Using field-based analogues to characterise the range of CO₂ storage options adjacent to intra-volcanic discoveries of the North Atlantic Igneous Province

Carbon capture and storage (CCS) plays an indispensable role in the quest to restrict anthropogenic warming to 1.5–2.0°C this century, as set out in the Paris Agreement. The majority of CCS projects are focussed on the injection of carbon dioxide (CO₂) into porous sedimentary rocks at greater than 1 km depth; these require impermeable overlying rocks to stop the CO₂ escaping to the surface. An alternative approach, however, involves the injection of CO₂ into reactive rocks (e.g. mafic or ultramafic lithologies) leading to carbonate mineralisation; this process permanently locks carbon away with minimal risk of it re-entering the atmosphere. The CarbFix project in Iceland has made significant strides in demonstrating the viability of this approach by injecting CO₂ into basalt lava flows. The project is, however, on a relatively small scale and there are uncertainties regarding the feasibility of scaling up this technology for widespread commercial use.

Large volumes of volcanic rock formations of the North Atlantic Igneous Province (NAIP) may be considered suitable for CO₂ storage adjacent to intra-volcanic discoveries, such as the volcanic rocks that encase the fluvial to shallow marine reservoir intervals of the Colsay Sandstone Member in the Rosebank Field. These volcanic rock formations vary significantly in terms of facies (various lava flow types, primary and reworked volcaniclastic rocks, etc.), mineral composition, porosity and permeability, all factors that need to be addressed in adopting the CarbFix approach. Alternatively, CO₂ could be stored in proven sub- and intra-volcanic conventional reservoirs where the volcanic rocks could act as primary or additional seals. The intra-volcanic discoveries have already driven research to understand, for example, sedimentation and drainage pathways in flood basalt provinces, reservoir architectures, and the effects of volcanic debris on reservoir properties and/or sealing potential. However, with the acceleration of the energy transition, research is now also looking at additional implications, such as reactivity, vis-a-vis CO₂ storage.

Fieldwork undertaken in the Ethiopian Flood Basalt Province (EFBP), the Faroe Islands Basalt Group (FIBG) and the East Greenland Plateau Lavas (EGPL) has begun to address these questions and is bringing together observations from beneath, within, and above these provinces. Developing detailed volcanic stratigraphies in the FIBG and EFBP has shown they share many common features. It has enabled us to understand the spatial and temporal distributions of the volcanic and sedimentary lithofacies and the influence of pre-existing structure on these variations. This is helping, not only to potentially predict the location of clean intra-volcanic, but also sub-volcanic reservoirs. Although the vast majority of interlava units are volcaniclastic in composition, detailed petrographic and heavy mineral studies are recording non-volcanic material preserved in these rocks. The provenance of this non-volcanic material is, for example, being deduced for the Faroese and Ethiopian samples and field/photogrammetric studies are highlighting the prevalence of channels and drainage pathways within the associated volcanic sequences. In East Greenland and Ethiopia, petrographic and geochemical analyses of crystalline volcanic clasts are also differentiating extraformational sources. In Ethiopia, mafic, alkaline, and felsic debris, or mixtures thereof have been recorded within mass flow and fluvial deposits preserved in interlava sedimentary sequences that are locally >200 m thick. The variability in composition has implications for drainage development within lava fields as well as reservoir/sealing properties (as captured in porosity-permeability, MICP, QXRD data) due to their differing susceptibilities to alteration/reactivity. An extreme example is observed in East Greenland, where subarkoses with porosities of >20% are interbedded with calcite-cemented volcaniclastic sandstones suggesting they not only underwent different diagenetic histories, but very little mixing occurred between the two systems. In comparable scenarios volcaniclastic rocks could act as baffles or barriers to CO₂ plume migration within subarkose reservoirs. The photogrammetric data are also enabling the detailed characterisation of internal lava flow architectures crucial in understanding their potential as CO₂ basalt reservoirs. These multi-disciplinary studies from a variety of analogous settings are beginning to elucidate the different CO₂ storage complex elements in volcanic provinces as well as providing parameters to fully assess their storage, sealing and carbonate mineralisation potential. This is in addition to constraining the types, scale, and controls on sedimentation patterns needed for de-risking exploration in these unconventional basins, including those of the UK continental shelf.