Origin And Resources Assessment Of Gas Hydrate In Okinawa Trough

Gas hydrate is a newly developed potential energy source in recent thirty years. Due to its big distribution area, large reservoir scale, intensive energy density, little combustion pollution effect, gas hydrate is believed to be an alternative fuel. Based on 717.7km-long high-resolution seismic data, 800.25km-long conventional multi-channel seismic data, 86 low atmosphere samples from"215"project, 90 subsurface sediment samples, 200 bottom water samples, more than 400 heat flow data from"126"project, and 124 bottom water temperature measurements from the Institute of Oceanography of Chinese Academy of Sciences, and light hydrocarbon test and C isotopic analysis for 11 samples from 4 Sites of ODP204,the regional geological analysis for gas hydrate formation conditions, the geochemical anomaly and geophysical indication analyses for possible gas hydrate occurrence as well as the special seismic data process and interpretation for the key seismic section are conducted in the study area, and discussion for possible origin of gas hydrate. After comprehensive integration, the strategic locations for gas hydrate potentials are preliminarily figured out in the continental slope of the East China Sea. Combined with the predecessors'achievements, the energy resource estimation for gas hydrate potentials is roughly calculated, and the possible gas sources and the formation mechanics are initially discussed. Main points are as fellows:1. Good geological settings for gas hydrate formation conditions in Okinawa troughThe Okinawa trough belongs to one typical back-arc basin of the pacific trench-arc-basin system in tectonics. Its average water depth is more than 1000m; the sedimentation accumulation is big (reaching 12000m); the sedimentation rate is high (the maximal amounts to 100-400m/Ma); the organic matter is rich. In the area with water depth greater than 500m, the bottom subsurface temperature is stable. Although the heat flow varies largely, the general heat flow is relatively low in the south of the trough. The topographical and morphological characteristics are various, and faults and diapirs are well developed. These characteristics are suitable for gas hydrate formation conditions.2. Gas hydrate geochemical anomalies are mainly located in the middle to south of the study areaThe ninety subsurface sediment acid-degassing analytical results show that methane anomalies are mainly located in the trough or continental slope, to the south of 27°30′and to the west of the trough axis. The anomalies are the most intensive and extensive around the Diaoyu Island. The 200 bottom-water hydrocarbon analytical results indicate that methane anomalies are mainly located in the two areas: one is around the Diaoyu Island and the other is in the middle of the Okinawa trough. Additionally according to the predecessors'mineral analyses of the trough sediments, carbonate crusts and ginger-shaped crusts, composed of ordinary calcite and high magnesium-content calcite, and sulfides are common in the middle to south of the trough. Hence it is suggested that the geochemical anomalies mainly occur in the middle to south of the trough, especially the intensive and extensive ones around the Diaoyu Island, possibly indicating the gas hydrate potential accumulations.3.CO₂ hydrate is probably an important type in Okinawa troughIn comparison with the origins of gas hydrate from ODP 204, gas hydrate possibaly has three kinds of gas supply in Okinawa trough: biogenic, pyrolysis and inorganic gas. The low atmosphere gas analytic results show that CO₂ is the major component and its contents are commonly high, about three times of the regional background, and the carbon isotope of CO₂ out of low atmosphere gases is a little heavy. This is coincident with the discovery of CO₂ hydrate near the black chimney by Japanese scientists with the use of deep submersible. Hence, I suggest that CO₂ hydrate probably be an important type of gas hydrate in the Okinawa trough.4.Geophysical indications for gas hydrate are obviousIn comparison with the typical geophysical characteristics of gas hydrate-bearing sediments, BSR, amplitude-blanking zone, velocity inversion, etc. are identified from the conventional and high-resolution seismic profiles in the continental slope and the Okinawa trough. After the further special seismic process such as AVO analysis on obviously anomalous geophysical profiles, 10 hydrate potentials or strategic prospecting areas are preliminarily selected, whose distribution range is about 410km long (BSR). Generally geophysical-indicated gas hydrate potentials have the following characteristics:1) The water depth of gas hydrate potentials is about 300-2625m; the deposit occurs 70-781m below seafloor and the quaternary serves as the reservoir;2) The morphology is characteristic of the trough plain and the continental slope;3) The major structural unit is the shelf-frontal depression;4) Gas hydrate-related anomalies are mainly within the south of the trough;5) Gas hydrate potentials are banded-distributing, parallel with the seafloor topography, which is related to water depth, gas source and faulting.5.Gas hydrate resource estimation is about 4000 billion cubic methaneThe method used for gas hydrate resource estimation is based on the conventional volume calculation. The gas hydrate distribution area and its thickness are figured out from the high-resolution seismic profiles in the study area; the porosity and the saturation are taken from the internationally statistic values. The calculated result indicates that the methane resource out of the gas hydrate potentials is about 4000 billion cubic in the Okinawa trough.Accordingly, from the regional geological settings, the geochemical anomalies and the geophysical indications, it can be seen that there exist the conditions and the possibility of gas hydrate formation and accumulation in the continental slope of the East China Sea; its potential resource is possibly highly enriched and the middle to south of the study area is probably the accumulation area. Therefore, from both the standpoints of resources and ocean rights and interests, it is necessary to further conduct the investigation and research-related work on gas hydrate in the Okinawa trough.