At the Iwafune-oki oil field which is located offshore Niigata Prefecture, multiple layers of sandstone forming a thick wedge exist at the lowermost portion of the Nishiyama Formation and a stratigraphic and structural combination trap is formed on the southwest nose of the Iwafune-oki anticline.
The sandstone is interpreted to have been deposited as turbidite under the upper to upper middle bathyal environment, from the results of the analyses of fossil foraminifera and lithofacies on the core samples. The examinations of the lithofacies based upon the classification proposed by MUTTI and RICCI LUCCHI (1972) indicate that the sandstone possess the characteristics of middle to outer fan.
Taking the information on paleocurrent and paleobathymetry into consideration, the writers experimented with modelling of a small submarine fan on the sandstone isopach map derived from the seismic records, well logs and geological information of the wells and the conclusion was that the fan must have grown northward.
It is inferred, based upon the examination of the lithology of the gravels contained in the sandstone, that the source material of the sandstone was supplied from the east or southeast mountainous area far from the offshore oil field. The same source may be said for the sandstone of the Shiiya and Nishiyama ages located around the Shintainai and Nakajo gas fields onshore, possibly through the same feeder channels.
