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Due to the depletion of conventional hydrocarbon resources, both China and Russia are giving more attention to the exploration and production of unconventional oil and gas resources, especially those generated and accumulated within source rocks. In an attempt to further understand the mechanisms of these resources, detailed mineralogical, lithological, and geochemical studies were performed to compare source rock samples from i) the Longmaxi Formation of the Lower Silurian in the Sichuan Basin (China), and ii) the Semiluksk Formation of the Frasnian Stage of the Upper Devonian in the Volga-Ural region of the East European Platform (Russia). The results showed that the main mineral of the source rocks of both formations is chalcedony that formed during an outpouring of deep fluids onto the sea bed. Simultaneous thermal analyses of this mineral indicated similar thermophysical properties to those of the hydrothermal-sedimentary chalcedony from jasper. Moreover, a direct relationship between the chalcedony content and the total organic matter content in the samples from the two formations was established. The presence of biophilic chemical elements in the siliceous component of the source rock samples indicate that high total organic content values were likely due to the presence of biophilic chemical elements in deep fluids, which led to the large-scale development of biota and subsequent accumulation of organic matter during sedimentation. The findings also revealed that the organic matter in the source rocks of the two regions was at different stages of maturity. The organic matter in samples from the Volga-Ural region was less mature and only at the early stage of oil generation, whereas the organic matter in samples from the Sichuan Basin reached both oil and gas generation stages. The Longmaxi Formation is already in the shale gas exploration and development stage, and the Semiluksk Formation could also be regarded as an exploration target for shale oil reservoirs.

The Fuling shale gas field in China is the largest shale gas field as well as the largest of its type discovered in any Lower Paleozoic formation. In this study, the geology and production of the upper and lower gas layers in the Fuling shale gas field are evaluated in terms of structure, shale quality, fault, initial production, and estimated ultimate recovery (EUR). The shale in the lower gas layer of the Jiaoshiba anticline is a high-quality reservoir, where the space is dominated by organic pores in kerogen, and the gas content is high. The shale gas wells reveal relatively high initial production and EUR. However, the shale in the upper gas layer of the Jiaoshiba anticline has reservoir space mainly composed of clay mineral pores and organic pores within bitumen, and the gas content is low. In terms of structure, primary gas migration may occur in the upper gas layer, resulting in free gas accumulation in the structural high, where the development effects are generally better than those in the structural low. The lower gas layer in the Pingqiao anticline, is the main interval for shale gas accumulation and development due to the high-quality shale. Under the influence of faults, the efficiency of exploration wells emplaced on top of the anticline is much lower a compared with those in the flanks. The residual synclines close to the Sichuan Basin, including the Baima and Baitao anticlines, are characterized by more recent uplifts, larger area, greater distance from the deep and large faults, and early fracture closure. Therefore, we recommend that the shale gas exploration and development should be carried out preferentially in areas close to the center of the residual synclines, featuring relatively high-pressure coefficient and moderate burial depth.