Gas-water distribution characteristics and control factors of Maokou Formation in Shunan (south Sichuan) area

Abstract：Located in Sichuan basin, Shunan area has an exploration area of 5.03×104 km2. The Maokou Formation in this area is composed of carbonate rock with low porosity and low permeability. Based on a summary and analysis of the geological, drilling and production data, this paper deals with the complexity of gas-water distribution. There is no uniform gas-water interface in the same anticlinal structure, and many gas-water interfaces may exist in one fracture-cave system; the exploitation of some fracture-cave systems are characterized by the process from producing pure gas through producing gas and water to producing pure gas again. Based on an analysis of the reservoir characteristics, the exploitation dynamic state and the evolution of the formation water, the authors formulated three gas-water distribution models, i.e., self-contained gas reservoir of structural-trap style in Songjiachang, anticline-fracture-cave composite trap pattern in Linantong and fracture-cave trap pattern in Hebaochang. In combination with geological background, it is thought that the hydrodynamic condition of different karst areas is the decisive factor of gas-water distribution, that the tectonic movement controls the regularity of the gas-water distribution, and that fluid potential indicates the activity of the water body.