Geochemical Characteristics of Crude Oil and Oil-Source Correlation of the Paleogene “Red Bed” in the South Slope of the Dongying Depression, Bohai Bay Basin, China

Abstract

The Paleogene Red Bed in the South Slope of the Dongying Depression is a set of red clastic sediments of the lower section of 4th member of the Shahejie Formation and the Kongdian Formation, and is the alternative strata for oil and gas exploration in this area. The Red Bed crude oil of the South Slope of the Dongying Depression is divided into four types according to the assemblage characteristics of biomarkers from 17 oil sand samples and 3 crude oil samples. The Type I crude oil is generated from the source rocks of the lower section of 3rd member of the Shahejie Formation, with low phytane and gammacerane concentration, Ph/ n-C18<0.6, relatively high ratios of Ts/(Ts+Tm) (>0.38) and hopanes/steranes (>3.4), high diasteranes concentration, and low C35 homohopane index. The Type II crude oil could be derived from the source rocks of the upper section of 4th member of the Shahejie Formation with abnormal high phytane and gammacerane concentration, Ph/ n-C181, relatively low ratios of Ts/(Ts+Tm) (<0.38) and hopanes/steranes (<2.5), low diasteranes concentration, and high C35 homohopane index. The Type III crude oil is mixed oil consisting of the Type II crude oil charged in the early stage and the Type I crude oil charged in the late stage and mainly of the Type I crude oil. The Type IV crude oil is characterized by C29 sterane predominance and this feature has not been found in the crude oils and source rocks of the study area, therefore this type of oil is identified as a mixture of the Type II crude oil and an oil type from a shallow lake-swamp facies. The fault throw of oil transporting faults controls the distribution of crude oils with different genetic types, and the upthrown side of the faults is the favorable zone for the Red Bed oil and gas accumulation.