Petroleum System of Baoshi Structure of Xihu Trough, East China Sea Shelf Basin

PQingsheng Zhang and Dag Nummedal
Colorado School of Mines, Golden, CO

The petroleum potential of the Xihu Trough of the East China Sea is a long-debated question. Good oil and gas were discovered in some wells, but other adjacent wells have been disappointing. Two questions arise from these mixed results. Is there a good source rock to generate enough oil and gas to make a big oil field? Do source, reservoir, and seal rocks, migrations and traps combine to form a petroleum system?

This research is focused on a new exploration target, the Baoshi Structure, in the Xihu Trough. Baoshi-1 Well has discovered a new interval below the productive Eocene Pinghu Formation, but no economic hydrocarbon was discovered. 3D seismic data show that Baoshi Structure is a NNE trending anticline-fault complex, which is superimposed on a monocline. Most Paleogene shales and coals are very thin, and most shales have very low organic content. The analysis of the organic content, type and maturity show that the best source rock is the newly discovered lower Eocene Baoshi Formation, which consists of dark shale with mostly Type III kerogen, formed in a lacustrine environment. The two major reservoir rocks are Upper Eocene deltaic fine-grained quartz sandstone with an average core porosity of 16.2% and an average core permeability of 509.3 md, and Oligocene fluvio-deltaic fine to coarse-grained sandstone with an average log porosity of 23.8% and an average log permeability of 354 md. Although the structures formed before the hydrocarbon migration, poor cap rocks and leaky fault seals resulted in hydrocarbon escape.