Geology of Lumut Balai Geothermal Field

Abstract

Lumut Balai Geothermal Field is operated by Pertamina Geothermal Energy, located in Penindaian Village, Muara Enim Regency, South Sumatra Province. Research in Lumut Balai has been conducted since 1993, exploration wells were drilled in 2008, followed by development wells, with a total of 29 wells. Geological model should provide ample information and interpretation of subsurface geological conditions such as the lithology distribution, alteration distribution, reservoir zone, heat source, and permeability. A combination of surface and subsurface data are required in order to construct a geological model. On the surface, outcrops provide information regarding lithology, alteration type, and geological structures, while manifestations provide fluid geochemistry data. Subsurface data are acquired from rock cuttings and core samples (which are then further analysed through petrographic analysis and XRD analysis), drilling parameter, and borehole image logging. The methods above results in an interpretation of Lumut Balai geological model. Lumut Balai is a geothermal system located within the Lumut Balai caldera, composed of andesite, andesite breccia, basaltic andesite, basalt, limestone, metasediment, tuff, and tuff breccia which are all altered to certain extents. There are five stratigraphic units, starting from the oldest to youngest are Tertiary Basement Unit, Pre-Old Lumut Unit, Pre-Caldera Unit, Caldera Unit, and Post-Caldera Unit. Alteration zones in Lumut Balai could be classified into smectite+chlorite zone, silica+chlorite zone, and chlorite+epidote zone. Productive faults in Lumut Balai are Air Ringkih Fault with NE-SW trend, Air Udangan Fault trending NE-SW, Old Lumut Caldera, and Air Gemuha Besar which trends N-S.