Influence of foraminifera on formation and occurrence characteristics of natural gas hydrates in fine-grained sediments from Shenhu area, South China Sea

Marine gas hydrates accumulate primarily in coarse-grained, high-permeability layers; however, highly saturated natural gas hydrates have been discovered in the fine-grained sediments of Shenhu area, South China Sea (SCS). This may be explained by key factors, such as the great abundance of foraminifera shells. In this paper, by analyzing the SCS foraminifera structure and performing hydrate formation experiments in the foraminifera shells, the contribution of foraminifera to hydrate accumulation in the SCS was investigated from a microscopic point of view. Simulations of hydrate formation were carried out in both pure SCS foraminifera shells and the host sediments. Pore structures in typical foraminifera were studied by use of micro-focus X-ray computed tomography (CT) and scanning electron microscopy (SEM). Hydrate growth and occurrence characteristics in the foraminifera shells were observed in-situ. The results showed that the presence of foraminifera significantly enhanced the effective porosity of the SCS sediments. Moreover, while the hydrates grew preferentially in the chambers of the coarse-grained foraminifera by adhering to the inner walls of the foraminifera shells, no apparent hydrate accumulation was observed in the fine-grained or argillaceous matrix. These findings provide a basis for further studies on the accumulation mechanism of hydrates and physical properties of hydrate reservoir in the South China Sea.