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

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 (Ro), and Rock-Eval Tmax, and by an abnormally low temperature/Ro/Tmax 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+/SCn 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.