INTRODUCTION

Gas hydrate is a crystalline solid，formed of water and gas.It is stable within a certain high-pressure low-temperature range and can exist in permafrost regions and continental margins where sea depth exceeds 300m.Methane in gas hydrate contains perhaps twice as much organic carbon as all fossil fuels on earth，therefore gas hydrate may become a major energy resource.Small changes in temperature or pressures can cause breakdown of hydrate，so gas hydrate has significant effect on sea floor sediment stability，causing collapse and landslide.The release and absorption of large volumes of methane from sea-floor hydrates may have had major impacts in modifying the Earth's climate.Gas hydrate is an important topic for study because of above three reasons［1］.(https://www.daowen.com)

Gas hydrates were found in pipe-lines in 1930s，but the naturally occurring gas hydrates were first recognized in frozen sediments under permafrost of West Siberia in 1960s.In 1970s，gas hydrates in marine sediments were discovered.Gas hydrate in deep marine settings has been inferred from seismic data［2］and confirmed with drilling［3］.Recent development of ODP（O-cean Drilling Project）and seismic surveys have revealed that gas hydrates are widely distributed in the worldwide continental margin sediments.Since 1990s，studies of gas hydrates develop vigorously.ODP Leg 164drilled three holes at the Blake Ridge on the continental margin off southeastern North America and penetrated the predicted depth of the BSR（Bottom simulating reflectors）into the sediments below in 1995［4］.Solid gas hydrate samples were recovered from these sites.MITI（Ministry of International Trade and Industry）of Japan has launched a fiveyear plan（1995～1999）for exploration and development of offshore methane hydrate resources in Nankai Trough region.Geophysical survey has almost been completed and in 1999，an exploration well with depth of about 3000m was drilled into methane hydrate bearing sediments of Nankai accretionary prism.