From Hydrocarbon to CO₂ Storage: Unveiling the Potential of the Miocene Lille John Member in the Danish North Sea

To achieve net-zero carbon emissions by 2050, gigatonnes of CO₂ must be captured and stored in the subsurface. Screening and exploration of prospective storage sites have thus gained momentum in recent years. The Miocene-age Lille John Member in the Danish Central Graben represents a promising, yet underexplored, CO₂ storage candidate due to its lack of commercial hydrocarbon potential. This study integrates high-resolution 3D seismic data, core analyses, and wireline logs within a sequence stratigraphic framework to characterise the depositional environment within the targeted Miocene interval. Seismic attributes such as RMS amplitude and spectral decomposition are used to define the three-dimensional architecture of the geobodies and evaluate their potential for CO₂ storage by comprehending reservoir distribution, heterogeneity, and connectivity. The reservoir consists of two unconsolidated sand units, informally termed the lower and upper sand units, separated by a mudstone interval. The lower sand unit represents a basin floor fan emplaced by gravity flows during the falling stage systems tract, while the upper unit comprises unconfined gravity flow deposits associated with the lowstand systems tract. The reservoir sands of the Lille John Member are predominantly localised in the southeastern portion of the Central Graben at depths suitable for storing supercritical CO₂. Theoretical P50 storage capacity is estimated at approximately 1108 million tonnes for the lower sand unit and 51 million tonnes for the upper unit. Heterogeneities such as silt beds, mudstones, and carbonate concretions may act as flow baffles, enhancing storage efficiency through plume dispersion, residual trapping, CO₂ dissolution, and geochemical interactions. This study situates the Lille John Member within a broader regional framework by integrating a larger 3D seismic dataset with advanced seismic interpretation workflows, extending beyond the scope of previous investigations. The results provide new insights with implications for unlocking CO₂ storage potential in analogous depositional settings.