Magnetotelluric survey for exploration of a volcanic-rock reservoir in theYurihara oil and gas field, Japan

The Yurihara oil and gas field is located on the southern edge of Akita Pre fecture, northeastern Japan. In this area, drilling, surface geological sur veys and many seismic surveys have been used to investigate the geological structure. Wells drilled into the Nishikurosawa Basalt Group (NBG) of Mioce ne age found oil and gas reservoirs at depths of 1.5-2km. Oil and gas are n ow being produced commercially and further exploration is required in the s urrounding areas. However, since the neighbouring areas are covered with yo ung volcanic products from the Chokai volcano, and have a rough topography, the subsurface distribution of the NBG must be investigated using other me thods in addition to seismic reflection. According to the well data, the re sistivity of the NBG is comparatively higher than that of the overlying sed imentary formations, and therefore the magnetotelluric (MT) method is expec ted to be useful for the estimation of the distribution of the NBG. An MT survey was conducted along three survey lines in this area. Each line trended east-west, perpendicular to the regional geological strike, and wa s composed of about 25 measurement sites. Induction vectors evaluated from the magnetic field show that this area has a two-dimensional structure. The evaluated resistivity sections are in agreement with the log data. In conc lusion, we were able to detect resistive layers (the NBG) below conductive layers. The results indicate that the NBG becomes gradually less resistive from north to south. In the centre of the northern line, an uplifted resist ive area is interpreted as corresponding to the reservoir. By comparison wi th a seismic section, we prove the effectiveness of the integration of seis mic and MT surveys for the investigation of the morphology and internal str ucture of the NBG. On other survey lines, the resistive uplifted zones are interpreted as possible prospective areas.