Sedimentological and Oil-Geological Considerations on the Sandstones of the Shiiya and the Nishiyama Stages in the Northern Part of Higashikubiki, Niigata Prefecture

Abstract

The writer reported that the sandstones in this area were composed of material supplied from the topographical high place by mass flowes such as turbidity currents, and that thick sandstone developed in WNW-ESE and -SE directions decreasing thickness and turbulence from the central to the marginal (remote) area (Part 1, Jour. Jap. Assoc. Petrol. Techn., Vol. 30, No. 2, 1965).Porosity of the sandstone decreases with increasing median diameter and standard deviation of grains, and with increasing bed thickness. Therefore sandstones in the marginal area are more porous than in the central area, but thier capacity as oil reservoir is large in both the central and the middle areas, and small in the marginal area.Permeability of the sandstone decreases with decreasing porosity and increasing standard deviation of grain size, but has no relation to the median diameter. Permeability of the sandstone decreases when bed thickness increases, and the sandstones in the marginal area are more permeable than those in the central area.Thus the sandstones in the middle area as well as in the central area are suitable for reservoirs, while those in the marginal area are more porous and permeable in spite of their thinness.The sandstone fabrics control the directional permeability. Permeability of the sandstone is maximum in the direction parallel to the current flow which carried sand grains at the time of deppsition and minimum in the direction perpendicular to bedding plane.It is concluded that these characters should be taken into consideration in petroleum exploration and exploitation works.