Molecular And Stable Isotopic Geochemical Characteristics Of Crude Oils From Jiyang Depression

Donging and Zhanhua Depressions,as one of the most petroliferous tectonic units in Jiyang sub-basin,have extremely high exploration potential.The oil-gas and source rock correlations have been highly complexed in this area because the Bohai Bay Basin had gone through multiple tectonic movements,with complicated oil and gas generation,migration,and accumulation processes.According to previous researches,the oil and gas of Donging and Zhanhua Depressions were mainly derived from Es4,Es3,and Es1 intervals of the Shahejie Formation in Paleogene.The study of oils in Dongying and Zhanhua Depressions with focus on vertical and horizontal correlations in Jiyang sub-basin plays a significant role in oil exploration,because a better understanding of distribution of reservoirs and source rocks is of great importance to predict oil-gas reservoirs and sweet spot of exploration.In this study,a total of 102 oil samples were systematically analyzed using organic geochemical methods of GC/MS and GC-IRMS,with references of previous researches.19 biodegraded oils within the 102 samples were dealt with urea adduction before n-alkanes and isoprenoid alkanes can be used to measure compound-specific carbon isotope,to avoid the influence of UCM.This study shows that the features of geochemical compositions,saturated and aromatic fractions,and compound-specific carbon isotopes of the n-alkane and iso-alkanes of the 102 oil samples,as well as discussed and interpreted the origins of organic matter and depositional conditions of the crude oil samples in the two depressions.The content of saturated fractions in Dongying Depression is higher than that in Zhanhua Depression,whereas the content of aromatic fractions is similar in both depressions.The content of polar fractions in Dongying Depression is higher than that in Zhanhua Depression,while the content of asphaltene fractions in Dongying Depression is lower than that in Zhanhua Depression.The CPI index is less than 1.2without odd-even number preference in all the oil samples in both Dongying and Zhanhua Depressions.The n-alkanes range from C₁₂ to C₃₇,with the main peak of C_23-243-24 or C_25-275-27 in both depressions.Phytane preference occurs in oil samples from Dongying Depression,whereas phytane preference or pristane preference exhibits in some oil samples from Zhanhua Depression.The oil samples show diverse level of biodegradation in the two depressions although both contain biodegraded oil samples.The biodegraded oil samples in Dongying Depression maximize at phytane,with the second peak at pristane or other isoprenoid alkanes,whereas the biodegraded oil samples in Zhanhua Depression peak at C₃₀ hopane,with low abundance of isoprenoid alkanes.This suggests that the products of biodegradation are diverse,as a result of different local conditions,microorganisms,and oil types.Origins of organic matter and depositional conditions of the source rocks of the oil samples in both depressions were interpreted on the basis of geochemical features of saturated,aromatic,and compound-specific carbon isotopic fractions.Organic matter in source rocks of oil samples in Dongying Depression were mainly derived from algae,and minor from mixtures of higher terrestrial plant and algae,with the source rocks deposited in reducing condition and fresh water-saline lake environments.Organic matter in source rocks of crude oil samples in Zhanhua Depression were mainly derived from mixtures of higher terrestrial plant and algae and minor from algae or higher terrestrial plant,with the source rocks deposited in reducing condition and brackish-saline lake environments.This study classified the crude oil samples from Dongying and Zhanhua Depressions into five oil families I-V and four oil families I-IV,respectively,using R language on the basis of principal component analysis and cluster analysis of 35selected geochemical parameters from 31 non-biodegraded crude oil samples in Dongying and Zhanhua Depressions,respectively.The results of oil-source rock correlation suggest that the Oil Family I,IV,and V in Dongying Depression were likely derived from Es4 source rocks,the Oil Family III were likely originated from Es3 source rocks,and the Oil Family II were mainly from mixtures of source rocks in both Es4 and Es3.The results also suggest that the Oil Family I in Zhanhua Depression were likely derived from Es4 source rocks,the Oil Family II and IV were likely originated from Es3 source rocks,and the Oil Family III were mainly from mixtures of source rocks in both Es4 and Es3.Biodegraded oil samples in Dongying and Zhanhua Depressions can be grouped into diverse oil families through comparing compound-specific carbon isotope values of the biodegraded samples and mean values of compound-specific carbon isotopes of oil samples in different oil families.This comprehensive study combined R computer language,principles of statistics,and organic geochemical analyses of crude oil samples,in order to systematically classify the oil families of crude oils in both Dongying and Zhanhua Depressions.The result of this study shows the methodology is scientific,simple,and effective,and thus can be widely used in the study of classification of oil families of crude oils in other basins.This may provide reference of classification of oil families and assist in oil-gas exploration and production in the future.