highest in the region) reveal that sediments there only reach pre-oil window to early oil window thermal maturities. However, the trough is potentially a major source of hydrocarbon fuel, in the form of methane clathrate. Nevertheless there is no commercial exploitation. At depth in the ocean bottoms, in some cases water can form an ice-like solid structure that has methane trapped in its crystalline lattice, forming gas hydrates. The source of water for the formation of gas hydrates frequently comes from the dewatering of a subducting slab as well as the overriding plate (Muramatsu et al., 2006). Gas hydrates nearest the trough appear to be sourced mainly from dewatering associated with subduction, while with increasing distance from the trough the sourcing is more a result of lateral movement of methane enriched waters (Muramatsu et al., 2006). This was determined by drilling a series of boreholes and measuring the concentration, as well as radiometric age determination of the halogen elements iodine, bromine, and chlorine (Tomaru et al., 2007). The age determination of the iodine indicated multiple methane sources. It has been estimated that convergent margins may contain up to two-thirds of the total gas hydrate volume on the Earth (Kastner, 2001). The Nankai Trough has been described as containing a large amount of gas hydrates and is one of the best studied sites of gas hydrate formations (Collett, 2002; Saito et al., 2007). The information concerning the gas hydrates in the Nankai Trough was initially published in 2000 by the Japan National Oil Corporations. The data in the news release came from a series of boreholes what were started in the late 1990s. In this area, the main sedimentological controls for the accumulation of gas hydrates are the sand-rich areas of the trough (Collett, 2002). Well coring indicates the presence of at least three gas hydrate zones. Krason, 1994, estimated that there are 0.42 to 4.2×1012 cubic meters of methane within the gas hydrates. Seismically, the high bottom simulating reflectors are considered indicative of gas hydrates (Colwell et al., 2004). Methane-rich horizons have been identified as areas of higher attenuation of sonic frequencies (10 to 20 kHz) and only slight attenuation of seismic frequencies (30 to 110 Hz) (Matsushima, 2006). Thermal history The Nankai accretionary complex is an area with high heat flow. Its thermal history is complex due to multiple heating events or property changes. IODP Expeditions drilled the accretionary