Lahendong geothermal field, Indonesia : geothermal model based on wells LHD-23 and LHD-28

Abstract

The Lahendong high-temperature geothermal field is tectonically located in the northern neck arm of Sulawesi Island. The field is within the depression of the Pangolombian caldera with clear fault structures in the eastern part but relatively open to the west. Permeability structures on surface are mostly fault controlled with a dominant NW-SE and NE-SW directions. The field is a liquid dominated system divided into two reservoirs, the hotter southern one and the colder Northern one. A total of twenty eight wells have been drilled into these reservoirs. A total 60 MWe are produced from nine wells. This study focuses on two wells in this system, LHD-23 and LHD-28. The main emphasis was on analysing the cuttings with binocular, petrographic and fluid inclusion microscopes, XRD, and then microprobe and chemical analysis with ICP equipment on selected samples. The strata are divided into three main formations: Post Tondano is the youngest, then the Tondano unit which is the pyroclastics of a catastrophic eruption during the collapse of the large Tondano depression and lastly the Pre-Tondano succession. The strata are predominantly made up of basaltic andesite lavas, tuffs and breccias. Chemical analysis, however, shows that a considerable part of the Pre-Tondano series is rhyolitic in composition, indicating a different volcanic evolution. The depth of the Tondano unit in the Lahendong wells show irregularitites which may suggest a subsidence within the Pangolombian caldera structure