Seismic Characteristics and Hydrocarbon Accumulation Associated with Mud Diapir Structures in a Superimposed Basin in the Southern South China Sea Margin

Mud diapirs are significant structures often associated with hydrocarbon migration and accumulation as well as gas hydrate formation in sedimentary basins. A total of 30 mud diapirs were observed in the northwestern Zengmu Basin in the southern South China Sea based on 2D multichannel seismic data. The structures are distributed along an NW-SE trend near the shelf break of the Kangxi depression in the Zengmu Basin. The mud diapirs were divided into the following according to their vertical shapes, intrusion heights, and effusive activities: tortoise, piercing, and nozzle types, which represent different evolutionary stages of diapirism. The mud diapirs in the study area suggest that the driving forces behind the mud diapirs considerably exceed the rock rupture limit of the overlying strata. Diapir formation can be divided into two steps: the accommodation of a large amount of mud in the Zengmu Basin and the movement of plastic mud induced by gravity-driven flow and regional tectonic compression. Combining seismic interpretations with analyses of the regional geology and tectonic subsidence, the current study proposes that rapid subsidence in the Kangxi depression sufficiently accommodated the vast amount of sediments deposited since the Middle Miocene, which provided favorable conditions for the growth of mud diapirs. Furthermore, the N-S directional stress field in the Zengmu Basin and the gravity slide northward along the southern slope of the depression further facilitated the development of mud diapirs since the late Miocene. The mud diapirs in the southern South China Sea margins were accompanied by the accumulation and release of overpressured fluid.