Genetic Mechanism of Low-Maturity Oil in the Qingyuan Tectonic Belt,Baoding Sag,Jizhong Depression
A significant amount of hydrocarbons has been discovered in recent years in the Qingyuan Tectonic Belt of the Baoding Sag,within the Jizhong Depression of the Bohai Bay Basin.However,the origin and prima-ry source rock of the discovered oils remain under debate.Conventional geochemical methods,combined with negative ESI FT-ICR MS,were employed to investigate the origin and formation mechanism of the crude oils.The majority of the crude oils collected from the Qingyuan Tectonic Belt can be classified as heavy oil,charac-terized by low wax(average 8.12％),low sulfur(average 0.81％),low Pr/Ph(0.14-0.24),high sterane/hopane ratio(3.3-5.6),high relative abundance of gammacerane and triaromatic sterane(40％-61％),and low sterane isomerization(C29 sterane ααα20S/(S+R)=0.23-0.28).These characteristics suggest a strongly reducing,brackish-saline,and/or water-stratified paleoenvironment during the deposition of the source rocks.Oil-oil and oil-source correlations reveal similar geochemical characteristics between the crude oils in the Lixian Slope of the Raoyang Sag and those in the main body of the Qiyuan Tectonic Belt.The crude oils show a close genetic affinity with the source rocks from the first member of the Shahejie Formation(Es1)in the Lixian Slope.It is suggested that the main source rocks of the crude oils had a burial depth of at least 3,000 m.No correlation was observed between the crude oils and the source rocks in the adjacent Baoding Sag,due to their relatively low hydrocarbon generation potential and thermal evolution.The crude oils were enriched in O1 and O2 species,particularly those with equivalent double bond equivalent(DBE)of 1 and 5(mainly fatty acids and steric acids,respectively),which could decarboxylate into hydrocarbons at low temperatures.We suggested that both the relatively low thermal evolution(in the early stage of oil generation window)and the enrichment of algal lipids in the Es1 source rocks are responsible for the less mature oils in the Qingyuan Tectonic Belts.
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