Geochemical characteristics of gas/water in mixed genesis coalbed methane reservoirs and their application for gas production prediction

Abstract

In mixed genesis gas (MGG) areas, varying proportions of microbial and thermogenic gases lead to more intricate geochemical composition distributions, but the critical geochemical differences between MGG and single genesis gas (SGG) areas have been rarely reported. Moreover, coalbed methane (CBM) production can be predicted by hydrogeochemical parameters in SGG areas due to stronger hydrologic sealing associated with higher gas content, but the applicability of this approach in MGG areas remains uncertain. This study analyzes geochemical compositions of CBM/groundwater from the western Fukang area, Southern Junggar Basin, NW China, and discovers that determining CBM genesis in MGG areas cannot rely on a single component, as biogenic CO2 and thermogenic CH4 can be determined in the same gas sample. Unlike SGG areas, since ultra-high total dissolved solids (TDS) negatively affect microbial gas generation, minimum SO42− content is found in regions with a medium TDS, and cumulative gas production (CGP) shows poor correlation with most hydrogeochemical parameters in MGG areas. Our findings demonstrate that components with a single genesis or parameters changing monotonically during gas/water migration can be applied for gas production prediction in MGG areas after clarifying CBM generation-enrichment and groundwater recharge-runoff processes. This study proposes the microbial gas replenishment coefficient (CO2 concentration/heavy hydrocarbon concentration) for predicting CGP in the study area, with CGP generally exceeding 200 × 104 m³ when the coefficient is greater than 6. Our study offers insights into the special geochemical characteristics in MGG areas, and provides a novel approach for predicting CBM production based on geochemical compositions.