite crystals, blade‐shaped, rosette‐like clusters, and a lump of bright spheroid‐shaped haematite/goethite). The arid to semi‐arid conditions together with the oxidizing and alkaline interstitial water favoured the noted mineral assemblage within the Gercus Formation.

Origin of chert nodules in the Ediacaran Doushantuo Formation black shales from Yangtze Block, South China

Abstract Chert nodules are widely developed in Ediacaran deposits of South China, yet the mechanism responsible for their formation remains disputed. Petrological and geochemical studies of chert nodules and host shale of the Ediacaran Doushantuo Formation of the Yangtze Block, South China, were carried out to more fully assess silicification of these deposits. Doushantuo chert nodules display a concentrically layered internal structure dominated by quartz and lesser concentrations of calcite, carbonate fluorapatite, organic matter, pyrite, and sphalerite. Individual layers become thinner from nodule centers to edges. Our results suggest that (1) Doushantuo host black shale accumulated under persistent anoxic bottom water conditions and (2) chert nodules formed as a consequence of the anaerobic oxidation of methane focused along sulfate-methane transition zones at shallow burial depth. Nodules likely originated from porous proto-nodules that consisted of 13C-depleted authigenic calcite produced by the anaerobic oxidation of methane. Decaying of organic matter within nodules helped to create porosity that was filled by authigenic calcite and also released phosphate and zinc ions to pore water, thus favoring precipitation of carbonate fluorapatite and sphalerite. Silica supplied principally from the diagenesis of clay minerals of the host shale cemented the nodules and replaced early formed calcite. The progressive infilling of residual porosity by 18O-depleted pore fluids associated with increasing burial depth is reflected in the strongly negative δ18Ocarb values documented from the nodules. Thus, the formation of Doushantuo chert nodules reflects a protracted diagenetic history initiated by bacterial sulfate reduction and anaerobic oxidation of methane close to and within the sulfate-methane transition zone and continued in association with the diagenetic release of silica from host shale as well hydrocarbon generation with increasing burial depth.

Thermal and exhumation histories of the northern subalpine chains (Bauges and Bornes—France): Evidence from forward thermal modeling coupling clay mineral diagenesis, organic maturity and carbonate clumped isotope (Δ47) data

AbstractAssessing the thermal evolution of sedimentary basins over time is a major aspect of modern integrated basin analysis. While the behavior of clay minerals and organic matter with increasing burial is well documented in different geological and thermal settings, these methods are often limited by the temperature ranges over which they can be precisely applied and by the available material. Here, we explore the emergent Δ47 clumped isotope geospeedometry (based on the diffusional redistribution of carbon and oxygen isotopes in the carbonate lattice at elevated temperatures) to refine time‐temperature paths of carbonate rocks during their burial evolution. This study provides a reconstruction of the thermal and exhumation history of the Upper Cretaceous thrust belt series in the western subalpine massifs (Bauges and Bornes, French Alps) by a new approach combining for the first time available data from three independent geothermometers. The investigated area presents two zones affected by contrasting thermal histories. The most external zone has undergone a relatively mild thermal history (T < 70°C) and does not record any significant clay mineral diagenetic transformation. By contrast, the internal zone has experienced tectonic burial (prealpine nappes) in response to thrusting, resulting in overheating (T > 160–180°C) that induced widespread clay mineral diagenetic transformations (progressive illitization from R0 to R1 and R3 illite‐smectite mixed‐layers), organic matter maturation (oil window) and Δ47 thermal resetting with apparent equilibrium temperatures above 160°C. The three employed geothermal indicators conjointly reveal that the investigated Upper Cretaceous rocks have suffered a wide range of burial temperatures since their deposition, with a thermal maximum locally up to 160–180°C. High temperatures are associated with the tectonic emplacement of up to 4 km of prealpine nappes in the northern part of the studied area. Finally, a forward thermal modeling using Δ47, vitrinite reflectance and clay mineral data, is attempted to precisely refine the burial and exhumation histories of this area.

Low-Temperature Clay Mineral Dehydration Contributes to Porewater Dilution in Bering Sea Slope Subseafloor

Widespread diagenesis of clay minerals occurs in deeply buried marine sediments under high-temperature and high-pressure conditions. For example, the smectite-to-illite (S-I) transformation has been often observed in sediments at in situ temperatures above ~60°C. However, it remains largely unknown whether such diagenetic processes naturally occur in relatively shallow and low-temperature sediments and, if so, what the consequences are of any related chemical reactions to the geochemical characteristics in the deep biosphere. We evaluated the possibility of naturally occurring S-I transformation at temperatures below 40°C in continental slope sediments of the Bering Sea by examining porewater chemistry, clay mineralogy, and chemical composition of clay minerals measured to ~800 m beneath the seafloor (mbsf) in core samples acquired during Integrated Ocean Drilling Program Expedition 323. In porewater from these cores, chloride concentrations decreased with increasing depth from 560 mM near the seafloor to 500 mM at ~800 mbsf; δ18O increased from 0‰ to 1.5‰; and δD decreased from –1‰ to –9‰. These trends are consistent with the addition of water derived from S-I transformation. The discrete low Cl– spikes observed between ~200 mbsf and ~450 mbsf could be attributed to the dissociation of methane hydrate. X-ray diffraction analysis of the clay-size fraction (<2 µm) showed an increase of illite content in the I/S mixed layer with increasing depth to 150 mbsf. This increase may imply the occurrence of S-I transformation. The decrease of Fe3+/Fe2+ in the clay-size fraction with increasing depth strongly suggests microbial reduction of Fe(III) in clay minerals with burial, which also has the potential to promote the S-I transformation. Our results imply the significant ecological roles on the diagenesis of siliciclastic clay minerals underlying the high-productivity surface seawater at continental margins.

Clay mineral diagenesis and source rock assessment in the Bornu Basin, Nigeria: Implications for thermal maturity and source rock potential

An understanding of the mineral and organic matter modifications during burial are critical in assessments of potential source rocks in petroleum exploration studies. Here, we examined the relationships between clay mineral diagenesis and organic thermal indicators (vitrinite reflectance and Rock-Eval Tmax) to constrain the thermal maturity and hydrocarbon potential of source rocks in the Mbeji-1 well in the northeastern part of the Cretaceous/Tertiary Bornu Basin. X-ray diffraction mineralogical data show that the <2 μm clay-sized fraction consist of mixed layer illite/smectite (I/S), kaolinite, illite, and Fe-chlorite clay mineral assemblages. The major diagenetic changes in the clay mineral evolution are transformation of random (R0) to ordered (R1) illite/smectite (I/S) mixed layer between ∼2500 m and ∼3640 m depths, decrease in smectite in I/S with burial, and appearance of chlorite at ∼2500 m. Total organic carbon and Rock-Eval pyrolysis parameters (HI, S2, Tmax) indicate presence of lean shales consisting of gas prone to inert terrestrial organic matters. Source rocks at shallower depths below 2500 m are immature while those below this level within lower Fika Formation to upper Gongila formations are marginally matured with respect to hydrocarbon generation. The variation between I/S transformation and VR for the Mbeji-1 well shows a delay in illitization process compare to organic matter maturity. We considered that other than temperature, insufficient and variable potassium availability, and short-heating time resulted in retarding the smectite illitization reaction relative to the organic matter parameters (vitrinite reflectance and Tmax). These differences further demonstrates that organic matter responds quickly to increasing temperature than mineral reactions at short heating time in young sedimentary basins.

The Sanrafaelic remagnetization revisited: Magnetic properties and magnetofabrics of Cambrian-Ordovician carbonates of the Eastern Precordillera of San Juan, Argentina

Systematic rock-magnetic and magnetofabric studies were carried out on samples from twenty-three palaeomagnetic sites distributed on the La Laja, Zonda, La Flecha, La Silla and San Juan Formations, which constitute a thick middle Cambrian to early Ordovician carbonate sequence exposed in the Eastern Precordillera of Argentina. Previous palaeomagnetic studies on these rocks showed that this succession is characterized by a recent full overprint in the lower levels and a post-tectonic Permian remagnetization associated to the widespread Sanrafaelic event in the upper part. Our investigation revealed that the fluctuations of the magnetic properties are stratigraphically (lithologically) controlled. Anisotropy of magnetic and anhysteretic susceptibility measurements defined consistent fabrics along the entire section that switch progressively from “inverse”, at the bottom, to “normal”, at the top, with “intermediate” fabrics occurring mainly at medium levels. Degree of dolomitization significantly affects many rock-magnetic parameters, but appears unrelated to the presence of the Permian remagnetization, which is determined to reside in magnetite despite the complex magnetic mineralogy shown by the studied carbonates. Hysteresis cycles of rocks affected by the Sanrafaelic remagnetization are governed by ferromagnetic fractions showing a clear difference respect to those not affected and characterized by the dominance of paramagnetic or diamagnetic signals. The magnetic fabrics and mineralogical characterization rule out a thermoviscous origin and suggest a chemical remagnetization originated in the authigenic formation of magnetite for the Sanrafaelic overprint. X-ray diffraction analyses indicate that clay minerals are virtually absent in the whole succession with no traces of illite in any sample, discarding burial diagenesis of clay minerals for the origin of the remagnetization. Lack of late Palaeozoic magmatic rocks near the study area difficults correlation of this event with hydrothermal brines as well as casts serious doubts on any effect produced by an assumed geothermal anomaly associated with the Permo-Triassic Choiyoi magmatic province. The original model of remagnetization associated to chemically active fluids expelled from the San Rafael Orogen towards the foreland still holds as a viable mechanism.

Multistage dolomitization in the Qal'eh Dokhtar Formation (Middle‐Upper Jurassic), Central Iran: petrographic and geochemical evidence

The late Jurassic Qal'eh Dokhtar Formation lithologically comprises three parts, from bottom to top, a lower sandstone unit, middle shale unit and an upper carbonate unit, which extend in a N–S direction over a wide area to the east of the Shotori Range and west of the Lut Block (Central Iran). This succession was deposited on a mixed carbonate–siliciclastic ramp. Carbonate rocks of the Qal'eh Dokhtar Formation vary from undolomitized, to partly dolomitized, to completely dolomitized. Field observations from two measured sections (the type section, 955 m thick, and the Sorond section, 639 m thick), combined with detailed petrographic and geochemical analyses, revealed the diverse types of dolomite in this formation. Five types of replacement dolomite and one type of dolomite cement were distinguished. Replacement dolomites (RD) consist of: (1) fine crystalline planar‐s (RD1); (2) medium crystalline planar‐s (RD2); (3) medium crystalline planar‐e (RD3); (4) coarse crystalline planar‐s (RD4); and (5) coarse crystalline planar‐e (RD5). Coarse crystalline planar dolomite cements (DC) were observed in low abundance and filling dissolution voids and fractures. Variation in dolomite types is mainly related to early to late diagenetic processes leading to changes in composition of the dolomitizing fluids. Replacement dolomites are non‐stoichiometric (Ca43‐56–Mg34‐45) with Sr, Mn and Fe concentrations of 41–138 ppm, 168–919 ppm and 5000–21000 ppm, respectively. These dolomites are characterized by δ18O values ranging from 0.0 to –11.8 ‰ VPDB and δ13C values of +1.1 to +3.2 ‰ VPDB. These values are depleted in δ18O relative to the postulated values for late Jurassic dolomites precipitated in equilibrium with seawater, while δ13C values are within the range of Jurassic seawater dolomite values. Fluid inclusion data of RD4, RD5 and DC yield homogenization temperatures of 72 to 118 °C. Based on petrographic, fluid inclusion microthermometric data and geochemical results, the replacement dolomites in the Qal'eh Dokhtar Formation are interpreted to have formed in the subsurface at shallow to intermediate burial depths. These dolomites were then recrystallized at increased burial depths and temperatures. Seawater was the major source of Mg2+ for early diagenetic dolomite (DR1), while Mg2+ for late diagenetic dolomites was provided from diagenesis of clay minerals in shales and mechanical compaction of mudstone in the Qal'eh Dokhtar Formation. The dolomite cement is postdated by coarsely crystalline mosaic calcite cement indicating that diagenetic fluids eventually became undersaturated with respect to dolomite and oversaturated with respect to calcite. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Clay mineral diagenesis in Cretaceous clastic reservoirs from West African passive margins (the South Gabon Basin) and its impact on regional geology and basin evolution history

The Gabon coastal region located along the western African margin hosts several sedimentary basins that developed prior, during and after the opening of the South Atlantic Ocean. A range of diagenetic processes controlled the distribution of clay paragenesis shown to be of high importance for the understanding of the basin's burial history and geotectonic development. Materials acquired for this study stem from the siliciclastic fluvio-lacustrine-deltaic, petroleum bearing, Early Cretaceous Dentale and Gamba formations cored by two respective wells at depths of ~1500m. Sampled materials were analysed by X-ray diffraction, automated electron microscopy, and inductively coupled plasma mass spectrometry in order to reconstruct eogenetic and mesogenetic variations directing the formation of clay assemblages in the basin. The clay contents in both cores consist of authigenic mixed-layer minerals like illite-smectite, chlorite-smectite and berthierine-chlorite, and some minor detrital illite/mica and chlorite. I-Sm and C-Sm phase chemistry implied that the original dioctahedral (montmorillonite to beidellite) and trioctahedral (saponite) smectite precursors formed out of acid volcanic feedstock during eogenesis. Different magmatic fractionation degrees, from rhyodacite to trachyandesite, reflected in the uniform REE curves of volcanic glass conform to an active geotectonic development of the Cretaceous margins of Africa. Mesogenesis led to the decrease of smectite and formation of mixed-layered phases; I-Sm composition showed maximal burial depths of sediments to be ~1000m and ~500m deeper than today for Gamba and Dentale sediments, respectively. Besides, temperature, a major role in the formation of mixed-layer minerals had the porosity of sediments and geochemistry of smectitic precursors. Thus, Fe-Mg smectite showed higher thermal stability and lower rates of transformations in non-expanding clays during burial compared to Al-rich smectite. Different burial histories of sediments from the two cores can be attributed to rift-related normal faulting and subsequent differential denudation.

Diagenetic and Paleoenvironmental Controls on Late Cretaceous Clay Minerals in the Songliao Basin, Northeast China

AbstractSedimentary and diagenetic processes control the distribution of clay minerals in sedimentary basins, although these processes have seldom been studied continuously in continental sedimentary basins. The Songliao Basin, northeast China, is a large continental, petroleum-bearing basin, and provides a unique study site to understand the sedimentary and diagenetic processes that influence clay assemblages. In this paper, the clay mineralogy of a 2500 m-thick Late Cretaceous (late Turonian to Maastrichtian) terrestrial sedimentary succession (SK-1s and SK-1n cores), retrieved by the International Continental Scientific Drilling Program in the Songliao Basin, was examined. The objective was to determine the diagenetic and paleoenvironmental variations that controlled the formation of clay mineral assemblages, and to determine the thermal and paleoenvironmental evolution of the basin. The results from both cores show that illite is ubiquitous through the succession, smectite is frequently encountered in the upper strata, and ordered mixed-layer illite-smectite (I-S), chlorite, and kaolinite are abundant in the lower strata. Burial diagenesis is the primary control on the observed decrease of smectite and increasing illite, I-S, and chlorite with depth. Observations of clay-mineral diagenesis are used to reconstruct the paleotemperatures and maximum burial depths to which the sediments were subjected. The lowermost sediments could have reached a maximum burial of ~1000 m deeper than today and temperatures ~50°C higher than today in the latest Cretaceous. The transition of smectite to I-S in the SK-1 cores and the inferred paleotemperatures provide new constraints for basin modeling of oil maturation at elevated temperatures in the Songliao Basin. Authigenic kaolinite and smectite are enriched in sandstones with respect to the coeval mudstones from the SK-1n core, as a result of early diagenesis with the participation of primary aluminosilicates and pore fluids. In the upper part of both SK-1 cores, variations in smectite and illite were controlled primarily by paleoenvironmental changes. Increases in smectite and decreases in illite from the late Campanian to Maastrichtian are interpreted as resulting from increasing humidity, a conclusion consistent with previous paleoenvironmental interpretations.

Comparing clay mineral diagenesis in interbedded sandstones and mudstones, Vienna Basin, Austria

Abstract There is no consensus about the rate and style of clay mineral diagenesis in progressively buried sandstones v. interbedded mudstones. The diagenetic evolution of interbedded Miocene sandstones and mudstones from the Vienna Basin (Austria) has therefore been compared using core-based studies, petrography, X-ray diffraction and X-ray fluorescence. There was a common provenance for the coarse- and fine-grained sediments, and the primary depositional environment of the host sediment had no direct effect on illitization. The sandstones are mostly lithic arkoses dominated by framework grains of quartz, altered feldspars and carbonate rock fragments. Sandstone porosity has been reduced by quartz overgrowths and calcite cement; their pore-filling authigenic clay minerals consist of mixed-layer illite–smectite, illite, kaolinite and chlorite. In sandstones, smectite illitization progresses with depth; at 2150 m there is a transition from randomly interstratified to regular interstratified illite–smectite. The overall mineralogy of mudstones is surprisingly similar to the sandstones. However, for a given depth, feldspars are more altered to kaolinite, and smectite illitization is more advanced in sandstones than in mudstones. The higher permeability of sandstones allowed faster movement of material and pore fluid necessary for illitization and feldspar alteration than in mudstones. The significance of this work is that it has shown that open-system diagenesis is important for some clay mineral diagenetic reactions in sandstones, while closed-system diagenesis seems to operate for clay mineral diagenesis in mudstones.