Mixing scenario of a vagarious oil in the Dongying Depression, Bohai Bay Basin

An oil from the third Member of Shahejie Formation (Es3) in well C104 from the Dongying Depression, Bohai Bay Basin, exhibits a vagarious mixing scenario of considerably distinctive maturities. Detailed molecular geochemical analysis was performed on eight oil samples and ten source rock samples from the Paleogene Shahejie Formation and Mesozoic strata by gas chromatography?mass spectrometry analysis. Biomarkers such as terpanes, steranes and aromatic steroid hydrocarbons are abundantly preserved in the C104 oil, with the predominance of high-molecular-weight biomarkers for each class of components, such as pentacyclic terpanes and tricyclic terpanes, regular steranes and pregnanes, suggesting that the C104 oil is ?immature?. However, the high values of maturity parameters derived from aromatic hydrocarbons consisting of MNR, DNR, TMNr, TNR1, TNR2, MPI-3 and MBPI indicate that the C104 oil has been highly mature. Hence, the C104 oil from the Es3 reservoir in the Dongying Depression witnessed multiple episodic charges of distinctive maturities and sources with highly mature source interfused into the immature fractions. Based on oil-oil and oil-source correlations, the ?immature? fractions of the C104 oil are mainly from the source rocks of the Es4 Member in the Shahejie Formation, while the ?highly mature? fractions of the C104 oil possess the possibility of Mesozoic sources. Potential highly mature source rocks in the Mesozoic strata probably remain undiscovered in the Dongying Depression, Bohai Bay Basin.