GEOCHEMICAL EVIDENCE FOR OFFSHORE GAS HYDRATES GENETICALLY RELATED TO ON-LAND MUD VOLCANOES IN SW TAIWAN

Taiwan is located at the plate boundary between Eurasian plate and West Philippine plate. Mud volcanoes, which are believed to be the products of accretionary prism due to collision, are commonly found in on-land of SW Taiwan. According to available seismic data, many active mud volcanoes were also observed in offshore SW Taiwan. Meanwhile, many bottom simulating reflectors (BSR) can also be identified, which indicates potential submarine gas hydrate deposits may exist in this area. Since the diapiric structures can be continuously extended from offshore towards on-land in SW Taiwan, local geologists suggested that on-land mud volcanoes may be genetically related to offshore mud volcanoes and gas hydrate deposits.

For better understanding their relationship, fluid samples of representative mud volcanoes in SW Taiwan were collected for geochemical analyses. According to the data of chlorine concentrations and oxygen isotopes, some of on-land mud volcanoes exhibit compositions which fall in the mixing trend between published compositions of gas-hydrate waters and seawater. This indicates that on-land mud volcanoes are genetically related to offshore gas hydrate deposits. Furthermore, some of them exhibit higher d¹⁸O value than the mixing line. It implies that these fluids may be derived from the deeper sources which have been experienced serious water-rock interaction during diagenesis processes.