Description: In summer 2017, the ICDP SUSTAIN project (Surtsey Underwater volcanic System for Thermophiles, Alteration processes and INnovative concretes), drilled three cored boreholes (Table 1) through Surtsey at sites ≤10 m from a cored hole obtained in 1979. Drilling through the still hot volcano was carried out with an Atlas Copco CS1000 drill rig, whose components were transported by helicopter to Surtsey and re-assembled on site. The first vertical borehole, SE-02a, was cored in HQ diameter to 152 meters below surface (m b.s.) during August 7-16. It was terminated due to borehole collapse. A second vertical (SE-02b) cored borehole was then drilled in HQ diameter to 192 m during August 19-26. Wireline borehole logging in SE-02b was performed August 26. The anodized NQ-sized aluminum tubing of the Surtsey Subsurface Observatory was installed in SE-02b to 181 m depth on August 27. A third borehole, SE-03, angled 35° from vertical and directed 264°, was drilled from August 28 to September 4 and reached a measured depth of 354 m (~290 m vertical depth) under the eastern crater. The core is HQ diameter to a measured depth of 213 m and NQ diameter from 213-354 m measured depth. The core traverses the deep conduit and intrusions of the volcano to a total vertical depth of 290 m b.s. Seawater drilling fluid for boreholes SE-02a and SE-02b was filtered and doubly UV-sterilized at the drill site. No mud products were employed while coring SE-02a, while small amounts of attapulgite mud were used in SE-02b and SE-03. Core samples for geochemical analyses of pore water and microbiological investigations were collected on site from all three boreholes. About 650 m of core was transported by helicopter to Heimaey, 18 km northeast of Surtsey, to a processing laboratory where the core was scanned, documented, and described. Additional core processing has taken place at the Náttúrufraedistofnun Íslands, the Icelandic Institute of Natural History in Gardabaer, where both the 1979 and 2017 cores are stored.

Description: The aim of the Iceland Deep Drilling Project is to drill into supercritical geothermal systems and examine their economic potential. The exploratory well IDDP-2 was drilled in the Reykjanes geothermal field in SW Iceland, on the landward extension of the Mid-Atlantic Ridge. The Reykjanes geothermal field produces from a 〈300 °C reservoir at 1 to 2.5 km depth and is unusual because it is recharged by seawater. The well was cased to 3000 m depth, and then angled towards the main up-flow zone of the system, to a total slant depth of 4659 m (~4500 m vertical depth). Based on alteration mineral assemblages, joint inversion of wireline logging, and rate of heating measurements, the bottom hole temperature is estimated to be about 535 °C. The major problem encountered during drilling was the total loss of circulation below 3 km depth and continuing to the final depth. Drilling continued without recovering drill cuttings, consequently spot coring provided the only deep rock samples from the well. These cores are characteristic of a basaltic sheeted dike complex, with hydrothermal alteration mineral assemblages that range from greenschist to amphibolite facies, hornblende hornfels, and pyroxene hornfels, allowing the opportunity to investigate water-rock interaction in the active roots of an analog of a submarine hydrothermal system. As they have not yet been sampled, the composition of the deep fluids at Reykjanes is unknown at present. Cold water is currently being injected with the aim of enhancing permeability at depth, before allowing the well to heat up prior to flow tests planned for early 2019. The well has at least two fluid feed zones, a dominant one at 3.4 km depth and a second smaller one at 4.5 km. Extensive geophysical surveys of the Reykjanes Peninsula completed recently allow correlation of geophysical signals with rocks properties and in-situ conditions in the subsurface. Earthquake activity monitored with a local seismic network during drilling the IDDP-2 drilling detected abundant small earthquakes (ML ≤ 2) within the depth range of 3–5 km. A zone at 3–5 km depth below the producing geothermal field that was generally aseismic prior to drilling, but became seismically active during the drilling. The drilling of the IDDP-2 has achieved number of scientific and engineering firsts. It is the deepest and hottest drill hole so far sited on an active mid-ocean spreading center. It penetrated an active supercritical hydrothermal environment at depths analogous to those postulated as the high temperature reaction zones feeding black smoker systems.

Description: Cementing operations in wellbores, especially for long casings, are often challenging and prone to deficiencies when not properly planned and executed. While exploring for and exploiting of geothermal resources at temperatures above the critical point of water was attempted in different drilling projects in recent years, the well design, and especially the procedure to run and cement long production casings became a key challenge for drilling engineers. For the first time, a reverse cementing job for a 2.97 km long production casing in a high-temperature geothermal well could be monitored and analyzed using a combination of permanently installed distributed fiber optic and electronic sensors as well as conventional well logging equipment. Data from the permanently installed sensors were used to monitor and evaluate the cementation process as well as the onset of the cement hydration. Based on the data, the understanding of downhole fluid dynamics during cementation could be improved. Our analysis suggests that the cement was diluted during cement placement and partly lost into the formation. These findings can help to better prepare for future drilling ventures under similar downhole conditions.

Description: Alteration of basaltic glass and in situ mineral growth are fundamental processes that influence the chemical and material properties of Earth’s oceanic crust. These processes have evolved at the basaltic island of Surtsey (SW Iceland) since eruptions terminated in 1967. Here, subaerial and submarine lapilli tuff samples from a 192 m-deep borehole drilled in 2017 (SE-02b) are characterized through petrographic studies, X-ray powder diffraction analyses, and SEM–EDS imaging and chemical analyses. The integrated results reveal (i) multi-stage palagonitization processes in basaltic glass and precipitation of secondary minerals from matrix pore fluids, (ii) multi-stage crystallization of secondary phillipsite, analcime and Al-tobermorite in the vesicles of basaltic pyroclasts and (iii) variations in palagonitization processes as a function of thermal and hydrological domains. Although temperature appears to be an important factor in controlling rates of secondary mineralization, the chemistry of original basaltic components and interstitial fluids also influences reaction pathways in the young pyroclastic deposits. The integration of systematic mineralogical analyses of the 50-yearold tuff from one of the most carefully monitored volcanic sites on Earth, together with temperature monitoring in boreholes since 1980, provide a reference framework for evaluating mineralogical evolution in other Surtseyan-type volcanoes worldwide.