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The petroleum system in the deep water of the Toyama Trough to the southwest offshore of the Sado Island in the Japan Sea was considered based on the analytical results of METI Sado Nansei Oki S and D drillings and their preliminary surveys.The middle Miocene Lower Teradomari and Nanatani Formations indicate good source rock potential 0.90 -2.98%TOC . The kerogen type is a mixture of Type II and III. The bitumens include organic matter from both marine and terrestrial sources, notably characterized by very abundant oleanane like the case of oils in the Kubiki area onshore Niigata.The oil sample collected from the Shiiya Formation of METI Sado Nansei Oki S is characterized by its high oleanane content. A seeped oil sample on the sea bottom, although heavily degraded, also indicates similar oleanane content and carbon isotope characteristics that can be correlated to the oil.The source rock of the oil is thought to be the mudstone of the Lower Teradomari and Nanatani Formations from the analytical results of the petroleum source potential, carbon isotopes and biomarker indices, especially the ratio of oleanane to norhopane content. The contribution of marine organic matter to the oil seems to be slightly higher than that to the mudstone in the Lower Teradomari and Nanatani Formations of the well. It suggests that the oil was derived from the source rocks of the same horizon that deposited in the basin center in comparison with the well location.The methane hydrate samples taken by preliminary sea bottom survey are referred to be thermogenic origin from the carbon isotope ratio. It can be isotopically correlated to headspace gas samples collected at 1900m and deeper zone of METI Sado Nansei Oki D well. This means that the deeper gas migrated to the surface very quickly without the appreciable input of shallow biogenic gas.Fluid inclusion analysis implies that the generated hydrocarbon was once trapped in the lower Teradomari Formation at METI Sado Nansei Oki D, then it migrated upward to the Shiiya sandstones through large faults and fi nally accumulated in the structural crest of the Shiiya horizon METI Sado Nansei Oki S . Part of the hydrocarbons leaked from the Shiiya Formation through a number of minor faults to form methane hydrates and many Direct Hydrocarbon Indicators DHI that are observed with three dimensional 3D seismic data.The kitchen area of the oil is estimated to be a syncline that locates west of the wells and is buried deeply along the Toyama Trough. It is suggested that the kitchen area provided hydrocarbons to deep water large structures and possibly to the onshore areas such as Kubiki Oil Field. The conditions required for oil accumulation in the Shiiya sandstone include the presence of deep faults for migration pathways and the absence of shallow fault that may leak hydrocarbons. Therefore 3D seismic is necessary to evaluate such geological condition.

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