1997 Volume 62 Issue 1 Pages 59-68
Magnetotelluric (MT) data were obtained along a line which is almost perpendicular to the strike of the geologic structure in the Higashi-Kubiki area, Niigata prefecture, Japan. The resistivity structure analyzed from a two dimensional inversion of the MT data is consistent with the geologic structure estimated from other survey results. We interpreted the resistivity structure in comparison with the MITI Higashi-Kubiki well data such as resistivity, porosity, temperature, vitrinite reflectance and altered-mineral contents. The results are as follows.
(1) Resistive layers at or near surface correspond to the Uonuma formations, which were not altered.
(2) Very conductive layers of about 1 to 3Ωm at a depth range from 1 to 3km correspond to the Nishiyama and the Shiiya formations. The low resistivity is primarily due to high contents of smectites which were produced by diagenesis. Because the permeability of rocks containing plenty of smectites is generally low, the conductive layers may form cap rocks.
(3) Below the conductive layers, resistivity increases with depth. The relatively resistive zone of about 3 to 100Ωm corresponds to the Upper Teradomari, the Lower Teradomari, the Nanatani formations and the Green Tuffs. According to the logging data of some nearby wells, this resistivity range agrees with a vitrinite reflectance range from 0.5 to 1.2%. Therefore the resistive zone is interpreted as source rocks of this area.
(4) Very resistive layers of more than 300Ωm at deeper than about 10km are probably pre-Tertiary basement rocks or intrusive rocks, which contain probably no oil and gas.
