Comparison of main reservoir characteristics between deep coal-rock gas of the No.8 coal seam of the Upper Paleozoic Benxi Formation and tight sand gas reservoirs, Ordos Basin

The Changqing Oilfield Company of PetroChina has achieved significant advances in exploring deep coal-rock gas within the Ordos Basin, discovering that methane (CH₄) largely occurs in coals as both adsorbed and free gas, which contributes to the formation of self-sourced coal-rock gas reservoirs. In this study, we comprehensively investigate the characteristics of coal-rock gas reservoirs represented by the No.8 coal seam of the Benxi Formation. The results indicate that the Ordos Basin contains 10 coal seams from top to bottom. The Nos. 5 and 8 coal seams, among others,predominate, exhibiting stable distribution and great thickness. Compared to shallow coalbed methane (CBM), the deep coal-rock gas occurring in the No.8 coal seam of the Paleozoic Benxi Formation is characterized by high formation pressure, high formation temperature, high gas content, high gas saturation, and high free gas content. Specifically,these coal-rock gas reservoirs exhibit average formation pressure ranging from 22 to 35 MPa, suggesting high formation pressure. However, their formation pressure coefficients vary between 1.0 and 1.1, indicating a normal pressure system. The coal-rock gas reservoirs present average formation temperatures ranging from 67 to 92 ℃ and an average geothermal gradient of 2.94 ℃/km, suggesting normal geothermal gradients. The composition of coal-rock gas in these reservoirs manifests an average CH₄ content of 96.92 %, average C₂H₆ content of 0.61 %, average CO₂ content of 0.85 %, and average N₂ content of 1.47 %, establishing these reservoirs of dry gas type. The formation water in these reservoirs has total dissolved solids (TDS) concentrations ranging from 41644 to 89776 mg/L (average: 62228 mg/L), indicating primary CaCl₂-rich formation water. Unaffected by shallow groundwater, the coal-rock gas reservoirs exhibited high gas production rates in tests and stable production at a low pressure drop rate. A systematic comparison between deep coal-rock gas reservoirs in the No.8 coal seam of the Benxi Formation and tight-sand gas reservoirs in the 8^th member of the Xiashihezi Formation (He 8 Member) and the 1st member of the Shanxi Formation(Shan 1 Member) in the Sulige gas field reveals similarities in major reservoir characteristics, including reservoir type, reservoir pressure and temperature, gas composition, formation water properties, and production capacity. These similarities will provide a valuable guide for future large-scale exploitation and production capacity construction of deep coal-rock gas.