AAPG Bulletin, V. 82 (1998), No. 7 (July 1998), P. 1372-1384.

Geology, Compositional Heterogeneities, and Geochemical Origin of the Yacheng Gas Field, Qiongdongnan Basin, South China Sea¹

Fang Hao,² Sitian Li,² Yongchuan Sun,² and Qiming Zhang³

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

¹Manuscript received September 23, 1996; revised manuscript received July 3, 1997; final acceptance February 3, 1998.
²Department of Petroleum Geology, China University of Geosciences, Wuhan 430074, Hubei, People’s Republic of China.
³Institute of Petroleum Exploration and Development, Nanhai West Oil Corporation, Zhanjiang 524057, Guangdong, People’s Republic of China.

This project was supported by the National Natural Science Foundation of China. We are appreciative of collaboration with and enthusiastic support from Shixiang Cai, Zhonglian Hu, Baojia Huang, Lintao Li, and many other co-workers at the Institute of Petroleum Exploration and Development, Nanhai West Oil Corporation. We thank J. K. Whelan, B. J. Katz, Lyle Baie, and Ken Wolgemuth for constructive review of this manuscript. 

Abstract

The Yacheng gas field is located in the footwall of the No. 1 fault, the boundary fault between the Yinggehai and Qiongdongnan basins. The main reservoir is the fan-delta sandstones in the Lingshui Formation. The seals are Meishan For m ation shales near the No. 1 fault and Lingshui Formation shales away from the No. 1 fault. All strata are normally pressured in the gas field except for the Meishan Formation. The Meishan Formation is overpressured near the No. 1 fault in the gas field and in the adjacent Yinggehai Basin. Away from this fault into the Qiongdongnan Basin, the overpressure diminishes. An obvious thermal anomaly occurs below 3600 m in the gas field. This anomaly, characterized by an abrupt increase in drill-stem test and fluid-inclusion homogenization temperatures, vitrinite reflectance (R_o), and Rock-Eval T_max, and by an abnormally low temperature/R_o/T_max gradient, diminishes away from the Yinggehai Basin. The gases and condensates have abnormally high aromatic hydrocarbon contents and show obvious heterogeneities. Away from the No. 1 fault, the C₂₊ hydrocarbon content and C₂₊/SC_n increase; carbon dioxide content decreases; d¹³C values for methane, ethane, and carbon dioxide become lighter; the heptane and isoheptane values decrease; and the relative contents of aromatic hydrocarbons, both in C₆/C₇ light hydrocarbons and in the condensates, decrease. Such heterogeneities reflect the reservoir-filling process and origin of the gas field. The gas field was charged from both the Qiongdongnan and the Yinggehai basins. Hydrocarbons sourced from the Qiongdongnan Basin have relatively low maturities, whereas hydrocarbons from the Yinggehai Basin have relatively higher maturities and seem to have been in association with hydrothermal fluids. The hydro thermal fluids from the Yinggehai Basin, in which methane, ethane, carbon dioxide, and especially aromatic hydrocarbons dissolved under the high-temperature and high-pressure subsurface conditions, migrated along the No. 1 fault and caused the abnormally high concentration of aromatic hydrocarbons, as well as the thermal anomalies in the gas field, especially near the No. 1 fault.