Thick silicic tuff sequences and silicic intrusives are found in deep wells drilled at the Muara Laboh and Rantau Dedap Geothermal Fields, Sumatra, Indonesia. Petrographic and petrophysical investigation of cuttings, core, gamma ray and...
moreThick silicic tuff sequences and silicic intrusives are found in deep wells drilled at the Muara Laboh and Rantau Dedap Geothermal Fields, Sumatra, Indonesia. Petrographic and petrophysical investigation of cuttings, core, gamma ray and image logs were used to understand the silicic stratigraphic controls on reservoir permeability. Regional geologic constraints and limited U-Pb zircon ages show that silicic explosive volcanism most likely occurred mainly during the Miocene to Plio-Pleistocene, and locally continuing into the Holocene. Wells in both fields show that silicic tuff sequences reach thicknesses of 500 to >1000 meters, and silicic intrusive complexes intrudes to reservoir depths. In Muara Laboh, thick silicic tuffs are found in a basin generated between major strike-slip fault segments. The dominant rock type is variably welded to non-welded silicic ash-flow tuff with a variety of devitrification textures. A long-lived sheared and altered granite-granodiorite-microdiorite intrusive complex representing multiple magmatic episodes (96 to 20 Ma) occurs in the SW sector of the field. In Rantau Dedap, similar silicic tuff sequences are found as caldera fill deposits overlain by debris flows. A weakly altered, poorly deformed granite to granodiorite intrusive complex occurs at depth. The silicic volcanics have high resistivity in image logs, fine fragmental textures (tuffs) to massive textures (intrusives), and high gamma ray counts (65 to 200 API) in both fields. Fracture intensity of the thick silicic tuff sequences increases with welding, primary devitrification, and possibly the thickness and cooling history of individual eruptive units. Major feed zones are associated with faulted lithological contacts and very limited at thick silicic tuffs in Muara Laboh. Fluid entries at Rantau Dedap are most abundant in the relatively thin Upper Rhyolite Tuff and underlying Dacite Tuff, and near the basal contact of the thick Lower Dacite Tuffs with intrusions. Permeable zones are encountered at the margin of the intrusive complexes in both fields.