Clay Mineral Transformation Research Articles

Evolution of mechanical properties of organic-rich shale during thermal maturation

Accurate assessment of the mechanical properties of organic matter, clay matrix, and bulk shale during maturation remains a challenge. Here, we aim to assess the mechanical properties of organic-rich shale during maturation using a combination of nanoindentation methods and various geochemical analyses, i.e., mineral composition, mass loss rate, chemical structure of organic matter, and Rock-Eval analyses. Results show that the evolution of mechanical properties of organic matter in shale during maturation can be divided into: the main oil-generation stage, and the condensate oil and gas generation stage. The stiffening of organic matter in the shale is mainly due to increased aromaticity and condensation of aromatic groups. The clay matrix experiences a slight decrease in hardness and Young’s modulus at low maturity levels due to the generation of liquid hydrocarbons. However, overall, the clay matrix becomes stiffer as the shale matures due to shale dehydration, expulsion or cracking of liquid hydrocarbons, transformation of clay minerals, and hardening of organic matter. The Young’s modulus and hardness of bulk shale generally increase with increasing maturity. This is closely related to the hardening of organic matter and clay matrix, as well as the development of the more compact and dense microstructure in the shale.

Diagenesis of marine-continental transitional shale from the Upper Permian Longtan Formation in southern Sichuan Basin, China

Abstract Upper Permian Longtan Formation transitional shale has become an important exploration layer, but the Longtan Formation shale (LFS) has a complex mineralogical composition, which affects the subsequent diagenesis and diagenetic evolution, and restricts the subsequent geologic exploration of shale gas. In this article, the observation of drilling cores, argon ion polishing-scanning electron microscope, Vitrinite reflectance, and X-ray diffractometer were used to analyze the type and characteristics of diagenesis of the LFS and clarify the stage of diagenetic evolution. The results show that the main diagenesis in the LFS is compaction, cementation, thermal maturation of organic matter (OM), dissolution, and transform of clay minerals. Among them, OM hydrocarbon generation, clay mineral transformation, and dissolution are pore-enhancing diagenetic events. Compaction and cementation are pore-reducing diagenetic events. The transitional and marine shales have similar characteristics of diagenesis, but there are big variations in the diagenesis of OM hydrocarbon, authigenic quartz, and siderite. The complex depositional environments of the marine-continental transition environment have resulted in a variety of rock types, which in turn influenced the diagenesis types and diagenetic evolution process. Compared with the transitional shales of the Shanxi and Taiyuan formations in the South China North Basin, the LFS are characterized by high clay content, low quartz content, complex mineral compositions, and a higher degree of thermal evolution.

Exploring microwave activation as a novel method for activating coal gangue: Focus on microwave activation mechanisms and hydration characteristics of cementitious supplementary materials

To support the decarbonization of the cement industry, coal gangue (CG) has potential as a supplementary cementitious material. The calcination process significantly impacts the phase transformation of clay minerals and the pozzolanic activity of CG. This study involved calcination of CG at 600 ℃ and 800 ℃ using both microwave and thermal activation methods. Employ XRD, XPS, FTIR, TG, and SEM to examine the impact of calcination mechanism on the crystal structure, element binding energy, chemical bonds, microstructure, and mineral composition of CG. The results indicate that the transformation of kaolinite in CG to metakaolin is completed around 600 °C. Microwave activation increased the reactive aluminate content in CG, enhancing pozzolanic activity and promoting secondary hydration with cement. This process generated additional C-(A)-S-H gels and AFm phases, which filled matrix pores, increased system density, and improved mechanical properties. The optimal activation method for CG was identified as microwave activation at 600 °C.

Characteristics of middle-deep reservoirs and the genesis of pore development in the southern slope of the Nanpu sag in the Bohai Bay Basin, China

To clarify the distribution of favorable reservoirs in the middle and deep layers of the southern slope of the Nanpu sag and identify relatively high-quality reservoir sections, the characteristics of the reservoirs in the middle and deep layers of the southern slope of the Nanpu sag were studied by means of core analysis, thin section casting, scanning electron microscopy, and analytical tests. Research shows that the middle and deep reservoirs on the southern slope of the Nanpu sag are mainly lithic feldspathic sandstone and that the debris is mainly magmatic rock. The diagenesis processes are mainly compaction, cementation, and dissolution. The deformation of plastic particles, brittle microcracks on the surface of rigid clastic particles, calcareous cementation, quartz overgrowth, and feldspar dissolution are common. The pores are co-developed by primary pores and secondary pores, in which the primary pores are more common in the third member of the Dongying Formation and the first member of the Shahejie Formation, whereas the secondary pores are more common in the first member of the Shahejie Formation. The genesis of pore development is discussed. It is considered that the anticompaction components, chlorite film, and hydrocarbon filling play major roles in the formation of primary pores, whereas atmospheric water leaching, diagenetic evolution, thermal evolution of organic matter, and clay mineral transformation play major roles in the formation of secondary pores.

Using iron speciation to track hydrocarbon migration in bleached rocks and its implications for reservoir reconstruction

Bleaching of red rock beds by hydrocarbons can provide valuable insights for hydrocarbon migration and reservoir transformation. This study investigated the bleaching effects of hydrocarbons on various rock types in the Dushanzi mud volcanic area. Bedrock and bleached samples of different lithologies, including mudstone, sandy mudstone and sandstone, were systematically collected and analyzed for their iron species, mineralogical and chemical compositions using Mössbauer spectroscopy, X-ray diffraction and X-ray fluorescence, respectively. The results indicate that the iron species of the studied samples were dominated by para-Fe3+ (from 44% to 76%), with variable contents of mag-Fe3+, para-Fe2+ and pyr-Fe2+. When compared to the related unbleached rocks, the bleached rocks exhibited a higher proportion of reduced Fe (para-Fe2+ and pyr-Fe2+). The pyr-Fe2+ in the sandstones and sandy mudstones was significantly higher than that of the mudstone interlayers. The observed variations in iron species were consistent with changes in mineralogical and chemical compositions, which can be correlated with the petrological properties of rocks. The increase of quartz and decrease of feldspar and carbonates in the bleached rocks could be attributed to the dissolution of feldspar and the subsequent secondary enlargement of quartz. Geofluids containing crude oils and natural gases preferentially dissolved K-feldspar over albite. The bleached rocks had a significant decrease in the proportion of kaolinite and a slight decrease in the proportion of chlorite, while the proportions of smectite and illite were increased. The dissolution of K-feldspar could provide sufficient K+ for the formation of illite, and the appearance of smectite in the bleached samples might be related to the dissolution of albite in an alkaline environment. The dissolution of carbonates and feldspar, along with the transformation of clay minerals, had a notable positive effect on the formation of secondary porosity, facilitating hydrocarbon migration. These findings provide valuable insights into the interactions between hydrocarbons and different lithologies, informing the impact of hydrocarbon migration on reservoir reconstruction in similar geological settings.

In-depth insights into transformation mechanism of synthetic saponite to amorphous silica under acid attack

Amorphous mesoporous materials derived from synthetic saponite (Sap) by acid treatment have been used in catalysis for several decades. Uncovering the transformation process of Sap from layered to amorphous structure under acid attack is beneficial to optimize the structure of Sap-derived mesoporous materials, yet such transformation process remains unclear. Herein, a magnesium-Sap was synthesized by hydrothermal method and then treated with sulfuric acid. By studying the changes in phase composition, morphology, chemical state of elements and surface area of the synthetic Sap after acid attack, a specified transformation mechanism of synthetic Sap to amorphous silica is clarified. The experimental results indicate that after leaching of Mg and Al cations from the Mg/AlO6 octahedral sheets of Sap during acid attack, the retained SiO4 tetrahedral sheets of Sap were transformed into amorphous short-range ordered silica phases and nanoscrolls with pseudohexagonal side pores. The transformation process of synthetic Sap under acid attack can be divided into leaching of metal ions from octahedral sheets of Sap to form layered silica particles, splitting and crimping of surface nano-silica layers from the formed silica particles, and completely amorphous transformation. The distortion of SiO bonds in SiO4 tetrahedral sheets during acid attack might be the driving force for such transformation. This work provides specific transformation process of synthetic Sap under acid attack which is conductive to the controllable preparation of acid-treated Sap derivates, and has certain significance for understanding of structural transformation of clay minerals with MgO6 octahedral structure.

Evolution of pore structure and diagenetic stages of late Permian shales in the Lower Yangtze Region, South China

This study examines the evolution characteristics of shale pores in the Late Permian Longtan Formation located in the Lower Yangtze region of South China. The complete process of thermal evolution of the samples was achieved through thermal simulation experiments while identifying qualitatively the pore types and their development characteristics using field emission-scanning electron microscope (FE-SEM). Quantitative analysis of pore size distribution was conducted through mercury intrusion capillary pressure (MICP), nitrogen (N2) and carbon dioxide (CO2) adsorption experiments. The paper comprehensively analyzed the pore evolution characteristics and controlling factors of shale in the study area, with an analysis of diagenetic differences. Findings reveal that during the pore evolution process, both morphology and pore size are influenced by thermal maturity. The pore volume is dominated by mesopores and macropores, while the specific surface area is mainly dominated by mesopores and micropores. Thermal evolution promotes the formation of micropores and mesopores but hinders the development of macropores. Moreover, clay minerals transformation and mineral dissolution make certain contributions to the development of micropores. The diagenesis in the study area is controlled primarily by the pyrolysis of organic matter. Pyrite and clay minerals are the first to dissolve, followed by calcite and quartz. Five stages of evolution characterization have been identified from low-mature stage (Ro = 0.88 %) to overmature stage (Ro = 3.35 %) in combination with diagenesis. The development of pore structure and its influencing factors vary across different stages. The influencing factors mainly include hydrocarbon generation from organic matter, compaction, mineral transformation, and dissolution. The process of hydrocarbon generation in organic matter occurs throughout the entire pore evolution process, resulting in the development of numerous micropores and mesopores. Compaction primarily impacts pore development during the early diagenetic stage, causing a substantial transition of primary mineral pores from macropores to mesopores. Mineral transformation and dissolution take place during and after the middle diagenetic stage. The former governs the development of mesopore-sized clay interlayer pores, while the latter primarily generates micropores.

Utica/Point Pleasant brine isotopic compositions (δ7Li, δ11B, δ138Ba) elucidate mechanisms of lithium enrichment in the Appalachian Basin

Global Li production will require a ∼500 % increase to meet 2050 projected energy storage demands. One potential source is oil and gas wastewater (i.e., produced water or brine), which naturally has high total dissolved solids (TDS) concentrations, that can also be enriched in Li (>100 mg/L). Understanding the sources and mechanisms responsible for high naturally-occurring Li concentrations can aid in efficient targeting of these brines. The isotopic composition (δ7Li, δ11B, δ138Ba) of produced water and core samples from the Utica Shale and Point Pleasant Formation (UPP) in the Appalachian Basin, USA indicates that depth-dependent thermal maturity and water-rock interaction, including diagenetic clay mineral transformations, likely control Li concentrations. A survey of Li content in produced waters throughout the USA indicates that Appalachian Basin brines from the Marcellus Shale to the UPP have the potential for economic resource recovery.

Evaluation of sweet spots for a tight sandstone reservoir: A quantitative study of diagenesis in the fourth member of the Oligocene Huagang Formation, Xihu Depression, East China Sea Shelf Basin

Reservoir sweet spot evaluation is very important for tight sandstone gas exploration and development. Due to the characteristics of large burial depth, strong diagenesis heterogeneity, and prominent importance of diagenetic facies, sweet spot evaluation is the focus and difficulty of unconventional oil and gas exploration and development. By combining with core observation, conventional core analysis, clay x-ray diffraction analysis, bulk rock x-ray diffraction analysis, powder grain size analysis, thin section quantitative statistics, fluid inclusion homogenization temperature, scanning electron microscopy, scanning electron microscopy mineral quantitative evaluation, well logging data, and pre-stack seismic data, integrated simulation approach of seismic diagenetic facies and diagenetic numerical simulation was used to simulate porosity evolution and spatial distribution of tight sandstone of second submember (Smbr 42) of the fourth member (Mbr 4) of the Oligocene Huagang Formation in the Xihu Depression in the East China Sea Shelf Basin, and to evaluate reservoir sweet spot distribution. Based on mineral texture relationship and diagenetic sequence of Smbr 42 sandstone compaction, clay mineral transformation, calcite cementation, quartz cementation, and dissolution, five diagenetic facies were identified. Based on various seismic elastic parameters and core-derived diagenetic facies, the distribution of seismic diagenetic facies was interpreted by seismic diagenetic facies prediction method. Seismic diagenetic facies distribution can provide a “hard-constraint” three-dimensional diagenesis heterogeneity model for diagenetic history reconstruction, and seismic diagenetic facies distribution provides constraints on grain size and quartz grain content distribution in diagenetic numerical model. Based on the detailed consideration of input parameters of burial history, thermal history and diagenetic history, integrated diagenetic numerical simulation was used to reproduce porosity evolution and spatial distribution of Smbr 42 sandstone, and was applied to reservoir sweet spot evaluation of tight sandstone. Smbr 42-1 sublayer developed type III reservoir sweet spot and non-sweet spot. Smbr 42–2 to Smbr 42-6 sublayers mainly developed type II1, type II2, type III, and minor type I reservoir sweet spot. Simulation results were consistent with the locations of multiple sets of drilled gas layers and proven gas reservoirs.

Nitrogen isotopes of released gas from rock crushing and implications to origins of molecular nitrogen in Lower Cambrian overmature shale gas in South China

The origin and formation mechanism of N2-rich Lower Cambrian shale gas remain ambiguous, posing serious exploration risks in the Lower Cambrian shale gas in South China. In this study, high-vacuum-pulsed electromagnetic and helium-filled ball-mill rock-crushing methods were developed to measure the released gas contents, compositions and nitrogen isotopes of N2 in shale-crushed gas from Lower Cambrian organic-rich shales in South China. Key factors in N2 enrichment in shale gas and its origins were investigated. Results showed that mineral compositions and thermal maturity affected the contents and compositions of shale-crushed gas. Carbonate cementation, and feldspar dissolution and reprecipitation, formed isolated pores filled with migrated bitumen and thus generated high gas contents with larger CH4/N2 ratios of shale-crushed gas. The thermal maturation of Lower Cambrian organic-rich shale can be further divided into the early and late stages of overmaturation with a boundary of 3.4–3.5%EqVRo. The contents and CH4/N2 ratios of shale-crushed gas continued to increase at the early stage of overmaturation, and then a rapid decrease at the late stage of overmaturation (>3.4–3.5%EqVRo), reflecting the release of large amounts of N2 by intensified aromatization of organic matter. An empirical equation of TONb = 0.0138 × TOC with R2>0.98 was used to estimate the relative contribution of inorganic nitrogen (TINb) and organic nitrogen (TONb) to the bulk of nitrogen content in the overmature Cambrian shale. The EqVRo-δ15NN2 plot allows identification of N2 derived from thermal decomposition of ammonium-bearing clay minerals or residual kerogen. The N2 generated at the early stage of overmaturation (including at wells LX03, EYY1, SNY1 and W001-4) in Lower Cambrian shale gas is characterized by δ15NN2 values ranging from −2 to +2‰, mainly reflecting thermal deammoniation and decomposition of ammonium-bearing clay minerals. In contrast, the N2 generated at the late stage of overmaturation (DQ outcrop) in Lower Cambrian shale gas is characterized by δ15NN2 values greater than +4‰, indicating thermal cracking of pyridinic nitrogen and pyrrolic nitrogen. The N2 generated from thermal transformation of ammonium-bearing clay minerals and pyridinic nitrogen and pyrrolic nitrogen is the main source of N2-rich shale gas in the Lower Cambrian shale of South China.

KAOLIN FORMATION IN YATAĞAN (MUĞLA) LIGNITE DEPOSIT

The 30-km-long and 10-km-wide, coal-bearing Yatağan Basin is located in SW Anatolia, close to the eastern coast of the Aegean Sea, Türkiye. The basement consists of Menderes Massif metamorphics in the NW part of this intermontane basin, c. 80-km2-large area (Turgut lignite deposit). The Neogene and Quaternary sedimentary filling comprises fluvioterrestrial, limnic and telmatic sediments which contain a mineable coal seam up to 15 m thick, unconformably overlain the basement. Two fresh and six weathered samples were picked up from the Menderes Massif gneiss outcrops of the catchment area. Eight sedimentary rocks, inorganics over and underlying the coal seam, were obtained from four borehole cores. All samples were examined under the optical microscope; XRD, SEM-EDX and ICP-MS analyses were later performed. This study aims to examine the mineralogical content and geochemical processes of the sedimentary rocks from different formations, namely Sekköy and Turgut Formations, which are over- and underlying the lignite horizon, to assess the clay mineral formations and transformations. Gneiss samples contain mainly quartz, plagioclase, K-feldspar, muscovite and biotite with a lesser amount of tourmaline, chlorite, garnet, apatite, zoisite, zircon and Fe-oxides. The -2 µm fraction consists mainly of illite, with fewer smectite and kaolinite. Sericitizitation of feldspar is the main product in gneisses. Coal over-and underlying sedimentary rock samples contain mainly quartz, plagioclase, K-feldspar, muscovite, and biotite with less pyrite and iron oxides. The -2 µm fraction of the sedimentary rock samples consists of variable clay mineral contents. Kaolin content is higher in acidic conditions with an opposite correlation to smectite occurrences. The geochemistry of Menderes Massif gneisses shows that it gave felsic material to the catchment area, whereas the geochemistry of sedimentary rocks is not coherent in all cases. Geochemical parameters like CIA and PIA proved that weathering processes of the source materials prevailed during the generation of these sedimentary units.

Diagenetic Evolution Sequence and Pore Evolution Characteristics: Study on Marine-Continental Transitional Facies Shale in Southeastern Sichuan Basin

Diagenesis and pore structure are essential factors for reservoir evaluation. marine-continental transitional facies shale is a new shale gas reservoir of concern in the Sichuan Basin. The research on its diagenesis pore evolution model has important guiding significance in its later exploration and development. However, the current research on pore structure changes, diagenesis, and the evolution of marine-continental transitional facies shale is not sufficient and systematic. In order to reveal the internal relationship between pore structure changes and diagenesis, the evolution of marine-continental transitional facies shale was tested by X-ray diffraction, field emission scanning electron microscopy, low-pressure gas adsorption, nuclear magnetic resonance, and the diagenetic evolution sequence and nanopore system evolution of Longtan Formation shale was systematically studied. The results show that the Longtan Formation shale underwent short-term shallow after sedimentation, followed by long-term deep burial. The main diagenetic mechanisms of the Longtan Formation shale include compaction, dissolution, cementation, thermal maturation of organic matter, and transformation of clay minerals, which are generally in the middle-late diagenetic stage. The pore structure undergoes significant changes with increasing maturity, with the pore volumes of both micropores and mesopores reaching their minimum values at Ro = 1.43% and subsequently increasing. The change process of a specific surface area is similar to that of pore volumes. Finally, the diagenetic pore evolution model of Longtan Formation MCFS in Southeastern Sichuan was established.

Widespread development of bedding-parallel calcite veins in medium–high maturity organic-rich lacustrine shales (Upper Cretaceous Qingshankou Formation, Northern Songliao Basin, NE China): Implications for hydrocarbon generation and horizontal compression

Bedding-parallel calcite veins (BPCVs) occur widely in Upper Cretaceous lacustrine shales of the Qingshankou Formation in the Northern Songliao Basin, China. Research on the formation processes and geological significance of BPCVs has focused on the sources of fluid, timing of formation, initiation, dilation and growth mechanisms, and on relationships with hydrocarbon generation. In this study, we addressed these issues through a comprehensive study of drill cores containing BPCVs using petrographic observations, isotope geochemistry, rare earth element (REE) analysis, fluid inclusion analysis, in situ U–Pb dating, and basin modeling. BPCVs were observed in medium–high maturity shales with high organic matter contents with heterogeneous compositions. Similarities in δ13C, δ18O, 87Sr/86Sr, and REEs contents between BPCVs and the host shale indicate that locally derived formation fluids driven by chemical gradients were involved in BPCV formation. The in situ U–Pb age dating (70 ± 8.9 Ma and 70.6 ± 9.7 Ma) constrained BPCVs formation to the late Cretaceous, corresponding to a burial depth of ∼1600 m, indicating that the BPCVs formed in the oil window. The latter is also supported by fluid inclusions-derived temperatures (∼76–109 °C). BPCV formation included vein initiation and dilation, and the microstructures, e.g., domal and pincer structures, sigmoidal calcite veins, conjugate en-échelon calcite vein arrays, as well as mechanical crystal twins of the BPCVs, all indicate that tectonic compression influenced fracture development as also indicated in other basins where beef occur. Fluid overpressure caused by hydrocarbon generation, tectonic compression, and clay mineral transformation (mainly smectite to illite conversion) was responsible for vein dilatation. Antitaxial fibrous crystals with smooth crystal boundaries and high aspect ratios, containing sinusoidal solid inclusions as well as median planes, indicate that crystallization force promoted vein growth. Thus, BPCVs can be used as a good indicator of hydrocarbon generation, and horizontal tectonic compression.

THE ROLE OF INTERACTIONS BETWEEN BACTERIA AND CLAY MINERALS IN PEDOCHEMICAL PROCESSES

The review considers bilateral interactions between bacteria and clay minerals and their influence on the chemical composition and chemical processes in soils. The participation of bacteria in the weathering, transformation and synthesis of new clay minerals is shown. These interactions contribute to changing the water-retaining properties of soils, ensuring their fertility, and contributes to the cycles of nutrients, inorganic and organic pollutants. Various processes of direct and indirect influence of phyllosilicates on microbiological activity and microbial diversity in soils and the possible biogeochemical role of these processes are described. Obviously, the prevalence of interaction types between bacteria and clay minerals is determined by the First Biogeochemical Principle formulated by V.I. Vernadsky

Microstructural observations of clay-hosted pores in black shales: implications for porosity preservation and petrophysical variability

AbstractTypical unconventional gas/liquid plays of China were studied using field-emission/focused ion beam scanning electron microscopy (SEM) for clay mineralogy and microstructural development. The SEM microstructural investigations of clay-rich shale and mudstone reservoirs provided significant information about clay mineral type, size, distribution and aggregates, which allows for interpretations regarding porosity preservation and petrophysical variability. The major clay-hosted porosity types are interparticle clay pores, intraparticle clay pores and aggregate pores. Interparticle clay pores occur in open spaces of the clay aggregates and include four subtypes: (1) elongated pores, (2) packed pores, (3) jagged pores and (4) card-house pores. Intraparticle clay pores are present within clay particles and have mostly secondary origin. These pores are diagenesis dependent and are restricted to secondary illite particles during the transformation of clay minerals from smectite to illite. Intraparticle clay pores constitute only isolated porosity and could not contribute to hydrocarbon molecule storage and migration. Aggregate pores were predominantly encountered in association with organic–clay and pyrite–clay aggregates. In places, organic–pyrite–clay aggregates can also display polymerization, but they do not contribute significantly to overall porosity and permeability. Combining SEM morphological analyses with the software ImageJ is critical in clay microstructure and porosity analyses via semi-quantitative characterization of the 3D pore surface, 2D pore profile, pore quantity, pore size, areal porosity, etc. These visual and semi-quantitative results highlight the significance of jagged pores and pyrite–clay aggregate pores in shale gas/liquid reservoirs because they may be important facilitators of gas storage and transmission.

Transformation of clay minerals in technogenic processes associated with radioactive waste disposal

Transformation of clay minerals in technogenic processes associated with radioactive waste disposal

Clay mineral transformation mechanism modelling of shale reservoir in Da’anzhai Member, Sichuan Basin, Southern China

Shale reservoirs often undergo intense clay mineral transformation, which plays a crucial role in the formation and evolution of pores. The reservoir lithofacies types of Da’anzhai Member in the Sichuan Basin are complex, the heterogeneity is strong, and the transformation mechanism of clay minerals is unclear, limiting the understanding of reservoir diagenesis and reservoir formation mechanism. In this study, we selected the typical shale reservoir in the Da’anzhai Member of the eastern Sichuan Basin and innovatively introduced the multiphase fluid-chemical-thermal multi-field coupled numerical simulation technique to focus on the dissolution, precipitation and transformation laws of diagenetic minerals in the shale reservoir. We calculated the transformation of diagenetic minerals and their physical response under different temperatures, pressure and fluid conditions and identified the main controlling factors of mineral transformation in shale reservoirs in the study area. The results show that the transformation of smectite to illite in the Da’anzhai Member is a complex physicochemical process influenced by various factors such as temperature, pressure, fluid, and lithology. The increase in temperature can promote illitization until the critical temperature of 110°C–115°C, below which the conversion rate of smectite to illite increases as the temperature increases. However, when it is higher than the critical temperature, the degree of illitization decreases. In specific K-rich fluids, organic acids significantly affect the conversion of clay minerals in the Da’anzhai Member of the formation. The acidic fluid promotes the dissolution of minerals such as K-feldspar and releases K+, thus provides the material basis for illitization. The research results provide theoretical support for the diagenetic and formation mechanism of the shale reservoir in the Da’anzhai Member of the Sichuan Basin and even for the efficient exploration and development of shale gas.

Lacustrine Shale Diagenesis—A Case Study of the Second Member of the Funing Formation in the Subei Basin

Shale diagenesis differs from that of sandstone and carbonate rocks with regard to the type, evolution stage, and evolution mode. The quality of shale reservoirs is closely linked to the extent of diagenetic evolution. This study identifies the types and characteristics of shale diagenesis using thin sections and scanning electron microscopy (SEM) observations. The stages of shale diagenesis are determined by analyzing organic matter evolution and clay mineral transformation and establishing a diagenetic evolution sequence. This paper describes the comprehensive diagenetic evolution of organic matter, clay minerals, clastic particles, and carbonate minerals to determine the diagenesis types, diagenetic sequences, and pore evolution occurring during diagenetic evolution. The results show that the diagenesis types of shale in the second member of the Funing Formation include compaction, dissolution, cementation, metasomatism, dolomitization, syneresis, and transformation of clay minerals, as well as thermal evolution of organic matter. The middle diagenetic A stage is prevalent, with some areas in the early and middle diagenetic B stages. The shale underwent a diagenetic evolution sequence, including the collapse and shrinkage of montmorillonite interlayers in the early stage; the rapid formation and transformation of illite and smectite mixed layers, massive hydrocarbon generation of organic matters, and dissolution of unstable components in the middle stage; and the occurrence of fractures filled with gypsum, quartz, ferrocalcite, or other authigenic minerals in the later stage. Dissolution pores and fractures are the dominant shale reservoirs of the second member of the Funing Formation in the Subei Basin. The results provide new insights into understanding the formation and evolution of reservoir spaces during shale diagenesis and information for the exploration and development of lacustrine shale oil and gas.

Heat-Induced Pore Structure Evolution in the Triassic Chang 7 Shale, Ordos Basin, China: Experimental Simulation of In Situ Conversion Process

The reservoir properties of low–medium-maturity shale undergo complex changes during the in situ conversion process (ICP). The experiments were performed at high temperature (up to 450 °C), high pressure (30 MPa), and a low heating rate (0.4 °C/h) on low–medium-maturity shale samples of the Chang 7 Member shale in the southern Ordos Basin. The changes in the shale composition, pore structure, and reservoir properties during the ICP were quantitatively characterized by X-ray diffraction (XRD), microscopic observation, vitrinite reflectance (Ro), scanning electron microscopy (SEM), and reservoir physical property measurements. The results showed that a sharp change occurred in mineral and maceral composition, pore structure, porosity, and permeability at a temperature threshold of 350 °C. In the case of a temperature > 350 °C, pyrite, K-feldspar, ankerite, and siderite were almost completely decomposed, and organic matter (OM) was cracked into large quantities of oil and gas. Furthermore, a three-scale millimeter–micrometer–nanometer pore–fracture network was formed along the shale bedding, between OM and mineral particles and within OM, respectively. During the ICP, porosity and permeability showed a substantial improvement, with porosity increasing by approximately 10-times and permeability by 2- to 4-orders of magnitude. Kerogen pyrolysis, clay–mineral transformation, unstable mineral dissolution, and thermal stress were the main mechanisms for the substantial improvement in the reservoir’s physical properties. This study is expected to provide a basis for formulating a heating procedure and constructing a numerical model of reservoir properties for the ICP field pilot in the Chang 7 shale of the Ordos Basin.

Quantifying the Microdistribution of Different Fluid Types in the Multiscale Pore Throat Structure of Low-Permeability Conglomerates

The volumes of different fluid types and the pore throat structure in low-permeability reservoirs are crucial parameters for understanding flow mechanisms and predicting production performance. The previously applied methods for distinguishing different fluid types are mainly based on T2 spectra under saturated, centrifugal, and heat-treated conditions. However, for rocks with different mineral compositions and pore structures, the critical temperature between capillary-bound fluid (CAF) and clay-bound fluid (CLF) differs from 60 to 100 °C. Moreover, the alteration of temperature can cause the transformation of clay minerals and a change in a nanopore structure. Herein, an improved method that integrates nuclear magnetic resonance (NMR) tests (n-dodecane-saturated, centrifugal, and spontaneous imbibition treatment conditions) and multicycle mercury intrusion porosimetry, was proposed for quantitatively evaluating the microdistributions of different fluid types, classifying the pore throat system, and analyzing the relationship between pore throat structure and different fluid types in low-permeability conglomerates. Based on the fractal characteristics and mercury intrusion/extrusion characteristics of pore throats at different scales, the pore throat system in these conglomerates was divided into macropore throats (>1 μm), mesopore throats (0.2–1 μm), and micropore throats (<0.2 μm). Macropore throats are connected to intergranular pores (>100 ms); mesopore throats are mostly connected to intergranular pores and intragranular dissolved pores (>10 ms); and micropore throats are connected to intragranular dissolved pores (<10 ms) and clay-related pores (<3 ms). The proportions of movable fluid, CAF, and CLF in the low-permeability conglomerates are 20.4–57.4%, 6.67–32.2%, and 36.0–47.4%, respectively. The movable fluids are mainly distributed in macropore throats and mesopore throats. CAF mainly exists in the micropore throats (pore throat radius <0.2 μm and pore size <10 ms). CLF is mainly distributed in pores with T2 < 3 ms, that is clay-related pores. The improved method of fluid typing is suitable for low-permeability or tight clastic reservoirs (excluding shale).