The geochemical characteristics, origin, migration and accumulation modes of deep coal-measure gas in the west of Linxing block at the eastern margin of Ordos Basin

The deep coal-measure gas, including coalbed methane (CBM), tight gas (TG) and shale gas (SG), has attracted widely attention because of its great potential for natural gas development and multiple benefits. To determine the gas origin, migration and evolution in the west of Linxing block, 56 coal-measure gas samples from different horizons and depth were collected. The gas components and stable isotope compositions were measured, and the results show that methane is the main composition of coal-measure gas, distributed between 78.83% and 96.67%. The average carbon dioxide and nitrogen concentrations are 1.99 and 1.56 vol %, respectively. Coalmeasure gas has apparent dry and wet gas coexist characteristics, with drying coefficient values ranging from 0.821 to 0.989. The 813C(CH4) and 8D(CH4) are between -41.81%o and -26.65%o and -224.1%o to -180.5%o, with an average value of -36.23%o and -206.5%o, respectively. The coal-measure gas in the study area is mainly coal-type gas mixed with a small amount of oil-type gas. The gas origin is mainly thermogenic, and in Benxi Formation and Taiyuan Formation there is inorganic CO2 due to the magma intrusion. The vertical and lateral migration processes of natural gas were observed controlled by dynamic isotope fractionation effect and migration fractionation effect. Based above results, two types of reservoir forming modes of natural gas, i.e., "near-source reservoir-forming mode" and "lower-source rock generation, upper-reservoir storage mode" were established. The process of gas accumulation has experienced four evolution stages. The comparative analysis of our data with that of other previously reported study fields, permit a more robust evaluation of the geochemistry, origin, migration, and evolution of deep coal measures gas in this area.