Top and lateral seal assessment for CCS in Dunlin Group sandstones within the Tusse fault block, Horda Platform

 

Osmond, J.L.1, Mulrooney, M.J.1, Skurtveit, E.2, 1 & Braathen, A.1

 

1 University of Oslo, Oslo, Norway, johnathon.osmond@geo.uio.no

2 Norwegian Geotechnical Institute, Oslo, Norway

 

Demonstration of the full-scale CCS value chain along the Norwegian Continental Shelf is scheduled to begin in 2024 as a result of both Longship and Northern Lights projects. Subsurface geological characterization and results from well 31/5-7 recently confirmed favorable storage conditions at the Aurora storage site (exploration license EL001), which is located roughly 11 km south of the Troll West hydrocarbon field and 8 km east of the Tusse Fault Zone. Supercritical CO2 will be injected into the Lower Jurassic Dunlin Group storage complex comprised of Johansen and Cook Formation sandstones, which are sealed by overlying Drake Formation mudstones and shales. Confidence is relatively high with respect to the geology controlling successful CCS operations at Aurora, however, little work has been done to extrapolate satellite storage potential of the Dunlin Group throughout the Horda Platform. This is especially true for the envisaged top and lateral seals that are critical for reliable containment of the injected CO2, as previous contributions instead focused on understanding the Johansen and Cook storage formations. In order to utilize local infrastructure and encourage further development of the Horda Platform as a European CCS hub, we present an assessment of potential top and lateral seals in the neighboring Tusse fault block east of Aurora and underneath the Troll East field. Relevant horizons and faults defining three potential storage traps are mapped using well logs and 3D seismic data, and are used to build a local structural model. Thickness maps indicate that the Drake top seal is present throughout the study area, but thickness decreases from over 125 m at the Aurora site to approximately 65 m along the eastern edge of the Tusse fault block. Log readings from nine wells within the Tusse fault block also suggest adequate seal properties for CO2 storage, particularly in the lower part of the Drake Formation. All three traps are dependent on lateral fault seals, however, their nature is primarily determined by the amount of displacement occurring along them and the regional stratigraphy. A number of small faults (throw < 50 m) displace the traps, creating local zones where up-thrown Johansen and Cook sandstones are juxtaposed against Drake mudstones. Contrastingly, displacements along larger trap-bounding faults (e.g., the Tusse Fault Zone) exceed 75 m, juxtaposing up-thrown Dunlin Group sandstones with thick Upper Jurassic Viking Group sandstones (Troll reservoir). These larger faults, therefore, must provide continuous membrane seals at sandstone-on-sandstone juxtapositions, and shale gouge ratio (SGR) analysis suggests that their membrane seal potential is favorable for retaining injected CO2. Assuming good regional reservoir quality within Johansen and Cook formations, these results infer that ample seals are present for leveraging Tusse fault block traps as supplemental CO2 storage sites in the Horda Platform.