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Understanding pore structure evolution in lacustrine shale systems provides critical insights for marine shale reservoir characterization. This study presents an integrated petrological and petrophysical analysis of a representative lacustrine shale succession, employing low temperature nitrogen adsorption (LTNA), whole rock X-ray diffraction (XRD), and scanning electron microscopy (SEM). The study shows that (1) Clay-dominated pore systems evolve through distinct pathways compared to marine shales, with illite/smectite mixed-layer minerals generating abundant mesopores through diagenetic transformation. (2) Organic matter- dominated pores display limited connectivity due to Type III kerogen characteristics and hydrocarbon generation-induced pore occlusion, contrasting with marine shale systems dominated by Type II kerogen. (3) Comparative analysis demonstrates that lacustrine shales preserve 30–40% higher micro-mesopore volumes than their marine counterparts under similar thermal maturity conditions, attributed to enhanced clay mineral diagenesis in freshwater environments. These findings provide a new framework for understanding pore structure development in non-marine depositional systems and provide valuable analogs for marine shale reservoir evaluation, particularly in transitional marine-lacustrine basins.

Lacustrine shale is an important target for the exploration of unconventional oil and gas in China beyond marine shale gas. However, the formation environment of lacustrine shale differs from that of marine shale, resulting in a different reservoir composition, organic matter, oil and gas content, and hydrocarbon mobility. In this study, the Chang 7 shale of the Yanchang Formation in the Ordos Basin was used to analyze the effect of volcanic activity on the paleoproductivity and preservation conditions during the formation of lacustrine shale. The results show that algae and bacteria were developed before the eruption. After the eruption, the number of bacteria declined, but the increased prosperity of algae reflects that the volcanic activity enhanced ancient productivity. The sulfate generated by volcanic activity promotes bacterial sulfate reduction, and the produced H 2 S leads to a strong reducing environment in the waterbody, which is conducive to the preservation of organic matter. Organic geochemical analysis shows that the black shale in the shale strata has a high total organic carbon (TOC) content and strong hydrocarbon generation potential, whereas the tuff has a low TOC content and can scarcely generate hydrocarbons, indicating that the tuff deposited by volcanic activity cannot be considered as effective source rock. In terms of storage space, shale is mainly laminar and dispersed, and it includes organic and inorganic pores. The development of organic pores is affected by thermal maturity, whereas inorganic pores mainly occur between detrital particles and crystals. Tuff is mainly supported by heterogeneous matrix and associated with alteration. Its pores include inter- and intragranular mineral pores. The development of tight sandstone pores is affected by compaction, cementation, and dissolution, which mainly consist of intra- and intergranular pores. The Chang 7 lacustrine shale generally contains oil, but different lithologies have different oil drainage efficiencies. Sandstone and shale exhibit the best and worst oil drainage efficiency, respectively. It is mainly affected by the pore size distribution, fluid properties, and rock wettability. Therefore, the development of shale oil should mainly focus on lacustrine shale formations with interbeds. The mutual dissolution of organic matter and hydrocarbons in the shale section leads to the poor mobility and difficult development of hydrocarbons.

The Sichuan Basin is rich in shale oil and gas resources, with favorable geological conditions that the other shale reservoirs in China cannot match. Thus, the basin is an ideal option for fully “exploring petroleum inside source kitchen” with respect to onshore shale oil and gas in China. This paper analyzes the characteristics of shale oil and gas resources in the United States and China, and points out that maturity plays an important role in controlling shale oil and gas composition. US shale oil and gas exhibit high proportions of light hydrocarbon and wet gas, whereas Chinese marine and transitional shale gas is mainly dry gas and continental shale oil is generally heavy. A comprehensive geological study of shale oil and gas in the Sichuan Basin reveals findings with respect to the following three aspects. First, there are multiple sets of organic-rich shale reservoirs of three types in the basin, such as the Cambrian Qiongzhusi Formation and Ordovician Wufeng Formation-Silurian Longmaxi Formation marine shale, Permian Longtan Formation transitional shale, Triassic Xujiahe Formation lake-swamp shale, and Jurassic lacustrine shale. Marine shale gas enrichment is mainly controlled by four elements: Deep-water shelf facies, moderate thermal evolution, calcium-rich and silicon-rich rock association, and closed roof/floor. Second, the “sweet section” is generally characterized by high total organic carbon, high gas content, large porosity, high brittle minerals content, high formation pressure, and the presence of lamellation/bedding and natural microfractures. Moreover, the “sweet area” is generally characterized by very thick organic-rich shale, moderate thermal evolution, good preservation conditions, and shallow burial depth, which are exemplified by the shale oil and gas in the Wufeng-Longmaxi Formation, Longtan Formation, and Daanzhai Member of the Ziliujing Formation. Third, the marine, transitional, and continental shale oil and gas resources in the Sichuan Basin account for 50%, 25%, and 30% of the respective types of shale oil and gas geological resources in China, with great potential to become the cradle of the shale oil and gas industrial revolution in China. Following the “Conventional Daqing-Oil” (i.e., the Daqing oilfield in the Songliao Basin) and the “Western Daqing-Oil & Gas” (i.e., the Changqing oilfield in the Ordos Basin), the Southwest oil and gas field in the Sichuan Basin is expected to be built into a “Sichuan-Chongqing Daqing-Gas” in China.

The global energy demand is increasing rapidly, and it is imperative to develop shale hydrocarbon resources vigorously. The prerequisite for enhancing the exploitation efficiency of shale reservoirs is the systematic elucidation of the occurrence characteristics, flow behavior, and enhanced oil recovery (EOR) mechanisms of shale oil within commonly developed nanopores. Molecular dynamics (MD) technique can simulate the occurrence, flow, and extraction processes of shale oil at the nanoscale, and then quantitatively characterize various fluid properties, flow characteristics, and action mechanisms under different reservoir conditions by calculating and analyzing a series of MD parameters. However, the existing review on the application of MD simulation in shale oil reservoirs is not systematic enough and lacks a summary of technical challenges and solutions. Therefore, recent MD studies on shale oil reservoirs were summarized and analyzed. Firstly, the applicability of force fields and ensembles of MD in shale reservoirs with different reservoir conditions and fluid properties was discussed. Subsequently, the calculation methods and application examples of MD parameters characterizing various properties of fluids at the microscale were summarized. Then, the application of MD simulation in the study of shale oil occurrence characteristics, flow behavior, and EOR mechanisms was reviewed, along with the elucidation of corresponding micro-mechanisms. Moreover, influencing factors of pore structure, wall properties, reservoir conditions, fluid components, injection/production parameters, formation water, and inorganic salt ions were analyzed, and some new conclusions were obtained. Finally, the main challenges associated with the application of MD simulations to shale oil reservoirs were discussed, and reasonable prospects for future MD research directions were proposed. The purpose of this review is to provide theoretical basis and methodological support for applying MD simulation to study shale oil reservoirs.

Methane has been rising rapidly in the atmosphere over the past decade, contributing to global climate change. Unlike the late 20th century when the rise in atmospheric methane was accompanied by an enrichment in the heavier carbon stable isotope (13C) of methane, methane in recent years has become more depleted in 13C. This depletion has been widely interpreted as indicating a primarily biogenic source for the increased methane. Here we show that part of the change may instead be associated with emissions from shale-gas and shale-oil development. Previous studies have not explicitly considered shale gas, even though most of the increase in natural gas production globally over the past decade is from shale gas. The methane in shale gas is somewhat depleted in 13C relative to conventional natural gas. Correcting earlier analyses for this difference, we conclude that shale-gas production in North America over the past decade may have contributed more than half of all of the increased emissions from fossil fuels globally and approximately one-third of the total increased emissions from all sources globally over the past decade.

As one of the most important constitutes of shales/mudstones, quartz has received increasing interests in the last decades, because productive shale gas successions are generally rich in quartz content. This study critically documents quartz types, silica source for quartz cementation and effect of quartz cementation on reservoir quality in the Lower Paleozoic shales, Middle Yangtze region, South China, including the Lower Cambrian Niutitang Formation and the Upper Ordovician – Lower Silurian Wufeng-Longmaxi formations. Our results suggest that high-resolution scanning electron microscopy combined with cathodoluminescene techniques are necessary for identifying quartz types in shales. Integrations of high-resolution imaging technique and detailed geochemical analysis are able to document silica source for quartz cementation and silica diagenetic processes. Six types of quartz can be identified in the Paleozoic shales, primarily including detrital quartz silt, siliceous skeletons, quartz overgrowth, microcrystalline quartz (matrix-dispersed microquartz and aggregated microquartz), silica nanospheres and fracture-filling quartz veins. Dissolution of siliceous skeletons provides the principal silica sources for authigenic quartz formation in the Paleozoic shales. Authigenic quartz has dual effects on porosity development. Quartz overgrowth definitely occupies interparticle pores and possibly squeeze spaces, whereas aggregated microquartz can form rigid framework that is favorable for generating and preserving intercrystalline pores and organic pores.

The study of shale mechanical characteristics on the microscopic scale can provide a theoretical basis to evaluate shale's macroscopic mechanical properties and fracability. Quartz is a rigid mineral that exists extensively in shale, and quartz of various types exerts different influences on the quality of shale oil/gas reservoirs due to their different microscopic occurrence states. However, this difference lacks rigorous quantitative evaluation and mechanism explanation. Therefore, using the finite element numerical simulation method, three classes containing different models were set up based on microscopic occurrence states of quartz observed under a scanning electron microscope to solve these problems. When quartz content is high (about more than 45%), the effect of quartz grain size and occurrence mode on shale compressive strength is gradually significant, exceeding the influence of quartz content on shale compressive strength, and this provides scientific evidence that authigenic microcrystalline quartz (Qm) featured by small particle size and distributed in an aggregate state can effectively enhance shale compressive strength and thus protect the primary intergranular pores of shale. Quartz content has the most significant influence on shale brittleness, and Qm cannot greatly improve shale brittleness, which is inconsistent with previous opinions. The abundant brittle minerals rather than Qm particles are the “underlying cause” of good brittleness of shales rich in Qm. The shale containing more quartz particles of small size can form long hydraulic fractures and thus form a larger volume of complex fracture network.

In recent years, multi-stage hydraulic fracturing for shale oil reservoirs is developing toward a shorter cluster spacing and a larger number of clusters per stage. The objective is to improve the stimulation performance by reducing the fracturing stages and creating more hydraulic fractures in terms of cost-efficiency in geological engineering. Field tests demonstrate the uneven distribution of fluid volume among clusters and only a few perforation clusters contribute to productivity. The extreme limited entry (XLE) completion is an effective method to improve the efficiency of perforation clusters. However, the influence of several controlling parameters on the cluster effectiveness during XLE completion remains elusive. Therefore, a 3D field-scale hydraulic fracturing model for XLE completion based on the lattice method is developed for the case of shale oil reservoirs of Changqing oilfield in China. In this model, perforation friction, geological factor such as stress difference between adjacent clusters, and filtration of fracturing fluid are considered. The numerical results are compared with field data (i.e., injection pressure and cluster efficiency) obtained from the fracturing construction and the step-down test. The influences of injection rate, cluster spacing, cluster number, and stress difference between adjacent clusters on the fracture morphology, cluster efficiency and injection pressure of XLE completion are discussed in detail. The results show that perforation friction is the most critical factor that affects the efficiency of perforation cluster during XLE completion. A guide chart of cluster efficiency under various differential geological stresses between adjoining clusters is proposed. According to this guide chart, the required perforation friction under different cases can be obtained, and then the related perforation parameters (i.e., perforation diameter and number) can be determined. The research results provide theoretical guidance for optimization of XLE completion in geological reservoirs.