Geological Study of the Natural Gas Resources in the Southern Okinawa Islands

The natural gas resources, probably of disolved-in-water type, exist in the Tertiary Shimajiri Group of Southern Okinawa Islands. From the detailed study of surface geology and biostratigraphy, writers have proposed following stratigraphic succession. Writers have also studied Shimajiri Group from the economic point of view, and the following facts were recognized. 1) The thickness of the Shimajiri Group attains to about 2500 meters in this area. Shimajiri Group chiefly consists of massive siltstone, and is open sea deposits with a montonous cycle of sedimentation. 2) From the investigation of geostructural and biostratigraphical evidence, this area are divided into three blocks: northern, middle, and southern districts. Each block is bordered by the fault, NW-SE in direction. Geological structures of northern and southern districts indicate monoclinal. On the other hand, in middle district complicated several foldings are recognized. 3) The sedimentary environment of Shimajiri Group indicate a relatively stable bathyal to outer neritic condition, and is most favorable as the mother rock of natural gas, but the very fine sandstone might have been a little permeable as the reservoir rocks.