SUBSURFACE HYDROTHERMAL ALTERATION AT THE ULUBELU GEOTHERMAL FIELD, LAMPUNG, SOUTHERN SUMATRA, INDONESIA

摘要

The liquid dominated Ulubelu geothermal field is situated in Lampung, southern Sumatra, Indonesia. The field lies in steep terrain between 300 and 1600 m above sea level (a.s.l.) but mostly from 700 to 800 m. The host rocks are mainly Quaternary andesitic pyroclastics and lavas. The field is at the southern end of the Great Sumatra Fault. The basement comprises pre-Tertiary volcanic rocks. Samples from 3 wells, namely UBL-01, UBL-02 and UBL-03, were studied and geophysical data, made available by Pertamina including gravity and resistivity results, were used. Hydrothermal minerals display a distinct zonation of both clay and non-clay minerals. They show that the rock-altering fluid was produced from a liquid dominated reservoir. There are four hydrothermal mineral zones, namely: smectite, mixed-layer clays, chlorite and chlorite-epidote. The smectite zone is present at shallow levels (from near surface to 650 m a.s.l.). The mixed-layer clay zone occurs at 500 to 650 m (a.s.l.). The chlorite zone occurs at 100 to 500 m (a.s.l.). The chlorite-epidote zone occurs deeper than 100 m (a.s.l.). The measured formation temperatures are generally lower than those indicated by the mineral geothermometers (chlorite, epidote, wairakite and others) by about 20 deg to 60 deg C and the fluid inclusion geothermometers by 10 deg to 100 deg C. Cooling may have occurred although the measured well temperatures may be too low, as wells were not thermally stable when the measurements were made. The permeability of the reservoir is indicated by the presence of albite and adularia. These minerals mostly indicate higher permeable zone and coincide with the location of the production zones.