Natural CO2 Sequestration in Basalts from Offshore Norway: Observed Variations in Calcite Precipitation 

Abstract

Basalt presents large potential reservoirs for carbon mineral storage. CO2 can react with the minerals in the rock and precipitate carbonates, thereby becoming stored for geological timescales. Experimental studies and pilot injections in Iceland and the US have shown the feasibility of the method. However, there are still open questions related to how and where the CO2 flows and reacts in basaltic reservoirs. Samples of basalt from offshore mid-Norway containing naturally precipitated calcite have therefore been studied as natural analogues to such systems. This study aims to characterize where and under what conditions the calcite precipitated. The 30 samples were collected during IODP Expedition 396 (holes U1571A and U1572A) and are from a potential storage site called the Skoll High on the Vøring Margin. Scanning Electron Microscopy (SEM) and Electron Microprobe Analyzer (EMPA) were used to characterize the mineralogy and microstructures of the samples and identify different types of calcite precipitation. The groundmass of the basalt contains mostly clinopyroxene, plagioclase, and Ti- and Fe-oxides and is presently devoid of olivine. The basalt also contains large amounts of Mg- and Fe-rich clays. Other secondary minerals include calcite and minor amounts of sulfide and Ca-rich zeolites. All vesicles are either completely filled with clay or have a ca. 10–30 µm thick clay coating on their surface. In 16 out of the 32 studied thin sections, some of the clay-bearing vesicles were later completely or partially filled with calcite (Type I calcite). Calcite also precipitated in secondary pore spaces created as minerals weathered (Type II calcite, 5 samples). In two samples of seemingly impermeable basalt from the interior of a lava flow, calcite is present as ca. 10–70 µm wide crystals in and around clinopyroxene in the groundmass of the basalt (Type III calcite). Microprobe analysis of three representative samples indicates that Type I calcite contains the least and Type III calcite contains the highest amounts of Mg and Fe. Together with its type of occurrence, this composition suggests that Type III calcite partially replaces the pyroxene and could indicate an in-situ dissolution-precipitation reaction while there are no clear signs of in-situ dissolution and precipitation for Types I and II calcite. In conclusion, calcite is most commonly precipitating in the vesicles of the basalt but the CO2-rich fluid may also penetrate and react with the seemingly impermeable basalt groundmass and massive lava flow interiors. This suggests that there is a micro-pore network that is important for fluid flow in the basalt. A combination of micro-CT (computer tomography), Hg porosimetry, He pycnometry and gas adsorption with CO2 and N2  will, therefore, also be used to characterize the surface area, porosity and connectivity of the micro-pore network of the samples. In addition, the precipitation of calcite often appears passive, without coupled dissolution of local minerals. Instead, factors such as transport and fluid mixing, partial pressure, pH, alkalinity and temperature likely control precipitation. U/Pb dating and clumped isotope thermometry will also be performed on the calcite to constrain the timing and temperature during precipitation.