Distal accumulation of leaked gas from deep overpressured zone：the case of the Yanan Sag, Qiongdongnan Basin, South China Sea

Abstract

In the overpressured Yanan Sag of Qiongdongnan Basin, South China Sea, four structures contain varying gas volumes despite overall similarities in geological conditions. One notable difference, however, is their varying pressure characteristics. We investigated the nature and cause of the overpressure as well as its relation to natural gas migration-accumulation in the four structures. Well logs, drill-stem tests (DSTs), and mud weights, reveal two zones of overpressure in the YC21-1 and YC26-1 structures that contain no economic gas reservoirs. There is one zone of overpressure in the YC13-1 structure, whereas the YC13-4 structure is hydrostatically pressured. Cross-plot analysis of density and sonic logs indicates a mild to moderate overpressure section in the Neogene strata, interpreted as resulting from disequilibrium compaction and clay diagenesis.The Pliocene to Quaternary is considered to be a key period for strong overpressure formation in the Yanan Sag. Fast subsidence and burial may have induced intense gas generation by maturation and vertical compaction disequilibrium in the Oligocene strata of the central Yanan Sag. The resulting high pressure would have been transferred to the Oligocene reservoirs in the YC21-1 and YC26-1 structures in the slope belt. Fluid-inclusion homogenization temperature (Th) in combination with modeled burial curves indicates that gas filled the four structures during this period, suggesting that gas migration was associated with overpressure. Fluid-inclusion phase reflecting gas saturation and abnormal excursion of vitrinite reflectance (%Ro) suggest that leakage related to caprock fracturing resulted in gas accumulation failure in the high-pressure YC21-1 and YC26-1 structures. 13C depletion of methane in the YC13-1 and YC13-4 structures away from the sag center suggests that the current gases probably result from gas exsolution from formation waters. These gases are high-to over-mature, and thus were mainly derived from the deeper, central Yanan Sag rather than the local source rocks. Therefore, we infer that the gas leaked from the deep high-pressure zone in the sag center, then migrated in solution until a point at which PVT conditions would allow exsolution. Subsequently, the free gas would have migrated into the normally pressured reservoirs in the slope belts where it finally accumulated. Compared to the YC13-4 structure, the overlying Neogene mudstones in the YC13-1 structure are overpressured, which may serve to increase the seal capacity and explain the higher gas reserves in this area.