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The Origin of Heavy Oil in Sanhecun Oilfield, Jiyang Depression, East China


Snahecun oil field located in Jiyang depression is a new discovered heavy oil field. It is unusual that the oils in deep Shahejie formation with better preservation conditions has higher density and viscosity than that in shallow Guantao formation. Geochemical analysis of 26 oil samples from different depths were carried on to clarify their source and cause of densification. Oil-source rock correlation results from biomarkers suggest that oils in shallow Guantao formation are derived from 3rd number of Shahejie formation in Bonan Sag and Gunan Sag, characteristic of lower gammacerane/C30-αβ-hopane (<0.25), higher ααα20R-C27/ααα20R-C29 (>1.5) and lower C21–22/C27–29 sterane (<0.07). Biomarker thermal maturity parameters indicate that they were expelled within the oil generation window. The oils in Guantao formation have undergone light to severe biodegradation ranging from levels 2 to 6 (PM level). With the increase of biodegradation degree, the saturated hydrocarbon content in crude oil gradually decreased (from 50.48% to 28.05%), while the content of resins and asphaltenes increased continuously (from 27.71% to 38.50%). Biodegradation is the main cause of oil densification in Guantao formation. The oils in Shahejie formation are from the 4th member of Shahejie Formation in Bonan Sag with higher gammacerane/C30-αβ-hopane (>0.5), lower ααα20R-C27/ααα20R-C29 (>1.5) and higher C21–22/C27–29 sterane (>0.14). Compared with shallow heavy oils in Guantao Formation, the oils in Shahejie formation have lower saturated hydrocarbon content (6.51%~16.84) and higher content of resins and asphaltenes (47.03%~55.81%). However the oils in Shahejie Formation were only slightly degraded (PM level 2~3), biodegradation is not the main cause of oil densification. Maturity assessments showed that oils in Shahejie formation were expelled at the early mature stage with high resins and asphaltenes content. Although oils in Guantao Formation suffered more serious biodegradation, they still have relative high content of saturated hydrocarbon than those in Shahejie Formation. Therefore, different cause of densification is the main reason for the unusual characteristic of heavy oils in Sanhecun oilfield.