THE LATE PERMIAN SEDIMENTARY ENVIRONMENT AND ORGANIC REEFS OF SICHUAN BASIN

The finding of Jiantianba reef in Lichuan of Hubei province at the end of the 1970's the gas firld in Jiannan reef with high output, and the sponge reef's (emergent and buried ) continuously discovered at the beginning ofr the 1980's in the east Sichuan and northeast Sichuan areas, provide models fol search of new types of gas field. Since more than ten years, over thirty organic reefs(mainly made of sponge and hydra ) have been found successi-vely in the Permian strata of south Guizhou, Yuannan, Guanxi, east Sichuan and west Hubei. Sixteen of them(including Honghua ua reef in Kaixian ) distribute in group and in row along the east Sichuan and west-Hubei areas, A number of reef gas fields was confirmed primarily by drilling in the east Sichuan and west Hubei areas which promoted the authors to study the ree f forming mechanism regionally of some typical organic reefs ( Kaixian Honghua Lichuan Jiantianba, Ziyun Shitouzhai, etc. ) .In recent years, after research of reef-forming sequence and the distribution, it was concluded that the basement made of platform basin sediments ( rich in silica) and arenitic banks is the necessary environment for organic reef form-ation. Based on the geochemical and thermodynamic property of silica, anomalous supplement and particular sedimentary environment (restricted shallow sea ) were discussed it was proposed that the silica-rich platform basin in restricted shallow sea from the late Permian Wujiaping to Changxing bank in platform open shallow sea, is mostly reef-forming environment of basin-bank in heritance and of bank-reef association.