In order to study the controlled factors and variation of gas content in deep low-rank coal reservoirs, taking No.4 coal of Jurassic Yan’an Formation in Huanglong coalfield as an example, collects the production data and prepares the coal samples. Carrying out the coal rock and coal quality test and multi-temperature methane isothermal adsorption experiment to explore the influence of different temperature and pressure conditions on coal adsorption capacity, and analyzes the variation characteristics of gas content with burial depth combined with geological conditions. The results show that the sedimentary, structural and hydrogeological conditions have a certain influence on the formation and preservation of coalbed methane. The gas content increases with the increase of coalification degree and vitrinite content, and decreases with the increase of inertinite content, ash yield and volatile yield. With the increase of moisture, the gas content increased first and then decreased. When the pressure is less than 4 MPa, the adsorption capacity increases about 4-4.7m³/t with the increase of pressure, and the increasing trend slows down when the pressure is greater than 4 MPa. The adsorption capacity decreased more obviously with the increase of temperature when the temperature increased from 25°C to 35°C. Based on the prediction model of saturated adsorption capacity of deep coal reservoirs, it is found that there is a gas content critical conversion depth 800-900 m. The gas content shows a process of rapid increase (900 m) with the increase of burial depth. Below 800 m, the positive effect of reservoir pressure is dominant, and the gas content increases with burial depth. Above 900 m, the negative effect of reservoir temperature is dominant, and the gas content decreases with burial depth. The study results provide a theoretical basis for the development of deep low-rank coalbed methane resources.

Gas content is a key parameter for coalbed methane (CBM) resource evaluation and production potential assessment. The accurate prediction of gas content in deep coal reservoirs is more challenging than in shallow reservoirs because both adsorbed gas and free gas must be considered. In this study, continuous logging data from the deep No. 8 coal seam in the Daning–Jixian block were integrated with measured gas content-related parameters to construct a quantitative logging interpretation framework for deep CBM reservoirs. First, the relationships between logging parameters and Langmuir volume, Langmuir pressure, porosity, and water saturation were analyzed. Then, multiple linear regression models were established to predict these key intermediate parameters, which were subsequently used to calculate adsorbed gas, free gas, and total gas content. The model was further applied to the DJ 52 well area for spatial prediction. The results show that the total gas content ranges from 24.49 to 32.90 cm³/g. The high-gas-content area is mainly located in the north-central part of the study area, whereas the southern part shows relatively lower gas content, partly due to the influence of coal seam thickness and reservoir property heterogeneity. The proposed method provides an interpretable and practical approach for deep CBM gas content evaluation using logging data.