GEOLOGICAL SIGNIFICANCE OF METHANE HYDRATE EXISTENCES AROUND JAPANESE ISLANDS AND THEIR IMPLICATIONS CONCERNING TO ENVIRONMENTS AND RESOURCES

Recently, the existence of gas hydrates in the deep sea ocean around the Japanese islands has become clear very much. Many results of the deep sea drilling and bottom simulating reflectors (BSR) which was recorded on the seismic reflection profiling records suggest the great amount of natural gases can be expected in the gas hydrates. Then, most of the methane hydrates exist in the fore-arc basins inside of the trenches and troughs such as the Nankai Trough, where it is normally difficult for conventional oil and natural gas fields to exist. Considering the environments of geological and geochemical conditions for the hydrocarbon generation in these fore-arc basins, the total organic carbon quantity per a certain unit volume of sediments is small, and the temperature which is necessary for the thermo-genetic methane origin to mature is generally low. However, it is under the environments which can generate a great deal of the natural gases of bio-genetic origin in the thick sediments with rapid sedimentation rates where remarkable subsidence occurred by crustal tectonic movements. While, uplifts of the trench slope breaks have made the bases of hydrate stability zone migrate up, and these conditions prefer to concentrate, seal and store the methane hydrate up in the shallower sediments. Generation of methane hydrates has taken place in the thick sediments for a long time. As a result, the hydrates have been able to store up a great amount of underground methane. Then, many faults formed by the tectonic events can sometimes become a good methane migration path for generation and store of gas hydrates. However, there are many traces which indicate the tectonic events can often be a trigger of submarine sliding which causes the methane hydrate collapse. These events may emit great amount of methane gases and may become a great important element of the short time climate change. How the hydrocarbons stored up in the hydrates can be produced avoiding these hazard are the present important research issues.