Title: 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.
The data on the 24 aquifers in both wells show a relationhip with the stratigraphy and fractures, but
some relations are inconclusive due to limited data. Evidence of aquifers associated with secondary
porosity is also present. The neutral pH hydrothermal alteration is dominant in the system with quartz,
calcite, chalcedony, wairakite, epidote, wollastonite and actinolite and clays. These are divided into
five alteration zones of smectite zone, smectite-chlorite, chlorite-illite, chlorite-illite-epidote, and
epidote-actinolite. Fluid inclusions provided a good control on the likely formation temperatures in
the wells, and showed the temperatures falling on the boiling point curve for both wells. Acidic fluid
was encountered in well LHD-23. A specific search was made in order to find acid alteration and
locate the source of acidic fluid entering the well. The acid alteration included kaolinite, pyrophyllite,
diaspore and sulphur. The most likely location for the acid aquifer was at about 1000 m in the well,
slightly below the production casing, and is possibly related to the fault structure F-6.
Depth to the first occurrence of epidote for all wells shows an N-S alignment of the geothermal
system, and possibly also a NW-SE alignment in the northern part. This compares well with the
geophysical MT-resistivity survey done in the area. These may indicate an effect of N-S structural
control (e.g. F-6) and NW-SE (e.g. F-2 and F-9).