The structure and formation of diapirs in the Yinggehai–Song Hong Basin, South China Sea

The occurrence of shale diapirs in the Yinggehai–Song Hong (YGH–SH) Basin is well documented, as is their association with big petroleum fields. In order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high-resolution two- and three-dimensional seismic reflection data, as well as drilling data that cover the diapirs in YGH–SH Basin. As many as 18 diapirs were identified and are arranged in six N–S-striking vertical en échelon zones. On seismic reflection sections gas chimney structures, diapiric faults and palaeo-craters are genetically linked with the process of diapirism. Here we use geophysical and geological observations to propose a three-stage model for diapirism: initiation, emplacement, and collapse. During these three stages, different diapiric structure styles are formed, which we describe in detail. These include buried diapirs, piercing diapirs and collapsed diapirs. We link the diapirism to activity on the offshore continuation of the Red River Fault, as shown on our high-resolution seismic reflection data, which is also related to a high paleogeothermal gradient caused by crustal thinning. We also recognize the role of loading by the very large volume of sediment eroded from the edges of the Tibetan Plateau and delivered by the Red River to the basin.