The Þeistareykir Geothermal Field, NE Iceland: Sub-surface Structural Analysis Based on Borehole Televiewer Imaging

Abstract

The Þeistareykir geothermal field is located in Northeast Iceland, within the active Þeistareykir volcanic system, one segment of the northern volcanic zone (NVZ). Geothermal field development during the last 10 years have made available a comprehensive dataset of acoustic borehole Televiewer (BHTV) for 9 of the 18 production wells in the area, alongside standard downhole wireline log and cuttings data. These datasets have been analyzed regarding types, distribution and orientation of fractures. Fracture orientation vary considerably in between the wells despite the rather small field size of Þeistareykir. Results from composite geological and geophysical borehole data analysis provide improved structural constraints for sub-surface modelling of the field in comparison to fluid flow. Three distinct fissures and faults trends were analyzed; a dominating NNE-SSW trend at surface, with a secondary NNW-SSE component, striking parallel to the Tjarnarás Fault zone, and a minor E-W trend observed to the south of the geothermal field. The different orientation of fractures was studied between wells, which provided a base for a division of the field into four sub-areas. Their structural characteristics are discussed and combined with a result from spinner logging. One of the sub-areas; located near the Tjarnarás Fault, shows very variously striking fractures as well as abnormally high injectivity indices.