The interlayer oxidation zone-type Mengqiguer uranium deposit in the southern Yili Basin is a typical sandstone-hosted uranium deposit in northwest China, and the lower member of the Jurassic Xishanyao Formation is its main ore-hosting stratum. However, mineralogical and geochemical responses to redox evolution in the deposit have not been systematically constrained. In this study, we carried out detailed petrographic observation, X-ray diffraction analysis, electron probe microanalysis, and whole-rock geochemical analyses on samples from the interlayer oxidation zone in the lower member of the Xishanyao Formation. Kaolinite and illite are the dominant clay minerals in the deposit, with higher contents in oxidation zones than in transition and unaltered zones, while the illite–smectite mixed-layer content shows the opposite trend. The main uranium minerals are uranium oxides and coffinite. U, S and organic carbon are enriched in the transition zone, while the Fe3+/Fe2+ ratio increases with the oxidation degree. Comprehensive analysis on clay minerals shows that the ore-forming fluids evolved from acidic oxidized meteoric fluids to weakly alkaline reduced fluids; the uranium was mainly derived from the leaching of uraniferous sandstone. The formation of the deposit is controlled by sedimentary facies, tectonic uplift, organic–inorganic fluid interaction and redox reaction. This study provides detailed mineralogical and geochemical evidence for the metallogenic mechanism of interlayer oxidation zone-type uranium deposits, and has important guiding significance for uranium prospecting in the Yili Basin.

The bulk mercury concentrations were determined in samples of fresh mud volcanic clays from 31 mud volcanoes of the Kerch‒Taman mud volcanic province. Its concentration ranged from 20 to 640 μg/kg. It is shown that the mercury concentrations do not depend on the size of the mud volcanic edifice or its activity. At the same time, it was noted that mud volcanic clays with higher density are characterized by higher Hg concentrations, which may reflect an increase in the concentration of mercury-bearing sulfides. Correlations were established between mercury concentrations in the clays and estimated formation temperatures obtained using hydrochemical geothermometers, Tmax values for clay kerogen, concentrations of HCO3, Li, B, Si in water, as well as δ18O(H2O) and δ13C(CH4). These relationships show that the observed [Hg] variations in the clays can be explained by different depths of the volcanic “roots.”. A hypothesis is put forward that the observed variability in mercury concentrations may not only reflect differences in its concentrations in the vertical section of the Maikop deposits, as previously thought, but also be a consequence of the fluid enrichment with this element as a result of the lithogenesis of clay rocks. The latter can be provided by smectite illitization, which is activated at temperatures >80–100°C and, due to desorption from the clay material, enriches mud volcanic fluids with a complex of chemical elements, including mercury.

The formation of oil-bearing pores in tight tuff has attracted considerable attention from petroleum geologists since the discovery of industrial oil.Devitrification may be an important cause for the formation of these pores; however, the relevant geological circumstance for devitrification still remains unclear. This study tries to decipher the formation of devitrification pores in the tight tuff in the Tiaohu Formation,the Santanghu Basin, Xinjiang, NW China. The result shows that the oil-bearing pore size in tuff is mainly in the range of micrometers to a few nanometers, and the porosity is mainly distributed between 0.10% and 26.71%; the permeability is mainly distributed between 0.17 and 1.20 mD. After high-temperature soaking, the oil-bearing tight tuff illustrated devitrification under both acidic and alkaline circumstances, with glassy tuff showing the greatest variation in porosity, followed by crystal pyroclast glassy tuff, while the mudstone tuff and silicified tuff show relatively small variations in porosity. The 140 °C threshold marks the optimal thermal window for devitrification-driven porosity: it coincides with the smectite–illite transition and the main hydrocarbon-generation stage in the Santanghu Basin. Porosity in all tuff varieties peaks at this temperature, recording a net gain of up to 16.31%; above 140 °C, porosity declines progressively. Devitrification proceeds in three successive stages: (1) neo-mineral nucleation, (2) metasomatic replacement, and (3) dissolution.

The South Sakhalin mud volcano (Sakhalin Island, Russia) emits HCO3-Cl/Na-Mg water, emanates CO2 prevailing over CH4 in the gas phase, and extrudes mud bearing five carbonate mineral species. The study focuses on the distribution, diversity, and origin of the carbonate minerals from the mud volcano (MV) ejecta, in terms of carbon cycle processes. The data presented include a synthesis of field observations, compositions of MV gases and waters, chemistry of carbonate minerals, as well as stable isotope geochemistry of MV waters (δ13С, δD, and δ18O) and carbonates (δ13С and δ18O). The sampled MV waters are isotopically heavy, with δ18O = +5.7 to +7.5 ‰ VSMOW, δD = −18.0 to −11.0 ‰ VSMOW, and 13С (δ13СDIC = +6.9 to +8.1 ‰ VPDB). This composition may be due to the dilution of basinal water with dehydration water released during the diagenetic illitization of smectite. Carbonates in the sampled mud masses belong to three genetically different groups. Mg-rich siderite, (Fe0.54–0.81Mg0.04–0.30Ca0.05–0.23Mn0.00–0.08)CO3, disseminated in abundance throughout the mud masses, coexists with common calcite and sporadic ankerite. The trace-element chemistry of Mg-siderite, as well as the oxygen (δ18O = +34.4 to +36.8 ‰ VSMOW) and carbon (δ13C = −1.3 to +0.6 ‰ VPDB) isotopic signatures, confirms its authigenic origin. Siderite formed during early diagenesis of the Upper Cretaceous sandy and clayey marine sediments mobilized by mud volcanism in the area. Another assemblage, composed of dawsonite, siderite, and vein calcite (±kaolinite), represents altered arkose sandstones found as few fragments in the mud. This assemblage may be a marker of later CO2 flooding into the sandstone aquifer in the geological past. The trace-element chemistry, particular morphology, and heavy C (δ13С = +5.5 to +7.0 ‰ VPDB) and O (δ18О = +39.1 to +39.5 ‰ VSMOW) isotope compositions indicate that aragonite is the only carbonate species that is related to the current MV activity. It crystallized in a shallow reservoir and was maintained by СО2 released from rapidly ascending liquefied mud and HCO3-Cl/Na-Mg-type of MV waters.

This study provides high-resolution pore-fluid profiles of surficial sediments (down to 40 cm) from four submarine mud volcanoes (MVs) of the Olimpi Mud Volcano Field (OMVF) in the Eastern Mediterranean, including the Gelendzhik, Heraklion, Moscow and Milano MVs. Here, we present major ions (Na + , K + , Mg² + , SO 4 ² - , Cl - ), sulfide, methane, dissolved inorganic carbon (DIC), δ¹³C DIC , ammonium, phosphate and silicate concentrations. These results were evaluated in relation to both early and deep diagenetic processes shaping pore-fluid chemistry. The four MVs can be classified into two geochemical groups: Gelendzhik and Heraklion, dominated by deep-sourced hypersaline fluids from Messinian salt dissolution and Moscow and Milano MVs, characterized by pore fluids largely reflecting seawater-derived compositions. In the hypersaline group, signatures of deep processes persist such as smectite–illite conversion at Heraklion and ammonium and methane upward migration, demonstrating that near-surface pore fluids retain the imprint of deep diagenesis. Organic matter oxidation via sulfate reduction (OSR) and anaerobic oxidation of methane coupled to sulfate reduction (AOM-SR) were also active, even within the hypersaline environments of the Gelendzhik and Heraklion MVs, as evidenced from stoichiometric ratios of ΔDIC and ΔSO 4 2- and δ 13 C DIC isotopic data. In the hypersaline Gelendzhik MV, the diagenetically added DIC, representing the isotopic signature of mineralized organic matter, is estimated at −53.1‰, further indicating active AOM-SR under extreme salinity. Overall, our findings demonstrate that deep-sourced fluids shape near-surface pore-fluid chemistry, generating pronounced heterogeneity among MVs and provide rare geochemical evidence of microbial resilience in hypersaline submarine environments.

The Sahara and Arabian deserts are traditionally thought to be the source of settled dust in the Middle East. Although there have been suggestions of evidence for contributions from local sources, isotopic and clay mineral data have not yet confirmed this. The current study adds the neodymium and strontium isotopic signals as well as the mineralogical composition of the clay fraction to earlier investigations of trapped Holocene eolian sediments in archeological structures carried out in the northern Negev, Israel, and the Petra region, Jordan. Identifying the Holocene and present-day dust source areas and weighing the relative importance of local and remote resources are the objectives of the current study. Illite-smectite (IS) predominates the clay fraction in the Negev, and kaolinite (Ka) in Petra. Clay minerals of both regions form a continuous line of two populations between dust end members of about 75% IS in the Negev and about 75% Ka in Petra. Petra and the Negev have εNd values of −8.8 to −14.1 and −5.1 to −8.6 and 87 Sr/ 86 Sr ratios between 0.7082 and 0.7119 and 0.7085 and 0.7134, respectively. The 87 Sr/ 86 Sr ranges in the Petra and Negev regions are rather similar, indicating that carbonate rocks from both local and remote sources have had a significant role. It may be inferred that the distinctive εNd values of the Petra eolian sediments must be generated from the local Paleozoic sandstone, even in the absence of information on the isotopic composition of any of the local rocks. Dust from the semi-distant Petra region, the remote Mesopotamia source area, and local sources all have an impact on Negev isotopic signals, but not the Sahara. This is because the significant contribution of coarse-grain local materials masks fine-grain materials from distant sources.

Effects of smectite-illitization and hydrocarbon generation on the pore structure: a case study from the continental shales in China

The genetic mechanism of low‐resistivity oil reservoirs in the Guantao Formation of the Bohai Sea is complex, and understanding the role of clay minerals and pore structures in forming low resistivity remains unclear. This study employed multiscale digital core technology to integrate multisource digital core data. Combined with flow characteristic‐based upscaling technology, microscale and nanoscale digital cores were reconstructed to calculate parameters such as porosity, permeability, and formation factor. At the same time, mercury intrusion and seepage experiments were simulated. The research reveals two key mechanisms underlying the influence of clay minerals and pore structures on low‐resistivity oil reservoirs: first, the additional conductive effect of clay. Among clay minerals, illite–smectite mixed‐layer minerals exhibit the strongest conductivity, which is the key factor contributing to reduced resistivity, followed by illite, kaolinite, and chlorite in decreasing order of additional conductive capacity. The second is the regulatory role of pore structures. Kaolinite fills intergranular pores to form a complex micropore system, resulting in high irreducible water saturation in the reservoir, which is the primary cause of low‐resistivity oil reservoirs. This study clarifies the influence mechanisms of clay minerals and pore structures on resistivity, provides key technical support for fine geological modeling and optimization of oil and gas reservoir development schemes, and holds significant practical value for reducing exploration risks and improving oil recovery efficiency.

ABSTRACT Organic maturation and clay mineral‐based estimates of the maximum paleotemperatures experienced by the Precambrian/Cambrian sediments on the East European Craton (EEC) are in disagreement. To resolve this conflict and reconstruct thermal histories of these sedimentary rocks we used three low‐temperature thermochronometers: zircon and apatite (U–Th)/He (ZHe, AHe) and apatite fission track (AFT) methods, complemented by AFT modelling of thermal histories. At the cratonic edge (Podillya), the ZHe ages are partially reset, implying that the AFT ages are fully reset, and that the maximum paleotemperatures were in the range of 160°C–200°C, similar to earlier estimates based on the degree of illitization of smectite. Comparison with analogous data for the overlying Silurian bentonites identifies the edge zone of the former Devonian/Carboniferous Variscan foreland basin. In the Podillya region the modelling of thermal history of the Cambrian/Ediacaran strata indicated exhumation in Carboniferous time. In the cratonic interior (Volyn, Belarus, and Lithuania), the AFT ages are totally or partially reset, implying the maximum paleotemperatures close to 120°C, which confirm earlier estimates based on the degree of illitization of smectite, but are higher than the estimates based on the organic geochemistry. The relatively short mean confined tracks lengths (13.6 ± 0.4 to 10.8 ± 0.4 μm), and their varied standard deviation values (2.1–0.9) indicate slow exhumation within the partial annealing zone.

Carefully designed oedometer and triaxial tests were carried out on reconstituted illite–smectite rich Lucera clay to reach specific states of the critical state soil mechanics (CSSM) framework, which were then investigated at the microscale using mercury intrusion porosimetry and scanning electron microscopy. The combination of mechanical and microscopy tests allowed further insight into the relationship between porosity and CSSM. The pore size distributions at the points selected are monomodal, and unique for a given state, independent of the stress path history. Lines representing the loci of normally consolidated states and critical states were drawn in terms of mean pore size against mean effective stress, but, unlike in CSSM, for the stress range investigated, the lines seem not to be parallel to each other. Examination of pore orientation using a fabric index shows that for this clay, very high isotropic stresses need to be applied to attain an isotropic fabric, while a constant fabric index seems to define the critical state line. Overall, states on the K0 normal compression line and critical state line correspond to unique particle size distributions and fabrics that are independent of the paths taken to reach them and solely dependent on the stress levels, a framework that is consistent with CSSM.

Analytical solutions for thermal-hydraulic-mechanical-chemical modeling of smectite illitization in bentonite buffers for nuclear waste disposal

The transformation of smectite to illite documents multi-scale water–rock–hydrocarbon interaction dynamics. Current studies predominantly emphasize the influence of inorganic systems on this process, while overlooking the dynamic regulation by organic matter and the synergistic effects of multiple controlling factors under actual geological conditions. In this study, we conducted integrated semi-open pyrolysis experiments on natural samples from the Chang-7 Member and hydrothermal experiments using synthetic analogs. The illitization process of smectite was characterized through XRD analysis and SEM observations, while organic geochemical testing was employed to track the corresponding thermal evolution of organic matter. The semi-open pyrolysis results reveal that significant changes in illite–smectite (I/S) mixed layer minerals and illite content/morphology occur above 320 °C, which coincides with the critical threshold for extensive organic matter evolution. Thermal degradation of organic matter generates pore space, thereby enhancing water–rock interactions involving clay minerals. This demonstrates the co-evolution of organic matter and smectite, and indicates that temperature indirectly influences illitization by regulating organic matter thermal evolution. The hydrothermal simulation experiments demonstrate the early-stage characteristics of illitization. Unlike long-term geological evolution, K+ under experimental conditions primarily originates from the aqueous medium due to kinetic constraints on feldspar dissolution. Notably, organic matter regulates K+ partitioning dynamics—increased organic matter content hinders K+ incorporation into smectite interlayers, thereby suppressing the illitization process. Cross-system experimental analysis reveals that organic matter exhibits temporally dependent dual functionality, serving both mediating and modulating roles within inorganic diagenetic systems. This study delineates diagnostic-stage-dependent mechanisms governing smectite illitization through multifactorial synergistic interplay, establishing a predictive framework applicable to organic-rich systems exemplified by the Chang-7 Shale.

This study aimed to investigate vertical variations in dissolved organic matter (DOM) properties, humus (HS) composition, humic acid (HA) characteristics, and clay mineral dynamics, with a particular focus on the vertical distribution of HS components and mineral composition across Dark-brown, Meadow, and Paddy soil profiles. Results indicated that: (1) DOM in all three soil types was predominantly endogenous, primarily derived from microbial metabolism with minimal contributions from plant residues. (2) Vertical trends in DOM carbon content (CDOM) were specific to soil type: in Dark-brown soil, CDOM slightly increased from the Ap to Bt layer, followed by a sharp increase in the C layer; Meadow soil exhibited a significant decrease in CDOM in the AB layer but remained relatively stable in other layers; Paddy soil showed a consistent decline in CDOM with increasing depth. (3) HS and its fractions exhibited vertical variability: Paddy soil showed higher HS content in surface layers; carbon contents of water-soluble substances, HA, and humic-extracted acid (CWSS, CHA, and CHE) decreased with depth in Dark-brown and Paddy soils, whereas they remained relatively stable in deeper layers of Meadow soil. (4) HA characteristics, including C/N ratio, functional groups, and aromaticity, were influenced by both depth and soil type: the Ap2 layer of Paddy soil effectively restricted the downward movement of organic matter; Fe3+ complexation played a key role in HA stabilization in Dark-brown soil; Meadow soil exhibited transitional HS properties. (5) Clay mineral assemblages were dominated by 2:1 type minerals (illite, smectite, illite–smectite interstratifications), showing distinct vertical weathering patterns: illite content decreased with depth due to hydrolysis, while proton-driven dissolution promoted kaolinite formation in surface layers, particularly in Dark-brown soil 2:1 minerals enhancing organic–mineral complexation in Meadow soil. The findings of this study provided a scientific basis for optimizing soil carbon pool management and offer insights into organic–mineral interactions that can enhance organic matter sequestration in agricultural soils.

Definitive evidence for the microbially induced smectite-to-illite (S-I) reaction has previously been shown using culture experiments with pure clay minerals, whereas recognition in nature remains difficult. Here, we investigated the microbially induced S-I reaction in natural sediments during laboratory compression and applied new and previously used techniques that can positively identify the products of this reaction. We performed resedimentation experiments without (control experiment) and with the Fe reducing bacteria Shewanella oneidensis MR-1 (microbially amended experiment) added to natural sediments collected from the Ursa and Brazos-Trinity regions in the Gulf of Mexico during Integrated Ocean Drilling Program Expedition 308. Following these experiments, subsamples were collected and analyzed with bulk and clay fraction X-ray diffraction, micro X-ray fluorescence, electron microprobe elemental mapping, and energy dispersive spectroscopy (EDS) spot analyses at the µm-scale. We found in the microbially amended experiments of both sediment samples that (1) clay fraction XRD scans and quantitative analyses revealed layer collapse, permanent K fixation, and decreased expandability in mixed layered illite-smectite indicating progressive illite formation, (2) electron microprobe mapping and EDS spot analyses both showed a decrease in Si, increases in Al/Si and K, and no change in Al, and (3) layer charge calculated using EDS spot data increased relative to the control experiments. The control and amended experiments of both sediments displayed little to no change in bulk elemental compositions. Our results indicate that the microbially induced S-I reaction occurred to a relatively small, yet systematic, degree in the amended experiments of both sediment samples and that electron microprobe elemental mapping, EDS spot analyses at the µm-scale, and clay fraction mineralogy can be used to infer this reaction in natural sediments, whereas bulk elemental compositions may not. This research will help provide a path forward in recognizing the microbially induced S-I reaction in natural settings and assist in understanding elemental cycling during early diagenesis, sediment pore fluid overpressures, and fault zone behavior.

In the exploration of unconventional petroleum resources in the Nanpu Sag of China, several tight oil sandstone reservoirs have been identified; however, their physical properties display pronounced heterogeneity. Using methods such as scanning electron microscopy (SEM), thin-section petrography, X-ray Diffraction (XRD), and high-pressure mercury intrusion, this study analyzed the mineralogical, petrological, and reservoir characteristics of the tight oil sandstone reservoirs in the second member of the Dongying Formation in the Nanpu Sag. This study also examined the relationship between the heterogeneity of the pore networks in the tight oil sandstone reservoirs and their fractal dimensions. The results indicate that as the fractal dimension (Df) of the tight oil sandstone reservoirs increases, their permeability decreases exponentially. The Df is strongly linked to pore morphology: larger Df values correspond to smaller pore sizes, more complex pore shapes, and greater pore heterogeneity. Additionally, variations in Df are closely linked to mineralogy: lower quartz content and higher clay content, particularly abundant illite–smectite mixed layers and illite along with reduced kaolinite, are associated with higher Df values. These findings highlight the complex, irregular nature of pore structures in tight sandstones and demonstrate that integrating high-pressure mercury intrusion analysis with fractal theory provides an effective approach for quantitatively characterizing their heterogeneity.

ABSTRACT Provenance studies are widely used in foreland basin settings to evaluate temporal changes in sediment source areas, pinpoint the composition of outcropping units during deposition, and shed light on the interactions between source-area composition, depositional environments, tectonics, and paleoclimate. We present a case example from the Miocene foreland basin of the Austral–Magallanes in southern Patagonia, Argentina (46.5–47.9° south latitude), providing provenance studies of conglomerates, sandstones, and mudstones, combined with previously published detrital-zircon U–Pb geochronology and sedimentological data. Based on this information, we document a change in volcaniclastic source areas consistent with an unroofing sequence during the main phase of Andean tectonic uplift. The studied Miocene beds are up to 1000 m thick and represent a synorogenic environmental transition from shallow marine (El Chacay Formation, ∼ 20–18 Ma) to fluvial–alluvial (Río Zeballos Group, ∼ 18–12 Ma), and ultimately to fluvial (Santa Cruz Formation, ∼ 18–14 Ma) paleoenvironments. Litharenite and feldspathic litharenite thin-section samples (n = 51) plot within the magmatic-arc and recycled-orogen tectonic fields. Six conglomerate samples indicate a similar setting. Furthermore, X-ray diffraction analysis of 36 mudstone samples identifies a well-crystallized smectite clay mineral, commonly associated with illite, mixed-layer illite–smectite, chlorite, minor calcite, and zeolites, suggesting the alteration of labile volcanic material from orogenic sources. Our compositional evidence, together with previous detrital U–Pb zircon data, characterizes the studied unroofing sequence exposing Mesozoic to Paleozoic rocks, and serves as a case study for other orogenic basins, where the progressive tectonic exhumation of source areas plays a dominant role.

ABSTRACT Smectite is an important component of siliciclastic reservoirs in many petroliferous basins around the world and has numerous industrial and environmental applications. Beyond its ability to preserve or degrade porosity and permeability in sandstones, smectite also acts as a precursor for other diagenetic cements, such as illite and zeolites. While numerous studies have explored the role of smectite as a precursor for the formation of illite and chlorite, the controls and mechanisms governing the formation of authigenic smectite coatings from feldspar precursors are rarely investigated. In this study, the authigenic formation of smectite coats and their subsequent illitization were studied using a series of hydrothermal experiments. The starting material was modern quartzo-feldspathic sediments from the sandy intertidal flat of the Anllóns estuary, NW Spain. The experimental solutions were 0.1 M NaCl, KCl, CaCl2·2H2O, MgCl2·6H2O (SF1, pH 5.69), natural estuarine water (EW, pH 7.85), and 0.1 M Na2CO3 (SF2, pH 11.20). The experiments were conducted at temperatures ranging from 50°C to 250°C, over the durations ranging from 168 to 336 hours, with a fixed fluid/sediment ratio of 10:1. The results revealed distinct variations in the synthesized mineral phases, depending on the experimental fluid composition. Detrital K-feldspar dissolution commenced at lower temperatures (< 100°C), and the subsequent formation of authigenic smectite coats occurred between 150°C and 250°C in both SF1 and EW. Geochemical evaluation revealed that SF1 produced more Mg-rich smectite with intermediate compositions, while EW produced mostly Al-rich smectite due to the relatively lower Mg content in the estuarine water. Smectite formation occurred through dissolution–crystallization and was subsequently illitized via a mixed-layer illite–smectite intermediate phase with increasing temperatures. Illitization began at 200°C, with required K+ supplied by the dissolution of K-feldspar. The processes of smectite formation and its illitization released abundant silica, which precipitated as quartz overgrowths. The resulting authigenic grain coatings were chemically and morphologically similar to those found in natural sandstone reservoirs. As temperature increased (from 150 to 250°C), the thickness of these coatings grew from 1.9 µm to 49.2 µm, and surface coverage expanded from 24.3% to 96.2%. In contrast to SF1 and EW, SF2 yielded entirely different mineral phases of halite and chabazite (zeolite), attributed to the high alkalinity and Na content of the solution. These findings highlight the potential of hydrothermal experiments to simulate burial diagenesis in marginal marine settings, which can aid in reservoir quality prediction, critical for energy exploration and transition.

ABSTRACTThe presence of clay grain coats retards syntaxial quartz overgrowths, leading to well‐preserved porosity in deeply buried sandstones. However, the genetic link between clay coat formation and epitaxial quartz outgrowths remains poorly evaluated. This study investigates the formation and reaction pathways of clay grain coats in Permian tight sandstones from the northern Ordos Basin, China, and evaluate their influence on epitaxial quartz cementation and reservoir quality. Petrographic and mineralogical analyses reveal that the clay coats are dominated by illite or mixed‐layer illite/smectite clay minerals, consisting of inner smectitic cutans and outer illitic rims. Smectitic cutans, enriched in Fe, Ti and K, form tangentially to detrital grains through mechanical clay infiltration and syn‐depositional volcanic matrix input. Illitic rims, higher in K and depleted in Fe concentrations, result from a two‐stage illitization process: (1) initial and progressive smectite illitization at elevated temperatures, releasing silica that promotes quartz precipitation and (2) advanced smectite illitization induced by K‐feldspar dissolution during organic acids influx, enhancing illitic coats growth and quartz cementation. The identification of clay rim‐associated and serrated authigenic quartz supports the close genetic link between illitic grain coats formation and epitaxial quartz cementations. Fluid inclusion microthermometry reveals two generations of quartz cements at 60 to 100°C and 100 to 140°C corresponding to the two‐stage illitization process. Lower δ13CPDB values (−16.05 to −9.35‰) of calcite cements suggest carbon sourcing from organic acids' thermal decarboxylation. Geochemical modelling and mass balance calculations demonstrate that the silica released during illitization is largely re‐precipitated as localized quartz cements in a closed system. These findings reveal the dual role of illitic grain coats on reservoir quality evolution. While they limit porosity modification by inhibiting syntaxial quartz overgrowths, their growth decreases permeability by occluding pore throats. This study enhances the understanding of clay coat dynamics and their implications for reservoir quality in tight sandstone.

Continental lacustrine shales, which are distinct from marine shales in reservoir architecture, pose challenges for resource evaluation due to their complex multi-scale controls. The Middle Jurassic Qianfoya Formation in the Langzhong–Yuanba (LZ-YB) area of the northeastern Sichuan Basin represents a critical continental shale gas target, yet systematic studies of its reservoir quality drivers remain limited. Through integrated sedimentological, geochemical, and petrophysical analyses of core samples, three dominant sedimentary facies are identified: blocky, banded, and laminated, reflecting depositional energy variations. The formation shows favorable hydrocarbon potential with an average total organic carbon (TOC) content of 1.85% and is mineralogically dominated by clay minerals (illite and illite–smectite) and felsic components. Semi-deep lacustrine facies, especially clay-rich lithofacies, demonstrate superior reservoir quality due to higher total organic carbon content and pore networks dominated by silty intergranular pores, interlayer pores in clay minerals, and intragranular pores in pyrite and microfractures, contrasting with marine shales where organic matter-hosted pores prevail. Key controlling factors include organic matter-clay mineral synergy, depositional environment (anoxic conditions, freshwater influx, and terrigenous input), and diagenetic processes such as clay transformation and recompaction. Laminated facies exhibit optimal reservoir quality compared to blocky or banded types, with positive correlations between clay-organic content and storage capacity. These findings highlight the coupled depositional–diagenetic controls on continental shale reservoirs, providing critical insights for global exploration of analogous lacustrine shale systems.

Authigenic minerals in shale are products of the co-evolution of organic and inorganic components, affecting the heterogeneity of shale reservoirs. However, due to their fine granularity and complex rock composition, studies on these minerals in shale are still insufficient. This research focuses on the lacustrine shales from the upper sub-member of the fourth member in the Eocene Shahejie Formation, Dongying Sag, East China. Utilizing core samples, thin sections, scanning electron microscope, X-ray diffraction, elemental geochemistry, and organic geochemistry, we systematically characterized the features and origins of authigenic minerals. The results identified several typical authigenic minerals, including authigenic quartz, framboidal and euhedral pyrite, ferroan dolomite, kaolinite, chlorite, and albite. Authigenic quartz is predominantly diagenetic silica formed through smectite illitization, acidic dissolution of K-feldspar, and alkaline dissolution of detrital quartz. Pyrite is a product of microbial sulfate reduction, with framboidal pyrite forming during an early diagenetic stage under conditions with sufficient solute supply and euhedral pyrite forming during a later stage under conditions with insufficient solute supply. Ferroan dolomite originates from the precipitation of Fe and Mg during smectite illitization, with slight contributions from the acidic dissolution of chlorite and calcite. Kaolinite stems from the acidic dissolution of K-feldspar, while chlorite results from the transformation of kaolinite. Albite primarily arises from the alkaline alteration of anorthite and K-feldspar. Most non-clay authigenic minerals likely enhance reservoir quality by slightly reducing the effects of compaction, whereas authigenic clay minerals typically exert detrimental effects on reservoir properties. This study constrains the genesis of authigenic minerals to assess their influence on reservoir quality in lacustrine shale.