Coupling Adsorption Mechanisms and Geological Evolution: A Case Study of Coalbed Methane Accumulation in Fukang Area, Junggar Basin, China

The unclear coupled temperature–pressure control mechanism of deep coalbed methane (CBM) occurrence and the poorly understood transformation law of adsorbed and free gas during geological history have severely restricted the accurate evaluation of CBM resources in the southern margin of the Junggar Basin. Taking the No. 42 main coal seam in the Fukang mining area as the research object, this study combines isothermal adsorption experiments, thermodynamic analysis, and basin numerical simulation to identify the methane adsorption characteristics of coal and their main controlling factors, establishes a CBM content prediction model coupled with coal maceral composition and temperature–pressure conditions, and reconstructs the complete CBM accumulation evolution process over geological time. Isothermal adsorption experiments were conducted at 30 °C over 0–8 MPa (6 pressure points) for all seven core samples, and variable-temperature adsorption experiments were performed on representative sample No. 42-5 at 30, 50, 70, and 90 °C over 0–22 MPa (10 pressure points per temperature). The results show that the average equilibrium moisture-based Langmuir volume of coal samples in the study area is 22.36 m3/t, and the adsorption capacity is significantly positively correlated with fixed carbon content. Pressure dominates the positive adsorption effect in the low-temperature and low-pressure range, while temperature dominates the negative adsorption effect in the high-temperature and high-pressure range. The total gas content first increases and then decreases with depth, reaching a peak of 12.56 m3/t at 900 m; free gas accounts for a larger proportion than adsorbed gas below 3600 m. The central-southern main area of the study area, with a gas content of 11–13 m3/t, is a favorable exploration target. The accumulation evolution four stages: pure adsorption, free gas occurrence, excessive gas expulsion, and uplift adjustment. The proportion of free gas reached its peak during the maximum burial period in the Late Cretaceous. After the Paleogene uplift, hydrocarbon generation ceased, and the total gas content remained at 13.16 m3/t. The coupled research method of “experimental determination–model prediction–evolution simulation” established in this paper realizes cross-validation of CBM content through multiple methods, and can provide theoretical support for deep low-rank CBM accumulation research and resource evaluation.