Following the marine shale gas, the continental shale gas also presents great exploration potential in China. However, few studies focused on the high-over-mature stage (HOMS) continental shale gas in terms of mineral composition, pore structure, and gas-bearing properties, as well as their comparisons with successfully marine shale gas in the same mature stage, which restricts its better potential evaluation and “sweet spots” suggestion. In this paper, the lower Cretaceous Shahezi Formation in Xujiaweizi fault depression, and the lower Silurian Longmaxi Formation in Pengshui Area, were selected as the target areas for the study of continental and marine shale gas in the HOMS. By means of thin sections, scanning electron microscopy, nitrogen adsorption, high-pressure mercury injection, and methane isothermal adsorption, a systematic comparison was carried out to reveal the differences in reservoir characterizes and gas-bearing properties, and summarize their controlling factors and “sweet spots” distribution. The type and distribution of organic matter (OM) dominate the differences between marine and continental shale in the HOMS. Compared with Longmaxi marine shale with OM of type I–II1 kerogen, there are fewer biogenic siliceous, lower proportion of organic pores, and more inorganic pores (including dominant clay related pores, quartz intergranular pores, and dissolution pores) in Shahezi continental shale with OM of type III–II2 kerogen, corresponding to larger average pore size and smaller specific surface area (SSA). Quartz content, porosity, and SSA are mainly controlled by TOC for marine shale, while those are controlled by deposition and burial depth for continental shale. Plain marsh shale, adjacent to a coal seam, is favorable for the development of cemented quartz grains and the limited conversion of mixed illite/smectite (I/S) to illite, which can effectively restrain the impact of burial depth on the proportion of intergranular and dissolution pores, porosity, and pore size for continental shale. I/S, rather than OM, mainly contributes to the methane adsorption capacity in continental shale, of which the adsorbed gas content is obviously lower than that of marine shale. Due to the higher pressure coefficient (PC), porosity and total gas content of marine shale still increase with increasing the burial depth, while those of Shahezi continental shale decrease rapidly due to the slight increase of PC with depth. Some exploration potential in organic-rich thick shales should be expected in the Shahezi Formation, which are developed in plain marsh or delta front with burial depths in the range of 2600–3600 m.

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Numerous large gas fields with large proven reserves occur in the deep formations of Songliao Basin, NE China. However, some challenges remain as follows: (1) the main source rock is poorly studied and (2) the origins and genetic types of the natural gases are controversial. In this study, these problems can be addressed by the research of the paleoenvironments during shale deposition, the organic matter sources, the source rock potential, the gas origin and genetic type, and gas–source correlations. The redox conditions of the water column during the Huoshiling to Yingcheng depositions were transitional (suboxidizing to subreducing) environments, and the Huoshiling deposition corresponded to reducing conditions. The paleosalinity of water was brackish to fresh from Huoshiling to Yingcheng depositions. The organic matters of the Huoshiling–Yingcheng shales mainly originated from terrigenous and/or lacustrine materials. The shales are evaluated as the fair to good source rock potential, and the Huoshiling is the best. The gases in the Huoshiling and Shahezi were mainly organic and thermogenic origins, with some inorganic origins. The gases of organic origins are mainly the mixed sources of coal-derived and oil-associated gases and are predominant products of secondary cracking, suggesting the well gas exploration potential in deep formations of Songliao Basin. The gases in the Shahezi formation are predominantly sourced by the Shahezi shales, indicating an approaching-source accumulation.

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