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Characteristics and Genetic Origin of the Saline Lacustrine Oil: A Case From the Shahejie Formation in the Wenmingzhai Oilfield, Dongpu Depression, China


Significant commercially important amounts of oils been found in saline lacustrine layer of Wenmingzhai Oilfield, Dongpu Depression, however, a more thorough analysis of the hydrocarbon-generating parent material and hydrocarbon genetic origin is required. Geochemical analysis methods, including GC/MS, QGF(Quantitative Grain Fluorescence),TSF(Total Scanning Fluorescence) and vitrinite reflectance(Ro) are used to reveal the crude oil characteristics and genetic origin. We observed that the normal paraffin hydrocarbon distribution of crude oil was featured with typical even/odd predom, which may be result from the saline lacustrine environment. While, the terpane distribution of crude oil shows enrichment of gammacerane and predominance of C35-hopane to C34-hopane, which presents the hydrocarbon-generating parent material deposited in strong reducing and hypersaline waters. The Pr/Nc17-Ph/Nc18 plate indicates that the matrix are all type I and type II organic material that formed in saline lacustrine facies. Meanwhile, regular sterane of “reverse L type” with C27 < C28 < C29 also shows that the crude oil hydrocarbon-generating parent material are mainly derive from hydrobios. From the west to the east, the QGF half peak wideth (Δλ) of different oil samples gradually decrease with the depth increase, which presents that the oil quality lighten gradually as buried depth increase. The αββ/(ααα+αββ)ratio of C29-sterane of different oil samples mainly ranged from 0.25 to 0.3. While the vitrinite reflectance(Ro) of source rock in Wenmingzhai Oilfield distributed among 0.4%~0.7%, which is evidently lower than the crude oil maturity in the study area and shows that the crude oils in Wenmingzhai Oilfield are not from local deep hydrocarbon source rocks. The results show that the origin material of hydrocarbon in Wenmingzhai Oilfield deposited in saline lacustrine environment, the hydrocarbon-bearing material of which are mainly from hydrobios and the kerogen types are mainly type I and typeII. The crude oils are not mainly from local source but are mixture of different sources, which is of significant meaning for further research about saline lacustrine crude oil and would be instructive for the further petroleum exploration in the area.