Two Petroleum Systems Charge the YA13-1 Gas Field in Yinggehai and Qiongdongnan Basins, South China Sea

Abstract

Restoring burial history, correlating maturities and biomarkers of gas-condensate source rocks, measuring fluid inclusions, and stable carbon isotope data show that two petroleum systems charge the YA13-1 gas field in Yinggehai and Qiongdongnan basins in the South China Sea. The first source came from the Yanan depression in Qiongdongnan basin, at a depth of 4450 m, with the critical moment of 5.8 Ma. The second source came from the Yinggehai basin, at a depth of 4700 m, with the critical moment of 2.0 Ma. The YA13-1 drape anticline trap formed at 10.5 Ma, which was earlier than gas migration and accumulation. The Meishan Formation, with 7-10 MPa (1015-1450 psi) expelling pressure and 49-66 MPa (7105-9570 psi) overpressure combined, effectively seals YA13-1 gases. The reservoir of Lingshui Formation sands has average values of 14.9% porosity and 213 ´ 10-3 µm2 permeability because the weathering of the feldspathic sands at the end of the Oligocene, the preserving of K-feldspar and quartz overgrowths, and dissolution of carbonate cements by organic acid enhanced the reservoir quality. Consequently, trap formation coincided with gas generation, migration, and accumulation.