Enrichment factors and strategic target areas of medium- to low-rank coal-rock gas/coalbed methane in Xinjiang, China

To solve the problems of the poor understanding of enrichment factors, unclear exploration targets, and challenging selection of favorable areas for medium- and low-rank coal-rock gas/coalbed methane resources in Xinjiang, this paper, based on the coal-measure whole petroleum system theory, examines the main controlling factors of coal rock gas/coalbed methane enrichment and further discusses the exploration targets and favorable areas, through extensive coal petrology and coal quality analysis, gas content measurements, and well-seismic data interpretation. The study shows that the low thermal maturity, with vitrinite reflectance ( R o ) commonly below 0.8%, is the primary reason why the actual gas content is significantly lower than the hydrocarbon generation capacity in basins such as the Junggar Basin. In addition, the coal-forming age and maceral composition characteristics also exert important controls on gas content and storage capacity. Accordingly, two exploration strategies are proposed: seeking relatively higher coal ranks and elevated geothermal gradients, and targeting older (especially Paleozoic) coal-measure strata. Further, five major exploration targets are identified: (1) post-coalification high geothermal gradient zone; (2) early deep burial and late uplift tectonic belt; (3) Upper Paleozoic coal measures with high thermal maturity; (4) coal seams with high vitrinite content; and (5) coordinated development area of the coal-measure whole petroleum system. Depending on the distribution of coal-measure strata, structural characteristics, and coal rock properties of various basins in Xinjiang, three practical exploration areas are defined: the Southern Piedmont Structural Belt and stable central region of the Junggar Basin, the Wenjisang Structural Belt and the Hongtai Slope of the Tuha Basin, and the Northern Kuqa Structural Belt of the Tarim Basin. Additionally, six peripheral strategic replacement areas are identified: Heshituoluogai, Yili, Yanqi, Santanghu, Kupu and Fujin basins. The study provides a scientific basis for selecting favorable zones to advance the large-scale exploration and effective development of coal-rock gas/coalbed methane resources in Xinjiang.