ABSTRACT: A geophysical model and the subsurface geology at the Ulubelu Geothermal Area, Lampung, Indonesia

A gravity low anomaly occurs in the north central part of the Ulubelu geothermal area, in Lampung Province, Indonesia that lies between the Great Sumatra Fault and the Semangka Fault at the southern end of Sumatra. The area is situated in steep terrain, from 300 m to 600 m RSL, surrounded by young volcanoes. Gravity and resistivity data and samples recovered from three wells (UBL-1, UBL-2 and UBL-3) were kindly provided by Pertamina for our study.

A gravity low, interpreted by 2-D modelling, it is associated with a graben. A resistivity low shows the field is large but with still unlocated boundaries to the south that may be associated with up-flows of hydrothermal fluids. The presence of veins with adularia, calcite, hematite and two-phase inclusions, together with vapour inclusions, suggests that the altering fluid was of neutral pH, alkali chloride water and boiling locally.

Hydrothermal minerals in the reservoir, including adularia, albite, wairakite, epidote and clays occur in samples from all three wells. These show distinct zoning. Four hydrothermal mineral zones occur, namely: smectite, mixed layer clays, chlorite and chlorite-epidote. The presence of adularia and albite suggests higher permeability occur generally from 100 to -150 RSL.

Fluid inclusion geothermometry studies of anhydrite, calcite and quartz yield equivalent wt. % NaCl values (0.0 to 0.9 %) that are close to that of pure water. The homogenisation temperatures are between 200° and 250° C in samples from depths between 400 and 920 m (300 to -250 RSL) although the downwell temperatures are now lower than the fluid inclusion homogenisation temperatures by 10° to 100° C.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia