Geothermal Resource Exploration along the Great Sumatera Fault Segment in Muara Laboh: Perspectives from Geology and Structural Play

摘要

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 consist of Mt. Bangko, Mt. Patah Sembilan, Mt. Anak Patah Sembilan, Mt. Kapur, and Mt. Gunung Kerinci. Flow patterns are generally from the eruption centers along the Siulak fault in the south towards the north. Field geologic mapping results indicate 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 fragments in the tuffs yield ages of ～34 to 41 ka. Debris flows underlying the tuffs may be related to sector collapse and debris avalanches from Patah Sembilan crater, but further work is required to confirm this. This provides a likely minimum age of the sector collapse. 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 capping of clay 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 v