Methane Adsorption Characteristics of Marine-Continental Transitional Shales Based on the Experimental Study of Shanxi Formation of the Lower Permian in the Ordos Basin

In order to study the methane adsorption characteristics of the sea–land transition phase, the shale of Shanxi Formation of the Lower Permian in the Ordos Basin is analyzed in terms of organic geochemical characteristics, pore structure, and methane adsorption capacity (MAC). The total organic carbon (TOC) content ranges from 0.78 to 14.40 wt % (6.26 wt % on average). The organic matter belongs to type III kerogen and is in the overmature stage. The main content of these samples is quartz, followed by clay minerals. Under the experimental condition of 60 °C, the Langmuir volume (V_L) and Langmuir pressure (P_L) of the studied shale samples were in the range of 0.85–5.54 cm³/g and 0.95–4.46 MPa, respectively. V_L is positively correlated with micropore volume (PV_micro), micropore specific surface area (SSA_micro), and TOC content in two stages. Correlation will be better at higher TOC (TOC >5%). Only in samples with low TOC content (TOC <5%), the clay mineral content shows a weak positive correlation with V_L. In addition, the MAC showed an inverse correlation with temperature and a positive correlation with pressure. Overall, TOC content, microporous structure, pressure, and temperature are the main factors controlling the MAC of sea–land transition phase shales. Based on the Langmuir model, the functional relationship between MAC and TOC content, clay mineral content, and depth was established by comprehensively considering the factors affecting the MAC of shales. As pressure plays a major role in shallow buried depth, an increase in depth is accompanied by an increase in pressure, which also means that the MAC grows rapidly until it reaches a peak value. However, the role played by temperature gradually strengthens, and the inhibition of methane adsorption by the temperature becomes more and more pronounced as the depth continues to increase, ultimately causing the MAC to continue to decline once it has reached a peak value. The maximum MAC is between 1020 and 1340 m.