Fission-track dating using both internal and external surfaces of zircons was carried out for ten core samples from the Miocene formations in the Ayukawa-Yurihara district, Akita Prefecture. These samples were collected from the Yashiozawakawa, Nishikurosawa and Onnagawa Formations of eight wells drilled for the petroleum exploration in this area.After the detail examination for grain ages of each sample using histogram, age spectrum and radial plot, fission-track ages are interpretated. The results are summarized as follows.1 Although external fission-track age 20.1 1.3 Ma of andesite in the upper part of the Yashiozawakawa Formation shows the eruption age with some possibility of rejuvenation, internal fission-track age 16.4 0.6 Ma shows the thermal event which probably corresponds to the start of the graben-fill basalt activity.2 The most reliable internal fission-track age 12.2 0.4 Ma of acidic tuff in the Nishikurosawa Formation is in harmony with the biostratigraphic age of this formation. The external fission-track age 16.6 0.8 Ma indicates that the provenance of zircons is probably the Hatamura Formation.3 The most reliable fission-track age of tuffs between the O-1 and O-2 benthonic foraminiferal assemblages, 8.6 0.4 Ma of the Ayukawa-kita well, is consistent with Sr isotopic age about 9 Ma of the O-2 assemblage. The age of the O-1 assemblage is estimated 8.2 Ma based on K-Ar ages of the overlying Fukuyama andesite. Therefore, the O-1 assemblage is correlated with Blue zone in Niigata Prefecture, and has a possibility of regional correlation marker.4 This study indicates that the fission-track dating using both surfaces and more than one samples of the same horizon is a valuable method for the use of few and expensive cores.

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The producing reservoirs of the Ojiya oil eld, Niigata Prefecture, which belong to the Uonuma Group in the northern part（Yamaya area）and the Haizume Formation in the southern part（from Jigokudani to Kirikubo areas） , are correlated to the Nishiyama stage in the Niigata well stratigraphy.Head space gases from the MITI † Oguni explorator y well and surface seepage gases are mainly thermogenic origin based on their carbon isotopes, whose maturity is estimated to be 1.0 to 1.1 ％ Ro equivalent, and were probably generated in the Lower Teradomari Formation below 4,000 m in depth.As oil properties such as speci c gravity and distillation test result are clearly different between the Yamaya oil and the others（the southern oil） , the Yamaya pool is separated from the others. The southern oil geochemically resembles the Sagara oil, Shizuoka Prefecture, which indicates that the oil were generated in the deeper part of the basin, migrated upward through faults, laterally migrated passing through the gas showing inter vals in the MITI Oguni well, and accumulated. The Yamaya oil was probably migrated through the reverse fault in the Nishiyama Formation of the Ojiya SK-1 and SK-2 wells.