Drilling activity and subsurface geology of the MTL-1 shallow well in the Mataloko geothermal area

Abstract

A shallow exploration well MTL-1 was drilled in October 1999 in the Mataloko geothermal field of the Ngada District of Flores Island by the Volcanological Survey of Indonesia (VSI) as a part of the Research Cooperation Project on the Exploration of Small-scale Geothermal Resources in the Eastern Part of Indonesia (ESSEI). The drilling encountered a sudden steam flowing at the 103.23 m depth, while the target depth was 250 m. The surface temperature of the steam was 115 °C whereas the wellhead pressure of 3 barg was measured during the well shutdown. The steam generated from a deeper part of the Mataloko geothermal reservoir escaped to the surface throughout the Wae Luja fracture zone near the well. The subsurface stratigraphy varies between andesitic tuff, pyroxene andesite, andesitic tuff breccias and andesite, which are hydrothermally altered to medium temperature (120 -190℃） of argillic type with 2 -15 % swelling clay.