From mid-1993 through early 1998 Unocal, under a Joint Operation Contract with Pertamina, conducted an extensive exploration program designed to locate and evaluate commercial geothermal systems within the Sarulla Block of North Sumatra, Indonesia. The exploration program included geological, geochemical, and geophysical surveys, the results of which were used to target 13 deep wells. This exploration program resulted in the discovery and appraisal of three new geothermal systems: Eastern Sibualbuali, Silangkitang, and Namora-I-Langit, with combined proven reserves of 330 MW for 30 years. Four wells were drilled in the Eastern Sibualbuali geothermal field, which is located on the extinct Sibualbuali andesitic stratovolcano and is cut by several major strands of the Great Sumatra Fault. The wells were all productive, finding a geothermal system with a maximum temperature of 267?C and with production zone temperatures of 218-248?C. Strong lateral and vertical temperature gradients found in the system are attributed to channeling of fluids along strands of the Great Sumatra Fault. Five wells were drilled in the Silangkitang field, located in the Sarulla graben along the main trace of the Great Sumatra Fault. Two of the wells found a strong upflow zone with fluid temperatures in excess of 310?C. A large-diameter well drilled in this zone is capable of producing fluid sufficient for 50 MW equivalent of generation, and has a maximum flowing wellhead pressure of more than 60 bar. As at Sibualbuali, the permeability of the Silangkitang system is also strongly controlled by the Great Sumatra Fault. Four wells were drilled in the Namora-I-Langit field, which is located several kilometers west of the main trace of the Great Sumatra Fault in an area of extensive thermal activity. These wells were all highly productive, encountering a large, high permeability geothermal system. The wells all produced fluids with temperatures in excess of 260?C, with a maximum measured temperature of 276?C. Three of the wells produced neutral Na-Cl brine, but the fourth produced a low-pH Na-Cl- SO4 fluid. The permeability of the Namora-I-Langit system, in contrast to Sibualbuali and Silangkitang, appears to be widely distributed and is not directly controlled by the Great Sumatra Fault.

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