Geological factors controlling the formation, stability, and distribution of gas hydrates were investigated by basin analysis of the Nankai Trough region. The Nankai Trough is located beneath the western Pacific, south of the Japanese islands Shikoku and central Kyushu. Geological, geophysical, and geochemical data from the region were assembled and critically evaluated to develop consistent interpretations of the relationships of geological environments and gas hydrates. This study was performed for the US Department of Energy Morgantown Energy Technology Center by Geoexplorers International, Inc., as part of a worldwide evaluation of 21 offshore sites where the presence of gas hydrates has been confirmed or inferred. 60 refs., 26 figs., 1 tab.