Hydrogeology of the Krafla geothermal system, northeast Iceland

Abstract

The Krafla geothermal system is located in Iceland's northeastern neovolcanic zone, within the Krafla central volcanic complex. Geothermal fluids are superheated steam closest to the magma heat source, two‐phase at higher depths, and sub‐boiling at the shallowest depths. Hydrogen isotope ratios of geothermal fluids range from −87‰, equivalent to local meteoric water, to −94‰. These fluids are enriched in 18O relative to the global meteoric line by +0.5–3.2‰. Calculated vapor fractions of the fluids are 0.0–0.5 wt% (~0–16% by volume) in the northwestern portion of the geothermal system and increase towards the southeast, up to 5.4 wt% (~57% by volume). Hydrothermal epidote sampled from 900 to 2500 m depth has δD values from −127 to −108‰, and δ18O from −13.0 to −9.6‰. Fluids in equilibrium with epidote have isotope compositions similar to those calculated for the vapor phase of two‐phase aquifer fluids. We interpret the large range in δDEPIDOTE and δ18OEPIDOTE across the system and within individual wells (up to 7‰ and 3.3‰, respectively) to result from variable mixing of shallow sub‐boiling groundwater with condensates of vapor rising from a deeper two‐phase reservoir. The data suggest that meteoric waters derived from a single source in the northwest are separated into the shallow sub‐boiling reservoir, and deeper two‐phase reservoir. Interaction between these reservoirs occurs by channelized vertical flow of vapor along fractures, and input of magmatic volatiles further alters fluid chemistry in some wells. Isotopic compositions of hydrothermal epidote reflect local equilibrium with fluids formed by mixtures of shallow water, deep vapor condensates, and magmatic volatiles, whose ionic strength is subsequently derived from dissolution of basalt host rock. This study illustrates the benefits of combining phase segregation effects in two‐phase systems during analysis of wellhead fluid data with stable isotope values of hydrous alteration minerals when evaluating the complex hydrogeology of volcano‐hosted geothermal systems.

Isotope systematics of Icelandic thermal fluids

“…Oxygen and hydrogen isotopes are commonly used in geothermal studies to identify the source(s) of thermal fluids and to define the physical and chemical properties of the geothermal reservoir. The pioneering work of Craig (1963), Friedmann et al (1963 and Árnason and Sigurgeirsson (1968) followed by Árnason (1976, 1977) led to the extensive work on δD values of precipitation and non-thermal and thermal water in Iceland in the 1960s and 70s, followed by subsequent regionally restricted studies on δD and δ O (Ólafsson and Riley, 1978;Arnórsson, 1985;Darling and Ármannsson, 1989;Sveinbjörnsdóttir et al, 1986;Ármannsson et al, 2014;Pope et al, 2015;Stefánsson et al, 2016c). Tritium has also been measured in some low-temperature thermal waters (Stefánsson et al, 2005).…”

“…In contrast, the effect of fluid-rock interaction on δD values are less well understood. The effect is usually assumed to be minimal, except in the case of very high rock to water ratios (e.g., Pope et al, 2015).…”

Micromechanics of brittle faulting and cataclastic flow in Mount Etna basalt

“…A systematic investigation of basalt deformation and failure mode is therefore of fundamental importance not only for understanding the dynamics of volcanic systems but also for other applications related to engineering geology [Jiang et al, 2014], geothermal systems [Ásmundsson et al, 2014;Fowler et al, 2015;Pope et al, 2015], reservoirs in volcanic environments [Wu et al, 2006;Ólvasdóttir et al, 2015], and CO 2 sequestration [Matter et al, 2009;Khatiwada et al, 2012]. Numerous studies have been conducted on the elastic and transport properties of basalts, and how they evolve with hydrostatic loading [Vinciguerra et al, 2005;Adelinet et al, 2010;Fortin et al, 2010] and cyclic uniaxial compression [Heap et al, 2009].…”

Multidisciplinary database of permeability of fault zones and surrounding protolith rocks at world-wide sites

“…• enhanced geothermal reservoirs 57,58 , • geothermal exploration and production in natural hydrothermal areas, including fault-hosted geothermal reservoirs 59,60 , • faulted volcanic and sedimentary reservoirs where the fault conduit-barrier effects are complicated by and permeable stratigraphic units 61,62 , • upflow of thermal waters in natural hot geothermal areas 63 , Radioactive waste repositories. The hydrogeological investigations were motivated by characterization of proposed radioactive waste disposal sites, usually located in low-porosity fractured and faulted metamorphic, plutonic, and volcanic rocks (in total 61 datasets).…”