Review on Deep Coal Measure Gas Accumulation and Its Geological Effects of Efficient Coproduction
Dameng Liu, Zheng Zhao, Yidong Cai, Shaobo Xu, Feng Qiu, Fengrui Sun
Abstract
Coal measure gas (CMG), as a vital component of unconventional natural gas, is a clean energy source with substantial reserves. While its integrated exploration and development potential is considerable, it also presents significant challenges. For deep CMG reservoirs with better resource potential and coproduction prospects, key geological challenges and research hotspots have included the distinctive pore structure and gas-bearing characteristics, the gas genesis, migration patterns, and accumulation mechanisms of CMG, as well as the orderly and efficient coexploitation models. These aspects have been systematically reviewed in this study. First, by clarifying the criteria for defining deep CMG, the differences in micropore structure and gas-bearing capacity among different types of deep CMG reservoirs were compared. Second, based on the geochemical characteristics of gases in various types of coal measure reservoirs, the origin, sources, migration, and accumulation mechanisms of CMG were elucidated. Finally, the geological effects of efficient coproduction of deep CMG were discussed. The results indicate that within deep coal measure composite reservoirs, tight sandstones, mud shales, and coal seams are predominantly characterized by macropores, mesopores, and micropores, with pore connectivity and organic matter content increasing in that order. A method for calculating the in situ gas content of deep coalbed methane (CBM) and shale gas (SG) was established by evaluating the density and volume of the adsorbed phase. Within the same coal measure strata, the differences in migration behavior among gas components result in CBM generally having lower CH 4 and N 2 contents but higher CO 2 and C 2+ contents compared to SG and tight sandstone gas (TSG). In addition, CBM exhibits higher carbon isotope abundances in both heavy hydrocarbons and CO 2 . Three main gas migration modes have been identified: upward migration along faults, lateral migration through sand bodies, and migration facilitated by vertical fractures. The fluid pressure system serves as the fundamental geological unit for multilayer coproduction, where parallel and serial production layer combinations offer multiple technological pathways through tailored reservoir stimulation strategies. Furthermore, promising directions for future research include constructing the in situ gas content predictive models of deep coal measure reservoirs, quantitatively analyzing fluid interactions between organic and inorganic reservoir components, and developing integrated CO 2 “enhanced permeability-displacement-storage” technologies in deep coal measure reservoirs.