Abstract

Sedimentary rocks of the Niigata basin, located on the Japan Sea side of central Japan Island, range in age from middle to recent Miocene. Petroleum production from this basin comes mainly from “deep sea sandstones” of the Shiiya Formation (late Miocene). One of these sandstones, called Minami-Aga Sandstone, is the main producing sand of four oil and gas fields in this basin.

Studying the lithology and thickness change of the Sandstone led the author to the conclusion that the Sandstone was deposited as a submarine fan, and fits Walker’s model of submarine fan deposition. The petrographic study of the pebbles and sand grains revealed that the source area of the turbidite was to the south. It flowed to the north, having the mouth of the feeder channel around Muramatsu. The Sandstone shows “upper-fan” facies at Kuwayama gas field, “mid-fan” facies at Minami-Age oil field (about 5 km north of Kuwayama gas field), and “lower-fan” facies at Higashi-Niigata oil and gas field (about 19 km north or Minami-Aga oil field) and at Aga-Oki oil field.

Detailed lithological examination of the sandstone in Minami-Aga oil field revealed that the sandstone is the product of two different types of turbidity current. The lower part is composed of pebbly sandstone and conglomerate and suddenly thins out towards the top of the anticline. Sand being supported and transported by several occurences of grain flow (high concentration turbidity current; Walker 1978), was deposited on the eastern part of Minami-Aga anticline which was growing up at that time, and filled up the channel by quick sedimentation. On the other hand, the upper part is composed of an alternation of thin sandstone and thin claystone, and the claystone ration increases towards the top of the anticline. It was mainly supported and transported by fluid turbulence (low concentration turbidity current; Walker 1978), and formed “fan-lobe”. The former is more in nature of a petroleum reservoir than the latter.