Structural mapping and subsurface geology compared to thermal gradient and the geothermal system in Mödruvellir, SW-Iceland

Abstract

The Mödruvellir geothermal field is a low-temperature geothermal resource located in southwest Iceland. Exploration in this region started in 1978 by drilling shallow gradient wells and using them for temperature measurements. At present there are two production wells and 22 gradient wells in the area. This study is based on an investigation of the surface geology, analyses of gradient well data and subsurface geology. Stratigraphical units in this area belong to the sequences of Late Tertiary to early Quaternary and approximately 480 m of basalt with intercalations of hyaloclastites are exposed in this area. Several faults and fractures were plotted on geological map. These faults have two main trends, NE-SW and ENE-WSW, some were observed and measured in the field while others were deduced from examination of aerial photographs. The geothermal gradient method was applied to locate a geothermal anomaly and a thermal map showing temperatures at 50 m depth in the gradient wells revealed two anomalies that are related to a local structural pattern in this area. Five different temperature zones were revealed on the local geothermal gradient map, the highest temperature zone on the map was within the highest gradient zone. Subsurface geology in this area was based on an analysis of drill cutting samples of two vertical wells, and the main rock types that comprise the subsurface geology of the studied wells were basaltic lavas and minor hyaloclastites. Correlation between subsurface lithological unites showed that displacement of these units are related to the normal dip of topography in this area. Based on this study the first conceptual model for the Mödruvellir low-temperature geothermal system is proposed.