Subsurface geology and hydrothermal alteration of well HE-53, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-53 is one of the exploration wells drilled in Hverahlíd geothermal field south of Hellisheidi and Hengill central volcano. The well is drilled directionally to a depth of 2507 m, but the current study focuses on the first 1350 m. Methods used during the study included rock cuttings binocular analysis, thin sections, XRD analysis and fluid inclusions along with the use of drilling data and geophysical wireline logs. The well has two major and five minor feed points. The feed point at 1180 m depth appears to be the largest and most important indicated by alteration minerals as well as temperature profiles. The well is comprised of alternating basaltic lava sequences and hyaloclastite formations. The first one is a postglacial basaltic lava sequence in the upper 100 m followed by a thick hyaloclastite formation containing basaltic tuff, glassy basalt, and basaltic breccia down to approximately 760 m. Variably thick basaltic lavas occur as well as possible intrusions in the lower part of the well. The hydrothermal alteration mineral distribution with an unaltered zone close to the surface, a zeolite smectite zone at greater depth and finally a high temperature actinolite zone at the bottom of the well suggests a continuous temperature rise with depth. Depositional sequences of alteration minerals revealed only prograde alteration. The comparison of alteration minerals with the formation temperature of the well suggests that the well is currently heating. The downhole temperature of the well, as confirmed by temperature of homogenization of fluid inclusions, alteration minerals and direct measurements, reaches about 300°C (at 1350 m).