Productive Structural Geology in Volcanogenic System: A case study of Lumut Balai Geothermal Field, Indonesia

Lumut Balai is geothermal field that occurs in the caldera system, named Lumut Tua Caldera, approximately 9 km in diameter. Located in Penindaian Village, Muara Enim Regency, South Sumatra Province. Lumut Balai is one of the fields operated by Pertamina Geothermal Energy. The volcanism in Lumut Balai is closely related to the Sumatra Fault Systems activity. Surface data such as 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 analyzed through petrographic analysis and XRD analysis), drilling parameters, and borehole image logging. These provide information and interpretation of subsurface geological conditions such as lithology distribution, alteration distribution, reservoir zone, heat source, and permeability. The methods above result in an interpretation of the 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, meta-sediment, 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, Lumut Tua Caldera and Gemurah Besar which trends N-S. This study aims to confirm the productive geological structure in Lumut Balai and gives better understanding for future field development, such as well targeting for makeup wells and reservoir simulation.