Prospecting Method Of Gas Hydrate In ODP Legs

Since no gas hydrate sample has been found in China and our sampling has been restricted in certain depth, the evaluation and development of the prospecting method we use in China relies on data from those areas in which gas hydrate samples have been found.The purpose of the research in this paper is to summarize the geological conditions and geophysical,geochemical characteristics of gas hydrate zone in ODP Legs in which gas hydrate has been sampled, such as Leg164,Leg204 and Leg308, herein a new model of occurrence and distribution of gas hydrate has been brought forward to guide the gas hydrate prospecting in Qiong Dongnan Basin and Xisha Trough of South China Sea. And meanwhile the data of Site996 which were analyzed by acidolysis method were compared with the original ODP data which were mostly analyzed by headspace free gas method, therefore the feasibility and the validity of acidolysis method has been verified with the comparison between the data of acidolysis method and headspace free gas method.Firstly the geological background of these Legs has been studied, such as the water depth of gas hydrate areas and the seafloor physiognomy, conformation and aggradations, the composing of formation, which have been compiled into a geological condition table later on, herein the primitive model has been brought forward, in which turbidite has been introduced;After summarizing the function of BSR and turbidite in this model, it's found out that BSR and turbidite have significant effect on this new model as the markers for gas hydrate.The characteristics of this new model are as following: the velocity of turbidite is lower than that of BSR which is underlying turbidite; the density,heat conductivity,peak shear strength and resistivity are higher than these of background rock stratum while the porosity is lower by 10%; Gas hydrate is sampled in turbidite or in area adjacent to turbidite; methane infiltrates upwards through turbidite while other hydrocarbons move breadthwise along turbidite.Then this new model has been proven valid again with those geochemical indexes of ODP Leg164;I've requested 81 samples related to gas hydrate from IODP repositories and done some chemical testing with acidolysis method, compared with the original data of ODP, it's indicated that acidolysis method's a workable method to study anomaly and seek for gas hydrate which can show more distinctive anomalies than head space free gas method. And finally the cause of formation of hydrocarbon in Blake Region has been discussed in which a complex cause of formation was indicated.