Deep shale gas reservoirs are vital for the future development of China’s natural gas industry. Presently, China has achieved preliminary industrial exploitation in this regard, as evidenced by the successful drilling of several high-yielding wells, the delineation of the second gas production growing area with estimated gas-in-place of around one trillion cubic meters and gas production of around ten billion cubic meters, and innovative breakthroughs in research on shale gas enrichment pattern together with exploration and exploitation technologies. These have facilitated the large-scale, effective shale gas production growth in China. Meanwhile, the United States has achieved industrial exploitation of four major deep shale gas blocks, leading to constant rise of the shale gas production from deep reservoirs reaching 313.2×10⁹ m³ in 2021, which accounts for up to 41 % of its total natural gas production. Through systematical summary, we determine six major shale gas enrichment characteristics for deep marine reservoirs: (1) deepwater shelf deposits in a strong reducing environment, which are favorable for organic matter enrichment and preservation; (2) high-quality reservoirs with stable thicknesses and a continuous distribution in large scale; (3) prevalent ultra-high pressure with good sealing capacity of faults; (4) well-developed organic pores and fractures, resulting in favorable reservoir physical properties; (5) superior gas-bearing property of deep shales where shale gas resources are available; and (6) a high proportion of free gas in deep shales, leading to high single-well production in the initial stage. Despite these characteristics as well as advancements in the exploration and exploitation of deep shale gas reservoirs in China, three challenges are posed in the study along with corresponding countermeasures for profitable shale gas extraction from deep reservoirs. Prospects show that deep marine shale gas reservoirs in the Sichuan Basin hold discovered shale gas in place of (3～5)×10¹² m³, suggesting potential gas production growth of (30～50)×10⁹ m³. It is suggested to persist in tackling key problems, and accurately build a “transparent geological body” for shale reservoirs by adhering to the philosophy of maximizing producing reserves. Furthermore, we should focus on the optimal engineering techniques and production systems to maximize single-well estimated ultimate recovery (EUR), to continually reduce exploitation costs and consistently surpass current shale gas production limits, with the ultimate purpose of driving further progress in China’s shale gas industry.

Temperature and pressure conditions of deep shale are beyond experiment range, and the amount of adsorbed gas is difficult to determine. To predict the adsorbed gas content of deep shales under formation conditions, isothermal adsorption experiments and model building were conducted on shale samples from Longmaxi Formation in China. A temperature-dependent adsorption model based on the Langmuir equation is proposed, which can be well-fitted by observed isotherms with a high correlation coefficient. Based on the fitted parameters at 303.15 K, the isothermal adsorption curves at 333.15 K, 363.15 K, and 393.15 K are predicted, showing a good agreement with experimental curves available. Compared with previous prediction methods, the biggest advantage of the proposed method is that it can be carried out only based on one-time isothermal adsorption experiment. Based on the predictions, the downward trend of the excess adsorption curves will slow down under high temperature and pressure conditions, and when the pressure reaches a certain level (> 80 MPa), the temperature has little effect on the excess adsorption capacity. While for absolute adsorption, the gas adsorption reaches saturation much slowly at high temperature, it can also reach saturation under formation pressure. Under the burial depth of marine shale, temperature plays a major role in controlling the adsorbed gas, resulting in the decrease of adsorbed gas content in deep shale, and its ratio will further decrease as the depth increases.