Geochemistry of petroleum systems in the Niuzhuang South Slope of Bohai Bay Basin: Part 3. Estimating hydrocarbon expulsion from the Shahejie formation

Based on the distribution and geochemical characteristics of potential source rocks in the Niuzhuang and Guangbei sags and the Niuzhuang South Slope, Bohai Bay Basin, the quantities of hydrocarbons expelled from the Eocene-Oligocene Shahejie Formation were evaluated using a mass balance approach. The results confirm that most of the hydrocarbons in the study area were generated and expelled from the Shahejie Formation source rocks in the Niuzhuang Sag. The algae-rich, saline-brackish water lacustrine, Es4 oil shales of the Shahejie Formation from the Bamianhe to Wangjiagang region along the Niuzhuang South Slope exhibit excellent potential for early hydrocarbon generation, but they are not the major source for proven oil reserves in the Bamianhe region. The source rocks for most of the known oil reserves in the study area occur in the Es4 mudstones in the deep part of the Niuzhuang Sag. Thus, the commonly assumed origin of so-called “immature oils” from source rocks shallower than the conventional oil window in this area needs revision.