New View on Hydrocarbon Accumulation Characteristics of the North Sag, North Depression of the South Yellow Sea Basin

The South Yellow Sea basin is a large size Mesozoic and Cenozoic sedimentary basin. Hydrocarbon exploration in this basin has been carried out for more than four decades, but no commercial hydrocarbon discovery has been obtained so far, with only one oil bearing structure Zhucheng 1-2 has been discovered. There are a lot of basic geological problems to be solved in this basin, including whether the excellent quality source rocks are well developed, whether hydrocarbon has been generated from the source rocks, and whether hydrocarbons can migrate into reservoirs. Based on evaluation results on source rocks, the hydrocarbon accumulation characteristics in the North Sag have been studied with fluid inclusion technique in this paper. The result showed that the second member of Taizhou formation is the main source rock, being semi-deep lacustrine facies, and distributes widely. The main source rocks have mature and large amount of oil and gas have been expulsed from the main source rocks. The hydrocarbon generation center was located in Well Block ZC-A. Hydrocarbon has migrated along faults and filled into the Eocene Dainan Formation. The results show that there were two phases of hydrocarbon charge at least in the Dainan formation based on the study results of fluorescence observation of fluid inclusion and the microscopic temperature measure. The first phase hydrocarbon charge happened in about 3 Ma, and the second charge happened at present.