Overview of the Lahendong Geothermal Field, North Sulawesi, Indonesia: a Progress Report

Abstract

Lahendong is a geothermal field located in the northern part of the North Arm of Sulawesi. It forms part of the Sangihe volcanic arc. Re-examination of cores and cuttings recovered from some of the 17 drillholes to depths of 2500m, as well as re-interpretation of air photographs and field checking has enhanced our understanding of the geology and hydrothermal alteration of the field. The thermal manifestations are of steam-heated-type, and their distribution is mainly controlled by the NE – SW trending faults. However the hydrothermal alteration of cores and cuttings shows that the thermal fluid is water of near neutral pH. The deep reservoir is hosted by volcanic rocks of Pleistocene age and volcanic-sedimentary rocks of Pliocene age which are intruded by diorite dykes. The hydrothermal fluid-rock interactions have produced clays, calcite, anhydrite, pyrite, iron oxide, quartz, epidote, prehnite, pumpellyite, adularia, secondary albite, and possibly tourmaline. The hydrothermal mineral paragenesis suggests that at least 3 stages of alteration have taken place. The earliest stage formed mono-mineral veins and cavities and the second and the third stages yielded veins and cavities with more diverse mineralogy and texture. However, the hydrothermal mineral geothermometers suggest that the productive reservoir has not undergone significant temperature change since the minerals formed. This paper is a progress report of research aiming to determine the subsurface geology and to reveal the evolution of the Lahendong hydrothermal system.