A multidisciplinary approach for subsurface investigations of the Goshogake geothermal field, Japan

Located on the western flank of the Akita Yakeyama Volcano lies the Goshogake hydrothermal system of Tohoku, Japan. Goshogake is situated between the magmatic volcanic arc and the hydrocarbon province in the back-arc of Japan. This system exhibits a variety of surface manifestations with temperatures ranging between 33°C to 97°C and low, acidic pH values of approximately 2.5. Seepage sites include sulphur-rich fumaroles, localized possible oil-rich pools, active mud- erupting gryphons, small hydrothermal lakes, and clustered bubbling pool fields, implying along with its geodynamic position, a possible hybrid field.

Hydrothermal emissions emerge at the base of a narrow valley, where a N-S oriented fault system framing the Akita Yakeyama Volcano is suggested to occur. Intense seismicity caused by the active tectonics in the region relates to ongoing deformation. To investigate the interconnection between the emission sites and the reservoir(s) feeding them, a 3D Deep Electrical Resistivity Tomography study was conducted by deploying 25 Iris Fullwavers across the Goshogake system. The results were combined with drone-derived thermal photogrammetry, satellite images and geochemical analyses of the seeping fluids.

The inverted resistivity model reveals a conductive system that enables discrimination of the conduits feeding the vents. Geochemical analyses reveal the presence of H2S and CO2 dominated gas with mantle-derived isotopic signatures; the presence of minor quantities of CH4 in colder seepage sites, whether thermogenic or abiotic,remainunsolved.The detected H2 among the seeping gasses may result from the interaction of hydrothermal fluids with pyroxene andesite lava host-rocks.

Further analyses of the possible oil films aim to distinguish its origin with several potential scenarios. The first scenario involving the presence of shallow oil derived from the alteration of recent organic- rich deposits, or a second scenario involving deep oil originating from the thermo- metamorphism of deep-seated lacustrine deposits hosting the volcanic complex. If the latter is validated, Goshogake could well be Japan’s first example of a sedimentary-hosted geothermal system.