Hydrocarbon generation and expulsion of tertiary coaly source rocks and hydrocarbon accumulation in the Xihu Sag of the East China Sea Shelf Basin, China

Abstract

The widely developed Tertiary coal-bearing intervals in the Xihu Sag, East China Sea Shelf Basin in offshore eastern China, are hosted in the Middle-Upper Eocene Pinghu Formation (PHF) and Oligocene Huagang Formation (HGF). Based on field data and geochemical features, a new model for the hydrocarbon generation and expulsion is developed using Rock-Eval pyrolysis that was applied to quantify the potentials of coaly source rocks of the PHF and HGF throughout the Xihu Sag. The results suggest the coaly source rocks of the HGF and PHF are primarily composed of type II/III kerogen, and begin to expel hydrocarbon at 0.68 %Ro and 0.71 %Ro, respectively. Several hydrocarbon expulsion centers are located in the western slope zone for coals and the central anticline zone for mudstones, with the highest intensity of 3.5 × 106 and 24 × 106 t/km2 for coals and mudstones, respectively. Among all source rocks, the coals and mudstones contribute 2.5% and 97.5%, respectively, of the total expelled hydrocarbon, whereas the source rocks of the HGF and PHF provide 19% and 81%, respectively, of the total hydrocarbon. The study also shows that the natural gas discovered in the north of the central anticline zone and the north of the western subsag have mixed sources from the HGF and PHF while the light oil and condensate accumulated in other zones are primarily derived from the source rocks of the PHF. The model used in the Xihu Sag provides an efficient method for evaluating source rocks and identifying oil-gas sources.