Research progress of symbiotic accumulation of coal measure gas in China

Coal measure gas (CMG), referring to natural gas stored in coal measures, as well as its existence, exploration and production, has been highly concerned recently in natural gas sector in China, and pilot tests of which have been succeeded with some achievements. To provide new geological references, this paper discussed the research progress in CMG co-existence and pooling factors in the respects of the tightening mechanism of coal measure sandstone reservoirs, CMG co-existence and gas pooling assemblages, the superimposed CMG system, and so on. The following findings were obtained. (1) The particularities of CMG geological conditions are shown in three aspects: First, the occurrence and reservoir lithology of CMG are diverse, and the accumulation of sandstone gas in coal measures may be different from that of conventional sandstone gas. Second, the sedimentary environment of coal measures causes the frequent and thin interbedding with various lithologic reservoirs, strong cyclicity and complex gas-water relationship, and the Surat-type CMG is worthy of attention. And third, the sandstone reservoirs in coal measures are embedded in the wide overlying mudstones, the special source-reservoir match and composite gas reservoirs need to develop adaptive co-exploration and co-production technology. (2) The coal measures are rich in organic matters, and a large number of the organic acids are formed during gas generation from the source rocks, which are important factors for the densification of the sandstone reservoirs in coal measures. And the sandstone reservoirs in coal measures may be characteristic of self-generating and self-storing gas and adsorption to a certain extent, and their physical properties can be improved by organic detritus in the reservoirs. (3) The sedimentology of coal measures defines four generalities for favorable CMG accumulation, but effect of the gas-generating intensity, gas migration system, formation fluid energy and effective regional caprock thickness to the accumulation are variable. The natural gas generated in source rocks is re-allocated with a special migration system in a complex source-reservoir system, which is an important basis of CMG accumulation. (4) The superimposition of the gas-bearing system is one of the leading problems in CMG research. In recent years, the logging response identification technology of key strata and the superposition identification method of the gas-bearing system have been further developed. It is found that there are three typical types of fluid pressure curves in coal measures. At the same time, the CMG co-accumulation in the whole sense requires a certain threshold depth. Based on the progress above, the theory of CMG accumulation has been improved and deepened, which is helpful in providing more pertinent suggestions for the development of the joint CMG exploration and production technology.