Abstract:

Two sets of active source rocks, the saline lake facies in the upper part of the 4^th member of Shahejie Formation and the brackish deep lake facies in the lower part of the 3^rd member of Shahejie Formation, have been developed in Dongying depression.Based on analysis of crude geochemical features and reservoir fluid inclusion,hydrocarbon accumulations have been formed in two different periods.In the first period,at the end of Dongying epoch in Paleogene,the source rocks in the upper part of the 4^th member of Shahejie Formation in the depression center entered the oil threshold and began discharging hydrocarbons.Hydrocarbon generation was then disrupted due to the uplifting and erosion caused by Himalayan movement.The biomarker components in the crude indicate that oil maturity was low and the amount of hydrocarbons generated was limited in this period.In the second period,Neogene to Quaternary, along with successive and overall subsidence,the deeply buried source rocks both in the lower part of the 3^rd member and in the upper part of the 4^th member of Shahejie Formation would further be matured with intensifying hydrocarbon generation and relatively high oil maturity.Distribution patterns of crude oil generated in these two periods are obvious different.Oil reservoirs with low maturity on the edge of depression were mainly generated from the upper part of the 4^th member of Shahejie Formation and accumulated in the first period; while that with high maturity in the center of depression were generated from the lower part of the 3^rd member of Shahejie Formation and accumulated in the second period.Hydrocarbons in reservoirs in the transitional zone(slope zone)came from multi sources and accumulated in multi periods.

Key words: hydrocarbon source rock, period of hydrocarbon accumulation, hydrocarbon distribution pattern, Dongying depression