Reservoir characteristics and influencing factors of multi-lithofacies shales in the Lianggaoshan Formation, Northeast Sichuan Basin

The effect of various depositional parameters including paleoclimate, paleosalinity and provenance, on the depositional mechanism of lacustrine shale is very important in reconstructing the depositional environment. The classification of shale lithofacies and the interpretation of shale depositional environment are key features used in shale oil and gas exploration and development activity. The lower 3rd member of the Eocene Shahejie Formation (Es3x shale) was selected for this study, as one of the main prospective intervals for shale oil exploration and development in the intracratonic Bohai Bay Basin. Mineralogically, it is composed of quartz (avg. 9.6%), calcite (avg. 58.5%), dolomite (avg. 7%), pyrite (avg. 3.3%) and clay minerals (avg. 20%). An advanced methodology (thin‐section petrography, total organic carbon and total organic sulfur contents analysis, X‐ray diffraction (XRD), X‐ray fluorescence (XRF), field‐emission scanning electron microscopy (FE‐SEM)) was adopted to establish shale lithofacies and to interpret the depositional environment in the lacustrine basin. Six different types of lithofacies were recognized, based on mineral composition, total organic carbon (TOC) content and sedimentary structures. Various inorganic geochemical proxies (Rb/Sr, Ca/(Ca + Fe), Ti/Al, Al/Ca, Al/Ti, Zr/Rb) have been used to interpret and screen variations in depositional environmental parameters during the deposition of the Es3x shale. The experimental results indicate that the environment during the deposition of the Es3x shale was warm and humid with heightened salinities, moderate to limited detrital input, higher paleohydrodynamic settings and strong oxygen deficient (reducing) conditions. A comprehensive depositional model of the lacustrine shale was developed. The interpretations deduced from this research work are expected to not only expand the knowledge of shale lithofacies classification for lacustrine fine‐grained rocks, but can also offer a theoretical foundation for lacustrine shale oil exploration and development.

The shale oil resources in X Sag hold substantial potential. A thorough assessment of the shale pore structure is crucial for guiding the development and exploration of shale oil and gas in this area. In this study, high-pressure mercury injection and nitrogen adsorption experiments were conducted. By integrating advanced characterization techniques, the pore structure characteristics of shale in X Sag were systematically investigated, achieving multi - scale precise characterization from the micron - to the nanoscale. The nitrogen adsorption results indicate that nanoscale pores are well - developed in X Sag shale, yet the poor connectivity of the pores, and the pore - permeability parameters are small. The mercury injection high-pressure experiment further clarifies the connectivity and permeability characteristics of micron - scale pores. These findings offer a significant basis for the detailed evaluation of shale oil reservoirs in X Sag and also lay a theoretical groundwork for optimizing the exploration and development of shale oil and gas in this region.

Possessed of easy access to development and fair economic benefits, medium-high maturity continental shale oil and gas have become the focus of shale oil and gas study in the future. Shale oil and gas mainly occur in pores, but studies on the pore characteristics of shale oil and gas occurrence are by no means sufficient. Focused on shale from the Middle Jurassic Lianggaoshan Formation in Well TYX, Fuling block, southeast of Sichuan Basin where a breakthrough in shale oil and gas exploration was recently achieved, this study selects core samples and conducts a series of analyses, including vitrinite reflectance analysis, kerogen microscopic examination experiment, total organic carbon (TOC) content analysis, mineral composition analysis, gas content measurement, isothermal adsorption experiment, S1 content analysis, and others. The analyses are to identify the pore characteristics of the continental medium and high maturity shale oil and gas by virtue of scanning electron microscope (SEM) with Ar-ion milling and the image processing software ImageJ. The conclusions are drawn as follows: in terms of lithofacies, medium-high maturity continental shale oil and gas mainly occur in organic-rich clay shale and organic-rich mixed shale; with regard to material composition, shale oil and gas mainly occur in organic matter, illite-smectite mixed layers and illite. Shale adsorbed gas content accounts for at most 40% of the total shale gas content and shale free gas content takes up at least 60% of the total shale gas content. Pores of solid bitumen, solid bitumen-clay mineral complex mass, clay minerals, structured vitrinite, and funginite are mostly developed in shale. Among them, the first three types of pores are the main reservoir space in shale considering their large number, good roundness, medium pore diameter, fairly good roundness of pore edges, and the complex shapes which altogether contribute to the large surface porosity.

Abundant lacustrine shale oil resources have been discovered in the Eocene saline lake system, Yingxiongling Sag, Qaidam Basin. Compared with the Cretaceous Qingshankou Formation in the Songliao Basin and Triassic Chang 7 Member in the Ordos Basin, the lithofacies of Yingxiongling shale are more complex, featuring thinner single-layer and faster vertical variations. How to establish a reasonable lithofacies classification and evaluate the effectiveness of different lithofacies is the key to unlocking the giant resources of Yingxiongling shale oil. Based on the total organic carbon analysis, rock pyrolysis analysis, fluorescence thin section observation, oil saturation analysis, helium porosity measurement, high pressure mercury intrusion by scanning electron microscopy, focused ion beam-scanning electron microscopy, and nitrogen adsorption analysis, the organic geochemistry, reservoir property, and oil-bearing property were comprehensively analyzed. This paper proposed a lithofacies classification scheme of Yingxiongling shale oil and systematically evaluated the quality of representative lithofacies. The key findings were as follows: (1) Eight types of lithofacies were identified based on the rock structure and mineral composition, namely, thin-bedded/laminated dolomitic limestone, thin-bedded/laminated limy dolostone, thin-bedded/laminated mixed rock, thin-bedded sandstone, and laminated shale. (2) The types and combinations of lamination varied greatly among different lithofacies. Dolomite primarily formed through calcite shrinkage during the dolomitization process, resulting in the formation of abundant intercrystalline pores. (3) More organic matter was observed in the calcite laminae, which was deposited as a result of small calcite particle flocculation. (4) The combination of laminated dolomitic limestone and thin-bedded limy dolostone formed a favorable source–reservoir assemblage, which was the pay zone for horizontal wells. These understandings could further be helpful in understanding shale oil enrichment in the Qaidam Basin and could provide scientific guidance and technical support for the exploration and development of global plateau shale oil.

Shale oil and gas exploration potential in the Tanezzuft Formation, Ghadames Basin, North Africa

Study on the relationship between shale organic matter development and paleowater depth—A new understanding of the condensed section

Reservoir formation conditions and key technologies for exploration and development in Qingcheng large oilfield

Depending on the characteristics and the demands of exploration and development of continental shale oil reservoirs, Sinopec has carried out researches on technologies for such reservoirs, including optimized and fast drilling and completion, logging evaluation, and reservoir stimulation, etc. These new technologies, which have been successfully applied to 17 exploratory wells, strongly support the exploration and development evaluation of continental shale oil in a number of regions, such as Jiyang Depression, Biyang Sag, and Sichuan Basin,etc. And with them, a major breakthrough was achieved in the exploration of shale oil in Jiyang Depression. However, the shale oil engineering technologies of Sinopec still lag behind those of some foreign countries. Therefore, further research is still needed in technologies of multi-layer stereoscopic development by shale oil well factory, drilling of horizontal wells with ultra-long horizontal sections, and integration of shale oil geology and hydraulic fracturing. Sinopec’s research could thereby strive toward the perfection of the shale oil engineering technology system, the improvements in quality, speed, efficiency, and output, and the beneficial development of continental shale oil.

Shale oil and gas resources have become an alternative energy source and are crucial in the field of sustainable oil and gas exploration. In the Junggar Basin, the Permian is not only the most significant source rock, but also an important field in shale oil and gas exploration. However, there are significant differences in the effectiveness of source rocks in different layers. During the Permian, the Bogda region effectively recorded the transition from marine environments in the Early Permian to terrestrial environments in the Late Permian, providing a viable opportunity for studying the Permian source rock of the Junggar Basin. We conducted an analysis of the total organic carbon (TOC), Rock-Eval pyrolysis, vitrinite reflectance (Ro), and biomarker compounds of Permian source rocks around the Bogda Mountain. The results indicate that the Lower Permian strata were primarily deposited in a moderately reducing marine environment, with the main organic matter sourced from planktonic organisms. These strata are currently in a high to over-mature stage, evaluated as medium-quality source rocks, and may have already generated and expelled substantial quantities of oil and gas, making the Lower Permian hydrocarbon resources within the basin a noteworthy target for deep condensate oil and gas exploration in adjacent depressions. The Middle Permian Wulabo and Jingjingzigou formations were deposited in a moderately oxidizing marine–continental transitional environment with significant terrestrial organic input. The kerogen type is predominantly Type III, and these formations are presently in the mature to over-mature stage with low organic abundance and poor hydrocarbon generation potential. The Middle Permian Lucaogou Formation was deposited in a moderately reducing saline lacustrine environment, with algae and planktonic organisms as the primary sources of organic matter. The kerogen types are mainly Type I and II1, and it is currently within the oil-generation window. It is characterized by high organic abundance and evaluated as good to excellent source rocks, possessing substantial potential for shale oil exploration. The Upper Permian Wutonggou Formation was primarily deposited in a highly oxidizing continental environment with significant terrestrial input. The primary organic source comprises higher plants, resulting in Type III kerogen. These strata exhibit low organic abundance, are currently in the immature to mature stage, and are evaluated as poor source rocks with limited exploration potential. The information presented in this paper has important theoretical significance and practical value for oil and gas exploration and development in the Junggar Basin.

Exploration progress and geochemical features of lacustrine shale oils in China

Because clay minerals and pore-size distribution are important for oil and gas accumulation in lacustrine mudstone reservoirs, this study aimed to characterize the pore-size distribution in lacustrine mudstone, reveal the importance of clay minerals in gas sorption capacity (GSC), provide insights into the primary GSC controls for lacustrine mudstones, and discuss the shale oil and gas resource potential and possible risks in their future production. A total of 256 core samples from the lower Paleogene sequence of the Dongying depression in the Bohai Bay Basin in China were analyzed to determine organic richness and hydrocarbon generation potential. A series of analyses was conducted on a suite of 28 mudstone samples from the S121 well to investigate the mineral composition, pore-size distribution, and primary controls on GSC. Scanning electron microscopy and x-ray diffraction were used to observe the microscopic characteristics and identify various minerals. Nitrogen adsorption experiments at low temperatures and a methane isothermal adsorption test were conducted to study the pore sizes and gas adsorption capacities. Experimental results show that the majority of the samples were rich in type II and I kerogen with moderate to high hydrocarbon generation potential and are thermally mature in the main oil window. The contents of illite and mixed layer illite–smectite (I–S) are high (>40%) in these lacustrine mudstones. The pores were mainly thin, long, and bottleneck shaped and dominated by type IV and H2 hysteresis loop characteristics. The abundance of quartz, feldspar, illite, and mixed layer I–S all presented a positive correlation with their Brunauer–Emmett–Teller surface area and total pore volume, whereas the abundance of carbonate minerals (dominated by calcite) showed a negative correlation. The volume of micropores and clay minerals were the main factors controlling GSCs. The results highlight the importance of clay minerals, including the illite and mixed layer I–S, in the lacustrine mudstones because of their considerable GSC. High content of illite and mixed layer I–S in lacustrine mudstone is in favor of gas storage, whereas at the same time, it is not conducive to shale gas development because of its reducing the brittleness of shale. The Paleogene of Dongying depression has shale oil and gas resources; however, several key geological factors, including pore-size distributions, pore diameters, oil density, and low brittleness, may pose risks in shale oil and gas development.

Against the backdrop of global low-carbon energy transition, the green, economical, and efficient development of shale oil and gas resources faces a series of challenges in theoretical frameworks, technological costs, low-carbon innovation, and engineering management. To foster technological independence and advance low-carbon development through academic exchange, the 6th International Symposium on Shale Oil and Gas Exploration, Development, and Utilization Technology was held by Jilin University in Changchun, China, from November 7 to 9, 2025. This symposium gathered over 270 experts and scholars from more than 50 institutions worldwide featuring 61 presentations spanning geological theory, intelligent exploration, drilling and completion technologies, in-situ conversion, pyrolysis mechanisms, enhanced recovery, and low-carbon strategies. The discussions underscored a decisive shift towards intelligent, integrated, and green technological solutions, highlighting the critical role of artificial intelligence, nanotechnology, and carbon management in field advancement. This event significantly strengthened the industry-academia-research-application collaboration system, providing important momentum for achieving technological independence and driving the shale industry towards a sustainable and secure energy future. Document Type: Editorial Cited as: Deng, S., Sun, Y., Guo, W. Innovative technologies for shale oil and gas exploration and development. Advances in Geo-Energy Research, 2026, 19(1): 97-100. https://doi.org/10.46690/ager.2026.01.08

Hydrocarbon generation patterns are crucial for defining favorable exploration maturity intervals and targets for shale oil. The Permian Fengcheng shale and Lucaogou shale in the Junggar Basin, as significant targets for shale oil exploration, study on their hydrocarbon generation, expulsion, and retention characteristics are insufficient. This study establishes hydrocarbon generation patterns for the Fengcheng shale and Lucaogou shale using a semi-open thermal simulation system, combined with rock pyrolysis, vitrinite reflectance (Ro), total organic carbon (TOC) analysis, carbon isotope analysis, gas chromatography-mass spectrometry (GC-MS), mercury porosimetry, and nitrogen adsorption. Additionally, a multivariate regression model was employed to systematically evaluate the primary controlling factors of hydrocarbon expulsion ratio. Based on these results, favorable exploration maturity intervals and targets for the Fengcheng shale and Lucaogou shale were identified. The results indicate that the hydrocarbon generation potential of Fengcheng shale was superior to that of Lucaogou shale, whether considering gas yield or oil yield. However, Lucaogou shale began the thermal degradation earlier than Fengcheng shale and has a broader degradation window. The corresponding (TR) is slightly higher for Fengcheng shale. The Lucaogou shale exhibited significantly higher hydrocarbon expulsion ratios compared to Fengcheng shale, with a maximum hydrocarbon expulsion ratio 2.1 times that of Fengcheng shale. Mesopore volume and its connectivity were critical factors affecting the hydrocarbon expulsion ratio, whereas oil mobility and macropore volume have relatively limited effects. The optimal maturity interval for shale oil exploration as Ro = 1.0%–1.1% for the Fengcheng shale and Ro = 0.9%–1.1% for the Lucaogou shale. Exploration of the Fengcheng shale should focus on lithofacies assemblages, the dolomitic mixed rock-shale assemblages in the shore-shallow lake facies represent the primary exploration target. Exploration of the Lucaogou shale should focus on source-reservoir assemblages, with a higher reservoir-to-shale thickness ratio being the primary exploration target. These findings provide support for the exploration and development decisions of shale oil in the Junggar Basin.

Understanding pore growth is of great significance to investigating reservoir performance in shale-gas systems. However, different from the marine shale reservoir, the lacustrine shale reservoir is commonly rich in clay minerals, resulting in a complicated and poorly understood pore system. We have investigated the impact of coexisting clay mineral and organic matter on pore growth in the Lower Jurassic Da’anzhai Shale in the Northeast Sichuan Basin, West China, through performing total organic carbon (TOC) analysis, X-ray diffraction, field-emission scanning electron microscopy, focused ion beam-scanning electron microscopy (FIB-SEM), [Formula: see text] and [Formula: see text] adsorption experiment, and high-pressure mercury intrusion porosimetry. Our results indicate that the Da’anzhai Shale is dominated by clay-mineral-hosted pores, which are commonly filled or partly filled by pyrobitumen. Controlled by organic maceral, organic pores are poorly and heterogeneously developed in pyrobitumen, and minor or even no organic pores grow in vitrinite. Mesopore and macropore are popular in the Da’anzhai Shale reservoir with complex shapes, e.g., slit- or plate-like shapes combined with “ink-bottle” shapes, confirming a pore system dominated by clay-mineral-hosted pores. The weak positive correlation between the clay mineral content and the meso/macropore volume confirms that the clay mineral is a positive contributor to the storage space, and the weak negative correlation between the TOC and the mesopore volume suggests that infilling of pyrobitumen decreases the pore volume significantly. Similar correlations occur between specific surface area and clay mineral/TOC. FIB-SEM observation confirms that the pore system, e.g., the pore size, pore shape, and pore volume, is controlled by the coexisting clay mineral and pyrobitumen filling in a later stage. The calculated plane porosity of the initial inorganic pore and the unfilled inorganic pore in the Da’anzhai Shale is in the range of 3.66%–10.95% and 0.79%–1.46%, respectively, suggesting that 76.66% of inorganic pores is inactive due to pyrobitumen filling. All of this evidence suggests that pore growth in the Da’anzhai Shale is positively contributed by clay minerals, but it is negatively contributed by pyrobitumen filling. Further discussion suggests that pyrobitumen infilling between clay minerals in the Da’anzhai lacustrine shale can decrease the original pore volume significantly, which work together to govern the pore system in shale reservoirs.

Quantitative assessment of the sweet spot in marine shale oil and gas based on geology, engineering, and economics: A case study from the Eagle Ford Shale, USA