Accumulation features and mechanisms of high saturation natural gas hydrate in Shenhu Area, northern South China Sea

Abstract Based on the comprehensive interpretation of cores, loggings and 2D/3D seismic data of Shenhu GMGS3 drilling area in the northern South China Sea, the distribution characteristics, differential accumulation mechanism and reservoir forming mechanism of diffusion type natural gas hydrate with high saturation discovered from clayey silt reservoirs were investigated. The following findings are reached through the research: (1) Gas hydrate with high saturation often displays high resistivity, low interval transit time, and strong bottom-simulating reflectors (BSRs), and accompanies with fluid seepage phenomena beneath BSRs, such as mud diapiric structure and gas chimney. (2) The gas hydrate reservoirs are dominated by fine grained clayey silt sediments, and the reservoirs have higher porosity and permeability in local parts. (3) The gas hydrate is largely type I, whereas type II gas hydrate may exist below the type I gas hydrate. (4) The gas sources are mixed microbial and thermogenic gases, and the thermogenic gas originated from the deep formation in the center of Baiyun Sag migrated into shallow strata through faults, mud diapirs and gas chimneys, then was mixed with microbial gas in situ and continued to migrate until they accumulated in the temperature and pressure stability zone and formed diffusion type gas hydrate with high saturation finally. (5) The fluid migration system influenced and controlled the differential distribution of gas hydrate with high saturation.