Geophysical characterization of a fine-grained gas hydrate reservoir in the Shenhu area, northern South China Sea: Integration of seismic data and downhole logs

摘要

High concentrated gas hydrate deposits in fine-grained sediment are proved to be eligible for exploitation with the current technology. Nevertheless, new insights and more understandings of the accumulation of gas hydrate and characterization of hydrate reservoir in fine-grained sediment are required, which can facilitate the commercial production of gas hydrate in the future. Combining seismic data and downhole logs, here we investigate a fine-grained gas hydrate reservoir in the Shenhu area, northern South China Sea (SCS). Gas hydrate with saturations of 10-45% occurs in fine-grained sediment. Besides the typical bottom-simulating reflectors (BSRs) and high amplitude reflection below BSR, a high amplitude positive reflector (referred to as the top of gas hydrate reservoir reflector; TGHR) exists at the top of high concentrated fine-grained gas hydrate reservoir. According to the result of acoustic impedance inversion, the TGHR is considered to represent a transitional zone from the overlying water-saturated sediment to the underlying high concentrated gas hydrate occurrence zone (GHOZ). Discontinuous BSRs beneath the traditional BSRs are also discovered in the study area, which are interpreted as the seismic evidence of the SII gas hydrate. In the Shenhu area, fluid flow features (such as chimney structures and faults) are found on the ridges where gas hydrate occurs, indicating that gas migration plays a critical role in the formation of gas hydrate in the fine-grained sediment. Our results provide valuable geophysical indications for detecting the presence of gas hydrate in fine-grained sediments.