Geology Controlling Factors of the Top Reservoir; Muara Laboh Geothermal System Case Studies

Abstract

Top of reservoir (TOR) determination is one of the resource keys features of the geothermal resource and significantly impacts the well’s productivity (Casing Point Determination). It is defined from the conceptual model process and refined by the well temperature data. This paper will describe the geologic controls on the top of reservoir geometries of the Muara Laboh (ML). ML has located in step over-pull apart basins of the Great Sumatra Fault Zone segment. The features then accommodate the emplacement of quaternary to tertiary volcanic and intrusion. The geoscientific data from the existing conceptual model, well cuttings, cores, MeB test, and image log data were combined with the drilling parameter data (mud temperature & mud loses) to characterize the geology of the TOR. Drilling results show that wells in the NE sector have a shallower TOR. The TOR is located near the quaternary Patah Sembilan volcanic unit’s contact with the undifferentiated silicic formation. The NE sector TOR corresponds with the change of alteration type from smectite rich argillic zone (<180°C) to the transitional zone (Chlorite, Chlorite-Illite, Chlorite-Smectite formed at 180°C to 240°C). The epidote – chlorite rich propylitic zone (>240°C) identified underlaying the NE reservoir sector’s transition zone. MeB Index below 10 seems to be corresponding with the base of conductive of 5 Ohm, and the NE TOR location. The SW sectors TOR sits a deeper elevation (∼300msl), lies within the intercalated dacite-andesite volcanic unit, higher temperature propylitic alteration zone, with epidote, epidote-adularia-quartz, and open space veins were observed below the TOR. Similar to the NE sector, the drop of the MeB index to less than ten, which is also related to the lithology contact, shows the possibility that the TOR has dropped in the SW sector. Below the low MeB zone, calcite infilled the fractures and sometimes encapsulated the pre-existing epidote vein; calcite then decreases with increasing epidote vein and epidote-adularia vein occurrence. The observation then suggests a rapid boiling process in the SW sectors due to the dropping of water levels, allowing the late-stage calcite vein to seal off the permeability within the shallow reservoir and not allowing the circulation of geothermal fluids within the interval.