A Lithostatically Pressurized Clay Cap of the Vapor-dominated Reservoir Deduced from Hydrothermal Alterations of Borehole Cuttings in the Mataloko Geothermal Field, Flores Island, Eastern Indonesia

Abstract

X-ray powder examination on the alteration of the drill hole cuttings revealed a mineralogical zoning of kaolinite -α-cristobalite (I) -, zeolite (II) - and kaolinite (III) - zones toward the bottom of the well MT-1 in the Mataloko geothermal field, central Flores, eastern Indonesia. Zone II is further subdivided into the upper heulandite (ha) -, middle laumontite (IIb) - and lower wairakite (IIc) - subzones in descending order. A temperature profile in the crystallization process deduced from the phase equilibria of zeolite minerals and the kaolinite crystallinity index dramatically increases toward the deeper part exceeding the hydrostatic boiling-point curve of pure water and reaching 236°C at a depth of around 180m. This depth is ascribed to the maximum enthalpy point on the two-phase region of water, dividing the deeper vapor-dominated reservoir and shallower vapor condensation zone. The vapor condensation zone generally is 360m thick, but in the Mataloko area it was only half as thick, owing to lithostatic pressure. Abundant clay and zeolite minerals in Zone II might have formed an extremely impermeable zone, and the continuing excess heat supply might have generated a lithostatically-pressurized clay cap. The lithostatic pressure has since been released.