Abstract: Coal strata in Qinshui Basin are rich in natural gas resources and they have better physical condition for forming medium-large gas pools. Plenty of drilling results show that 3# coal bed and 15# coal bed are major absorbed gas beds, and free gas mainly exists in surrounding rocks of coal-bed fracture and coal-bed roof and floor. Absorbed gas and free gas coexist in the Basin and they are of the same origin, with similar temperature and pressure. So a coexistence system of absorbed gas and free gas comes into being, within which the absorbed gas and the free gas in the coal strata, the free gas in the coal strata and the free gas in surrounding rocks exchange dynamically and form a dynamic equilibrium system. Tectonic evolution and the change of temperature and pressure may lead to asymmetric migration of absorbed gas and free gas. According to the contact relations between coal beds and other rocks, the coexistence system could be classified into three types: the coal rock-roof type, the coal rock-floor type, and the confined type of coal rock. According to the lithology of surrounding rocks, the system could be further classified into the coal rock-sandstone type, the coal rock-mudstone type, and the coal rock-limestone type. Different coal rock-lithology associations vary in gas-bearing features. The paragenesis regularity of absorbed gas and free gas of the same origin in the coal strata south of Qinshui Basin lays theoretical basis for their rational exploitation.

Key words: Qinshui Basin, coal strata, absorbed gas, free gas, formation of pools of the same origin and paragenesis