| Title | Indonesian Volcanic Geothermal Systems |
|---|---|
| Authors | Manfred P HOCHSTEIN, Sayogi SUDARMAN |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Strato-volcanic systems, volcanic feeder (plug type) systems, advective versus convective fluid flow, geothermometry problems, unequilibrated fluids, |
| Abstract | Two groups of Indonesian volcanic geothermal systems can be recognised using fluid analyses and their volcanic (terrain) settings as discriminants. The largest group includes geothermal systems that are hosted by young strato-volcanoes. Manifestations are often sustained by advective flow of diluted condensates over the outer flanks and foothill region of volcanoes with high relief. The second, smaller group includes steeply dipping ‘volcanic feeder (plug-type) systems’ that are hosted by convective geothermal systems standing in moderate terrain and transfer some magmatic fluids to the surface (affinity with ‘vapour core’ structures). Subgroups are indicated for both groups About 17 Indonesian strato-volcano prospects had been explored until 2000, mainly by VSI and Pertamina groups. Mixed neutral pH and acidic manifestations were found in a few prospects close to the central summit sector (Sibayak reservoir, now developed for power production). Settings where acidic manifestations discharge up-slope with minor neutral pH springs occurring down-slope, are common. Normalised cation ratios (Na/K and Mg/Ca) of acidic springs are similar to those of down-stream springs and (andesitic) host rocks. The springs acquire their mineralisation by isochemical dissolution of host rocks. Outflows of neutral and quasi-equilibrated thermal fluids in the foothills have been reported for a few strato-volcano prospects (Sorik Marapi for example). At least 5 Indonesian ‘volcanic feeder (plug type)’ systems have been explored which are hosted by liquid-dominated reservoirs (Dieng) or natural 2-phase reservoirs (Patuha). Two prospects (Dieng and Lahendong) have been developed for electric power production. The normalized cation ratios of their acidic and neutral pH springs also reflect isochemical dissolution of shallow reservoir rocks. Volcanic geothermal systems of both groups discharge some magmatic water, some crustal fluids (CO2) and sub-crustal gas fractions. The study confirms that empirical, cation-based geothermometry can not be used for ‘strato-volcanic’ and ‘volcanic feeder (plug-type)’ prospects to infer their average reservoir temperatures using thermal spring data. |