Magnetotelluric survey for exploration of a volcanic‐rock reservoir in the Yurihara oil and gas field, Japan*

Abstract

The Yurihara oil and gas field is located on the southern edge of Akita Prefecture, northeastern Japan. In this area, drilling, surface geological surveys and many seismic surveys have been used to investigate the geological structure. Wells drilled into the Nishikurosawa Basalt Group (NBG) of Miocene age found oil and gas reservoirs at depths of 1.5–2 km. Oil and gas are now being produced commercially and further exploration is required in the surrounding areas. However, since the neighbouring areas are covered with young volcanic products from the Chokai volcano, and have a rough topography, the subsurface distribution of the NBG must be investigated using other methods in addition to seismic reflection. According to the well data, the resistivity of the NBG is comparatively higher than that of the overlying sedimentary formations, and therefore the magnetotelluric (MT) method is expected 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 was 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 conclusion, 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 resistive area is interpreted as corresponding to the reservoir. By comparison with a seismic section, we prove the effectiveness of the integration of seismic and MT surveys for the investigation of the morphology and internal structure of the NBG. On other survey lines, the resistive uplifted zones are interpreted as possible prospective areas.

2.5‐D inversion of frequency‐domain electromagnetic data generated by a grounded‐wire source

“…The targets that we are interested in are those of oil and geothermal exploration. In the investigation of volcanic-rock oil reservoirs (Mitsuhata et al, 1999) and geothermal reservoirs (Uchida, 1990), we often need to detect a resistive basement overlain by a thick conductive layer rich in clay minerals. The existence of the clay minerals decreases the permeability for fluids, and hence the conductive layer works as a cap layer for fluids flowing upward.…”

Volcanic settings and their reservoir potential: An outcrop analog study on the Miocene Tepoztlán Formation, Central Mexico

“…porosity and permeability) of a rock unit is important for assessing its reservoir potential, not only for the oil-and gas industry but also for a variety of industrial applications,. In volcanic or volcanic-derived rocks these data are used to determine (1) groundwater resources (Foster et al, 1985;Anderson, 1994;Flint and Selker, 2003), (2) oil or gas storage (Mitsuhata et al, 1999;Levin, 1995;Hinterwimmer, 2002), (3) geothermal reservoirs (Stimac et al, 2004;Arellano et al, 2005, Carranza et al, 2008, and (4) potential environmental impacts associated with waste disposal (Neretnieks, 1980;Neeper and Gilkeson, 1996). Volcaniclastic deposits are widespread in many convergent margin and rift basin settings that have hydrocarbon production or potential, but have traditionally been regarded as poor targets for exploration (Manville et al, 2009).…”

“…Volcanic rock reservoirs have been reported in many locations around the world, e.g. California (Nakata, 1980), Argentina (Sruoga et al, 2004), Russia (Levin, 1995), Japan (Shimazu, 1985;Sakata et al, 1989;Mitsuhata et al, 1999), Indonesia (Sembodo, 1973) and especially China (Wang et al, 1997;Chen et al, 1999;Jinglan et al, 1999;Luo et al, 1999Luo et al, , 2005Guo, 2001;Wang et al, 2003a-c;Wang et al, 2008;Feng, 2008), demonstrating the high potential for hydrocarbon exploration in volcanic and associated sedimentary rocks (Feng, 2006). A statistical analysis of the known volcanic gas reservoirs around the world was done by Zhang and Wu (1994;Tab.…”

Organic geochemical and fluid inclusion evidence for filling stages of natural gas and bitumen in volcanic reservoir of Changling faulted depression, southeastern Songliao basin

“…In the world, oil or gas volcanic reservoirs have been discovered (Sruoga et al, 2004;Mitsuhata et al, 1999;Levin, 1995;Sakata et al, 1989;Nakata, 1980;Sembodo, 1973). In the faulted depression of eastern China, there is also widely exiting oil or gas of Early Cretaceous in Xujiaweizi faulted depression of southeastern Songliao basin and Jurassic in western Liaohe depression of Bohaiwan basin volcanic rocks (Feng, 2008(Feng, , 2006Zhao et al, 2008;Zhang et al, 2006;Wu et al, 2003;Gu et al, 2001).…”