Distribution and geological controls on gas-bearing section of coal measure gases in Qinshui Basin

To study the characteristics of coupled accumulation and determine the geological control effects on coal measures gases (CMGs), coal measures of the Carboniferous-Permian Taiyuan and Shanxi formations at Qinshui Basin were selected as the target formations. Various methods including field data collection, field/lab measurements, and theoretical analyses were applied to described the spatial superposition of CMG reservoirs, quantitatively determine the multi-scale pore system, identify the spatial development pattern of CMGs gas-bearing section, and clarify the types of CMGs coupled accumulation and geological controls. Results show that coal measures deposited at the unique marine-terrigenous depositional environment was characterized by lithological diversity and cyclic superposition which was served as the potential basics for coupled accumulation and co-exploration/development of CMGs. The abundance of the organic matter gradually increased from the “inorganic reservoir” to “organic reservoir”, forming a continuous rock sequence without a natural boundary in target coal measures. Gas-bearing sections was characterized by a vertically intermittent distribution and the dominant coupled accumulation assemblages can be subdivided into: shale gas dominated coupled accumulation type, CBM dominated coupled accumulation type, and multiple CMGs coupled accumulation type. Obviously, the effective gas-bearing sections need the appropriate combination of source, reservoir and cap. Coal reservoirs directly controlled the distribution of effective gas-bearing section. Reservoir burial conditions restricted the possibility of an effective gas-bearing section. Moreover, coals were believed to be more favorable for independent coalbed methane accumulation, whereas shale gas and sandstone gas required extremely strict geological, spatial and temporal conditions. Additionally, both organic-inorganic fabric and physical characteristics limited the potential of effective gas-bearing sections. Inspired by the findings of this study, further studies on the coupled accumulation mechanism and the co-exploration evaluation system of CMGs should be continuously conducted.