Geophysical Indicators of Gas Hydrate in the Northern Continental Margin, South China Sea

Xiujuan Wang,Yiqun Guo,Yuehua Gong,Shengxiong Yang,Shiguo Wu

doi:10.1155/2011/359597

Abstract

Gas hydrate drilling results show that gas hydrate has a close relationship with strong bottom-simulating reflectors (BSRs) identified from seismic data in the Baiyun sag, South China Sea. The BSRs observed on seismic profiles at the crests of submarine canyons indicate the likely existence of gas hydrate. We calculate the acoustic impedance using constrained sparse spike inversion (CSSI), the interval velocity, and the seismic reflection characteristics such as reflection strength, instantaneous frequency, blanking, and enhanced reflection to demonstrate the presence of gas hydrate. Higher acoustic impedance and P-wave velocity were identified above the BSR. A remarkable low impedance, low frequency, and acoustic blanking indicated the presence of gas below gas hydrate stability zone. The occurrence of gas hydrate at the crests of canyons suggests that the abundance of gas hydrate in Baiyun sag may be due to the migrating submarine canyons providing the structural reliefs and the topographic ridges.

Highlights

Gas hydrates are ice-like crystalline solids and are composed of water molecules and hydrocarbon gas
Gas hydrates have been identified by the gas hydrate drilling exploration at the Shenhu area, Baiyun sag, Pearl River Mouth basin, South China Sea
High acoustic impedance, high frequency and enhanced reflections identified from the 2D and 3D seismic profiles at about 280 ms of two-way travel time below seafloor were caused by the presence of gas hydrate

Summary

Introduction

Gas hydrates are ice-like crystalline solids and are composed of water molecules and hydrocarbon gas (usually methane). They are distributed worldwide in the continental margin sediments and beneath permafrost [1, 2]. Bottom simulating reflectors (BSRs) identified from seismic reflection profiles are conventionally interpreted as indicators for gas hydrate beneath seafloor [3]. Gas hydrates-associated BSRs have been recognized from the seismic data of other geophysical studies, and their presences have been validated by drilling or coring either in accretionary wedges [4,5,6,7] or in the continental margin of the world [8,9,10,11]. The acoustic impedance inversion of seismic data, log to seismic correlation, and seismic attribute analyses were combined to delineate gas hydrate zone [17]

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