Carbonates of Middle Permian Qixia Formation and Maokou Formation had been gas-producing reservoir mainly in southern Sichuan Basin in the last century. It had been accepted that these reservoirs were fractured limestones, resulting in low gas ratio(5.80%) and failure to discover uncompartmentalized gas fields. Based on the investigations and analysis of 6 outcrops, 10 exploration wells, 500 thin sections, and other data of isotope, minor element, rare earth element, fluid inclusion and isotopic age get from 80 samples, three findings are presented as follows: (1) Dolostones with intercrystaline pores, dissolution-enlarged intercrystaline pores and vugs are widespread in Qixia Formation and Maokou Formation; (2) Dolostone reservoirs was predominantly constrained by shoal water carbonate sediments, and the dissolution in freshwater during early diagenetic processes, and the tectonic and hydrothermal events during burial; (3) The dolostone reservoirs are mainly distributed in the 2nd member of Qixia Formation in west and central Sichuan Basin, and in the 2nd and 3rd member of Maokou Formation in west, central and south Sichuan Basin. |