Efate Geothermal Project, Phase 1 : summary report

Phase I of the Pre-Feasbility study of the geothermal energy prospects of the island of Efate, Republic of Vanuatu, has involved geological mapping of the Project Area (250 km2 ) at 1:25,000, and assessment of the thermal springs and their regional context, principally by a thorough geochemical investigation. The islands of Vanuatu are part of the extensive island arc system of the SW Pacific. Efate island is composed of Early Pleistocene rhyolitic pumice breccias and bedded pumice tuffs overlain by extensive Recent and raised Late Pleistocene reef limestones, with two major centres of Pleistocene basaltic extrusion in the north and on the northerly offshore islands. The thermal springs are related to the major structural features of the island. Four groups discharge on or near the Eastern Boundary Fault of the Teuma Graben at temperatures ranging from 24 to 59C, resulting in a total excess heat discharge to the Teuma River of approximately 10 MW. Three groups of springs and other minor springs discharge near Takara on the north coast, in an ESE alignment, at temperatures between 36 and 71 0C. They have a maximum combined flow of approximately 110 1/s, and a maximum excess heat discharge of 18 MW. In addition, high ground temperatures up to 100C occur at Takara over an elongated zone of ground parallel to but south of the spring line over at least 2km2, accounting for an extra 2.5 MW heat loss. In the Teuma Graben the chemical and isotopic characteristics of the three southerly groups of springs suggest they have experienced a similar evolution involving complex mixing of three components, although significant differences indicate different depths of circulation and extents of reaction. The northernmost springs are quite unrelated and result from meteoric water involved in si I icate alteration reactions during circulation at modest depths and temperatures, probably not in excess of 65C. Highest discharge temperatures and highest predicted base temperatures are associated with the Mid Teuma springs, for which it is estimated that chemical equilibration occurs at 200-215C, either with tuffs of the Efate Pumice Formation, or with basalt intruded as dykes along the graben. The effects of deep thermal water circulation along the Teuma Graben may be masked by dilution with shallow groundwater in the limestones which overlie and may be expected to be in hydraulic communication with the Efate Pumice. If the thermal water is stored in the Efate Pumice and the heat flow is a regional anomaly then an extensive hydrothermal reservoir may exist at depth. If, on the other hand, basalt dykes intruded along the graben are the source of heat, then the reservoir may be more limited. Geophysical surveys are required to test which of these two possibilities is correct. At Takara the thermal springs indicate a mixing series for which the thermal component is identified as sea water that has reacted with basalt between 165 and 180C. Results from groundwater of 88C at a depth of 3 m in a borehole on Takara airfield are in close agreement. Correlation between rainfall, spring discharge rate, and spring temperature and chemistry suggests that the flow of thermal water is governed by piezometric pressure in Quoin Hill or Mt Sussunatarr, inland from Takara. This further implies that the heat source is also to the south, rather than associated with the more recent volcanism on the northerly offshore islands. A heat source associated with the major basaltic extrusive centres in the north of Efate is suggested. A hydrothermal reservoir in which temperatures of 165 to 180C are common, and over 200C may exist in parts, is demonstrated. It is possible that the reservoir is extensive, and associated with the Efate Pumice which is likely to have favourable hydraulic characteristics. Organic Rankine (Binary) Cycle technology offers the possibility of generating electricity from primary fluids at less than the predicted temperatures. Accordingly, a geophysical survey as Phase 2 of the Pre-feasibility study of the geothermal potentia1 of Efate is encouraged. The geophysical survey should define more precisely the shape of the basaltic intrusions, and indicate the depth and extent of thermal and hydrothermal anomalies associated with them, thereby showing the most favourable sites for deep exploration boreholes on Efate. Proposals include a reconnaissance resistivity survey, a gravity survey and a microearthquake survey. It is suggested that the resistivity survey employs a combination of conventional Schlumberger traversing techniques and an audio-magnetotelluric survey. The total cost is estimated at £225K, of which the highest priority, the resistivity reconnaissance, ac=ounts for £145K.