Hydrocarbon Expulsion Characteristics Of Sinian And Lower Paleozoic Source Rocks And Their Controlling Effects On Hydrocarbon Accumulations

As gas origins of Sinian and Lower Paleozoic in Sichuan basin are complex and the vertical distribution of accumulations is significantly controlled by source rocks,it is of great significance to comprehensively analyze the geological and geochemical characteristics,hydrocarbon generation and expulsion characteristics,and the control effects of source rocks on occurrence of oil and gas accumulations so as to provide references for the exploration and development of Sichuan Basin.In this study,source rocks including algal dolomite source rocks of Dengying formation,shales of the 3rd member of Dengying formation,and shales of Qiongzhusi formation are comprehensively analyzed,which demonstrate them to be effective source rocks contributing to gas reservoirs in the basin.Through the analysis of hydrocarbon generation and expulsion characteristics,it is found that the hydrocarbon generation history of the source rocks can be divided into five stages,namely the lower-maturity stage,the intensive hydrocarbon generation stage,generation cessation stage,re-generation stage and dry gas generation stage,with the corresponding historically accumulative hydrocarbon generation amounts of 43×1012m3,1.1×1012 m3 and 21×1012 m3 and accumulative hydrocarbon expulsion amounts of 38.5×1012 m3,0.97×1012 m3 and 18.4×1012 m3.It is suggested by gas-source correlation that Sinian and Cambrian source rocks commonly contribute to Sinian gas reservoirs while Cambrian source rocks are the main contributor of Cambrian gas reservoirs.There are three extensive hydrocarbon expulsion stages,corresponding to Triassic to Jurassic,Jurassic to Cretaceous,and Cretaceous to Quaternary.Fluid activities have been there since the late Triassic.Eventually,the quantitative characterization of source rocks is conducted,which demonstrates that the Gaoshiti-Moxi Area is generally in the favorable accumulation areas in principal accumulation periods with the possibility larger than 0.5 while the possibility of Ziyang-Weiyuan Area is slightly lower.