Numerical modeling of deep coalbed methane accumulation in the central-eastern Ordos Basin, China

Deep coalbed methane (CBM) has become one of the most significant potential sources of natural gas in China. However, the exploration and development of deep CBM in China is still in an initial stage, and its accumulation-forming characteristics require further study. Therefore, taking the No. 8 deep coal seam in the central-eastern region of Ordos Basin as an example, this study investigated the geologic characteristics of CBM accumulations to establish a numerical model. The evolution of the burial and accumulation of CBM in the area was reconstructed. The modeling results suggest that the No. 8 coal seam experienced continuous subsidence from the Late Cretaceous to the Triassic, alternating subsidence and uplift during the Jurassic, rapid burial throughout the Early Cretaceous, and continuous uplift since the Late Cretaceous. The coal reached its maximum maturity at the end of the Early Cretaceous. Furthermore, CBM generation in the region was divided into four stages of thermal events—biogenic and early thermogenic gas, cracking of light oil into gas, cracking of the remaining kerogen into gas, and hydrocarbon generation ceasing—which accelerated coal maturity and generation. The adsorption capacity presented an overall declining trend prior to the end of the Cretaceous, followed by a rapid increase since the Late Cretaceous. As for adsorption mass evolution, the CBM successively underwent unsaturated minor adsorption, unsaturated rapid-rising adsorption, saturated decreasing adsorption, and saturated rising adsorption. The in-situ gas mass was found to be controlled by a combination of generation, adsorption, and expulsion of hydrocarbons, with its present-day value being 9–29 × 10 4 t/km 2 and the corresponding gas volume per ton of coal being 12–28 m 3 /t. Moreover, free gas evolution initially showed an increasing trend, followed by a decline, ultimately accounting for 11%–28% of the total gas content.