AAPG Bulletin, V. 82 (1998), No. 5A (May 1998 Part A), P. 757-772.

Two Petroleum Systems Charge the YA13-1 Gas Field in Yinggehai and Qiongdongnan Basins, South China Sea¹

Honghan Chen,² Sitian Li,² Yongchuan Sun,² and Qiming Zhang³

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received January 29, 1996; revised manuscript received April 4, 1997; final acceptance November 14, 1997.
²Department of Petroleum Geology, China University of Geosciences, Wuhan City, Hubei Province 430074, People’s Republic of China.
³Nanhai West Oil Corporation, Zhanjiang, 524057, People’s Republic of China.

This project was supported by the National Natural Science Foundation of China (No. 4932064 and No. 49472119). We wish to acknowledge the many petroleum geologists from China Offshore Oil Nanhai West Corp. who, over the last five years, have helped us to collect data. We wish to thank Leslie B. Magoon and Kevin T. Biddle for constructive reviews and additional remarks. We greatly benefited from discussion with Fei Qi while we were preparing the manuscript; we also thank Quanmao Chen, who read the manuscript and made helpful suggestions. 

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.