Mixed-layer Illite Smectite Research Articles

Global cooling controls Eocene environmental change in the Lunpola Basin, central Tibetan Plateau: Evidence from salinity and weathering reconstructions

Detailed investigations of sedimentary rocks from the Tibetan Plateau interior, which has been shaped by plateau development and climate change, will provide new insights into its uplift history. However, few continuous environmental records with clear climatic significance and precise age controls from the Tibetan Plateau hinterland exist, particularly in the early Cenozoic. This shortage hinders a full understanding of the impact of uplift on regional climate change. In this study, we present detailed mineralogical, micromorphological and geochemical investigations of the middle–upper Eocene (~39.6–35.3 Ma) lacustrine sediments from the well-dated Dayu section in the Lunpola Basin, central Tibetan Plateau, to explore the paleoenvironmental evolutionary history of the middle–late Eocene from paleoweathering and paleohydrological perspectives. The results reveal that the regional weathering process continuously weakened, as indicated by a long-term decrease in the (smectite + mixed-layer illite–smectite)/(illite + chlorite) ratio. The quantitative reconstruction of salinity variations via clay boron-based paleosalimeters reveals long-term salinization of paleolakes from freshwater–mesosaline conditions (averaging 9.0 psu) at ~39.6–37.8 Ma to freshwater–polysaline conditions (averaging 13.4 psu) at ~37.8–35.3 Ma. The long-term continuous weakening of the weathering intensity and lake salinization collectively suggests a secular drying trend in the central Tibetan Plateau. This drying was consistent with the climatic evolution of the southeastern and northern Tibetan Plateau, which is attributed primarily to global cooling, with secondary contributions from the increasing distance from moisture sources in the Paratethys and regional tectonics.

Advanced smectite alteration and the role of accessory reactants at 180 °C: New experimental constraints on the stability of bentonite

Bentonite clay is commonly accepted as an appropriate material for backfilling the space between the radioactive canisters and the host rock of planned underground repository sites for waste disposal. Despite its favourable properties as a hydrodynamic seal, its long-term stability remains a concern. This experimental study of a Bavarian bentonite investigated advanced montmorillonite (smectite) illitisation at 180 °C in the presence of K-oxalate and/or the accessory minerals of pyrite and calcite. The formation of mixed-layered illite-smectite was quantified by X-ray diffraction Rietveld refinement and the crystal chemistry was determined by transmission electron microscopy. The results showed neocrystallisation of a celadonitic illite-smectite (up to 63% illite-layers) after treatment with KCl or K-oxalate with increased Al3+ in the tetrahedral sheets, reduced amounts in the octahedral locations and increased fixation of non-exchangeable K+ ions in response to the increased layer charges. K-oxalate complexation of Al3+ and enhanced dissolution of the smectite resulted in the formation of a possible intermediate amorphous phase and illite-smectite crystallites with additional vacant sites explaining the lower number of octahedral ions (<2 per formula unit). Adding 10% pyrite, 10% calcite, or 5% of each, did influence the degree of alteration to a recognisable degree in the presence of KCl, but the effects on the rate of illitisation were minimal when reacted with K-oxalate, which further increased illitisation by up to 6.4 times. Although batch reactor experiments do have limitations compared to complex repository conditions, our results do indicate that claystone host rocks low in organic matter should be favoured over organic-rich lithologies that are likely to contain oxalate ligands or similar catalysing compounds.

Illitization of montmorillonite in ammonium solutions under hydrothermal conditions

In diagenetic solutions, ammonium may be incorporated into smectites as an exchangeable cation and become fixed within the interlayer space of illites and other white micas. To study the potential impact of NH4+ on the smectite-to-illite transformation reaction, a series of hydrothermal experiments were carried out at 100, 150 and 200 °C, spanning reaction duration from 15 to 90 days, and two NH4+ concentrations (0.1 and 0.2 M). The solids resulting from these alteration experiments were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and transmission and analytical electron microscopy (TEM and AEM). The XRD analysis revealed that, under the specified experimental conditions, smectite incorporates NH4+ in the structure, leading to the formation of non-swelling layers, resulting in partial transformation to illite layers and producing packets of disordered illite/smectite (I/S). In addition, a minor XRD peak at ∼10 Å suggests the formation of discrete illite crystals. The FTIR spectra demonstrated the uptake of NH4+, with deformation bands observed at 1400 and 1430 cm−1, corresponding to exchangeable NH4+ in smectite and fixed NH4+ in high-charge layers, respectively. TEM analysis revealed that smectite particles exhibited wavy stacks comprising a few layers with abundant defects and lateral discontinuities. The interlayer spacing in these particles ranged from 12 to 15 Å and became thinner and more plate-like with increasing temperature and time. Moreover, they contained inclusions of 10–10.3 Å layers, either as discrete layers or in packets of several layers, indicating the formation of disordered mixed-layer illite-smectite. Lateral transitions from 12 to 15 Å to 10 Å layers were frequently observed and interpreted as reaction fronts due to local rearrangement. At 150 and 200 °C, isolated packets of 10 Å layers were identified as discrete illite crystals that precipitated directly from solution. Analysis of the chemical composition of individual particles revealed an increase in octahedral charge (MgVI for AlVI substitution) in smectite particles, followed by increase in tetrahedral charge in I/S particles. Interlayer NH4+ played a stabilizing role in the high-charge layers and favored the smectite-to-illite conversion process.

Petrological and mineralogical characteristics of “red material layer” in Patuha geothermal field, Bandung, West Java

Abstract Patuha Geothermal Field is located in Ciwidey, Bandung, West Java Province, with a total resource potential of approximately 400 Mwe (Megawatt electric). With such potential, seven power plants have been developed in Unit-1, capable of producing 60 MWe of electricity, and are currently being developed in stages for Unit-2. In the development of the Unit-2 power plant, there was a problem where collapse occurred around the drill hole area, especially since the color of the material that came out during drilling was red. We call this material the red material layer (a layer composed of red material). This problem also occurs in Unit-1. This research aims to characterize the type of red material layer to see why this problem occurs in this layer. We studied the petrological and mineralogical characteristics of the red material layer by cutting rock samples from six drill holes in Unit-1. The analytical methods used were megascopic observation, petrography, and XRD (X-ray diffraction) to determine the mineral content texture and identify the material’s origin, including clay minerals, alteration, and oxidation mineral aggregates. The petrological and mineralogical analysis found that the red-colored material is dominated by hematite, with some samples containing magnetite. We found four characteristic types of red layer material and four types of textures that appeared in iron oxide minerals: rim, vein, matrix replacements, and fragment replacements. We conclude from the visible vein texture that oxidation occurs due to oxygen penetration through gaps of a fault/fracture/joint. This oxidizes the area around the vein and then undergoes hydrothermal change. Meanwhile, those that do not have a vein texture are considered material resulting from an oxidation process on the surface, which is then deposited and undergoes hydrothermal change to contain oxide minerals such as hematite and magnetite. Apart from that, XRD observations also found high amounts of clay minerals such as chlorite, smectite, kaolinite, illite-smectite mixed layers, and chlorite-smectite mixed layers. The presence of these clay minerals is thought to cause a collapse in the area around the drill hole because its swelling and exfoliation properties are reactive to fluids, especially drilling fluids.

NH3-probe X-ray photoelectron spectroscopy method to characterize solid acidity of clay minerals

Accurate determination of solid acidity is essential for evaluating the catalytic activity of clay minerals. However, the conventional Fourier-transform infrared (FTIR) method using NH3 as the probe encounters challenges in measuring the solid acidity owing to the presence of water on the surface or within interlayer spaces of clay minerals as well as the influence of environmental water during the detection process. To mitigate water interference, a NH3-probe X-ray photoelectron spectroscopy (XPS) approach was adopted as an alternative to the FTIR method for the identification and semi-quantification of solid acidity of clay minerals. Following NH3 adsorption, the types of solid acid sites in clay minerals were identified through the chemical shift in N1s XPS spectra. The ratio of Brønsted and Lewis acid sites in a given sample and the relative concentration of acid sites across different samples were determined by examining the specific N1s peak area ratio. According to the XPS data, the ratios of Brønsted and Lewis acid sites in Ca-montmorillonite (SAz-2) and Na-montmorillonite (SWy-2) were nearly 1:1, whereas that for the illite–smectite mixed-layer mineral (ISCz-1) was 4. The acid site contents in SAz-2 were more than twice those of SWy-2. This work highlights the promising application prospects of the NH3-probe XPS method in solid acidity investigations of clay minerals.

Evolution of weathering intensity in the Qaidam Basin, northeastern Tibetan Plateau, since the middle Miocene: Insights from clay mineral records

Terrestrial silicate weathering records from the tectonically active Tibetan Plateau are crucial to explore the interactions among climate change, tectonic uplift and chemical weathering during the Cenozoic. However, the complex sedimentary facies changes and frequent occurrence of coarse lithologies related to intense tectonic uplift of the Tibetan Plateau have largely limited the reliability of bulk chemical weathering indexes to reveal the silicate weathering history. Here, we present detailed records of clay minerals and geochemical compositions from clay-sized (< 2 μm) sediments collected from the well-dated Huaitoutala section (ca. 15.3–1.8 Ma) in the NE Qaidam Basin to reconstruct the chemical weathering history of the NE Tibetan Plateau during the late Cenozoic. The relatively high (illite–smectite mixed layers + smectite)/(chlorite + illite) ratio indicates a strong silicate weathering intensity from 15.3 to 12.6. Ma, while a continuous decrease in this ratio since 12.6 Ma reflects a long-term decreasing intensity of silicate weathering. By comparing the clay mineral records with the climatic reconstruction and the tectonic events in the NE Tibetan Plateau, the results reveal that middle Miocene global cooling-related drying exerted a significant influence on the decreasing weathering intensity since ca. 12.6 Ma. Furthermore, the late Cenozoic episodic and persistent uplift of the NE Tibetan Plateau may have exerted an additional influence on the long-term decrease in weathering intensity. Specifically, the dramatic fluctuation in the silicate weathering intensity during ∼11–9.6 Ma is mainly due to uplift-induced provenance changes. Moreover, the fluctuating increase in the silicate weathering intensity during ∼8–5 Ma may have been caused by intensified East Asian summer monsoon precipitation. Our study suggests that both climate change and tectonic deformation controlled the chemical weathering intensity in the NE Tibetan Plateau during the late Cenozoic.

The dominant mineralogical triggers hindering the efficient development of the world's largest conglomerate oilfield

In the complex fluid environment of subterranean rocks, some mineral components are easily hydrated, which hinders the accumulation and utilization of hydrocarbon. However, the nature and distribution of these key substances remain unclear. This study aims to provide a fresh perspective on the substances that impede the storage and extraction of hydrocarbon of the world's largest conglomerate oilfield. Using a variety of qualitative, semi-quantitative, and quantitative methods, including X-ray diffraction, secondary electron imaging, elemental spectroscopy, and Tescan-Integration-Mineral-Analyzer (TIMA), our experimental results show: (1) For the first time, water-sensitive clay minerals on the surfaces of gravel particles in tight conglomerates in China have been identified. Although the matrix composition contains less than 4.2% clay minerals on average, certain gravels contain over 72.7% clay minerals. These clay minerals are primarily composed of smectite, which is highly water sensitive, followed by chlorite, which poses significant challenges to hydrocarbon extraction; (2) The distribution of clay minerals on a single gravel surface is relatively uniform, enabling their interaction with fracturing fluid and exacerbating reservoir degradation. The total content of the illite-smectite mixture in the conglomerate can reach approximately 12%, with skeleton particles (gravel) containing more than 30% of this mixture. There are significant differences in the degree of clayiness among different types of gravel, which can be classified as high clayey, medium clayey, or low clayey. The evenly distributed illite-smectite mixed layer on the gravel surface allows complete contact with the fracturing fluid, resulting in severe reservoir damage; (3) Gravel particles with a size range of 8.88 μm–10.73 μm contain the highest concentration of illite-smectite mixture, which is identified as a major factor contributing to reservoir damage.

Atomistic Computer Simulations of Uranyl Adsorption on Hydrated Illite and Smectite Surfaces

A quantitative understanding of the molecular-scale mechanisms of radionuclide sorption on different clay minerals is crucial for the development and safe implementation of geological nuclear waste disposal technologies. We apply classical molecular dynamics (MD) computer simulations to study the adsorption of uranyl on the external basal surfaces of two typical clay models. In the illite model, negative charge is primarily localized in the tetrahedral sheets, while in the lower-charge smectite model, the isomorphic substitutions are introduced in the octahedral sheet. The comparison of atomic density distributions at the clay surfaces and adsorption-free energies profiles as a function of distance from these surfaces demonstrates that overall U behavior at the basal clay surface is quite similar for illite and smectite. Uranyl is sorbed as a mixture of outer-sphere aqua complexes [UO2(H2O)5]2+ and hydrolyzed aqua complexes [UO2(H2O)4–5OH]+ on both surfaces. The structural and compositional differences between the models do not greatly affect the uranyl’s nearest coordination environment and are mainly reflected in the specific localization and orientation of the uranyl ions at both surfaces and in the magnitude of the adsorption-free energies. The observed quantitative characteristics of uranyl interactions with illite and smectite surfaces will help to better understand U behavior during the sorption process on clay minerals for the entire range of mixed-layer illite–smectite structures. A comparison of two versions of the ClayFF force field in the simulations made it possible to more accurately and quantitatively evaluate some subtle features of the uranyl–clay interactions and to obtain a more precise composition of uranyl complex with the modified ClayFF force field (ClayFF-MOH).

Mechanism of arsenic accumulation in sediments of the northern China: Insight from provenance, weathering processes and paleoenvironmental changes

Fluctuations in sedimentary As burial have been reported throughout geological timescales, but detailed weathering pathways governed by climatic evolution are poorly understood. To this end, we reconstructed various climatic stages utilizing lithologies, geochemical indicators, and dating data and illustrated the physical–chemical pathways that affect As accumulation and release. The results revealed six climate changes. The climatic evolution was a consequence of the relative intensities of the East Asian summer monsoon (EASM) and East Asian winter monsoon (EAWM). Higher sedimentary As, Fe, and Feclay concentrations are observed in the cold-arid climates and suggest As accumulation occurred due to strong physical erosion. Moreover, correlations between Feclay and clay minerals indicate favorable clay minerals during cold-arid climates (e.g., illite, chlorite, and illite–smectite mixed layer) enhanced the accumulation of Fe-rich clay minerals and then As burial. In warm-humid climates, intense biogeochemical processes prevailed and contributed to crystalline Fe enrichment, which might accelerate As release. This analysis shows climate shifts alter solid As partitioning in sedimentary basins and potentially result in significant impacts with respect to global As contamination.

Factors Controlling the Pore Development of Low-Mature Marine–Continental Transitional Shale: A Case Study of the Upper Permian Longtan Shale, Western Guizhou, South China

The Upper Permian Longtan Shale is a significant reservoir in western Guizhou. To clarify the main factors controlling the low-mature marine–continental transitional shale pore development in western Guizhou, pore types were classified with scanning electron microscopy (SEM), and the pore developmental stages and morphological structures were quantitatively characterized by nitrogen adsorption isotherm analyses. Additionally, the qualitative or semi-quantitative relationships between the pore developmental stages and the main controlling factors were established via geochemical analysis. The results showed that the Longtan Shale pores include intergranular pores, intragranular pores, organic pores, and microfractures. The intergranular pore structures were categorized into ink-bottle, slit, layered, and irregular types. The intragranular pores were found to be of the elliptical, nearly circular, ink-bottle, and irregular varieties. The organic pores were categorized into elliptical, bubble-like, and irregular polygonal variants. The microfractures were only of the elongated type. The clay-mineral-related intergranular pores were the predominant pore type. The organic pores were found to be poorly developed. The mesopores were predominant, followed by macropores. The shale pore diameters ranged between 1 nm and 100 nm, and they are characterized by multiple peaks. The specific surface area (SSA) was primarily provided by nanopores in the range of 5 nm to 10 nm, such that the smaller pores provided a greater contribution to the SSA, and they are more conducive to shale gas adsorption and accumulation. Clay mineral content was the dominant internal factor controlling pore development and the SSA, with the illite–smectite mixed layer being the most obvious controlling factor. While too low or too high clay mineral content is adverse to macropore development, brittle mineral content, carbonate mineral content, and total organic carbon (TOC) content are adverse to pore development and the SSA. Thermal maturity has no remarkable control effect on pore volume and the SSA of non-organic pores.

A relatively warm and humid Oligocene climate in the northeastern Tibetan Plateau based on a high-resolution clay mineralogical and geochemical record

The Oligocene was the earliest cool period in the Cenozoic, and thus its terrestrial climatic evolution is of significant interest but remains poorly constrained for Inner Asia. In this study, we obtained a high-resolution paleoclimate record based on clay minerals and silicate chemical weathering proxies of the clay fraction (<2 μm) in sedimentary rocks from the Xining Basin on the northeastern Tibetan Plateau. We compared these data with those for surrounding basins to reconstruct the Oligocene climatic evolution of the northeastern part of the plateau. Illite, chlorite, smectite (I/S; including irregular mixed-layer illite–smectite), and kaolinite are the four main clay minerals in the Oligocene strata. Multiple proxies indicated three characteristic climatic periods. During the late Eocene to early Oligocene, a weak silicate weathering intensity is indicative of an arid climate in the northeastern Tibetan Plateau. A stronger silicate weathering intensity and enrichment in smectite (I/S) and kaolinite indicate more warm and humid climate conditions during 32–25 Ma. During the Oligocene, before the high topography of the northern and northeastern parts of the Tibetan Plateau had developed, the relatively warm and humid climate conditions since 32 Ma were likely caused by enhanced transport of water vapor in response to relatively warm temperatures. The decrease in the degree of silicate weathering from 25 to 23 Ma was possibly caused by regional tectonism and global climatic changes. These relatively warm and humid conditions would explain the occurrence of large Oligocene mammals in the northeastern Tibetan Plateau.

Microscopic occurrence and movability mechanism of pore water in deep shale gas reservoirs: A typical case study of the Wufeng-Longmaxi Formation, Luzhou block, Sichuan Basin

Water in pore system (i.e., pore water) is of great significance for shale gas exploration and exploitation. However, the microscopic occurrence and movability mechanism of pore water is still unclear. In this study, the pore water of deep shale gas reservoirs of the Wufeng-Longmaxi Formation in the Luzhou block was quantitatively evaluated by a new theoretical approach. The microscopic distribution and movability of pore water were investigated by combining NMR technology and theoretical models, and then the influencing factors were discussed. The results show that the signal amplitude of dry shale originates from clay minerals and organic matter. For clay minerals, illite has the greatest influence on the signal amplitude, followed by illite-smectite mixed layer and chlorite. Pore water mainly exists in an adsorbed phase, with an average adsorbed ratio of 69.26%, and an average adsorption thickness (H) of 0.5392 nm. Adsorbed and free water coexist in pores of 3–100 nm, and have equivalent content and saturation in pores of 10 nm and 6–7 nm, respectively. The specific surface area and pore volume control the occurrence of adsorbed and free water respectively. With the increase in average pore size, the water-holding capacity of pore system weakens. Adsorbed water mainly occurs in pores related to organic matter and quartz, while free water tends to occur in clay minerals pores. The adsorption affinity of spherical pores is the strongest, followed by cylindrical and slit pores. The effective pore size threshold available for fluid flow in spherical, cylindrical, and slit pores is 6H, 4H, and 2H respectively. With increasing centrifugal pressure difference, the water saturation decreases, the adsorbed ratio and mobility increase. Shale with higher pore volume has stronger mobile potential and mobility. Clay minerals are essential for pore water movability, while organic matter and brittle minerals inhibit pore water movability. This study offers a novel approach for quantitatively characterize the occurrence and transport of pore water and potentially has wide application prospects.

Mixed-Layer Illite-Smectite Illitization under Supercritical CO2 Conditions

The long-term safe storage of CO2 in geological reservoirs requires the understanding of the impact of CO2 on clay-rich sealing cap rocks. The reactivity of the mixed layer of illite-smectite was investigated to determine the reaction pathways under conditions of supercritical CO2 (scCO2) conditions in the context of geological CO2 storage. A common clay (blue marl from the Guadalquivir Tertiary basin, southern Spain) was tested under brine scCO2 conditions (100 bar and 35 °C) for 120 and 240 h. The clay sample (blue marl) contains calcite, quartz, illite, smectite, and the corresponding mixed-layer and kaolinite. X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses were performed. The illitization of mixed-layer illite-smectite was observed by XRD and confirmed by a variation in the content of different elements (K, Mg, Na, Ca, and Fe) of the transformation, as well as an increase in the specific surface (SSA) of the clay (36.1 to 38.1 m2/g by N2, 14.5 to 15.4 m2/g by CO2 adsorption). Furthermore, these reactions lead to mineral dissolution and secondary mineral formation along the CO2–water–clay intercalations of the source rock were responsible for a change in porosity (7.8 to 7.0 nm pore size). The implications of illitisation, mineral destruction, and precipitation processes on CO2 storage and clay layer integrity should be explored before deciding on a geological storage location.

Clay Minerals in the Deep Reservoir of the Cerro Pabellón Geothermal System (Northern Chile)

This study focuses on hydrothermal alteration, and more specifically clays, in the geothermal reservoir of Cerro Pabellón in Andean Cordillera, Northern Chile. Although it is the first commercial geothermal power plant in South America with 48 MWe installed and an additional 33 MWe during 2022, the alteration mineralogy of the deep reservoir remains poorly investigated. Cuttings from two deep wells drilled between 500 and 3000 m depth were sampled. The fine-grained clay fraction was analyzed using X-ray diffraction to observe the mineral distribution and variations of crystal structure of the clays. Scanning electron microscopy coupled with energy dispersive spectroscopy allowed us to perform microanalysis of the clays. The results suggest a high heterogeneity of the reservoir in term of alteration: a pervasive alteration dominated by trioctahedral clays was observed in all the samples, whereas a fracture-controlled alteration was only observed locally, dominated by dioctahedral clays. Illite-smectite mixed layers with more than 90% illite typically occur in permeable fracture and fault as a typical clay signature. This study completed an integrated conceptual model of the high enthalpy reservoir (&gt;250 °C). Circulations of a neutral pH Na-K-Cl fluid occur in a wide fracture system connected to highly dipping and NW-striking faults and could extend beyond the Pabelloncito graben.

Study on characteristics of oil and gas occurrence and reservoir space of medium-high maturity continental shale—A case study of middle jurassic lianggaoshan formation in fuling block, southeast of sichuan basin, south China

Possessed of easy access to development and fair economic benefits, medium-high maturity continental shale oil and gas have become the focus of shale oil and gas study in the future. Shale oil and gas mainly occur in pores, but studies on the pore characteristics of shale oil and gas occurrence are by no means sufficient. Focused on shale from the Middle Jurassic Lianggaoshan Formation in Well TYX, Fuling block, southeast of Sichuan Basin where a breakthrough in shale oil and gas exploration was recently achieved, this study selects core samples and conducts a series of analyses, including vitrinite reflectance analysis, kerogen microscopic examination experiment, total organic carbon (TOC) content analysis, mineral composition analysis, gas content measurement, isothermal adsorption experiment, S1 content analysis, and others. The analyses are to identify the pore characteristics of the continental medium and high maturity shale oil and gas by virtue of scanning electron microscope (SEM) with Ar-ion milling and the image processing software ImageJ. The conclusions are drawn as follows: in terms of lithofacies, medium-high maturity continental shale oil and gas mainly occur in organic-rich clay shale and organic-rich mixed shale; with regard to material composition, shale oil and gas mainly occur in organic matter, illite-smectite mixed layers and illite. Shale adsorbed gas content accounts for at most 40% of the total shale gas content and shale free gas content takes up at least 60% of the total shale gas content. Pores of solid bitumen, solid bitumen-clay mineral complex mass, clay minerals, structured vitrinite, and funginite are mostly developed in shale. Among them, the first three types of pores are the main reservoir space in shale considering their large number, good roundness, medium pore diameter, fairly good roundness of pore edges, and the complex shapes which altogether contribute to the large surface porosity.

Depositional conditions of the Upper Cretaceous shales from the Anambra Basin, SE Nigeria: Constraints from mineralogy and geochemistry

Thirty-four core shale samples from the Upper Cretaceous Mamu Formation, southeastern Nigeria, were studied using XRD and geochemical analyses to constrain their provenance, tectonic setting, palaeoweathering and palaeoredox conditions during deposition. X-ray diffraction analysis revealed the presence of clay minerals (kaolinite, illite and illite-smectite mixed layer) and non-clay minerals (quartz and feldspar). The dominance of kaolinite and the high kaolinite/illite (K/I) contents indicate warm and humid climatic conditions during sediment accumulation. The total organic carbon (TOC) content of the shales ranges between 0.31 and 55.34%. The high Chemical Index of Weathering (CIW; mean∼98.8%), Chemical Index of Alteration (CIA; mean∼92%) and Plagioclase Index of Alteration (PIA; mean∼98.7%) for the shales, including a high content of kaolinite, indicate intense chemical weathering in the source area. The positive correlations between V, Cr, Ni, Co, Ga, Cd, Be, Sc, TiO2, MgO with Al2O3 and Ba with K2O suggest that these elements in the shales are mainly held in clay minerals. The elevated contents of Zr and the high Zr/Cr and Zr/Sc ratios imply that the shales were derived from predominantly felsic source rocks. However, TiO2/Zr and Y/Ni ratios suggest a derivation from an admixture of complex felsic and mafic basement rocks. The low average enrichment factor values of Mo (MoEF), V (VEF) and U (UEF), along with low concentrations of redox-sensitive trace element ratios (Mo/U, Ni/Co, V/Cr and Cu/Zn) and high TOC contents infer oxic conditions during deposition. Furthermore, Fe2O3–S-TOC relationships, along with DOPT, V/(V + Ni) and Fe2O3/Al2O3 ratios, suggest dominantly oxic to slightly dysoxic/anoxic conditions during the accumulation of the Mamu Formation shales. Binary plots of K2O/Na2O ratio versus SiO2 indicate a passive margin tectonic setting for the shales.

Akın ve Taşkonak Köyleri (Van) Çevresinde Yüzeyleyen Orta-Geç Miyosen Yaşlı Killi Kayaçların Mineralojik Özellikleri ve Paleoiklimsel İpuçları

Bu çalışmada Orta-Geç Miyosen yaşlı denizel ortamda oluşmuş olan Van Formasyonu killi kayaçlarının mineralojik özelliklerini belirlemek ve bu kayaçların oluştuğu Orta-Geç Miyosen boyunca çalışma alanındaki eski iklim koşullarını yorumlamak amaçlanmıştır. Çalışma alanı Van Gölü’nün güney batısındadır. Orta-Üst Miyosen kayaçlarından dört stratigrafi kesiti ölçülmüş ve yetmiş üç kilce zengin kayaç numunesi derlenmiştir. Numunelerin tümünün X-ışını difraksiyonu analizleri yapılmış, tüm kayaç bileşimlerinde kuvars, mika, feldispat, kalsit, amfibol, dolomit, kristobalit ve kil mineralleri; kil fraksiyonlarında ise illit, klorit, smektit, serpantin, karışık tabakalı klorit-smektit ve illit-smektit belirlenmiştir. Seçilmiş üç örnek üzerinde yapılan SEM ve EDX çalışmalarında serpantin, klorit ve smektit gözlenmiştir. İyi kristalli illit, smektit, klorit ve feldispat minerallerinin bir arada bulunmalarına dayanılarak, kırıntılı kayaçların oluştuğu süre boyunca bölgede soğuk ve kurak iklim koşullarının hakim olduğu yorumu yapılmıştır.

Palaeoclimatic and diagenetic controls based on clay mineralogy and organic matter distribution: The continental rift Cuyo Basin (Triassic), west‐central Argentina

AbstractThe Triassic Cuyo rift Basin is one of the renowned petroleum systems in South America that records distinctive alluvial, lacustrine and fluvial settings with volcanic input. The present study focuses on the clay mineral assemblages that represent the syn‐rift to post‐rift fills. Four clay mineral assemblages are defined based on X‐ray diffraction, optical and electron microscopy analyses. The illite/mica‐dominated assemblage is documented in the syn‐rift I (Cerro de las Cabras Formation). Such mineral phases are inherited and derived from the erosion of pre‐rift units, suggesting that physical weathering associated with a warm and semi‐arid climate played a leading role in their occurrence. Authigenic smectite clays are diagnostic in organic‐lean mudstones of the syn‐rift II (lower Potrerillos Formation) which point to alteration of pyroclastic material under a warm and semi‐arid to humid climate. A warm and humid climate prevailed during the deposition of lacustrine hydrocarbon source rocks (upper Potrerillos and Cacheuta formations). Organic‐rich mudstones show the illite/mica–kaolinite–smectite assemblage in conjunction with amorphous organic matter and restricted terrigenous elements. Diagenetic kaolinite booklets and bipyramidal quartz are attributed to the acidification of pore‐water related to kerogen transformation during burial diagenesis. Vitrinite reflectance values (ca 0.63%) confirm that organic‐rich sediments reached the earliest oil window; however, smectite is devoid of illite layers throughout the Triassic succession, probably related to the absence of potassium in the system and the high crystallinity of the smectite, which delayed the transformation of smectite into mixed layers illite–smectite. Red mudstones of the post‐rift II (the fluvial‐lacustrine Rio Blanco Formation) show the kaolinite‐dominated assemblage where the authigenic kaolinite is interpreted to be derived from the alteration of volcaniclastic detritus and indicative of warm climate with markedly wet periods.

Clay Mineral Assemblages in the Cretaceous Volcanogenic–Sedimentary Rocks of the North-Western Part of the Transition Zone from the Asian Continent to the Pacific Ocean

In this study, clay and non-clay minerals in the cement of Cretaceous volcanogenic–sedimentary rocks from the bottom of the marginal seas of the north-western Pacific Ocean and adjacent areas were studied. Corrensite and mixed-layer chlorite–smectite, rectorite and mixed-layer illite–smectite, chlorite, swelling chlorite (?), illite, kaolinite, smectite (?), calcite, ankerite, barite, gypsum, epsomite, zeolites (laumontite, analcime, and stilbite), cristobalite, and quartz were determined. The following are the indicative properties: (a) minerals: corrensite and rectorite; (b) associations: corrensite–chlorite, corrensite–chlorite–laumontite, corrensite–epsomite–authigenic calcite, and quartz–illite. Such minerals indicate that the thickness of the accumulated sediments in the studied basins could reach three to five kilometers and that the temperature of their formation could be higher than 150 °C. Transformations in the process of diagenesis and epigenesis occur in two directions: smectite–rectorite–mica, with an excess of potassium, and smectite–corrensite–chlorite, with an excess of magnesium. The chlorite–corrensite association may indicate conditions favorable for seawater evaporation, and the presence of laumontite in the corrensite–chlorite association suggests a periodic supply of calcium to the sedimentation basin. The illite–kaolinite association is probably associated with coal accumulation in epicontinental conditions and a warm humid climate in nearby areas. Periods of sedimentation, possibly associated with global climate events, were identified: 113–120, 110–113, 105–110, 93–95, 72–83 and 61–72 Ma. The established time intervals and mineral associations can serve as benchmarks for stratigraphic constructions in reconstructing the physicochemical, climatic parameters, and conditions of Cretaceous volcanogenic–sedimentary strata accumulation.

Eocene Sedimentary–Diagenetic Environment Analysis of the Pingtai Area of the Qaidam Basin

Based on the petrological characteristics and elemental geochemical analysis of core samples from the Pingtai area in the northern structural belt of the Qaidam Basin, this study shows that the clay mineral assemblage of Lulehe Formation sandstone is dominated by high contents of smectite, chlorite and illite, and does not contain illite–smectite mixed layers or kaolinite. The chlorite and illite in the Xiaganchaigou Formation decreased gradually and the smectite disappeared. In addition, illite–smectite mixed layers began to appear and kaolinite was not present. These results indicate that the diagenetic environment of the Pingtai area in the early Eocene was dominated by alkaline media poor in K+ and rich in Mg2+, Na+ and Ca2+. In the late Eocene, K+ content in the diagenetic medium increased significantly, and smectite began to transform into illite. From the early Eocene to the late Eocene, the overall climate and environmental characteristics showed a transition from cold and dry to a cold climate that alternated between dry and wet. The content changes of common oxides, such as CaO, MgO, K2O, Na2O, SiO2, Al2O3, Fe2O3 and TiO2, showed obvious correlation. Based on the content and ratio change tendencies of Sr, Ba, Cu, Zn, U, Th, Ce and other trace elements, combined with the variation characteristics of rare earth element contents, it is suggested that the Lulehe Formation was dominated by a dry and cold freshwater sedimentary environment, and that paleoclimatic conditions were relatively cold and arid during this sedimentary period. However, the climate in the sedimentary period of the Xiaganchaigou Formation was dominated by a cold environment alternating between dry and wet, which also reflected the finding that the global climate was mainly cold and dry in the early Eocene and gradually became warm and humid in the late Eocene.