Tracing early-charged oils and exploration directions for the Wenchang A sag, western Pearl River Mouth Basin, offshore South China Sea

Abstract

The Wenchang A sag is the largest petroleum producing sub-depression in the western Pearl River Mouth Basin, offshore South China Sea. The Eocene Wenchang Formation and the early Oligocene Enping Formation are the two major source rocks in the sag. The oil discoveries in this region mainly originated from the Enping Fm. How to objectively estimate the petroleum potential of the Wenchang Fm. and trace the Wenchang oils has long been the focus of research and exploration in this region. In this paper, inclusion oils obtained from reservoir sandstones, as well as typical reservoir oils and potential source rocks, were investigated by systematic geochemical methods. Oil–oil and oil–source rock correlations indicate that inclusion oils in the central and marginal areas of the Wenchang A sag had similar geochemical characteristics and mainly originated from the Wenchang source rocks. The reservoir oils in the central area are derived mainly from the Enping source rocks, while in the marginal areas they have mixed characteristics of both the Enping and Wenchang formations. Combining the tectonic-sedimentary evolution and petroleum generation histories, it is concluded that the two source rocks in the Wenchang A sag have generated oils, but in the central areas the main stage of oil generation of the Wenchang Fm. was much earlier than the formation time of the traps, resulting in the dissipation of most of the early generated Wenchang sourced oils. In the marginal areas, the main stage of oil generation of the Wenchang Fm. was later and the Wenchang oils not only could be captured as fluid inclusions in the reservoir quartz grains, but also migrated and accumulated into oil pools in which they mixed with the Enping sourced oils. The early lithological and structural traps in the central and marginal areas of the Wenchang A sag, especially in the northern part of the Shenhu uplift, are the most prospective areas for further exploration of the sag.