| Abstract |
Sumatra island is considered as the most potential location in Indonesia to host high-enthalpy geothermal fields in Indonesia. However, despite recent efforts to accelerate the development of new geothermal fields here, the results are not so good. There are some success stories, such as Sarulla, but some other projects are now struggling to discover economical reservoirs. It seems that complex geological setting of Sumatran geothermal fields, due to the influence of the Sumatran Great Fault, make them significantly different from the Java island cases, in which the majority of existing producing fields have been developed in Indonesia. While many geothermal companies rely much on geophysical survey, especially MT, here the author will discuss the importance of geological mapping program at the initial stage of geothermal exploration. Especially we focus on important aspects of mapping techniques that are considered critical, based on experience in several projects in Sumatra. Before the real field mapping program, pre-field stage is important to generate tentative lithological, structural, alteration and thermal anomaly maps that can be used for efficient field work campaign. Public-domain data such as topography (DEM) and geological maps are not adequate for this purpose. We need high-resolution DEM data at several meters ground resolution (typically radar), to be used as base map for appropriate desk study on lithological units and structural aspects of study area. Identification of monogenic volcanic centres (many are only several hundreds of meters in diameter) is needed as volcanism in Sumatran is not only present as stratovolcano but also many monogenic volcanic centres controlled by structures. Multispectral thematic remote sensing such as Landsat TM is proven in some cases to detect the distribution of clay and oxide minerals. Surface thermal anomalies can be detected mainly by the night thermal IR ASTER image. At the field survey stage, efficient field observation should focus on different facies of volcanic deposits (coherent vs. fragmental), volcanic stratigraphy of different volcanic units, structural geology, and alteration aspects of different units. As there are many Holocene ash fall deposits in Sumatra that covers the majority of Pleistocene volcanic units, we must observe the underlying rocks. Youngest ash and other volcanic products need to be mapped to evaluate the current activity of volcano. Hydrothermal alteration zones in Sumatra are not always related with currently active geothermal system, so that identification and study of the nature of alteration zones, including observation of quartz vein-float, is required to elucidate which alteration zones are directly related with active geothermal system. Structural geology data is typically difficult to interpret at the initial exploration program, but observation on distribution of thermal manifestations, dykes and epithermal veins may serve as good indication of the existence of extensional structures. Reliable geological information should then be incorporated in the integration of other data, especially geochemical and geophysical data, in order to better targeting the sites of exploration drilling. |