Reservoir characteristics and its physical origin of shale gas in coal measure in the Lower Permian Shanxi Formation in Huainan coal field

Shale gas in coal measures is one of the important types, and the shale in coal measures holds favorable ability of gas accumulation and enrichment potential. It has important significance of research for shale reservoir characteristics in coal measure strata to confirm the coexistence and co-exploration of unconventional gas. As a case of the Lower Permian Shanxi Formation in Huainan Coal Field, the results show that the thickness of the Shanxi Formation shale is about 72m, and the thickness of mudstone accounts for about 25%. The quartz mineral content varies from 29.2% to 36.7% in the mudstone with terrigenous origin. Shanxi Formation contains 0.37% to 8.87% of TOC. The organic matter is moderate mature with RO of 0.83%-1.32% and dominated by type Ⅱ2-kerogen. Porosity of the mudstone measured by Hg intrusion ranges from 0.87% to 4.29%, and pulse permeability ranges from 0.31×10-3μm2 to 0.91×10-3μm2 with an average of 0.54×10-3μm2. The linear relationship of porosity and pulse permeability is generally better. The development of micro-fracture observed in the mudstone includes micro-fracture extended at the edge of quartz grain, particle internal parallel linear fracture and bent irregular fracture of the internal clay minerals. The micro-porosity dominated by inter-granular and intra-granular pores, with pore size ranging from 2μm to 50nm. The promoting effect of TOC and thermal evolution to micro pore is not obvious, but the mixed-layer minerals of illite and smectite is the main contributor to pores with a large proportion in the composition of clay mineral content. Different types of sedimentary micro-facies could develop different lithic facies, and it is the sedimentary basis of the physical properties of shale gas reservoir from Shanxi Formation in the study area. ©, 2015, Science Press. All right reserved.