Total play system of natural gas in northern continental margin of the South China Sea

For the next exploration direction and integrated evaluation and optimization of targets for the northern continental margin of the South China Sea, this paper proposes the concept of the “total natural gas play system” based on the principles of systems theory. Integrating over 60 years of exploration achievements in the four major basins, the paper studies the basic geological conditions, hydrocarbon accumulation models and distribution characteristics of the system. With the core principle of “source-heat controlling natural gas and play-stratigraphy controlling accumulation”, it analyzes the distribution law of natural gas reservoirs covering “intra-sag, sag margin, extra-sag” and multi-stratigraphic sequences. The study shows that under the joint control of source and heat, the northern continental margin of the South China Sea can be divided into two major gas areas: the southern area dominated by coal-type gas and the northern area dominated by oil-type gas, with the former as the main body. Based on the distribution location of gas kitchens, the total gas plays are classified into three types: intra-sag, sag margin and extra-sag. In the oil-type gas area of the northern coastal zone, the proportion of intra-sag natural gas is relatively high; in the coal-type gas area of the southern offshore zone, the proportions of intra-sag and sag margin natural gas are relatively large; while the scale of gas accumulation in the extra-sag plays is relatively small. Finally, it is clearly pointed out that the southern offshore zone is the main direction for the next natural gas exploration in the northern South China Sea. Specifically, in the offshore zone, the intra-sag play and middle-deep layers of the sag margin play in the Yingzhong Sag should be focused for the Yinggehai Basin; the intra-sag play and sag margin play in the central depression are targets for the Qiongdongnan Basin; the middle-deep layers of the intra-sag play are targets for the Baiyun Sag of the Pearl River Mouth Basin. Furthermore, in the northern depression zone of the Pearl River Mouth Basin within the coastal zone, the main exploration directions include the middle-deep layers of the intra-sag play in the Huizhou Sag and the middle-deep layers of the intra-sag play in the Enping Sag; in the Beibu Gulf Basin, the main directions are the middle-deep layers of the intra-sag play in the Weixinan Sag and the middle-deep layers of the intra-sag play in the Haizhong Sag.