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The Reservoir Characteristics Under Control of Evaporative Fractionation and Biodegradation: A Case of QHD29-2 Oilfield, Bohai Bay Basin, China


QHD29-2 oilfield is located in the south end of Shijiutuo uplift, the southern of which is near by Shijiutuo uplift and its north part close to Qinnan depression. Its reservoir type is attached to the boundary fault and stratigraphic pinch out layered reservoir. In Bohai water, QHD29-2 is one of the few oil and gas field, and its hydrocarbon formations distribute widely, include Neogene and Paleogene. Based on analysis of various samples of oil and gas, the complexity of the reservoir is described and explained. The reservoir performance density and viscosity increases from shallow to deep, and the viscosity of bottom of the reservoir is 68940 mPa.s(50 °C). The viscosity shows a good correlation with asphalt and non-hydrocarbon. But there is abnormal high density and viscosity in part of the Neogene crude oil. Crude oils in QHD29-2 oilfield have medium-high gammacerane, and high 4,23,24-trimethyl triaromatic dinosterane contents and low-high 4-methyl-24-ethyl triaromatic sterane contents. Combined with evolution of Qinnan depression, the third member of Shahejie Formation (E2s3) and the first member of Shahejie Formation (E3s1) is the Main hydrocarbon source rocks. The research approves evaporative fractionation is the main cause of the overall characteristics of the oil and gas reservoirs. Lighter hydrocarbon feature shows evaporative fractionation is very obvious. But reservoir forming period is about 3Ma to 0Ma, when there is without massive erosion. And analysis of the natural gas of QHD29-2 oilfield shows the presence of a large amount of mantle derived carbon dioxide gas was charged, and inclusions shows hydrocarbon and carbon dioxide gas was charged in the same period. Therefore, the evaporation fractionation may be mainly caused by strong extraction mechanism of carbon dioxide gas. The research approves biodegradation is the main cause of anti law phenomenon of the Neogene oil. The chromatogram of crude oil (2010m) shows low carbon number of hydrocarbons loss and drum bag appears, which approves biodegradation of existence. But there is no biodegradation in deeper than 2000m. Analysis of thermal history show lower geothermal gradient, and the depth of biodegradation can reach 2050m (<80 °C) in QHD29-2 oilfield. In summary, the characteristics of QHD 29-2 oilfield which is density and viscosity increases from shallow to deep and the variation of shallow oil is caused by the interaction of evaporative fractionation and biodegradation.