Borehole geology and alteration mineralogy of well RN-12, Reykjanes geothermal field, SW-Iceland

Abstract

The current study focuses on high temperature geothermal well RN-12, which is located in the seawater-dominated geothermal system at Reykjanes field in SW Iceland. This is a vertical well, drilled in 2002 and reaching a total depth of 2506 m. The uppermost 1024 m are presented in this report. The well was kept offline until it was brought online for production in May, 2006 to feed the then-new 100 MW power plant. Drill cuttings were used to gather data from binocular and petrographic microscopes, and XRD and fluid inclusions analyses. Geophysical logs were used to corroborate conclusions derived from the cuttings. The lithology of well RN-12 contains hyaloclastite formations, such as tuff, breccia, tuff-rich sediments and pillow basalts in addition to basaltic lava flows and intrusions. The rock texture within these units range from fine-grained to medium-grained, even coarse-grained in some cases. Basalt intrusions were found at 360-396, 416-452, 460, 480-484, 500-510, 524-538, 542-576, 622-654, 708-752, 768-772 and 920- 932 m.

Two alteration groups were identified in the well, one based on the clay mineralogy zones, the other on the alteration mineral distribution zones. These sequences are: smectite-zeolite zone from 10 to 702 m; mixed-layer clay zone from 702 to 782 m; chlorite zone from 782 to 1024 m. The alteration mineral zones are: quartz zone from 70 to 260 m; prehnite zone from 260 to 610 m; epidote zone from 610 to 694 m; garnet zone from 694 to 762 m; and wollastonite zone from 762 to 1024 m. Thirteen small aquifers were identified in the well, all cased off, but three main feed zones were inferred at depths of 1000, 1300 and 2200 m. A fluid inclusion study from secondary quartz showed a homogenization temperature which ranges from 265 to 295°C. This temperature range correlates with the alteration and formation temperatures, which indicates that the well is in equilibrium with the geothermal system around well RN-12.