Assessing the potential for permanent carbon storage in volcano-sedimentary sequences offshore mid-Norway: initial results from IODP Expedition 396

International Ocean Discovery Program (IODP) Expedition 396 drilled 21 holes at 10 sites spanning the mid-Norwegian volcanic rifted margin in 2021. Six sites recovered volcanic sequences with one objective of the cruise being to appraise the potential for permanent carbon storage within the offshore volcanic sequences.

This study presents a core-log-seismic and reservoir appraisal of the Kolga High Site U1566 which penetrated a c. 120 m Seaward Dipping Reflector (SDR) sequence beneath a thin Quaternary sediment cover below c. 2100 m water depth. Coring with high average recovery > 65 % through the volcanic sequence revealed a variably altered basaltic lava flow dominated sequence with inter-layered volcaniclastic and siliciclastic sediments, the latter containing granitic clasts eroded from the nearby high. The base of the volcanic sequence was recovered in the cores and marks a sharp transition from sub-aerial lava flows into deeply altered granite.

Shipboard petrophysical and wireline data for Site U1566 are analysed to characterize the SDR sequence and integrated with available seismic data. Whole-Round Multisensor Logger data were collected on cores for natural gamma radiation, bulk density, magnetic susceptibility, and P-wave velocity at 2.5 cm spacing. 588 P-wave caliper measurements along with 2219 point magnetic susceptibility measurements were made on the working-half sections. In addition, a total of 102 discrete samples were taken for moisture and density (MAD) analysis including 2 cm cubes and 34 additional minicore samples which were also tested for ambient permeabilities. Wireline data including gamma, density, P- and S-wave velocity, resistivity, magnetic susceptibility and image log data (micro-resistivity and acoustic) were collected over the main volcano-sedimentary sequence enabling a comprehensive appraisal of the penetrated volcanic sequence.

The volcanic sequence is characterized by vertically stacked compound to simple lava flows showing asymmetrical log profiles with individual flow lobes rarely exceeding c. 3 m in thickness. Primary vesicular porosity exceeds 40 % in fresh unaltered flow margins and decreases to < 10 % in flow interiors and where alteration and secondary mineralization are pervasive. Matrix permeability ranges from microdarcies within flow interiors up to several 10’s of millidarcies within flow margins and up to 100’s of millidarcies within sediment interbeds highlighting significant vertical heterogeneity linked to facies development through the studied sequence. The presence of alteration and fracturing within the studied sequence significantly alter reservoir properties and form key elements, along with facies scaling, that must be incorporated into reservoir appraisal. This pilot study reveals clear reservoir potential within both lava flows and inter-bedded sediments offshore mid-Norway.