| Keywords |
Muara Laboh, Sumatera, geology, geophysics, tectonism, volcanism, fault, structure, permeability, geothermal system, vein paragenesis |
| Abstract |
We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to better understand their roles in controlling the Muara Laboh geothermal system. The oldest rocks in the Muara Laboh region are the metamorphic Paleozoic Barisan Formation. Late Oligocene to Middle Miocene volcanism is represented by the Painan Formation, consisting of mixed volcanic and sedimentary rocks of andesitic to dacitic composition. In the Middle Miocene, granitic and granodioritic rocks intruded the Barisan and Painan Formations. Undifferentiated Silicic Volcanic and volcaniclastic rocks consisting dominantly of dacitic to rhyolitic tuff and sediments, are widely distributed northwest, west and southwest of Muara Laboh. Evidence from exploration wells indicates that this rock sequence is present mostly in the western Muara Laboh basin. In the eastern Muara Laboh Basin Andesitic Volcanics were erupted to the SE of Muara Laboh at about the same time. These sequences are overlain by Quaternary andesitic volcanics over almost the entire Muara Laboh area. These products come from several eruption centers respectively from the NW to SE including Mt. Patah Sembilan and Mt. Anak Patah Sembilan. Flow patterns are generally from the eruption centers along the Siulak fault in the south towards the north. Field geologic mapping indicates that all of these volcanoes are composed dominantly of andesitic rocks and consist mainly of lava, tuff, breccia, lahar, and debris flow deposits. The more distal deposits consist dominantly of volcaniclastic equivalents of the eruptive products. The most recent volcanic deposits at Muara Laboh consist of andesitic to dacitic tuffs and debris flows. Carbon in the tuffs yielded ages of ~34 to 41 ka. Debris flows underlying the tuffs may be related to sector collapse and debris avalanches from Patah Sembilan crater, providing a likely minimum age of this event. The Muara Laboh geothermal system is situated within a pull apart basin along the NW-SE Great Sumatera Fault (GSF). About 8 km north of the prospect, the Suliti Fault segment juxtaposes uplifted metamorphic basement with young basin fill deposits. South of Muara Laboh, the Siulak Fault segment has accommodated the magmatic intrusion that provides geothermal heat sources. Cross sections constrained by mapping, gravity, and well data indicate two main grabens in Muara Laboh i.e. West Muara Laboh Basin and East Muara Laboh Basin. The proposed model is for an asymmetric basin system with a narrower and deeper western basin along the Siulak master fault, and a shallower but wider eastern basin within the main step over fault structure. Analysis of structures at surface and in borehole image logs show dominant trends of open fractures are N-S, NW-SE, and NE-SW. The NW-SE fracture trend is associated with the GSF and based on image logs is important in the deeper section of exploration wells (H pad well), while surface mapping found this orientation in areas near the main GSF (Suliti and Siulak Fault segments). The N-S structural trend is considered to be the step over fault trend, and associated with the pull apart basin structures generating a horst and graben system. This N-S set corresponds with extensional fractures that are interpreted as the most important in controlling permeability, fluid flow, and thermal discharges in Muara Laboh geothermal system. The NE-SW fault trend is interpreted to be antithetic to the GSF and is considered to be the youngest structure based on the field mapping data. The image log data also support this interpretation because NE-SW fractures are more abundant at shallower depth. An extensive clay cap overlies the Muara Laboh geothermal system and its outflow area, but thick clay on the eastern flank of the system is in part related to basin fill deposits. The commercial reservoir top conforms to the base of the conductor best near Idung Mancung fumarole, where it hosts a 240°C steam cap. This zone appears to have recently been heated and host quartz, wairakite and prehnite veins open space texture. Some of the edges of the MT conductor have also been shown to approximately parallel the traces of inferred faults. The top of the propylitic alteration zone does not typically conform to the base of the conductor because a zone of transitional alteration defined by mixed-layer clay and chlorite underlies the smectite cap that corresponds with resistivity of ≤ 7 ohm-m. The highest deep permeability in the area is associated with epidote-adularia veins with open space textures that produce fluid at ≥ 270°C, but permeability is locally reduced by late-stage infilling by calcite, quartz, and prehnite. |