Abstract
Enormous methane gas hydrate is expected to be in the surface sediments over accretion prisms of Nankai Trough. Methane in gas hydrate is generally of biogenic origin produced by microbial fermentation. However, thermogenic gas can contribute to the formation of gas hydrate especially in Nankai Trough, since thick sedimentary rocks more than 5000m are distributed below the possible gas hydrates in this area. In the present study, hydrocarbon gases trapped in the sedimentary rocks with and without calcite veins and carbonate concretions collected from the Paleogene Otonasigawa Group in the Shimanto Terrain, Kii Peninsula were preliminarily analyzed to evaluate the upward migration of thermogenic gas.
Hydrocarbon gases were recovered from the sedimentary rocks by whole rock pulverization method. The sedimentary rocks with calcite veins are generally rich in hydrocarbon gases showing that they are mainly present in fluid inclusions of calcite veins. Significant amount of hydrocarbon gases is also detected in carbonate concretions. These hydrocarbon gases generally contain significant ethane, showing their thermogenic origin. Calcite veins and carbonate concretions formed in sediments over the accretion prisms of Nankai Trough can trap thermogenic hydrocarbon gases migrating upward.
