The METI Hidaka-Trough Well was drilled offshore southern Hokkaido in 2019. The microfossil analysis was carried out to reveal geologic ages of the formations encountered in the well and to correlate them to the on-land formations. The microfossil datum planes allow the establishment of accumulation rate curve through Plio-Pleistocene. The curve shows non-deposition and/or eroded interval at the boundary between the Pliocene and Pleistocene, called submarine hiatus hereinafter. The hiatus ranges more than one million years based on the diatom biostratigraphy.

The unconformity at the same geologic age at around the base of the Quaternary is found in the Plio-Pleistocene Shiranuka Formation typically distributing along the coastal lines of the Atsunai and the Shiranuka, where the NPD8 and lower part of NPD9 are missing based on the diatom biostratigraphy. The unconformity was recognized in the middle of the Shiranuka Formation mainly composed of diatomaceous siltstone and the surface is overlain by a meter-scale conglomerate bed and tuff beds. Any significant difference of dip angles and strikes between the formations above and below the unconformity was not recognized. The implications of our findings include that the dual targets of hydrocarbon resources which are shallow gas above the hiatus and/or unconformity, and conventional thermogenic oil and gas beneath them, could be prospected in the study area.

Although the cause of development of the hiatus and the unconformity did not become clear, they are important traceable datum planes for correlation in the region. Further survey to estimate the lateral extent of the hiatus and the unconformity in the basin is the next issue.

Although the Niitsu oil field is one of three largest oil fields which were discovered before the Second World War in Niigata Prefecture, geological and petroleum engineering information on the field is limited. Petroleum geology including stratigraphy and geologic structure on the oil field was summarized using published papers and data in this paper. And the author presents some problems which remain to be not resolved.