Research on the formation model of gas hydrate deposits in the Shenhu Area,northern South China Sea

Abstract

The Shenhu area on the northern continental slope of the South China Sea is considered as a promising target for gas hydrate exploration and exploitation. Its special hydrate occurrence makes it difficult to explain and quantitatively describe its formation mechanism by using the deposition rates and fluid flow conditions at present. Its formation evolution mode and controlling factors are unclear yet. In this paper a dynamic model for the hydrate accumulation is constructed with two controlling parameters, i.e. the sedimentation rate and fluid water flow rate. The model is used to simulate the hydrate accumulation in the Shenhu area and testified with saturation data derived from salinity. Finally, a hydrate formation pattern in the Shenhu area is built based on the investigation of tectonic activities and the dynamics of hydrate accumulation in the area. The model indicates that Shenhu hydrates were developed from the hydrates formed in late tectonic activities during the late Pliocene Epoch-early Pleistocene Epoch. The gas hydrate formation in the Shenhu area has a typical dual pattern. In the first stage the hydrate grew in fractured sediments formed by tectonic activities 1. 5 Ma. The water flow rate was up to 50 m/ka, and abundant methane gas was carried into the hydrate stability zone and led to a rapid hydrate formation. The hydrate saturation went up to 20% in the duration of 40 ka. In the second stage of the following 1. 5 Mas, permeability of the sediment reduced sharply due to the sedimentation of clay silts. Fluid flux was 0. 7 m/ka and methane supply was inadequate. Thus hydrates could not form in shallow sediments and only grew slowly on the base of the hydrate stability zone, and then formed the feature of hydrate occurrence observed currently in the Shenhu area. And also the resource amount of hydrate continued reducing.