Sealing capability of the Eocene–Miocene Horda and Lark formations of the Nini West depleted oil field – implications for safe CO² storage in the North Sea

The Nini West depleted oil field in the Siri Canyon, Danish North Sea, is considered a potential storage site for safe sequestration of CO2 to fulfill Denmark's ambition to reduce CO2 emissions using carbon capture storage (CCS) technologies. The Paleocene Nini West sandstone reservoir is overlain by several caprocks forming a 900 m thick seal complex composed of the regionally distributed Eocene–Miocene Horda and Lark formations. The integrity and capacity of the seal complex has been evaluated using a multidisciplinary analytical approach integrating organic and inorganic compositional data, physical properties, wireline logs, and mud gas data. The seal complex consists of clay-dominated mudstones with a relatively homogenous lithology constituting a tight seal. Mud gas logs are introduced as a novel proxy for seal integrity in a CCS context, and they show only very limited vertical gas migration has occurred from the reservoir into the basal part of the seal. Average porosities range from ∼15–20% and both measured and modelled average permeabilities range from only 0.11–0.39 μD. The seal mudstones have narrow unimodal pore throat size distributions, and 40% of the samples have extremely small median pore throat radius r50. The seal has high supercritical (sc)CO2 displacement pressures, and it can hold back a vertical column of scCO2 of minimum ∼600 m providing a comfortable seal safety column-overhead. The current findings have relevant application to other depleted oil fields and saline aquifers with similar seals in the larger Central Graben area in the North Sea.