Dynamic accumulation of gas hydrates in the Shenhu area, northern South China Sea: A two-stage evolution model constrained by temporally varying fluid flow

Abstract

Gas hydrates occurring within the Pleistocene sediments of the Shenhu area, South China Sea, represent a dynamically evolving accumulation inherited from an “ancient” hydrate system. A hydrate-enriched layer with a maximum saturation of 48% is present near the base of the current gas-hydrate stability zone (GHSZ), above which hydrate saturation rapidly declines to nearly zero. This sharp transition cannot be explained by the previously estimated slow and nearly constant aqueous flow rate for the Pleistocene sediments. Geological evidence indicates that a substantial reduction in fluid flow occurred during the Early Pleistocene. To explore this process, the authors develop a coupled fluid-heat flow numerical model to simulate the dynamic evolution of the Shenhu hydrate system under temporally varying water-flow conditions. The model incorporates both the burial of “ancient” hydrates through sedimentation and the formation of “new” hydrates in younger deposits. Simulations with time-dependent flow rates can successfully reproduce the observed hydrate distribution in the Shenhu area. Under present sedimentation and fluid-flow conditions, the burial and dissociation of “ancient” hydrates beneath the GHSZ greatly outpace the formation of “new” hydrates within it, resulting in a continuous decline in the total hydrate inventory over time. These findings offer new insights into the long-term evolution of natural hydrate systems and have important implications for in-situ hydrate resource assessment.

Keywords

Gas hydrateCombustible ice / Clean energy / Shenhu area / Pleistocene sediments / Water flow rate / Sedimentation rate / Tectonic activity / Numerical modeling

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