Subsurface geology and hydrothermal alteration of well HE-4, Hellisheidi geothermal field, SW- Iceland

Abstract

Intense geoscientific studies are needed before exploitation of geothermal resources commences. The studies involve multiple fields of geoscience including geology, geochemistry and geophysics during the exploration, production and monitoring stages. Well HE-4 is amongst the earliest deeper exploration wells in the Hellisheidi geothermal field with a total depth of 2008 m. The well was directionally drilled in 2001 towards SW from a drill pad east of Reykjafell mountain. This report covers the upper 1200 m of the well where binocular microscope, petrographic microscope, XRD-analysis, fluid inclusion analysis and geophysical logs were applied to gather and interpret data. The stratigraphy of well HE-4 is governed by two major formations which are hyaloclastite basalt and basaltic lava. Hyaloclastite basalt formations (including basaltic tuff, basaltic breccia and pillow basalt) alternate and dominate in the well, representing volcanic eruptions beneath a glacier. Basalt formations of fine- and fine- to medium-grained crystalline basalt appear occasionally, representing interglacial lava flows and intrusive rocks.

Seven feed points were identified, two large ones located at 496 m and 1150 m and five smaller ones identified at 300 m, 580 m, 890 m, 900 m and 1100 m. Alteration zones are somewhat contradictory and are, therefore, based on two separate zones of clay minerals on the one hand and alteration mineral distribution on the other. Clay minerals show five zones within the well starting with an unaltered zone followed by a smectite zone at 82-642 m, which is interrupted by a thin layer of mixed layer clay at 450-458 m. A chlorite zone is then noted from 642-862 m, followed by a mixed layer clay zone from 862 to 1050 m and a chlorite amphibole zone from 1050- 1200 m. The distribution of alteration minerals shows five zones starting with an unaltered zone, followed by a zeolite zone, then quartz and wairakite, epidote, and wollastonite zones. Correlation of alteration temperature and homogenization temperatures with the formation temperature of the system indicates that the upper part of the system is in equilibrium, the middle part from 414 to 784 m is heating up, while the lower part of the system is cooling.