Hydrocarbon gas leakage from high‐pressure system in the Yanan Sag, Qiongdongnan Basin, South China Sea

Abstract

Despite high pressure being ubiquitous in the Yanan Sag of the Qiongdongnan Basin, South China Sea, its potential effect on gas migration‐accumulation remains unclear. In this study, a series of geochemical indicators combined with the origin of high pressure were used to elucidate the cause of gas accumulation failure in the overpressured YC19‐1 structure. Drill‐stem tests, mud weights, and well‐loggings reveal two vertical overpressure systems. Pore pressure in the deep Lingshui and Yacheng formations almost reaches a lithostatic magnitude, which was interpreted as a result of gas generation and disequilibrium compaction triggered by the Pliocene‐Quaternary rapid subsidence. A thermal anomaly above the high‐pressure system was evidenced by high homogenization temperature (Th) of fluid inclusion, abnormally high vitrinite reflectance, and a rapid transformation from kaolinite to illite. The high thermal field likely resulted from a cross‐stratal fluid migration from the deep high‐pressure system. Both methane‐bearing inclusions in sandstones and abnormal Rock‐Eval data of mudstones above the high‐pressure surface suggest an influx of external hydrocarbons. Based on primary aqueous inclusion Th and salinity, carbon and oxygen isotope compositions, and petrographic observations, widespread silica and carbonate cements near the overpressure surface were interpreted to be derived from extraformational materials carried by deep‐seated basinal fluids. Fluid inclusion Th coupled with burial model suggests that the gas leakage took place at circa 1.6 Ma, coinciding well with the stages of both gas generation and overpressure. These observations suggest an upward leakage of gas‐bearing fluids induced by pressure release, thus leading to the existence of only non‐commercial gas‐bearing aquifers in the overpressured YC19‐1 structure.