Research On Migration System Of Free Gas Of Coal-bearing Strata In Qinshui Basin

The low yields of single well in southern area of Qinshui basin led to unsatisfactoryeconomic benefits. In order to realize co-production of coal bed methan（eCBM）and free gas,it was necessary to make further researches on free gas. Based on existing studies, core, welllogging and other laboratory analysis data, the paper made analyses on accumulationconditions of natural gas in Qinshui basin firstly. Then taking Zhengzhuang-Fanzhuang Blockas an example, the paper discussed the influences of sedimentary and tectonic migrationbodies including sandstone body, fault, unconformity and collapse columns on gas migration.Based on the symbiosis and unidirectional conversion of CBM and coal roof tight sandstone（CRTS）gas reservoirs, this paper proposed symbiosis accumulation microcosmic system.After these researches, the paper established gas accumulation evolution models on the basisof regional tectonic evolution and predicted favorable commingled zone. The processesobtained were as follows:1. There were several kinds of gas source rock such as coal, limestone, mudstone inpalaeozoic strata and coal was dominant. Reservoirs including coal, carbonate and clastic rockprovided accumulation space for gas. Source, reservoir and cap rocks were superposed oneach other and made up several sets of favorable accumulation assemblages. The matchrelationship between trap and secondary hydrocarbon generation process was good. Theseconditions were beneficial to multi-type gas reservoirs accumulation.2. The lithology of clastic reservoir of coal-bearing strata in Zheng zhuang-FanzhuangBlock was mainly lithic quartz sandstone and quartz sandstone. Both of them had lowporosity and permeability and belonged to tight sandstone. According to marker bed includingcoal and limestone, Shanxi and Taiyuan Formation could be divided into three and seven sandsets. Each sand set only distributed local well field and had poor lateral continuity, whichwent against gas large-scale lateral migration.3. CRTS gas was mainly from coal adsorbed gas and both of them were in a system.When temperature and pressure changes led to decrease of coal adsorption capacity, someadsorbed gas changed into free gas through microcosmic pores.Combined with burial and thermal evolution history of Qinnan area, the paper made discussions on the conversion ofadsorbed gas into free gas in Guxiang block during Paleogene tectonic uplifting. Baed on themigration mechanism of gas in coal, the pore of coal was divided according to the theory ofmolecular movement.4. Among tectonic conducting bodies, Sitou fault and collapse column groups formedafter secondary hydrocarbon generation process. Gas contents near these structures were low,which explained that gas could transport through them. The space structure and lithologyjuxtaposition of unconformity in the top of Ordovician of Qinnan Area determined gasaccumulation below unconformity.5. Combined with regional tectonic evolution, the paper summed up gas accumulationevolution model. Based on the symbiosis between CBM and CRTS gas reservoirs, favorablecommingled zone was predicted.