Differences in accumulation patterns of low-rank coalbed methane in China under the control of the first coalification jump

The considerable variations in coalbed methane (CBM) production in different low-rank basins have attracted widespread attention. Moreover, the occurrence of the “first coalification jump” (FCJ) has increased the uncertainty in the accumulation law. Following systematic sample collections of different ranks and lithotypes, the variation of material composition, methane source, reservoir properties, occurrence state, and enrichment rule under FCJ control was studied. Low-rank CBM is mainly obtained from microbial gases abundantly found in lignite and subbituminous coal. The microbial methane of lignite and sub-bituminous coal accounted for about 86.46% and 77.83%, respectively. As its maturity increases, thermogenic gas can provide a supplement to methane in subbituminous coal. Lignite has a low degree of coalification and retains a large amount of primary plant tissue structure. It is characterized by high porosity and a high proportion of macropores. The low adsorption capacity and high porosity result in a substantially higher proportion of free gas in lignite reservoirs than in subbituminous coal reservoirs. The free gases in lignite reservoirs have stronger mobility and are easily enriched in the structural culmination. Owing to coalification, the subbituminous coal’s porosity is considerably reduced. Thus, micropores and transition pores dominate subbituminous coal. The methane adsorbed in subbituminous coal is primarily concentrated in the stagnant area in the syncline and mixed “thermogenic/microbial” genetic areas. It is difficult to use a unified model to explain the gas distribution. Therefore, taking typical types of basins (foreland, craton, and fault subsidence basins) as examples, we summarize the differences in low-rank CBM enrichment patterns under FCJ control and provide a basis for detailed exploration and efficient development of CBM.