| Title | Conceptual Model Update of the Sokoria Geothermal Field, Flores, Indonesia |
|---|---|
| Authors | Nicholas HINZ, William CUMMING, Gabe MATSON, Jill HAIZLIP, David SUSSMAN, Steven FERCHO, Amanda FISHBIN, Dhani SANJAYA, Birean SAGALA, Yan TANG |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Conceptual Model, Sokoria, Flores, Indonesia, Structural Geology, Volcano Hosted System |
| Abstract | At the Sokoria Geothermal Field on the island of Flores in Indonesia, five wells have discovered a 200 to 225°C reservoir and the integration of well results with supplementary geology, geochemistry and geophysics surveys have supported the revision of conceptual models to guide the development of the reservoir for power generation. Previous Sokoria conceptual models have included a proposed more than 250°C neutral hydrothermal system with an upflow located near the Mutubusa fumaroles at elevation 1280 masl on the southwest flank of the Kelimutu volcano, with an outflow extending below a low resistivity clay cap toward chloride hot springs 3 to 6 km to the south below 600 masl. Gas chemistry had suggested a relatively neutral water chemistry below the Mutubusa fumarole compared to the chemistry expected below the magmatic acid lakes 5 km to the northeast at the Kelimutu summit. Since 2017, PT Sokoria Geothermal Indonesia has drilled five directional wells to depths of 1650 to 2100 m from Pad-A at 1070 masl located about 1.4 km south of the Mutubusa fumaroles. To extrapolate reservoir parameters beyond those constrained by the temperature, production test and geoscience data from the wells, new surface surveys have been completed, including LiDAR-supported structural geology studies, surface alteration and lithology mapping, surface manifestation resampling, and MT resistivity and gravity surveys. The integrated interpretation of these data indicates a 200-225°C southward outflow, about 600 to 800 m-thick extending south from the Mutubusa fumarole area and overlying a linearly increasing (conductive) temperature gradient. The well stratigraphy consists of a thick sequence of Kelimutu volcanics. The shallow portions of the wells (0-600m) were drilled through tightly interbedded alternating andesitic tuffs and lavas. Within the intermediate and deep sections of the wells, primarily andesitic lithic tuffs and tuff breccias were encountered without the interbedding of lavas. The permeable zones of the Pad-A wells occur within these tuffs. The water table is relatively deep, at about 500 m below the surface. The new mapping facilitated by LiDAR coverage detected no significant Quaternary fault scarps; nor did it support the existence of a previously interpreted fault zone roughly following the axis of Loworia River, where extensive clay alteration is exposed 600 to 1200 m east of Pad-A. Coincident with the exposed alteration, the low resistivity zone becomes a thin veneer and gravity is much higher, and so this zone is now interpreted as dense relict high temperature phyllic/propylitic alteration that is close to the surface where an earlier smectite clay cap has been removed by erosion. A well targeted on this feature is likely to encounter cooler and lower permeability rocks than those found in the drilled outflow. The permeability encountered by the Pad-A wells is interpreted as associated with faults and fractures at a scale too small to be mapped at the surface and within tuffs of the Kelimutu volcanics. These conceptual elements are presented in maps and cross-sections that illustrate the resource conceptual models developed to analyze the uncertainty, risk and opportunity in the Sokoria resource. |