Porosity structures and local compositional variations associated with natural CO2 sequestration in basalts offshore Norway

Permanent storage of CO₂ as carbonates in basalt deposits utilises natural weathering reactions between silicate minerals and carbonic water. By studying how CO₂-enriched water has altered basalt in 43 samples from the Vøring Margin, offshore Norway, we demonstrate the pathways and reactions of CO₂ within a basalt reservoir.

The pore spaces occupied by carbonates in the samples were studied through μ-CT (computer tomography) studies on three minicores and detailed microanalyses on 32 thin sections via scanning electron microscopy (SEM), electron microprobe analysis, and electron backscatter diffraction (EBSD). Single crystal X-ray diffraction on 15 samples determined the type of carbonate minerals and precipitation ages were determined for two samples through U/Pb-radiomeric dating. δ¹⁸O and δ¹³C-isotope analyses were used to determine the fluid origin in 11 samples.

Three types of carbonate precipitation were characterised based on the pore structures they fill and the associated mineralogy. Type I carbonates, seen in 18 thin sections, fills vesicles, mainly along lava flow margins. SEM analyses showed that all vesicles are initially coated with a smectite layer which incorporates most of the Mg²⁺ and Fe²⁺ from dissolving silicate minerals. As pH and Ca concentrations increase, calcite precipitates in some vesicles. The secondary mineral assemblage and precipitation order fit with low-temperature alteration of basalt with pore water and atmospherically balanced CO₂ levels (pH≥8). Type II carbonates appear in seven thin sections as <500μm calcite crystals surrounded by clay minerals in the basalt groundmass. The porous carbonate crystals containing clay inclusions suggest formation through coupled dissolution-precipitation reactions. Thus, Type II carbonates result from the replacement of primary minerals like olivine with mostly clays and some calcite under similar conditions as Type I. δ¹⁸O and δ¹³C-isotope analyses indicate a meteoric fluid origin for the Type I carbonates and U/Pb dating of Type I and II calcites indicates precipitation occurred within 10 Ma after lava emplacement (47.3 ± 6.9 Ma and 43.3 ± 5.1 Ma, respectively). Type III carbonates are only observed in two thin sections and show partial replacement of clinopyroxene by calcite, following micro-fractures in the minerals. Near-complete replacement of olivine by calcite and some siderite through coupled dissolution-precipitation reactions is also observed. This indicates reducing conditions at a slightly higher CO₂ concentration (pH: 6-7) closer to what we expect in a CO₂ injection scenario. EBSD analysis of Type III calcite reveals no clear relationship between the crystal orientation of the calcite and the mineral it replaces. This suggests that chemical rather than crystallographic factors lead to the preferential dissolution of clinopyroxene and olivine over plagioclase. Microprobe analysis indicates varying trends in Ca-substitution within all three calcite types by either Mg (approximately 0-4.5%) or Mn (approximately 2-16%), likely linked to local variations in fluid chemistry or temperature within the reservoir.

In conclusion, the results show that CO₂ storage primarily occurs in vesicles along lava flow margins, but CO₂ can also migrate into the micro- and nano-pore networks of the basalts, enhancing storage potential. Coupled dissolution-precipitation reactions between silicate minerals and carbonates may further increase available storage space.