THE RESERVOIR OF THE RENDINGAN-ULUBELUWAYPANAS GEOTHERMAL SYSTEM

Abstract

The Rendingan-Ulubelu-Waypanas (RUW) geothermal system, is dominated by products of Quaternary and Tertiary volcanism (the Tanggamus volcanism) consisting mainly of andesitic lavas and pyroclastics. Thermal manifestations occur over a distance of more than 15 km within the Ulubelu graben and southwest of Mt. Waypanas. Six exploration wells encountered temperatures from 50 to 220 oC and penetrated rocks that mostly also occur at the surface. Microearthquake epicenters were determined using a random analysis method as velocity structures are not known. Gravity data were interpreted using ‘iterative forward modeling’ and deconvolution methods and gave consistent results. Ground magnetic data revealed the effects of deeper bodies. The Mt. Rendingan pyroclastics and andesite lavas, Mt. Kukusan basaltic andesite lavas and Mt. Kabawok pyroclastics are normally magnetised. The magnetic interpretation, together with results from the mapping of surface manifestations, microearthquake study and previous Schlumberger resistivity surveys, show that the RUW geothermal system is a single unit, covering an area of about 150 km2. Measured well temperatures and pressures, hydrothermal mineralogy, and the thermal characteristics of fluids trapped in inclusions indicate that in the central part of the system, perched meteoric water and steam condensate occurs above 250 m depth (450 m a.s.l.). Between about 250 m and 550 m depth (450 m and 150 m a.s.l.) the reservoir contains vapor with two phases occurring from about 600 m to 800 m depth (100 m a.s.l. to 100 m b.s.l.). Alkali chloride water with a near neutral pH and a low concentration of dissolved carbon dioxide occurs below 800 m depth (100 m b.s.l.). A pronounced temperature reversal in well UBL1 indicates an inflow of cooler water at about 700 m depth; this is probably meteoric water descending a fault zone. Convection occurs below 800 m depth, consistent with the presence here of high permeability indicating minerals (i.e. adularia and albite). The dimensions of the RUW geothermal system have changed spatially and temporally during its life, but the sequence, chronology and directions of the changes are incompletely known.