Geology, hydrothermal alteration and tectonic setting of the geothermal system at Bitra, Southern Iceland - evidence from exploration drilling
|Location||International Geological Congress,oslo 2008|
|Author||Mortensen, Anette K.; Franzson, Hjalti; Blischke, Anett|
|Holding Date||11 October 2008|
Geology, hydrothermal alteration and borehole logging data from three exploration wells within the geothermal system at Bitra, Southern Iceland, are presented. Bitra geothermal area is located east of and adjacent to the Hengill central volcano within the Hrómundartindur volcanic centre, which has been active for more than 100.000 years. Bitra is located within a tectonic setting comprising a volcanic rift zone characterised by extensional tectonics with NE-SW striking normal faults and fractures, and an active transform zone, the South Iceland Seismic Zone, characterised by N-S striking shear faults.
The three exploration wells at Bitra cover an area of ~2 km2 and reach to a maximum vertical depth of 1900 m, exposing strata of hyaloclastite and pillow lava formations formed during glacial conditions as well as sequences of lava flows formed during interglacial periods. The hydrothermal alteration reveals that a high temperature geothermal system reached shallow levels in the area before the system eventually cooled. The alteration progresses rapidly with depth from smectite-zeolite > mixed layer clays > chlorite zone within 250 m from surface. Alteration increases further with depth progressing through the chlorite-epidote alteration zone and in the western part of the area the epidote-actinolite alteration zone is reached at a depth of 800 m. This high alteration at such shallow depths points towards that the water level for the boiling point curve was at a point in time ~100 m above the present surface. This suggests that the geothermal system reached the highest temperatures during the last glacial, when the area was covered with a thick glacier. Analyses of fluid inclusions reveal that the temperature below 700-800 m in the geothermal system had been stable at 260-270°C before the system cooled. Today the upper 1000 m of the area has cooled and at 1000 m depth the temperature is ca. 200-210°C. On the other hand exploration drilling has revealed a significant temperature variation below 1000 m. In the eastern part of the geothermal field the temperature shows minor cooling and is ~270°C, while in the western part analyses of fluid inclusions indicates cooling to around 200-225°C, even in the lower explored parts of the reservoir. Equally, there is a considerable variation in permeability within the area with high permeability in the western part, i.e. where maximum cooling occurs, while the eastern part of the geothermal system, where cooling appears to be insignificant, permeability is much less. It is tentatively suggested that more intense alteration and subsequent cooling in the western part of Bitra geothermal system is caused by the combination of higher permeability due to interference between active transform zone shear faults and normal faults associated with the volcanic rift zone as well as changes in permeability with depth during glacial and postglacial conditions forming a confined zone of more intense heating/cooling.