Classification of Cap Rocks in Geothermal Systems

Abstract

The cap rocks in geothermal systems are divided into the primary type and the secondary one, and the latter is called the “alteration cap rock”. The alteration cap rock is impermeable altered layer in near-surface aquifer, formed by upflow of geothermal fluids. The alteration cap rocks are subdivided into porous sediment type, younger lava type and fracture type by the kind of near-surface aquifer. And they are subdivided into neutral type, H₂S related acid type and SO₂ related acid type by the kind of hydrothermal alteration. Many active geothermal areas explored for power station are classified by the cap rock type, in this paper. Porous sediment type; (Mori), Uenotai, Onikobe, (Oku-aizu), Takigami, Fushime Younger lava type; Otake, Hatchobaru, Oguni, Kirishima Fracture type; (Mori), Onuma (Sumikawa), Matsukawa, Kakkonda, (Oku-aizu) Neutral type; Mori, Onuma (Sumikawa), Kakkonda, Uenotai, Onikobe, Oku-aizu, Kirishima, Oguni, Fushime H₂S related acid type; Takigami, Otake, Hatchobaru SO₂ related acid type ; Matsukawa It appears that many active geothermal areas have alteration cap rocks, which are mainly com-posed of neutral alteration type. A alteration mineral is mainly montmorillonite. The alteration cap rocks have role of preventing deep hot waters from cooling, of keeping hot waters in the geothermal reservoirs, and of adsorbing of H₂S and CO₂ gases.