Reservoir Rock of Ulubelu Geothermal System, Indonesia

Abstract

Ulubelu geothermal field lays in a volcano-tectonic depression produced from combined Sula post-caldera depression and tectonism related to the Great Sumatera Fault at 700 to 900 meter above sea level, Lampung, Indonesia. The field associated with a dormant strato-volcanoe called Mt.Rendingan in the northern part, in which its pyroclastic products fills the graben structure to the southern part. As a sequence of graben filling formation, the lithology distributed evenly inside the Ulubelu graben boundary. Drilling data and well test result show a conformable trend between reservoir feed zone and ignimbrite (rhyolitic tuff) layer distribution as a highly permeable zone. With its unique pattern and characteristic, the ignimbrite layer is proven to be a productive reservoir zone controlled by both lithological facies distribution and rock fracturing. Targeting this layer will improve the success ratio of production wells in Ulubelu geothermal field. This stratigraphic approach is effectively applied in volcano-tectonic depression settings filled by pyroclastic materials. With the same regional tectonic setting, this lateral permeability concept might be suitable and relevant to be applied in other geothermal fields along the Great Sumatran fault.