Geothermal analysis of boreholes in the Shenhu gas hydrate drilling area, northern South China Sea: Influence of mud diapirs on hydrate occurrence

Abstract

Mud diapirs developed on the northern South China Sea continental slope are important indicators for gas hydrate exploration. The Guangzhou Marine Geological Survey (GMGS) of the Chinese Ministry of Land and Resources targeted mud diapirs and deployed gas hydrate drilling in the Shenhu area. Although an obvious bottom-simulating reflection (BSR) was detected below borehole number SH5, no gas hydrates were found at this borehole. Scientists encountered several contradictions when they analyzed the formation mechanism of hydrates related to mud diapirs. It was important to resolve this problem by sophisticated detection and thermodynamic analysis. In this study, we analyzed the thermodynamic mechanism of mud diapirs and their relationship to the occurrence of gas hydrates based on seismic profiles, mineral identification, sediment thermal conductivity measurements, and other available datasets. Our study concludes that gas hydrates were not detected at SH5 because a mud diapir pierced into the gas hydrate stability zone, leading to the decomposition of hydrates. The evolution of a mud diapir could be divided into three stages with a continuous geological process that controlled the accumulation of gas hydrates. The seismic profile and lithologic analysis showed that the mud diapir below SH5 was in the late stage of mud diapir evolution. The gas leakage system of the mud diapir was evident in the seismic profile, and the typical V-AMP (velocity-amplitude anomaly) structure displayed free gas accumulation under the BSR. The anaerobic oxidation of leaking methane promoted the formation of authigenic carbonate minerals. Thus, the deep layer of SH5 contained a relatively high percentage of carbonates and had a low thermal conductivity. In this stage, higher-temperature liquid migrated into the gas hydrate stability zone. The in situ temperature measured in this borehole was higher than the critical temperature of hydrate formation and decomposition. The liquid changed the temperature field of the region and altered the geothermal environment of hydrate accumulation, which led to the dissociation of hydrates.