TWO CONTRASTING GEOTHERMAL FIELDS IN SUMATRA, INDONESIA: MUARA LABOH AND RANTAU DEDAP

Abstract

Muara Laboh and Rantau Dedap share a tectonic setting but their different distances from the Intra-Arc Volcanic Barisan Mountains and Great Sumatra Fault (GSF) results in contrasting reservoir rocks, structure, and permeability. Muara Laboh is located in a pull-apart basin between two GSF segments and the reservoir is hosted dominantly in multiple granitic-dioritic intrusives, silicic volcanics, and intercalated andesitic-silicic volcanic rocks with minor sediments. Productive permeability is influenced both by NW-SE and NNE-SSW oriented faults and the geometry of intrusions, but overall the reservoir is oriented in the NW direction and parallel to the GSF. The reservoir top at Muara Laboh varies greatly in depth, and does not correlate well with the base of the conductor imaged by MT.
In contrast to Muara Laboh, Rantau Dedap is offset approximately 15 km east from the GSF and has a structure and stratigraphy that resembles the South Sumatra Back-Arc Basin. The reservoir in Rantau Dedap appears to be dominated by volcano-sedimentary rocks and includes a marine sequence that is absent at Muara Laboh. The reservoir permeability, in terms of the feedzone locations, appears highly influenced by the distribution of the brittle rhyolitic lavas that host intense fracturing. Reservoir rocks are faulted by NNW-SSE and NE-SW oriented structures that appear to be influenced by large basement faults. The reservoir is oriented in the NE direction, perpendicular to Muara Laboh and the GSF. In contrast with Muara Laboh, the reservoir top at Rantau Dedap is nearly flat. However, like Muara Laboh, reservoir top does not correlate well with the base of the conductor.