Geochemical characteristics and genesis of crude oils with complex physical properties in the eastern part of the southern gentle slope of Dongying Sag

Abstract

The crude oil in the eastern part of the southern gentle slope of Dongying Sag has complex physical properties, and various crude oils with different properties such as heavy oil, high sulfur oil, and high wax oil have developed. The complexity of the physical properties reflects the influence of the source rock properties, crude oil migration process, and secondary alteration after accumulation on crude oil characteristics. Through systematic analysis of geochemical data, the formation mechanism of crude oil with these special physical properties has been clarified. The result of hydrocarbon generation at an early stage from source rocks rich in high-sulfur kerogen deposited in a euxinic waterbody is the cause of high-sulfur crude oil with low maturity in Wangjiagang oil field. While the shallow reservoir burial depth and the resulting biodegradation is the mechanism of the formation of high-sulfur crude oil in Le’an oil field with higher maturity. Biodegradation is the main mechanism of the formation of heavy oil, and the recharge of hydrocarbons into the degraded crude oil reservoirs leads to the complication of crude oil density. Both the hydrocarbon generation of organic matter rich in terrestrial higher plants and phase-controlled fractionation during hydrocarbon migration lead to the generation of high wax crude oil.