Basic characteristics and key factors of gas accumulation in Yanchuannan coalbed gas field

The basic characteristics of the Yanchuannan coalbed gas field were analyzed in a case study of the No.2 coalbed in the Shanxi Formation. The gas field is featured by simple structure, stable thickness, low porosity, low permeability, low pressure gradient, high hydrocarbon potential, high gas content, and high thermal maturity. The relationships between faults and gas content, formation water, formation pressure gradient, the relationships between local structure and cleat development, coalbed gas content, and the characteristics of methane carbon isotopes were studied. Conclusions were drawn as follows. (1) Faults significantly controlled the enrichment of coalbed gas. Coalbed gas content decreased in fracture belts. (2) The central fault belt controlled the pressure gradient of coalbed and the distribution of formation water. (3) The coalbed gas was mixed self-sourced gas and thermogenic gas. Positive structure with certain amplitude is helpful for the enrichment of coalbed gas.