Hydrocarbon accumulation mechanism and resource prospect of HTHP natural gas reservoirs in Western South China Sea: a case study on the Ying-Qiong Basin

Many normal-pressure giant gas fields have been discovered in the Ying-Qiong Basin, South China Sea, e.g. X1-1 and Y13-1. However, the hydrocarbon accumulation mechanism of HTHP natural gas reservoirs are not figured out due to poor-quality seismic data of middle-deep layers, complex HTHP geological conditions and high-difficulty drilling and completion engineering. And as a result, the exploration discovery of HTHP natural gas in the Ying-Qiong Basin is seriously restricted. This paper is aimed at finding out the key theories on HTHP hydrocarbon accumulation mechanism of the Ying-Qiong Basin. A series of studies were carried out by using drilling, geological and seismic data, including hydrocarbon generation of source rocks under high temperature and pressure, natural gas dissolution experiment, sedimentation model of large reservoirs under high pressure and temperature, and sealing mechanism of cap rocks. It is indicated that the high temperature field in the Ying-Qiong Basin promotes the gas generation of organic matters, and the high pressure inhibits the gas generation of organic matters in the early stage, but promotes in the late stage. Under high temperature and pressure, natural gas can be exsolved in the form of free gas. In the Ying-Qiong Basin, large non-restricted gravity-flow submarine fan and restricted gravity-flow channel sand reservoirs are developed. In the Ying-Qiong Basin, the hydrocarbon accumulation and enrichment of high-pressure gas fields is controlled by overpressure sealing. Finally, the hydrocarbon accumulation pattern of HTHP natural gas reservoirs was established as “hydrocarbon generated by land-derived marine source rocks, gas accumulated in gravity flow reservoirs, gas transported along fractures/overpressure fissures and gas sealed by overpressure cap rocks”. The research provides theoretical bases for exploration breakthrough of HTHP gas reservoirs in tne Ying-Qiong Basin, and they are used as the reference for the discovery of large and medium high-pressure gas fields, such as X13-1/X13-2 and L25-1. In the Ying-Qiong Basin, a series of large, deep high-pressure sags are developed and the overall exploration degree of middle-deep reservoirs is lower, so the potential of natural gas resources is immense.