An updated conceptual model of the Travale geothermal field based on recent geophysical and drilling data

Abstract

an updated picture of the Travale field is given, based on geophysical and drilling data acquired since 1978. In deriving the model, extensive use is made of the geophysical data produced in the course of the EEC test site programme (1980–1983), particularly from seismic and time domain EM methods which allowed for penetrating thick and conductive cover formations and to match deep tectonic and hydrothermal alteration trends thought to indirectly characterize the geothermal reservoir. It is interesting to note that the geophysical operations coincided with a fairly busy drilling programme, including completion of six new wells by the field operator (ENEL). This led to correlations with geophysical data and made it possible to assess with a fair degree of reliability the lateral extent of the “useful” geothermal field, limited in the Mesozoic carbonate series by the 175°C isotherm. Within this area, which excludes the central graben area, productive zones are clearly associated with fracturing, the prediction of which remains in many instances a form of black magic! However, the geophysical methods provided valuable indicators of tectonics, particularly transverse faulting, and of active (or fossil) fluid convection. Because of the combination of a fracture-dominated reservoir and strongly conductive cap rocks, geophysics failed to identify the fluid signature but did produce a relevant description of the upper geothermal structure, in good agreement with drilling data. Recent seismic surveys have drawn attention to deep intra-basement reflectors, which could possibly be related to liquid-filled basement fractures. Additional information on reservoir and fluid characteristics is given to complement the predominantly geometric model derived from geophysical and drilling data.