Geological and hydrogeological controls on the accumulation of coalbed methane in the Weibei field, southeastern Ordos Basin

Abstract

Commercial exploration and production of coalbed methane (CBM) in the Weibei field, Ordos Basin, China has rapidly increased since 2010. The Weibei field has become one of the most productive CBM areas in China. However, relatively few studies have investigated the migration of gas and water in the coal reservoir and their controls on the gas accumulation. This study conducts stable isotope analyses and quality tests for groundwater samples, discusses the relationships between the fluid flow pathways and tectonics, and concludes by discussing the geological and hydrological controls on potential gas accumulation in the Weibei field.The coalbed groundwaters contain primarily sodium and bicarbonate and are effectively devoid of sulfate, calcium and magnesium. The groundwaters are typically freshwater, with total dissolved solids (TDS) values ranging from 814 to 2657mg/L. Differences in hydrogeology and structural geology divide the study area into four gas domains. In the northern Hancheng area, the predominant northwest flow of groundwater has resulted in higher gas content in the west (>12m3/t) than in the east (8–12m3/t), even though the coals in the east have high thermal maturity (2.1%–2.3% Ro). The area with the highest gas content (>16m3/t) is in the region near the downthrown side of the Xuefeng–Nan Thrust Fault in the northern Hancheng area, and the fault forms a barrier to the northwestward flow of groundwater. The area with the lowest gas content (<4m3/t) is in the southwest Heyang area, where there is no entrapment mechanism and the gas has been flushed out of the coals due to a reduction of hydrostatic pressure and active groundwater flow from the east. Structural and hydrodynamic mechanisms, especially the intensity of the hydrodynamic activity and the groundwater flow pathways, are important for gas accumulation in the Weibei field.