Thermal Regime, Interreservoir Compositional Heterogeneities, and Reservoir-Filling History of the Dongfang Gas Field, Yinggehai Basin, South China Sea: Evidence for Episodic Fluid Injections in Overpressured Basins?

Abstract

The Dongfang gas field, located in a diapir structure zone, is the largest gas field found in the Yinggehai Basin. A strong thermal anomaly caused by hydrothermal fluid flows occurs in the gas field, as evidenced from drill-stem test and fluid inclusion homogenization temperatures, Rock-Eval Tmax, vitrinite reflectance, and clay-mineral transformation profiles. Such a thermal anomaly suggests focused, rapid flow of deeply sourced hydrothermal fluids, which has shifted the threshold depth to the onset of hydrocarbon generation upward by about 500 m. The Dongfang gas field shows considerable variation in nitrogen and carbon dioxide content, with nitrogen content ranging from less than 5 to 31.2% and carbon dioxide content ranging from less than 1 to 88.9%. Hydrocarbon gases and associated condensates show high maturities and have been generated most probably from the Meishan and Sanya formations of Miocene age. Carbon dioxide in gases with CO2 content less than 10% is organic in origin, whereas carbon dioxide in gases with CO2 content higher than 10% is inorganic in origin and has been generated from high-temperature decomposition of carbonates. Most gases display negative d15N values. Gases with nitrogen content higher than 15% always contain organic CO2 (CO2d13C values lighter than -10o/oo,), and the nitrogen contents decrease as the d13C values for methane and ethane become heavier, suggesting an organic origin of the nitrogen gas generated in the catagenetic stage (source rock Ro <2.0%), before the significant thermal decomposition of carbonate took place. Systematic interreservoir compositional heterogeneities occur in the gas field, which, along with the thermal regime and fluid-inclusion homogenization-temperature measurements, give a clear suggestion of the reservoir-filling history: methane-dominated gases with relatively high nitrogen content and a small amount of organic CO2 accumulated first, and carbon dioxide-rich gases with hydrocarbon components of higher maturity injected into the reservoir later. Interreservoir compositional heterogeneities, which have the advantage of being unaffected by in-reservoir mixing processes, can be effective indicators of the field-filling history, especially when they are studied in combination with fluid-inclusion and reservoir-diagenesis analyses. The short-lived, transient nature of the thermal effect of fluid flow and the wide variation of the toluene/n-heptane values seem to suggest episodic fluid injections from the overpressured systems into the reservoirs.