Origin and hydrocarbon source of the Ordovician high wax content oil in Southern Slope Zone of Dongying Depression

Exploration has inevitably extended into deep layers with ever-increasing exploration for oil and gas. Oils from Ordovician burial hill are recently discovered from Ordovician burial hill in the south slope of Dongying Depression, Bohai Bay basin, and the oils are characterized by high content of waxy alkanes with a low concentration of biomarker and sulphur content. Several types of oils also existed in the south slope of Dongying depression, such as biodegraded oil, normal oils in Shahejie Formation and high wax oils in Kongdian Formation. Forty-three oil samples and twenty-three source rock samples are finely analysed by inclusion analysis, monomer hydrocarbon isotope analysis and absolute content of biomarkers. On the basis of isotope compositions, biomarkers and integrating the physical properties of the hydrocarbons including densities and PVT relationships, it can be speculated that the oils from Ordovician buried hill and Kongdian Formation have the same source, obviously different from the overlying oils from Shahejie Formation. Combined with maturity parameters, biomarkers, and inclusions analysis, it is speculated that the oils are a secondary hydrocarbon accumulation from the mixing of earlier formed oil and a late formed gas, and the oils of Ordovician and Kongdian bear a typical features of deep Es4 source rocks. Aquatic organism, higher plants and bacteria are the source of the waxy oil, and the gas invasion in geological history should be responsible for the formation of the high waxy oils. This study is helpful for unravelling hydrocarbons accumulation mechanisms and deep petroleum resources evaluation in this area. Besides, the research process in this paper can also be applied to other basins to explore for high wax oil reservoirs.