Abstract:

The Cambrian evaporite basin developed in Sichuan Basin covers an area of ~200000 km2; the saltbearing areas are mainly distributed in the central part of the evaporite basin, e.g. southeastern Sichuan, northwestern Guizhou, northeastern Chongqing, and northwestern Hubei (Jiannan). The main saltbearing strata in this area are the Lower Cambrian and Middle Cambrian (second and third series), which are different in various areas. According to the characteristics of saltbearing strata and its spatial distribution, the evaporite basin is broadly divided into two gypsumhalite accumulation areas (subbasins): the southeastern Sichuan subbasin and northeastern Chongqing subbasin. The main saltbearing horizon in southeastern Sichuan is mainly the Lower Cambrian Qingxudong Formation (midupper part of the fourth stage of the Cambrian); the saltbearing horizon in northeastern Chongqing is mainly the Middle Cambrian Qinjiamiao Formation (third series). Previously, many studies on lithofacies paleogeography and potash finding hydrochemistry had been done, but lack of the study on carbon and oxygen isotope analyzes for drilling samples of Cambrian carbonate in studying the evolution of salt basin. The present study chooses Cambrian carbonate samples from the three representative drillings (Dingshan No1 well, Lin No7 well and Jianshen No1 well) to analyze the carbon and oxygen isotope composition. We hold the opinion that the δ13C negative drift is related with marine regression and biological extinction events and δ13C positive drift is mainly effected by marine transgressive events. The frequently alternation of transgression and regression in Cambrian is good for the formation of sourcereservoircaprock system in Sichuan Basin, and the formation of ancient reservoirs. Southeastern Sichuan subbasin is a closed and strongly evaporation environment in Longwanniao formation(the middle and upper part of the fourth stage), forming the depocentres of gypsumhalite accumulation areas in JiangjinLuzhou area. The δ13C value and salinity of Jianshen 1 well is higher than that of Lin 7 well. Combing with Sichuan Basin lithofacies paleogeography research, we consider that from early to midCambrian the seawater was more and more highly concentrated from southeastern Sichuan subbasin to northeastern Chongqing subbasin, which is significant for potash finding and the study on hydrocarbon forming conditions in Sichuan Basin.

Key words: Sichuan Basin, Cambrian, carbon and oxygen isotope, saltbearing basin, sedimentary evolution