Coalbed methane enrichment model of low-rank coals in multi-coals superimposed regions: a case study in the middle section of southern Junggar Basin

The Middle Jurassic Xishanyao Formation in the central section of the southern Junggar Basin has substantial amounts of low-ranked coalbed methane (CBM) recourses and is typically characterized by multi superimposed coal seams. To establish the CBM enrichment model, a series of experimental and testing methods were adopted, including coal maceral observation, proximate analysis, low temperature nitrogen adsorption (LTNA), methane carbon isotope determination, porosity/permeability simulation caused by overburden, and gas content testing. The controlling effect of sedimentary environment, geological tectonic, and hydrogeological condition on gas content was analyzed in detail. The results demonstrate that the areas with higher gas content (an average of 8.57 m³/t) are mainly located in the Urumqi River-Santun River (eastern study area), whereas gas content (an average of 3.92 m³/t) in the Manasi River-Taxi River (western study area) is relatively low. Because of the combined effects of strata temperature and pressure, the gas content in coal seam first increases and then decreases with increasing buried depth, and the critical depth of the inflection point ranges from 600 m to 850 m. Affected by the changes in topography and water head height, the direction of groundwater migration is predicted from south to north and from west to east. Based on the gas content variation, the lower and middle parts of the Xishanyao Formation can be divided into three independent coal-bearing gas systems. Within a single gas-bearing system, there is a positive correlation between gas content and strata pressure, and the key mudstone layers separating each gas-bearing system are usually developed at the end of each highstand system tract. The new CBM accumulation model of the multi-coals mixed genetic gas shows that both biological and thermal origins are found in a buried depth interval between 600 m and 850 m, suggesting that the coals with those depths are the CBM enrichment horizons and favorable exploration regions in the middle section of the southern Junggar Basin. An in-depth discussion of the low-rank CBM enrichment model with multi-coal seams in the study region can provide a basis for the optimization of CBM well locations and favorable exploration horizons.