Geochemistry of petroleum systems in the Niuzhuang South Slope of Bohai Bay Basin—part 1: source rock characterization

Detailed mapping of the Tertiary strata of the Dongying Depression, Bohai Bay Basin reveals that oil shales and dark mudstones in the Es4 and Es3 members of the Eocene–Oligocene Shahejie Formation have excellent potential as petroleum source rocks. Regional distributions of total organic carbon content, kerogen type and thermal maturation indicate that abundant mature source rocks are present in the Niuzhuang Sag, whereas source rocks along the Niuzhuang South Slope are currently immature or marginally mature for the generation of conventional oils. This study examined the molecular characteristics for a number of biomarker compound classes, and related these to either source facies change or different burial and thermal maturation. An important observation made during this study is the significant variation in the relative distributions and absolute concentrations of a wide range of aliphatic and aromatic hydrocarbon markers in the rock extracts as a function of burial depth. This variation may potentially explain the uncertainty associated with the maturity assessment and genetic modeling for so-called “immature oils” that are usually defined on the basis of alkane biomarker isomerization ratios and the presence of biomarkers with “biological” configurations.