Geological and hydrological controls on the accumulation of coalbed methane within the No. 3 coal seam of the southern Qinshui Basin

Since 2006, the Zhengzhuang (ZZ) block has been part of a commercial coalbed methane (CBM) production area in the southern Qinshui Basin, which has become one of the most productive CBM areas in China. However, hydrogeological study of the migration gas and groundwater of the ZZ block has not been systematically launched. This study focuses on geochemical analysis, including quality analysis and stable isotope analyses for the water from the CBM production wells, discusses the groundwater flow pathways and their influencing factors, and evaluates the geological and hydrological controls on gas accumulation in the study area. The results show that the produced waters produced with total dissolved solids (TDS) values of 152.8-5389.49 mg/L are typically fresh water and brackish water. The water type is primarily bicarbonate and the dissolved ions are effectively devoid of sulfate, calcium and magnesium. With the low desulfurization coefficients of 0.24-5.63, the hydrochemical condition is conductive to CBM preservation. The predominant flow of groundwater originates from the southwestern source, which flows to the north basin center and then shifts to the east deep graben due to the drastic structural variation. Four hydrodynamic zones were distinguished through the differences in hydrodynamic conditions. In the southwest Zone I and the central Zone II, the gas contents are relatively low (13.17-24.53 m3/t) despite the high thermal maturity (Ro,max = 3.31%-3.97%). However, the stagnant Zone III in the north and the deep stagnant Zone IV in the southeast show comparatively high gas content (16.5-31.44 m3/t). The complex geostructures act synergistically with the intense hydrodynamic activities for gas accumulation. Three CBM enrichment modes were distinguished: hydrodynamic trapping at structure lows with high hydrostatic pressure; conventional trap of impermeable boundary; and hydraulic flushing and gas loss through the fault. These investigations may serve for CBM exploration and development in the study area.