Cretaceous hydrocarbon accumulation conditions in the southeastern East China Sea Shelf Basin

The exploration of oil and gas is very successful in the East China Sea Shelf Basin up to present. A number of oil and gas fields have been discovered in the Cenozoic, but no significant breakthrough has been made in the Mesozoic so far in this basin. In order to improve the efficiency of oil and gas exploration, we analyzed the Cretaceous hydrocarbon accumulation factors in the study area by using the theories and methods of petroleum system, which are summarized into a hydrocarbon accumulation model for the study area. The results show that the Cretaceous source rocks in the study area are mainly deposited in the Jilong Sag, dominated by dark mudstone of bay facies with high organic matter abundance as a good type of source rock. The reservoirs are mainly consisting of Mesozoic-Cenozoic sandstone, with primary pore and secondary fractures as the main reservoir space, and the Mesozoic sandstone has moderate porosity and permeability in general. The Cenozoic reservoirs have good performance, in which the sandstone of the Pinghu Formation in Eocene and Huagang Formation in Oligocene are the most important ones. The Mesozoic and Cenozoic caprocks are mainly mudstone. There are two sets of source-reservoir-cap assemblages. Controlled by tectonic evolution, the Jilong Sag may have experienced three periods of hydrocarbon generation, i.e late Cretaceous, Paleogene and Neogene-to-present, while other areas have only experienced two. Hydrocarbon conducting system is composed of sand bodies, unconformities, faults and fractures. There are mainly three types of hydrocarbon models in the Cretaceous of the southeastern East China Sea Shelf Basin. The results of this study have provided a basis for the evaluation of the Cretaceous oil and gas resources in the southeastern part of the East China Sea Shelf Basin.