Geological Factors for the Development and Newly Advances in Exploration of Gas Hydrate in East China Sea Slope and Okinawa Trough

Abstract

Abstract:

Up to now, East China Sea is the only marginal sea in which gas hydrate samples haven’t been revealed in the West Pacific. Based on the geological and geophysical data obtained both from our past several cruises and from other publications, geological factors including sedimentary sources, sedimentary thickness, source rock, sedimentation rate, temperature and pressure distribution that control gas hydrate development have been analysed. The paper points out that East China Sea has abundant sedimentary sources supplied mainly by Changjiang River and partly by Huanghe River which have the capability of transporting 1,498×10⁶ t of sediment each year from mainland China to East China Sea, Bohai Sea and Huanghai Sea. Several sedimentary centers have been clearly revealed along East China Sea Slope and west part of Okinawa Trough by our seismic data. Sediment thicknesses of those centers are all over 5 km with the earliest sediment layer since around Eocene or even Oligocene time. We believe that source rock is existing among the deeper part of the sediment center either Eocene layer or Oligocene layer which have already been confirmed as good quality source rock in East China Shelf Basin by drilling works, and hydrocarbons might be generated earlier from the source rock in those centers than in shelf basin due to higher temperature gradient in slope and trough area as comparing with that of the shelf area. Sedimentation rate along the slope and trough varies from several cm/ka to 400 cm/ka, with higher values mainly constraining in the sediment centers and delta area which is connecting to the end of slope canyons and distributing at the most west part of trough basin. Based on our measured sea floor temperature data, gas hydrates can only developed in the slope and trough area with water depth over than 600 m. Gas hydrates-stability-zone can be as thick as 650 m when take temperature gradient as 30 ℃/km, but thinning to northeast along the axis of Okinawa Trough and to northwest up to the slope as the water depth changing shallower and sea floor temperature changing warmer. Local geological structures like mud diapir, anticline, fracture system are quite common in the study area, and they mainly control the fluid migration forming a favorable environment for the gas hydrate development and preservation in the study area. Based on our analysis, and BSRs found on some seismic profiles, we believe that the reason for gas hydrates unrevealling in East China Sea as comparing with other marginal seas is mainly due to the relatively lower exploration level.

Key words: East China Sea Slope, Okinawa Trough, gas hydrate, geological factor, seismic exploration, advance

CLC Number:

P744.4
P736

LUAN Xi-wu, LU Yin-tao，ZHAO Ke-bin, SUN Dong-sheng, LI Jun. Geological Factors for the Development and Newly Advances in Exploration of Gas Hydrate in East China Sea Slope and Okinawa Trough[J]. Geoscience, 2008, 22(3): 342-355.

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