Genesis, source and charging of oil and gas in Lishui sag, East China Sea Basin

Based on the results of components and stable carbon isotopic analysis of gas and oil from the Lishui sag, East China Sea Basin, the oil and gas genesis, source and charge process are studied by combining the history of structural evolution, oil and gas generation and migration and accumulation. The study shows that there are two types of gases in the Lishui sag now, one is the gas with high CO2 content and low oil-type pyrolysis gas content found in Well WZ13-1-1, the other is the gas with high content of the mixture of coal-type gas and oil-type pyrolysis gas and certain content of CO2 found in Well LS36-1-1, LS36-1-2 and LF-1. Hydrocarbon gases are all organic thermal gas, in which high maturity oil-type gas was from Paleocene Yueguifeng Formation and charged in middle-late Eocene-early Oligocene, and low maturity coal-type gas was from Paleocene Mingyuefeng Formation and was charged in late stage. The CO2 in Well LF-1 is of organic origin, while CO2 in other wells were derived from the deep inorganic mantle source gas charged variously in Oligocene. Oil in Well WZ6-1-1 is the high maturity sapropel type oil from Yueguifeng Formation which was charged in middle-late Eocene-early Oligocene. Condensate in Well LS36-1 is low maturity humus type oil from Mingyuefeng Formation charged in late stage. Oil in Well LF-1 may be of mixed source and greater proportion of oil likely from Paleocene Yueguifeng Formation.