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The successful development of shale oil in China has claimed that laminated shale is a favorable lithofacies for the effective extraction of petroleum. Clarifying the role of laminae in shale oil generation, migration, storage, and enrichment is urgent and important. Starting from the describing and classifying of the lamina, the common methods and terms used to delineate lamina types are briefly summarized. The results of different schemes are often mutually inclusive, which prompted scholars to work towards a unified division scheme. The influencing factors of oil retention in shale systems, including organic matter (OM) type, total organic carbon (TOC) content, OM maturity, mineral composition, pore structure, and preservation conditions, are systematically discussed. Subsequently, comparative work on source rock quality, reservoir properties, and hydrocarbon expulsion efficiency of shales with different laminar structures is carried out. The comparison results of shale with different rock structures reveal that the laminated shale has a high expulsion efficiency. However, the strong oil generation capacity and superior storage space of laminated shale synergistically control the considerable amount of retained oil in the shale system. Especially the oil mobility of laminated shale is also considered because of great pore size and pore connectivity. The fine evaluation of laminar structure and prediction of laminar distribution has great significance for the selection of shale oil "sweet spot area" or "sweet spot interval". [ABSTRACT FROM AUTHOR]

The laminar structure of shale system has an important influence on the evaluation of hydrocarbon source rock quality, reservoir quality, and engineering quality, and it is receiving increasing attention. A systematic study of the lamina structure is not only of great scientific significance but also of vital practical importance for shale oil production. In this paper, the identification and description classification of shale laminae are first reviewed. Multiple scales and types indicate that a combination of different probe techniques is the basis for an accurate evaluation of shale laminar characteristics. The influence of laminae on shale reservoir, oil-bearing, mobility, and fracability properties is discussed systematically. A comparative analysis shows that shale systems with well-developed lamination facilitate the development of bedding fractures, thus improving the shale storage space. The average pore size and pore connectivity are also enhanced. These factors synergistically control the superior retention and flow capacity of shale oil in laminated shales. In such conditions, the high production of shale oil wells can still be achieved even if complex networks of fracturing cracks are difficult to form in shale systems with well-developed lamination. This work is helpful to reveal the enrichment mechanism of shale oil and clarify the high-yield law of hydrocarbons, so as to guide the selection of sweet spots. [ABSTRACT FROM AUTHOR]

Pore-fracture structure of shales is important to shale gas resource potential evaluation, exploration and development. However, investigations about the pore-fracture structure of shales were mainly focused on nano-submicron pores, while the contribution of micrometer-scaled pores and micro-fractures to pore system of shale reservoirs lack sufficient attention. Here, using a combination of field-emission scanning electronic microscopy (FE-SEM), gas adsorption (N 2 and CO 2) and CT scanning (Nano-CT and Micro-CT), the Longmaxi shale cored from JYA well, in the Fuling area, was selected to comprehensively evaluate the characteristics of the pore-fracture structure at different scales. The results show that there are diverse types of pores in the organic-rich shale, mainly consisted of organic pores, inorganic pores (interP pores and intraP pores) and micro-fractures. Many pores are ink bottle-shaped characterized by narrow necks and wide bodies, while some others are slit-shaped. Pore size range from 0.305 nm to 98.5 μm. Pores with diameter of 0.305–2 nm (micro-pores), 2–50 nm (meso -pores), 50 nm-2 μm (macro-pores) and 2–98.5 μm (micro-fractures) account for 33.59%, 36.28%, 14.04% and 16.09% of the total pore volume, respectively. Moreover, shale permeability and specific surface area were calculated to be 0.005 mD and 39.44 m2/g, respectively, based on the full-scaled pore splicing. About 92% of specific surface area is contributed by pores with diameter of 0.305–5 nm. Micro-fractures connect with each other and form net-shaped structure with great connectivity from observational results. The micro-fractures with size of 20–98.5 μm provide the main permeability (about 90%). Micro-pores and small meso -pores in shale reservoirs provide large adsorption space for adsorbed gas, while micro-fractures enhance the seepage capability of shale gas, which are favorable for shale gas accumulation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The storage space of Ediacaran Doushantuo shale was dominated by both organic matter pores and matrix pores associated with dissolution. • The Doushantuo shale reservoir quality is jointly determined by carbonate minerals and organic matter content. • Three enrichment modes were summarized for typical commercial shale gas reservoirs. • The exploration feasibility and reservoir forming scale of ancient shale were clarified. The Doushantuo shale in the Yichang area of South China, a newly discovered Ediacaran gas shale, is important in evaluating the potential of Precambrian source rocks. The shales, with great thickness (50 ∼ 130.5 m), high total organic carbon (average TOC is 1.64%), and moderate thermal maturity (Ro is 1.8%∼2.5%), have a favorable hydrocarbon generation potential. Multiple techniques, e.g., gas adsorption, scanning electron microscopy (SEM), nano- and micro-computed tomography (CT), were subsequently carried out to evaluate the pore systems. Both organic and inorganic pores are considered major storage spaces. The size of organic pores is generally < 30 nm, while inorganic pores are closely related to macro-pores. The full-scale pore size distribution revealed that the average pore volume ratio of micro-, meso-, macro-pores, and micro-fractures is 20.15%, 46.83%, 21.64%, and 11.38%, respectively. Macro-pores and micro-fractures also play a decisive role in shale gas enrichment except migration. The development degree of inorganic pores was determined by TOC and carbonate contents. Three enrichment models, i.e., Type I (continuous deep burial with limited uplift amplitude), Type II (significant burial depth with substantial uplift amplitude), and Type III (moderate burial depth with moderate uplift amplitude), were proposed based on the burial history of shale strata. The Doushantuo shale falls into Type II, and its favorable combination of generation, storage, and preservation conditions suggests promising exploration potential. This study demonstrates that the Doushantuo shale is likely to become a new set of strata for the deep exploration of shale gas in southern China. [ABSTRACT FROM AUTHOR]

Carbonate-rich shale in salt lakes has become a potential target for petroleum exploration. A full understanding of storage space development characteristics is the basis for the efficient development of this kind of shale. In this study, the complex mineral composition and multiple pore structure of a typical carbonate-rich shale from the Paleogene Xin'gouzui Formation in the Jianghan Basin were systematically analyzed. The mixed shale and dolomitic shale were the main lithofacies types in the study area. Both of them were dominated by interparticle (interP) pores. However, their pore sizes are noticeably different. Pores with a size of <100 nm are the main compositions of the mixed shale, with an average volume proportion of 78.24%. Conversely, the dolomitic shale develops mainly micro-sized pores and micro-fractures, with a ratio of 70.46%. The difference in pore structure for different types of shales is closely associated with mineral composition. When particles of different sizes are mixed, most of the interP pores are prone to be filled by fine minerals (e.g., clay), resulting in relatively small pore sizes for the mixed shale. Homogeneous carbonate minerals are favorable for the residual of larger size interP pores. However, recrystallization and secondary enlargement of carbonate minerals can also lead to a significant decrease in pore diameter of interP pores. With these results, a storage space development model for different lithofacies shales was established. This work is crucial to further reveal the reservoir mechanisms and hydrocarbon scales of an argillaceous dolomite reservoir. • Quantification of lacustrine shale pore structure from multiple scales and dimensions. • The mixing of minerals with different diameters is the main factor leading to the narrower interparticle pores. • Established a dynamic response model of pore structure to mineralogical changes. [ABSTRACT FROM AUTHOR]

Many sets of Paleozoic marine organic-rich shale strata have developed in South China. However, the exploration and development results of these shale formations are quite different. Based on the data of core experiment analysis, drilling, fracturing test of typical wells, the reservoir differences and controlling factors of four sets of typical marine organic-rich shale in southern China are investigated. The four sets of shale have obvious differences in reservoir characteristics. Ordovician–Silurian shale mainly develops siliceous shale, mixed shale and argillaceous shale, with large pore diameter, high porosity, moderate thermal maturity, large pore volume and specific surface area. Cambrian shale mainly develops siliceous shale and mixed shale, with small pore diameter, low porosity, high thermal maturity and smaller pore volume and specific surface area than Ordovician–Silurian shale. Devonian–Carboniferous shale has similar mineral composition to Ordovician–Silurian shale, with small pore diameter, low porosity, moderate thermal maturity and similar pore volume and specific surface area to that of Cambrian shale. Permian shale has very complex mineral composition, with large pore diameter, low to medium thermal maturity and small specific surface area. Mineral composition, thermal maturity and tectonic preservation conditions are the main factors controlling shale reservoir development. Siliceous minerals in Cambrian shale and Ordovician–Silurian shale are mainly of biological origin, which make the support capacity better than Devonian–Carboniferous shale and Permian shale (siliceous minerals are mainly of terrigenous origin and biological origin). Thermal maturity of Ordovician–Silurian shale and Devonian–Carboniferous shale is moderate, with a large number of organic pores developed. Thermal maturity of Cambrian shale and Permian shale is respectively too high and too low, the development of organic pores is significantly weaker than the two sets of shale above. There are obvious differences in tectonic preservation conditions inside and outside the Sichuan Basin. Shale reservoirs inside the Sichuan Basin are characterized by overpressure due to stable tectonic activities, while shale reservoirs outside the Sichuan Basin are generally normal–pressure. Four sets of marine shale in South China all have certain resource potentials, but the exploration and development of shale gas is still constrained by complicated geological conditions, single economic shale formation, high exploration and development costs and other aspects. It is necessary for further research on shale gas accumulation theory, exploration and development technology and related policies to promote the development of China's shale gas industry. [ABSTRACT FROM AUTHOR]

Tectonism significantly affects the pore characteristics of organic-rich shales. Here, two sets of shales with similar mineral compositions and total organic carbon (TOC) content were used to evaluate the response of pore structure to tectonic deformation through gas adsorption, FE-SEM, and Nano-CT. The stable region shales have superior porosity, pore volume, pore diameter, surface porosity of organic matter (OM), and pore connectivity. However, the deformed region shales show obviously low storage space, pore size, and connectivity. The OM pores of the stable region shales were well preserved due to the support of residual overpressure in the closed shale system. The larger pore size and better pore connectivity enable gas molecule to be much easier to diffusion and flow, thus leading to high yield after hydraulic fracturing. Conversely, the OM pores of the deformed region shales are commonly compressed and even closed, because of the absence of overpressure-associated supporting mechanism in a relatively opened shale system. Shale gas is difficult to transport in this kind of shale reservoir with poor pore structure parameters, resulting in limited yield. Furthermore, our work also provides 3D pore deformation information of shale reservoirs under complex tectonic episodes. • Shale pores were quantitatively evaluated in 2D and 3D under different tectonic preservation condition. • Tectonic deformation influences the shale pore structure and connectivity. • Great preservation condition is beneficial to gas accumulation and engineering renovation. [ABSTRACT FROM AUTHOR]

The distribution and development of faults and fractures in organic-rich shales are important factors for the accumulation or loss of shale gas. In this work, merged three-dimensional seismic data, automated fault extraction, imaging logging, rock core examination, micro-CT scans, and field emission scanning electron microscopy were used to precisely characterize the distribution of faults and fractures of the Wufeng-Longmaxi shales from the Zilichang Block, Jiaoshiba area. The effects of faults and fractures on the accumulation and loss of shale gas were analyzed. The results show that the microfractures of shale samples from the JYA well of the Zilichang Block mainly consisted of intergranular, intragranular, and contraction fractures, and the fracture porosity of the samples is approximately 1.24%. These fractures help to increase the storage space of the shale reservoirs and are conducive for shale gas accumulation. In micro-CT scans, the microfractures were linked in a network-like structure and generally had excellent connectivity. Therefore, the microfractures could enhance the seepage capacity of the shale reservoirs and help the shale gas to migrate to hydraulically induced fractures. As a result, the productivity of the shale reservoirs could be dramatically improved. Macroscopic fractures were found to play a key role in the storage of shale gas. Many natural fractures, mainly are faults and open fractures, have developed in the caprock strata that directly overly the main production layers shales. The shale gas direct seal conditions are poor, which is not conducive to shale gas accumulation. Therefore, the Wufeng-Longmaxi shales in the JYA well is a relatively open shale system, with a formation pressure coefficient and gas saturation value of 0.96 and 37%, respectively. The single-well hydraulic fracturing productivity of the JYA well is low, with an absolute open flow value less than 0.5 × 104 m3/d. • Multiscale faults and fractures were identified. • Faults and macrofractures cause the loss of shale gas. • Microfractures enhance the seepage capacity of shale gas. [ABSTRACT FROM AUTHOR]

Both the characteristics of lithofacies and tectonic movements are important factors in controlling the development, destruction, and preservation of pores in shale reservoirs. However, the main factors that control the structure of shale pores are complex and highly disputed, restricting the understanding of the mechanisms that lead to the accumulation of shale gas. In this study, mineral composition and geochemical analyses, high resolution field emission scanning electron microscopy (FE-SEM), and low-pressure gas adsorption (N 2 and CO 2) were conducted to evaluate the pore systems of different types shale in the Jiaoshiba area, Sichuan Basin. A total of three shale groups, including silica-rich high production shales, clay-rich low production shales, and silica-rich low production shales were identified on the basis of lithofacies and gas-bearing characteristics. The highest values of TOC, pore volume, and specific surface area are found in the silica-rich high production shales, with an average of 4.182%, 33.04 × 10−3 cm3/g, and 38.71 m2/g, respectively. The total pore volume and specific surface area in the clay-rich low production shales are 19.85% and 28.62% lower than that of silica-rich high production shales. This suggests that the types of lithofacies have a more prominent influence on the specific surface area of the shales, while the pore volume plays a smaller role. However, the pressure in silica-rich low production shales is often released due to strong tectonic deformation. The OM pores in the silica-rich low production shales therefore tend to be both compressed and closed, with low surface porosity and of round shape. The total pore volume and specific surface area of silica-rich low production shales are 34.63% and 22.0% lower than that of the silica-rich high production shales. This indicates that tectonic movement has a significant influence on the shale pore volume, while specific surface area has a smaller influence. • Effects of lithofacies and preservation conditions on shale pores structure are analyzed. • Lithofacies mainly controls specific surface area and adsorbed gas. • Preservation conditions mainly controls shale pore volume and free gas. [ABSTRACT FROM AUTHOR]

In situ catalytic upgrading of heavy oil decomposes viscous heavy oil underground through a series of complex chemical and physical reactions with the aid of an injected catalyst, and permits the resulting lighter components to flow to the producer under a normal pressure drive. By eliminating or substantially reducing the use of steam, which is prevalently used in current heavy oil productions worldwide and is a potent source of contamination concerns if not treated properly, in situ catalytic upgrading is intrinsically environmental-friendly and widely regarded as one of the promising techniques routes to decarbonize the oil industry. The present review provides a state-of-the-art summarization of the technologies of in situ catalytic upgrading and viscosity reduction in heavy oil from the aspects of catalyst selections, catalytic mechanisms, catalytic methods, and applications. The various types of widely used catalysts are compared and discussed in detail. Factors that impact the efficacy of the in situ upgrading of heavy oil are presented. The challenges and recommendations for future development are also furnished. This in-depth review is intended to give a well-rounded introduction to critical aspects on which the in situ catalytic application can shed light in the development of the world's extra heavy oil reservoirs. [ABSTRACT FROM AUTHOR]

Many sets of Paleozoic marine organic-rich shale strata are developed in southern China. However, the exploration and development degree of these strata are different. Cambrian shale and Ordovician-Silurian shale are two horizons with high degree of exploration, while Devonian shale and Carboniferous shale exploration is deficient. Based on XRD, FE-SEM, and gas adsorption experiment, pore development characteristics and controlling factors of Devonian and Carboniferous shale are investigated. There are mainly four lithofacies in Devonian and Carboniferous shale: mixed shale (M), carbonate/siliceous mixed shale (M-1), argillaceous/siliceous mixed shale (M-2), and argillaceous-rich siliceous shale (S-3). Reservoir characteristics of both two sets of shale strata are quite different. The averages of porosity, pore volume, and specific surface area of Devonian shale are 3.81 % , 9.7 × 10 − 3 cm3/g and 11.8 m2/g, while those of Carboniferous shale are 3.57%, 17.3 × 10 − 3 cm3/g and 19.8 m2/g. Thermal evolution (Ro) and tectonic preservation conditions are the main factors affecting the pore development. Carboniferous shale (Ro ≈ 2 %) is in the stage of producing a large number of organic pores. Devonian shale (Ro ≈ 3.5 %) is having difficulty preserving organic pores due to high thermal evolution. Meanwhile, Devonian shale (well GTD1) is strongly affected by tectonic movement; tectonic fractures and calcite veins are developed. Carboniferous shale (well GRY1) is in relatively stable area; tectonic fractures are not developed. Under the influence of compaction, the pore volume and specific surface area of Carboniferous shale are 78.3% and 67.8% higher than those of Devonian shale, respectively. This research can provide reference for clarifying shale pore development and evolution mechanism and similar shale gas exploration both in study area and around the world. [ABSTRACT FROM AUTHOR]

Early Cambrian time witnessed accumulation of high-quality organic-rich siliceous shale deposits that comprise an especially promising shale gas exploration target, especially in China. The present study of the Cambrian Niutitang Formation of the shelf area of the Upper Yangtze Platform utilizes mineralogical and elemental geochemical data to aide in the paleoenvironment reconstruction of these organic-rich deposits. We are especially interested in elucidating organic carbon and silicon enrichment mechanisms of the Niutitang Formation that will benefit shale gas exploration and development of these deposits. The Niutitang Formation can be subdivided into three intervals based on lithology, gamma ray (GR) signature, and TOC abundance. Enriched TOC (average = 6.7%) and nano-size authigenic microcrystalline quartz (Qn) contents of Interval I deposits likely reflect deposition in an environment that experienced persistent euxinic and elevated organic matter delivery associated with enhanced paleoproductivity driven by hydrothermal activity. Interval II shale is interpreted to have accumulated under euxinic-ferruginous conditions that experienced the effects of robust paleoproductivity and diminished terrigenous input related to persistently warm, humid climatic conditions. Enriched TOC (average = 4.9%) and moderate microcrystalline quartz content of these deposits were likely the result of redox conditions and elevated paleoproductivity. Interval III strata appears to have accumulated under oxygenated bottom conditions that experienced moderate paleoproductivity, brackish water and elevated terrigenous detritus input driven by the establishment of cold, dry climatic conditions. The combined results of these conditions are diminished TOC content (average = 0.7%) and enrichment of the sediment in terrestrial quartz. The elevated content of authigenic quartz appears to have diminished the reservoir capacity of Niutitang Formation shale. Interval II organic-rich siliceous shale deposits of Upper Yangtze Platform shelf areas, which have the comparable authigenic microcrystalline quartz content, reservoir performance, and greater organic matter abundance as the rift trough, appear to comprise the stratigraphic horizon with the greatest exploration and development potential. Results of the present study offer theoretical guidance for future shale gas exploration and development efforts in geologically similar regions. • The environment during the Niutitang shale sedimentation period are reconstructed. • Mechanism of organic matter and silica enrichment of the Niutitang shale are revealed. • Authigenic quartz significantly affects the reservoir capacity of the Niutitang shale. [ABSTRACT FROM AUTHOR]

• A detailed critical review of VAPEX techniques is provided. • The mechanisms and enhancement factors of VAPEX are analyzed. • The future directions of VAPEX recovery processes are discussed. VAPEX is the promising heavy oil/bitumen recovery method with solvent diffusion into heavy oil/bitumen as a primary driving mechanism. The estimated oil recovery factor ranges from 41 ∼ 75 % after utilizing variable vapour solvents, including propane, butane, methane, and carbon dioxide. Other contributing driving mechanisms revealed are capillarity and interfacial tension, with a limited knowledge of them, which emphasizes using programming language software for their analysis. Formulated analytical models were based on the equations' insertion of a constant molecular diffusivity coefficient and constant thickness of transition zone parameters, which are both functions of solvent concentration in heavy oil/bitumen, reflecting that there are study gaps which involve concentration dependence diffusivity parameters in the models to be fulfilled. Single or mixed solvents are utilized, with choices based on recovery performance characterization. Maximum recovery, in most cases, was found while solvent injection pressures approach their dew/vapor pressure with the possibility of asphaltenes precipitates, which might plug small pores and reduce recovery factor. Lack of solvent mixture phase diagrams and pressure composition relationships was noted in this review study. Simulation software does not entirely mimic the physical processes and faces challenges, including longer computing time due to the small grid block sizes used. A lack of actual field permeability was noted in most reviewed experiments and simulations. Oil viscosity, solvent injection pressure and rate, interfacial/capillarity effects, and reservoir permeability are the most explored factors that affect VAPEX, which needs sensitivity analysis for performance optimization. This critical review will help identify the next challenges and opportunities in the VAPEX technique for heavy oil and bitumen production in the future research. [ABSTRACT FROM AUTHOR]

The Late Ordovician mass extinction (LOME), the second-largest biodiversity crisis of the Phanerozoic, occurred in two discrete pulses, the causation of both of which has been linked to environmental stresses induced by the onset and termination of the Hirnantian Glaciation. Recent studies have inferred significant mercury (Hg) anomalies, mainly from the South China region, during the Ordovician-Silurian transition (OST) that links the LOME to massive volcanism–a hypothetical late Ordovician large igneous province (LIP). However, volcanism in South China during the OST was a dominantly regional phenomenon, even though coeval eruptions in Laurentia and Baltica were major events controlled by Caledonian tectonism. The potential linkage between regional arc or LIP volcanism and the LOME needs further investigation. Here, we examine Hg concentrations, Hg isotopes, and redox-sensitive elements in two deep-water successions (the YY2 and JY11 drillcores) of South China straddling the OST. Our analysis revealed no volcanic Hg anomalies and showed significant discordance between bentonite-bed distribution and the overall pattern of the OST redox evolution in South China. Also, Hg isotope data show relatively constant mass-independent fractionation (Δ199Hg = +0.13‰ ± 0.03‰), which implicates seawater sources of Hg enrichment during the OST. Hence, we propose that the LOME was caused not by massive volcanism, but rather by the expansion of anoxia triggered by long-term cooling and weathering P-induced marine eutrophication. [ABSTRACT FROM AUTHOR]

Volcanism is one of the most important geological processes that impact Earth's surface system. However, little is known about how a volcanic eruption in the past could cause environmental changes in the sea and its impact on organic matter (OM) accumulation due to the difficulties in gaining high-resolution sedimentary records. Here we conducted a high-resolution petrographic study and geochemical analyses including proxies reflecting the influx of volcanic and terrestrial material (Hg, Al, Fe, CIA), redox conditions (Mo, Fe HR /Fe T , Fe Py /Fe HR), paleoproductivity (Ba xs , Cu xs , Si xs), and biological variations (δ13C org , δ15N org) on a 25-cm long drilling core bracketing three cycles of volcanic activities from the Upper Ordovician Wufeng Formation in South China. This enables us to find, for the first time, the detailed process of environmental changes and different stages of OM accumulation induced by volcanism. Three nearly identical cycles have been identified, and each of them is comprised of a bentonite bed, a siliceous clay-rich mudstone interval, a radiolarian bed, and a siliceous mudstone interval with intercalations of thin pyrite. The marine redox condition shows the strongest anoxia and even euxinia on the eruption stage and began to decrease with the waning of the volcanic activities shown by Hg/TOC. The volcanic eruption will cause the microbial change that is likely reflecting the shift from nitrogen-fixation bacteria bloom to radiolarian bloom through the variation of bioavailable nitrate and some trace metals such as Cu. Of significance is that the volcanism-induced shift in microbial communities from prokaryotes to eukaryotes bloom is observed to impact the abundance of the sedimentary OM, a critical process leading to the wide occurrence of abundant oil and gas reservoirs in south China. • Volcanism produced a micro-lithofacies assemblage containing bentonite bed, siliceous clay-rich mudstone, radiolarian bed and siliceous mudstone. • Volcanism induced a shift in microbial communities from prokaryotes bloom to eukaryotes bloom. • Volcanism could trigger marine euxinia and enhance pre-existing anoxic conditions. • Post-volcanism oceanic environmental changes were closely linked to OAEs and OM deposition. [ABSTRACT FROM AUTHOR]