Study On Formation And Evolution Of Sinian Petroleum Pools In Leshan-Longnusi Paleo-uplift, Sichuan Basin

The laws of formation and evolution of Sinian gas pools in Weiyuan and Ziyang areas are clarified and the Sinian plays in Leshan-Longnusi Paleo-uplift are evaluated through the geochemical dissection of Cambrian source rock and Sinian reservoir rock of Gaoke 1 well, and on the basis of following researches: hydrocarbon source conditions, formation and evolution of Dengying reservoir, geochemical characters of reservoir fluids, structural evolution and trap formation in Leshan-Longnusi Paleo-uplift, and migration and accumulation of oil and gas in Dengying reservoirs.It is clearly pointed out that Sinian natural gas of Leshan-Longnusi Paleo-uplift originated from Qiongzhusi black shales, Lower Cambrian, through the biomarker correlation study in reservoir soluble bitumen and extracts of source rocks. Distribution of black shale is wide and thickness of black shale is big, abundance of organic matter is high and the type of organic matter is good, and the ability of hydrocarbon generation is strong. At present organic matter of source rock has already been in over-mature thermal evolution stage. The distribution of source bed thickness, sedimentary burial history, and the thermal evolution course of Cambrian source rock are common and different, because of being the different structure positions in Leshan -Longnusi Paleo-uplift. Source rocks are thick in South Depression which is southern from Zishen 1 well and the time of entering Oil window of organic matter is earlier. Source rocks are gradually thinner from Upper Slope (Wei 117 well) to Top of Paleo-uplift (Gaoke 1 well) and the time of entering Oil window of organic matter is later. The hydrocarbon creating peak period is different in various areas. The period of hydrocarbon supply is long. All of those show that the thermal evolution of source rock organic matter are closely associated with the evolution of paleo-uplift, the thermal evolution of source rock organic mattermhave been obviously controlled by structural evolution, sedimentary bural history and paleotemperature in Leshan -Longnusi Paleo-uplift. The thermal evolution courses of organic matter in Leshan-Longnusi Paleo-uplift are divided into three periods. The early hydrocarbon generation stage accompanies with the first burial period of Lower Cambrian source rock, the hydrocarbon generation stagnation stage accompanies with the rise period of structure, and the second hydrocarbon generation stage accompanies with the second burial period of Lower Cambrian source rock. In South Depression oil large generation took place in terminal of Silurian-Early Permian and in Upper Slope it took place in terminal of Triassic.The solid anthraxolites in Dengying carbonate reservoirs are residual matter of oil which had been originated from Qiongzhusi black shales and had been migrated into Dengying reservoir and had been later undergone thermal evolution in Dengying reservoirs. Through the analysis of their occurrence and evolution parameters, the solid anthraxolites are thought to be the results of thermal evolution under high paleotemperature. The physical property of Dengying reservoirs becomes bad because of solid anthraxolites extensively been filled into the early structure fissures and tar mat probably been formed, so that the migration and accumulation of oil and gas are later much difficult in Dengying reservoirs. On the other hand, the later structure fissures are likely the migration channels that lead oil and gas accumulation in Dengying reservoirs. It is the first time to point out that the different reflectance ratios of whole rock bitumen are the results of the different thermal evolution stages. Bitumen were filled into Dengying reservoir accompanying with oils having been migrated from source bed into reservoir bed, and continuously evolved in reservoir-inside and finally formed solid anthraxolites, as the residue of oil thermal evolution existing in leach holes, corroded hollows and corroded fissures of reservoir rock in the course of succeeding bury. Based on the analysis of natura...