John M. Millett

Permanent storage of gigatons of carbon in basalt requires thick, uncompartmentalized sequences that allow lateral dissipation of CO ₂ away from injection sites. Here we assess the presence of flow barriers by evaluating the variations in ⁸⁷ Sr/ ⁸⁶ Sr patterns in pore water, a method used extensively in sedimentary successions. Present-day pore water ⁸⁷ Sr/ ⁸⁶ Sr was measured from residual salts in core samples, while past water compositions were reconstructed from carbonate cements of different ages. We reveal smooth and uninterrupted trends in strontium isotopes through a 100 m thick basalt section indicating a common fluid reservoir unaffected by low-permeability massive lava flow interiors. The data further indicate that unconsolidated overburden sediments act as a good seal preventing seawater to mix with in situ pore water, and buoyant supercritical CO ₂ to leak. Finally, basaltic sequences form viable targets for CO ₂ storage and suggests that fractures may bypass low-permeability units to create a well-connected reservoir system.

This study presents the fluid flow testing results of 32 lava flow samples recovered from IODP Sites U1571/U1572 (145–315 mbsf) on the Skoll High, Vøring Margin. Few past studies have investigated the mid-Norwegian margin’s basalt reservoir and flow properties. Even fewer implement subsurface-like conditions to explore in-situ fluid flow and the possible response to changing pore fluids during injection. These aspects are critical for large-scale carbon sequestration. In this study, hydraulic properties including porosity and permeability were measured at ambient conditions for all samples along with multi-stress, multi-fluid core flooding experiments on a representative sample enclosed in an X-ray µCT scanner. Petrographic observations reveal varying degrees of alteration with secondary clays, including smectite, commonly lining pores and vesicles. He-porosities (4.2–45.6%) fall mostly below the theoretical percolation threshold of ~ 30% for randomly distributed spherical pores. Klinkenberg-corrected nitrogen-gas permeability (0.0038–361.9 mD) shows weak correlation to porosity and no apparent depth dependence. Further, brine permeability is four orders of magnitude lower than gas permeability, with no notable flow reduction during injection of CO ₂ . Rather than being stress-dependent, we argue that brine-induced swelling significantly reduced the accessibility of microstructural elements to fluids. The findings urge caution when using literature-reported, ambient gas measurements for basalts with similar levels of alteration as they may not accurately reflect in-situ fluid flow properties. Low matrix permeability of the moderately to highly altered vesicular basalts and prevalence of swelling clays may, without major contributions from fractures, limit the injectivity potential into the basaltic lava flows on the Skoll High, Vøring Margin.

The Mid-Norwegian volcanic rifted margin and its NE-Greenland conjugate formed in relation to continental breakup in the latest Palaeocene to earliest Eocene during the emplacement of the North Atlantic Igneous Province (NAIP). The development of the NAIP and opening of the North Atlantic occurred contemporaneous to the Paleocene Eocene Thermal Maximum (PETM) which corresponded to a rapid 5-6 °C global warming episode.The cause of this rapid global warming, explored as part of IODP Expedition 396, is thought to relate to the thermogenic gases released to the atmosphere via thousands of hydrothermal vents. The thermogenic gases were produced by contact metamorphism of carbon-rich sediments during widespread sill emplacement from the NAIP. The potential of hydrothermally-released greenhouse gases to influence climate depends strongly on the water depth at which they get released. Unless it is released in a shallow marine environment most methane will be oxidized before it reaches the atmosphere.Early results from IODP Expedition 396 have documented that at least one of the Mid-Norwegian hydrothermal vents was emplaced in shallow marine to potentially sub-aerial conditions. The aim of this contribution is to constrain further the paleo-water depth at which hydrothermal vents formed along the other parts of the mid-Norwegian volcanic rifted margin. This study focuses on an integrated workflow of quantitative geophysical and geodynamic analyses calibrated by new IODP drilling results and structural and stratigraphic observations. We use a 3D flexural-backstripping, decompaction and reverse thermal subsidence modelling to predict the palaeobathymetry and palaeostructure at keys stages of the syn- to post-breakup evolution that can be compared with palaeo-water depths estimated from biostratigraphic data. Results provide new constraints on the paleobathymetry of hydrothermal vent complexes required to confirm whether the global warming recorded by the PETM was triggered by the magma-rich continental breakup leading to the opening of the northeast Atlantic Ocean.

Widespread and large-scale volcanism associated with the North Atlantic Igneous Province (NAIP) erupted during the Paleocene and Early Eocene and covers vast regions of the conjugate North Atlantic rifted margins. The majority of the preserved volume occurs in the offshore sequences of the continental shelves of Greenland, the Faroe Islands, UK, and Norway. These vast mafic igneous rock volumes in the offshore NAIP have been proposed as a potential area for permanent CO2 storage. However, a wide range of factors influence the suitability of mafic rock masses to act as fluid reservoirs including perhaps most importantly the volcanic facies (and associated pore structure, permeability, and distribution), along with alteration state, fracturing, and reactivity. In this contribution we focus on assessing the nature, distribution, and reservoir potential of sub-aerial lava flows from available borehole data across the NAIP. Lava flows are chosen for focus as they represent the most favorable reservoir target identified to date whilst also often constituting the dominant facies within much of the accessible NAIP offshore sequences.Over 50 offshore boreholes have penetrated the lava sequences of the NAIP including both industry and scientific boreholes drilled from the 1970’s onwards. Boreholes have encountered anything from a single lava flow through to several 10’s of lava flows in volcanic sequences reaching over 2 km in cumulative thickness in the deepest industry wells such as Brugdan (6104/21-1) and Lagavulin (217/15-1z) in the Faroese and UK sectors respectively.A summary of the lava flow nature (simple, compound, thickness, core-crust ratios) and physical property ranges (P-wave velocity, density, resistivity) of the penetrated lava flow sequences is presented utilizing available core, sidewall core, wireline and drill cuttings data to support the facies appraisal.Borehole data and the level of possible interpretation vary significantly depending on the age, purpose, and importantly the drilling operations of the individual boreholes. Some of the most robust and complete datasets are represented by modern hydrocarbon industry boreholes, for example those from the Rosebank Field in the UK sector, and cored boreholes from recent IODP Expedition 396 drilling Mid-Norway. Results from Expedition 396 are presented to highlight formation evaluation approaches for NAIP lava flow sequences including assessing the effects of different lava flow facies on Net to Gross calculations and the impact of alteration and secondary alteration on wireline log responses.This study presents important new constraints on the nature, variability, and distribution of NAIP lava flows from extensive available borehole ground truth data forming a foundational study for the development of reservoir appraisal techniques in the province going forward.

While basaltic volcanism is dominant during rifting and continental breakup, felsic magmatism may be a significant component of some rift margins. During International Ocean Discovery Program (IODP) Expedition 396 on the continental margin of Norway, a graphite‐garnet‐cordierite bearing dacitic unit (the Mimir dacite) was recovered in two holes within early Eocene sediments on Mimir High (Site U1570), a marginal high on the Vøring Transform Margin. Here, we present a comprehensive textural, petrological, and geochemical study of the Mimir dacite in order to assess its origin and discuss the geodynamic implications. The major mineral phases (garnet, cordierite, quartz, plagioclase, alkali feldspar) are hosted in a fresh rhyolitic, vesicular, glassy matrix that is locally mingled with sediments. The major element chemistry of garnet and cordierite, the presence of zircon inclusions with inherited cores, and thermobarometric calculations all support an upper crustal metapelitic origin. While most magma‐rich margin models favor crustal anatexis in the lower crust, thermobarometric calculations performed here show that the Mimir dacite was produced at upper‐crustal depths (<5 kbar, 18 km depth) and high temperature (750–800°C) with up to 3 wt% water content. In situ U‐Pb analyses on zircon inclusions give a magmatic crystallization age of 54.6 ± 1.1 Ma, consistent with emplacement that post‐dates the Paleocene‐Eocene Thermal Maximum. Our results suggest that the opening of the Northeast Atlantic was associated with a phase of low‐pressure, high‐temperature crustal anatexis preceding the main phase of magmatism.

Lava flows form important fluid reservoirs and have been extensively exploited for water aquifers, geothermal energy, hydrocarbon production and, more recently, for carbon storage. Effusive subaerial mafic to intermediate lava flows account for vast rock volumes globally, and form reservoirs with properties dictated by well-known lava flow facies ranging from pāhoehoe through several transitional forms to ‘a’ā lava. These variations in flow type lead to critical differences in the pore structure, distribution, connectivity, strength and fracturing of individual lava flows, which, alongside lava flow package architectures, determine primary reservoir potential. Lava flow margins with vesicular, fracture and often autobreccia-hosted pore structures can have porosities commonly exceeding 40% and matrix permeabilities over 10 ⁻¹¹ m ² (>10 D) separated by much lower porosity and permeability flow interiors. Secondary post-emplacement physicochemical changes related to fracturing, meteoric, diagenetic and hydrothermal alteration can significantly modify reservoir potential through a complex interplay of mineral transformation, pore-clogging secondary minerals and dissolution, which must be carefully characterized and assessed during exploration and appraisal. Within this contribution, a review of selected global lava flow-hosted reservoir occurrences is presented, followed by a discussion of the factors that influence lava flow reservoir potential.

Source‐to‐sink sedimentary systems associated with volcanic rifted margins serve as important archives for basin development by recording lithospheric changes affecting the depositional systems. Distinguishing between sediment transport processes and their sediment source(s) can guide the interpretation of a basin's history, and thereby inform regional paleogeographic reconstructions. In this contribution, we integrate and utilize wireline geophysical logs, detailed petrographic observations from side‐wall cores, and seismic analysis to describe and decipher a Maastrichtian to Danian‐aged basin‐floor depositional system in the deep outer Møre Basin, mid‐Norwegian margin. Well 6302/6‐1 (Tulipan) is a spatially isolated borehole drilled in 2001 that penetrates Maastrichtian and younger strata. A succession of hitherto undescribed carbonates and sandstones in the outer Møre Basin was discovered. It is investigated for sediment transport, provenance, and depositional processes on the basin floor surrounded by structural highs and ridges. The strata from the lower parts form a basin‐floor apron consisting of redeposited carbonate sourced from a westerly sub‐aerial high. The apron transitions vertically from mixed siliciclastic and carbonate into a purely siliciclastic fan with intercalated sandstone and mudstone, providing a rare high‐resolution record of how depositional environments experience a complete shift in dominant processes. The development coincides with similar latest Cretaceous‐earliest Palaeocene sequences recorded south of this region (e.g., well 219/20‐1) and may have been influenced by regional uplift associated with the onset of magmatism in the Northeast Atlantic. This study improves our understanding of a late, pre‐breakup source‐to‐sink sedimentary system developed near the breakup axis of an infant ocean, and documents what is possibly the northernmost chalk deposit in the Chalk Group.

The Palaeocene–Eocene Thermal Maximum (PETM) was a global warming event of 5–6 °C around 56 million years ago caused by input of carbon into the ocean and atmosphere. Hydrothermal venting of greenhouse gases produced in contact aureoles surrounding magmatic intrusions in the North Atlantic Igneous Province have been proposed to play a key role in the PETM carbon-cycle perturbation, but the precise timing, magnitude and climatic impact of such venting remains uncertain. Here we present seismic data and the results of a five-borehole transect sampling the crater of a hydrothermal vent complex in the Northeast Atlantic. Stable carbon isotope stratigraphy and dinoflagellate cyst biostratigraphy reveal a negative carbon isotope excursion coincident with the appearance of the index taxon Apectodinium augustum in the vent crater, firmly tying the infill to the PETM. The shape of the crater and stratified sediments suggests large-scale explosive gas release during the initial phase of vent formation followed by rapid, but largely undisturbed, diatomite-rich infill. Moreover, we show that these vents erupted in very shallow water across the North Atlantic Igneous Province, such that volatile emissions would have entered the atmosphere almost directly without oxidation to CO₂ and at the onset of the PETM.

The Paleocene-Eocene Thermal Maximum (PETM) around 56 Ma was associated with 5-6 °C global warming, resulting from massive carbon release into the ocean–atmosphere system. One potential driver of hyperthermal conditions was the North Atlantic Igneous Province (NAIP), as both volcanic degassing and thermogenic volatile release during contact metamorphism during its emplacement were large potential emitters of carbon. Despite a broad temporal correlation between NAIP activity and the PETM, the exact relationship is obscured by multiple climate forcings, imprecise geochronological data, uncertainties in the timing and magnitude of volatile fluxes from volcanic and thermogenic sources, and limited availability from crucial NAIP localities that could constrain these unknowns. Here we present new seismic and borehole data for the Modgunn hydrothermal vent complex in the Northeast Atlantic (IODP Sites U1567-U1568). Stable carbon isotope stratigraphy and dinoflagellate cyst biostratigraphy reveal a negative carbon isotope excursion coincident with the appearance of Apectodinium augustum in the vent crater infill. Modern examples of submarine explosion craters suggest they have filled in within decades to centuries, so the preservation of the PETM onset within the Modgunn vent suggests an extremely close temporal correlation between the crate formation and the onset of hyperthermal conditions. Furthermore, the majority of the craters across the entire NAIP are likely to have vented in very shallow water, implying that the vast majority of emitted CO2 and CH4 gases directly entered the atmosphere during eruptions. These findings add considerable weight to the hypothesis that thermogenic degassing aided the initiation of the PETM.

Extensive magmatism during the NE Atlantic break-up generated about 6-10 million cubic kilometers of magma in the Paleogene. Magmatic degassing and explosive discharge of thermogenic gases by contact metamorphism is implicated in triggering the Paleocene-Eocene Thermal Maximum (PETM; 56 Ma). In 2022, the CAGE22-5 research cruise collected high-resolution seismic, sub-bottom profiler, and multibeam data across the central and northern Vøring Plateau to tie IODP Expedition 396 Sites U1571 to U1574. One high-resolution P-Cable 3D seismic cube (10.3 by 1.8 km) was collected on the Skoll High, covering both sites U1571 and U1572. The data were fast-track processed with a bin size of 6.1 x 6.1 m. Four Cenozoic sedimentary horizons and the Top Basalt horizon were subsequently interpreted, followed by horizon attribute analyses. The Top Basalt horizon reveals unprecedented details of the nature of the volcanic paleosurface. A faulted and eroded lava horizon characterizes the southeastern part of the cube at 150-250 m below sea floor (bsf) around the U1571 site location. In contrast, in the northwestern part of the cube, the Top Basalt attribute maps reveal a pitted surface and lobate structures with linear ridges with characteristic inter-ridge spacings of a few tens of meters. The pitted basalt surface is mapped in an area of about 5.5 km2 at depths of 250-350 m bsf and drilled by Site U1572. About 270 individual pits have been mapped with radius ranging from about six meters, within the horizontal resolution of the data, to about 50 meters with a mean radius of c. 16 meters. The holes are interpreted as rootless cones, which comprise volcanic craters resulting from the explosive reaction between lava flows which flow over water-saturated sediments. The dimensions of the rootless cones in the Skoll 3D survey are comparable with the field analogue of rootless cones mapped in the Myvatn lake area of NE Iceland. Unlike at Myvatn and at other field analogues, the Skoll3D data allow unique 3D imaging of the rootless cones revealing internal structures and geometries including the base of the cone structures. The cone depths vary from meters to a few tens of meters. The lobate structures are interpreted as separate subaerial lava flows with compressional ridges. These lava flows are similar to outcrop analogue lava flows and compressional ridges imaged by elevation model ISN2016 with a 2x2 m resolution near the Myvatn lake. In conclusion, the interpreted patterns of the basaltic pitted surface within the Skoll3D suggest a wet terrestrial paleoenvironment during effusive volcanism. Unlike in the eastern faulted area of the survey, the extremely well imaged pitted basalt surface of the Skoll3D area did not experience significant erosion, suggesting rapid subsidence and drowning of the paleo-land surface in a low-energy coastal environment. This study demonstrates the unprecedented detail that is possible when high resolution 3D seismic data acquisition is applied to buried volcanic landscapes and opens the potential for unique associated insights in the Northeast Atlantic and further afield.