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• The Muara Laboh geothermal system is a liquid-dominated, fracture-controlled reservoir showing some characteristics of both intrusion-related and fault circulation systems. • The developed reservoir has two distinct sectors of differing fluid chemistry and temperature. • Reservoir fluids generally have low salinity (∼400-1600 ppm Cl) with benign chemistry and low non-condensable gas (NCG) content (∼0.5 to 2 wt% in steam). • In the deep SW reservoir, fluids ascend along WNW to NNW and N to NE fracture, dikes, and stock margins. • In the shallower NE steam cap and outflow, N to NE steeply dipping fractures dominate, with bounding N to NNW faults. The Muara Laboh geothermal system is a liquid-dominated, fracture-controlled reservoir showing some characteristics of both intrusion-related and fault circulation systems. The developed reservoir has moderate to high-temperature (230–310 °C) with a long NNW outflow (160–230 °C) extending to the Sapan Malulong boiling chloride springs. Reservoir fluids generally have low salinity (∼400-1600 ppm Cl) with benign chemistry and low non-condensable gas (NCG) content (∼0.5 to 2 wt% in steam). The proven reservoir is divided into distinct SW and NE sectors. The principal deep upflow zone (270 to 310 °C) is located in the SW and is associated with Patah Sembilan volcano and satellite vents (Anak Patah Sembilan). The SW upflow is characterized by relatively low salinity (∼400-600 ppm Cl), whereas fluids in the NE emerge from a second upflow along faults and fractures at 240–250 °C (1200–1600 ppm Cl) and ascend to an initial-state steam cap near the Idung Mancung fumarole. The shallow NE part of the geothermal system is hosted mainly by Quaternary to Miocene Age andesitic to rhyolitic rocks, whereas the deeper SW part of the system occurs in a Mesozoic to Cenozoic Age plutonic complex and its host rocks that are cut by younger dikes. Rock porosity and permeability trends can be related to reservoir age, volcanic deposit type, and alteration history. Fracture permeability is high, whereas matrix porosity (<1-7%) and permeability are moderate to low and decrease with depth. The volcanic sequence is capped by tuffs dated at ∼34,000 to 41,000 years BP, incapsulated in debris flows. This sequence may have formed during and shortly after likely sector collapse episodes of the Patah Sembilan volcano. The shallow clay cap of the system wraps around the crater, suggesting that it was excavated during crater formation. Permeable fracture patterns and their origins differ somewhat between the deep SW and the shallow NE reservoir. In the deep SW reservoir, fluids ascend along fractured margins of stock and dike intrusions. Steeply dipping intrusions and open fractures strike from WNW to NNW, subparallel to the main Great Sumatra Fault trend, with a secondary set of N to NE fractures and intrusions parallel with the inferred maximum horizontal stress direction (SH Max). In the shallower NE steam cap and outflow, N to NE steeply dipping fractures dominate, with bounding N to NNW faults. Bulk rock alteration includes shallow argillic and transitional clay zones overlying phyllic and propylitic zones. High-T propylitic alteration (secondary amphibole) and rare potassic alteration (secondary biotite) are associated with intrusion. Vein paragenesis in the SW reservoir indicates that portions of the initially permeable propylitic zone have been sealed by calcite and quartz ± prehnite. Late calcite veins suggest ingress of steam-heated bicarbonate waters contributed to resealing of the system. These relationships suggest that the SW system underwent extensive boiling, fluid loss, and inflow of dilute bicarbonate waters, possibly related to sector collapse and partial excavation of the topseal of the system (now Patah Sembilan crater). [ABSTRACT FROM AUTHOR]