Formation and Fracture Characterization of the Muara Laboh Geothermal System, Sumatera, Indonesia

摘要

We describe controls on permeability of the Muara Laboh (ML) geothermal system. ML is situated in a pull-apart basin formed in a stepover between segments of the Great Sumatera Fault (GSF). Intrusion and volcanism occurred primarily along the southern fault segment. Dilation accommodated prolonged intrusion at depth and overlying brittle extension within the pull-apart basin. Borehole image logs (BIL) and cuttings and core data were integrated to constrain rock types and their structural properties and to infer the orientations of permeable fractures determined by PTS and PT logs. Trends of open fractures are NW and N to NE. NW structures are subsidiary to the GSF, whereas younger N to NE structures grew between GSF fault strands. Detailed fracture analysis at entries indicates that deep permeability in the SW reservoir is associated with intrusion contacts. Most dike and fracture orientations follow the GSF trend, but N and NE dikes and fractures are also observed in high permeability wells. In the shallow NE reservoir permeability is related to formation contacts and N and NE extensional structures in brittle lithologies (lava flows, breccias and devitrified tuffs). Adularia (Ad) and epidote (Ep) were deposited on fracture initiation and are found in open-space veins in the system upflows, whereas wairakite (Wai) and quartz ±prehnite (Qtz±Preh) dominate in the shallow steam cap region. An episode of cooler steam-heated inflow is represented by anhydrite ± calcite (Anhy±Cal) in the NE. In the deeper SW reservoir boildown and ingress of steam-heated water led to calcite and quartz precipitation that reduced permeability in the upper propylitic zone. Distribution of matrix porosity (Φ) and permeability (k) is tied to the history of fracture formation, infilling, and fluid sources and temperatures.