The Spatial-Temporal Evolution Mechanism of Stratigraphic-diffusive Methane Hydrate Reservoirs during Sedimentation Process on Passive Continental Slope Surface since Pliocene: Theory and Application

When biogenic methane produced in marine shallow sediments flows laterally into hydrate stable zones, there is likely to be formation of disperse stratigraphic-diffusive hydrate reservoirs that generally appear thin-bedded and disseminated. The stratigraphic-diffusive layers are also potential industrial exploitation targets because they have moderate abundance, wide distribution, and convenient mining maneuverability. Here, the Pearl River Mouth Basin in the northern slope area of South China Sea as a typical passive margin has been chosen to probe the principles of the accumulation mechanism and distribution pattern of stratigraphic-diffusive methane hydrates, determine the optimal degree of aggregation and spatial extension, and thus evaluate the potential recovery value. Based on the composite stratigraphic sequence, we theoretically described the occurrence characteristics of this type of stratigraphic-diffusive hydrate reservoirs. A two-dimensional model that couples sedimentation process, fluid flow and reaction kinetics has been established to gauge the influence of engineering geological conditions and other controlling factors since 5.3 Ma in this system. Several key parameters (including porosity, temperature, salt and hydrate content) at three different moments, 5 Ma ago, 1.5 Ma ago and at present, have been exhibited. The impacts of geothermal gradient and kinetic reaction coefficient on the evolution process have been discussed. Finally, the methane generated from local particulate organic carbons and different starting time is used to verify the rationality of this theoretical approach. Our two-dimensional model clearly depicts the occurrence and accumulation characteristics of local stratigraphic-diffusive hydrates, discloses the exploration value and exhibits the exploitation possibility.