| Abstract |
The hot fluid upflow at Awibengkok (Salak) appears to be controlled by deep intrusion along the general E-W trend of the Sunda Volcanic Arc. At shallow levels of the geothermal system the hot fluid upflow is localized along N- to NE-trending faults and fractures where subvolcanic intrusions (small stocks, dikes, sills) may be found closest to the surface. The NNE-trending Awi fault, which controls the locations of recent volcanic vents, plays an important role in segmenting the system, and allowing the descent of shallow fluids into the eastern portion of the geothermal reservoir. Similarly, the NNE-striking Gagak fault forms an important compartment boundary on the west. Together these structures bound a zone of extension, recent volcanism and subsidence that hosts a thicker and more permeable volcanic pile than the adjacent areas. Surface and subsurface fault and fracture patterns, determined from photo lineament interpretation, geologic mapping, and formation image logs indicate strongly anisotropic permeability aligned with the dominant N to NE fracture trend, dividing the field into a number of subcompartments that are locally connected by fractured aquifers and NW- and E-W-trending fractures. The local maximum principal stress is vertical, whereas the maximum horizontal stress is oriented approximately N-S to NNE. These structural trends are consistent with open fractures being optimally oriented relative to an approximately E-W minimum stress direction. Subsurface offsets on the Rhyodacite Marker (RDM) and basement rocks serve as the main basis for interpretation of reservoir structure. A structural map of the field on the top of the RDM unit shows a fault pattern that includes roughly N-S-, NE-, E-W-, and NW-trending structures, similar to that expressed by surface faults. The wide variety of evidences used to make this interpretation makes it likely that both active and inactive faults are represented. Interpretation of open to partially open fractures trends observed in borehole image logs also indicate multiple fracture sets, with N to NE fractures most common. However some wells also have prominent NNW to N, and NE to E-W trends. Most fractures are steeply dipping to vertical. Tracer return patterns also indicate preferential N to NNE fluid flow along this open fracture trend, along the strike of optimally oriented major faults or distributed fractures, or both. However well interference tests and pressure trends also indicate semi-permeable barriers compartmentalize the reservoir and define its western margin along this same trend. Initial-state temperatures and fluid chemical trends suggest that intersections of N to NE- and NW- or E-W trending faults facilitate fluid migration between adjacent compartments, especially along the southern and northern margins of the proven reservoir; however, these flowpaths were not validated by tracer returns. This may be because these faults represent sources of external recharge or, have relatively lower permeability than N-to-NE structures due to their unfavorable orientation within the present stress regime. There is also evidence based on entry distributions in wells that certain stratigraphic units, such as the RDM, and their contacts act as fractured aquifers or barriers to vertical fluid flow. Fractured aquifers appear to channel fluids laterally, and link to major fault-controlled pathways. Another prominent cluster of fluid entries is associated with the contact between the Lower Volcanic Formation and the underlying marine sedimentary package. This may be related to fractures developed in or enhanced at the top of the eroded basement rocks. Contact zones between sedimentary rocks and intrusions also commonly host permeability at the deepest levels of the system. |