Comparison of Marine and Continental Shale Gas Reservoirs and Their Gas-Bearing Properties in China: The Examples of the Longmaxi and Shahezi Shales

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–II1kerogen, 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–II2kerogen, 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.