Geology of coalbed methane reservoirs in the Southeast Qinshui Basin of China

The No. 15 coal seam of the Pennsylvanian Taiyuan Formation and the No. 3 coal seam of the Permian Shanxi Formation in the Southeast Qinshui Basin are relatively thick (> 3 m), laterally continuous, and are of anthracite rank (Ro = 2.2-4.5%). The depth of these coal seams is generally between 200 and 700 m. A tectonic-thermal event during the Jurassic to Cretaceous Yanshanian Orogeny controlled coalification and thermogenic hydrocarbon generation. Because the generation of late-stage bacterial gas, coalbed methane in the Southeast Qinshui Basin occupies two zones, which are the mixed thermogenic and late-stage bacterial gas zone near the basin margins, and the dominantly thermogenic gas zone in the structurally deep part of the basin. Basin hydrodynamics causes low gas content in the shallow subsurface (< 20 m3/t) and high gas content at the deep part of basin (> 20 m3/t). Because cleats are rare and are mostly filled with calcite, the permeability of the coalbed methane reservoir is controlled by shear fractures. The recharge and flow of groundwater not only control gas content, but also control reservoir pressure. Abnormally high pressure reservoir in the deep stagnant zone is a favorable area for coalbed methane exploration and development. The current coalbed methane wells are generally completed in the deep stagnant zone.