Non-marine shale oil is a new frontier. Recent exploration practices have made significant progress in different shale formations including in the Ordos Basin, Songliao Basin, Bohai Bay Basin, Junggar Basin, and Qaidam Basin. At the end of 2022, the geological reserves of shale oil, including the proven, controlled, and predicted reserves, reached 44×10⁸ t, and the production in 2022 was 318 ×10⁴ t. The exploration theory and technology of shale oil have also made a series of advances, such as innovating shale experimental testing and analysis technologies including organic matter type analysis and organic matter generation and expulsion experiments, reservoir characterization technology, shale oil occurrence state and oil content analysis, and pressure maintaining coring and on-site testing. These technologies can basically meet the requirements of shale oil related experimental testing. A series of new understandings has been obtained in the aspects of fine-grained sedimentation and organic matter enrichment mechanisms, terrestrial shale laminated structure and combination types, pore and fracture structure and storage capacity in reservoirs, and shale oil enrichment mechanisms, which guided the research on evaluation of selected areas and zones in the key areas of shale oil. Technologies such as logging evaluation of hydrocarbon source rock quality, reservoir quality, and engineering quality, analysis and quantitative prediction of rock physical sensitive parameters, multi-task learning of reservoir parameter prediction, anisotropic stress prediction, seismic geological orientation evaluation of horizontal wells, and comprehensive evaluation of enrichment layers (“sweet spots”) have been developed and promoted. This provides important and all-cycle technical support for shale oil reserves submission, sweet spot area selection, horizontal well deployment, and directional drilling warning and engineering retrofit after drilling completion. However, large-scale exploration and efficient development of non-marine shale oil still face many challenges. It is necessary to establish new research content and priorities, especially to upgrade research precision and strengthen microscopic research, solid/liquid/gas multiphase and multi-field coupling flow mechanism research, and interdisciplinary research, in order to establish a new discipline of shale oil accumulation.

In this work, the potential influences of grain-coating clays on water-CO₂-rock interactions in sandstones and subsequent ramifications for CO₂ storage were investigated using reactive transport simulations. The results indicated that, compared to pore-filling smectite, grain-coating smectite leads to significant pH decrease, increases in the CO₂-species concentrations, and decreases in smectite dissolution and the precipitation of secondary minerals. Moreover, it was revealed that smectite and chlorite coats dissolve preferentially over detrital K-feldspar being covered, while K-feldspar is dissolved preferentially over illite and kaolinite coats. While the mineral trapping mechanism is only important for smectite and chlorite coats, sandstone porosity is significantly reduced for chlorite coat but increased for the other three clay coats. The main causes of the differences between pore-filling and grain-coating scenarios for smectite and chlorite coats are ascribed to the inhibitory effect of clay coats on the growth of secondary quartz and the dissolution of clay. In addition to the above two factors, the decelerating effect of clay coats on the dissolution of K-feldspar is also important for illite coat; meanwhile, for the kaolinite coat, the dissolution of clay is less important and the other two factors are more critical. Furthermore, the coverage and thickness of clay coats, fluid flow rate, detrital grain size, detrital lithology, partial pressure of CO₂, and temperature may all impact the role of clay coats.

The Qingshankou Formation shale, Gulong Sag, Songliao Basin, has been gushing out oil in a rate stunning the Chinese oil industry. However, further prediction of sweet spots in the formation remains a challenge due to the highly uneven distribution of organic matter and confusion in the understanding of controlling factors. This study adopted the concepts of “hierarchy in sequence stratigraphy” and “transgressive-regressive (T-R) sequences”, with updated astronomical cycle research results to determine the accurate duration of sequences and re-establish an isochronous sequence stratigraphic framework specifically for lacustrine deep-water shale based on core, outcrop, and thin section observation, as well as seismic profile, well-logging, geochemical, and paleontological data analyses from a microcosmic to macroscopic scale. Subsequently, using modern lakes sedimentation as an analogy made it possible to propose qualitative and quantitative indexes for identifying paleo-water environment, and probe into the origin of the heterogeneity of organic matter enrichment under the sequence stratigraphic framework on the basis of the coupling relation among paleoproductivity, redox conditions, and sedimentation rate. The main conclusions are as follows: 1)There are four third-order sequences in Qingshankou Formation Shale, Gulong Sag, Songliao Basin. Among them,SQ1 and SQ2 consist of two T-R sequences of 13 parasequence sets (made up of 52 parasequences). The durations of parasequence and parasequence set are approximately 40 kyr and 170 kyr, respectively. 2) Three types of lithofacies, four types of laminae, five laminae combinations, eleven laminae combination patterns, and three sedimentary microfacies have been recognized under the sequence stratigraphic framework. T-R cycles control the vertical distribution of laminae combination, lithofacies and sedimentary microfacies. The argillaceous shales deposited in deep-lake stagnant water and mud flow are prospective lithofacies. 3) T-R cycles control the enrichment of organic matter and different orders of flooding surfaces are the favorable locations for organic matter enrichment. Parasequence set 2 in Gulong Sag and parasequence sets 1 to 4 in Sanzhao Sag have been evaluated to be the most promising exploration targets. This study aims to provide sedimentological evidence for shale oil target area and “sweet spots”prediction.

Traditional correlation analyses based on whole-rock data have limitations in discerning pore development determinants in shale oil reservoir, given the complex lithology of shale formations and intricate interdependencies (multicollinearity) among geological variables. In this study, mercury injection capillary pressure and digital analysis of scanning electron microscopy were employed to examine the macropore structures of both whole rocks and their constituent lithologies for the Upper Triassic Chang-7 shale of the Ordos Basin. Variations were observed among clay shale (shale primarily consisting of clay-sized mineral grains), massive siltstone and silty laminae within the Chang-7 shale. Through the combination of correlation analysis and scanning electron microscope digital technique, it was demonstrated that total organic carbon content primarily controls the level of macropore development, while lithology primarily governs macropore types and structures. Although quartz and pyrite exhibit correlations with macropore volume, they do not emerge as primary factors; instead, they appear interconnected to total organic carbon. Due to detrital mineral framework preservation during compaction, larger macropores are more developed in massive siltstones and silty laminae than in clay shale. Additionally, silty laminae, situated closer to the source rock and influenced by organic acids, exhibit a higher abundance of larger dissolution pores, potentially favoring shale oil development. This study overcomes traditional method constraints, disentangling multi-correlations, and providing new insights into shale macropore development mechanisms, potentially advancing shale oil exploration and production.

Occurrence and mobility of shale oil are prerequisites for evaluating shale oil reserves and prioritizing exploration targets, particularly for heterogeneous lacustrine shales. The Qingshankou Formation in the Gulong Sag, Songliao Basin is a classic lacustrine pure shale reservoir that contains abundant shale oil resources. The predicted geological reserves of the shale are 1.268 × 10⁹ t. In this study, field emission scanning electron microscope (FE-SEM), the modular automated processing system (MAPS), pyrolysis-gas chromatography (Py-GC), low-pressure nitrogen gas adsorption (LPNA), Soxhlet extraction, pyrolysis, and 2-D nuclear magnetic resonance (NMR) were integrated to describe the shale oil components, microscopic occurrence, mobility, and the effective pore size distribution. Meanwhile, the related controlling factors are discussed.

The shale oil in the Qingshankou Fm exists dominantly in the matrix pores of the clay minerals, with small amounts distributed in the intergranular pores of terrigenous clastic grains, intercrystalline pores of pyrite, intragranular pores of ostracod shells, and micro-fractures. Shale oil is distributed in the pore spaces of variable sizes in different lithofacies. The clay mineral-laminated shales are characterized by the broadest range of pore size and largest volume of pore spaces with shale oil distribution, while the ostracod-laminated shales have limited pore spaces retaining oil. Furthermore, the proposed integrated analysis evaluates the shale oil molecules existing in two states: movable, and adsorbed oil, respectively. The result illustrates that movable oil takes up 30.6%–79.4% of the total residual oil. TOC, mineral composition, and pore structures of the shale joint together to control the states and mobility of the shale oil. TOC values are positively correlated with the quantities of shale oil regardless of the state of oil. The mineral components significantly impact the state of shale oil. Noticeable differences in the states of oil were observed following the changing types of minerals, possibly due to their difference in adsorption capacity and wettability. Clay minerals attract more adsorbed oil than movable oil. Felsic minerals generally decrease the occurrence of total and adsorbed oil. Carbonate plays a positive role in hydrocarbon retention of all the shale oil states. As for the pore structure, the average pore size exerts a critical impact on the total, movable, and adsorbed oil content. The total pore volume and specific surface area of shales play a principal role in controlling the total yields and amounts of adsorbed oil. This research improves the understanding of the occurrence characteristics and enrichment mechanisms of shale oil in terrestrial pure shales and provides a reference for locating favorable shale oil exploration areas.