Geochemical characteristics of produced fluids from CBM wells and their indicative significance for gas accumulation in Daning-Jixian block, Ordos Basin

The Daning-Jixian block, the eastern edge of the Ordos Basin, is one of the most potential areas for CO2 geological storage, enhanced coalbed methane recovery (ECBM) exploration and production in China in recent decades. The ionic composition and total dissolved solids (TDS) of the produced water, coal organic matter maturity, molecular composition and carbon isotope characteristics of the produced gas were utilized to analyze the hydrogeological condition, CBM generation and migration characteristics in this area. The CBM enrichment patterns and the geological impacts on gas well production characteristics were revealed. The optimal area for CBM development and CO2 geological storage in the study area were also proposed. Dominated by the Xueguan reverse fault zone, the hydraulic unit in this area can be divided into two parts (i.e., the recharge-runoff zone in the east and the weak runoff-stagnation zone in the west). The thermogenic gas is dominating CBM genesis in this area. Secondary biogenic gas replenishment is only distributed in the eastern margin area, where the delta C-13(1) value is less than the thermal simulation results as an influence of hydrodynamic fractionation. Finally, two models of CBM formation and accumulation were proposed, 1) thermogenic CBM migrated by hydrodynamic and resorbed for preservation at impermeable fault boundaries; 2) thermogenic CBM trapped by fault and accumulated by hydrodynamic in slope zone. The gas production performance, generally increased from east to west, is mainly dominated by hydrogeological conditions. Generally, the west side of the fault zone is the enrichment and high-yield area for ECBM development and CO2 geological storage in the study area.