Comparative Subsurface Geology and Hydrothermal Alteration of Pad A and Pad E Wells in Sorik Marapi, North Sumatra, Indonesia

Abstract

The Sorik Marapi Geothermal Project (SMGP) is a greenfield area located within the administrative region of Mandailing Natal in North Sumatra, Indonesia. Fast-tracked exploration and development drilling operation in Sorik Marapi geothermal field commenced in late 2016, successfully completing 18 deep wells in 2018. The first well drilled is Well A-101 in Pad A (Area 1), situated close to the eastern flank of Mt Sorik Marapi and immediately west of the main Sumatran fault. The success of initial wells drilled on this pad encouraged exploration drilling in Pad E, about 4.5km NNE of the volcano. To date, Pad A holds seven directional wells, drilled to a maximum depth of 2100mVD. The wells penetrated three volcanic units; Sibanggor Volcanics - the oldest composed mostly of altered undifferentiated volcanics; Roburan Volcanics - a relatively thin layer of predominantly Dacitic rocks and Sorik Volcanics - the youngest andesitic rocks overlying all other formations. Similarly, the two wells drilled in Pad E (Area 2) encountered these same formations, but Roburan Volcanics is notably extensive in this area of the field. The dominant alteration mineral assemblage of A-series wells exemplified an alteration suite largely produced by neutral pH geothermal fluids. High-temperature alteration minerals are prevalent in the production zones of Pad A wells, where estimated formation temperature exceeds 250°C. On the other hand, Pad E wells showed low to medium ranked hydrothermal alteration mineralogy with subsurface temperature of less than 220°C. The drilling results from Pad A confirmed the existence of a high temperature geothermal resource in Sorik Marapi’s Resource Area 1. Contrarily, Pad E showed very low permeability and relatively cooler temperature, which revokes previous theory of a possible separate upflow in Area 2.