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Analogous Juxtaposition of Mixed Lithologies Against a Siliciclastic Hydrocarbon Reservoir and Proposed CO2 Storage Formation in the Norwegian North Sea


Thorough characterization and evaluation of potential seals is essential for derisking any geologic CO2 storage prospect. In particular, prospects with traps that require bounding faults to act as lateral seals for facilitating containment demand great attention. One such prospect, known as Alpha, is located within the tilted normal fault block of Smeaheia in the Norwegian North Sea. The Alpha trap is an elongate 3-way closure bounded on its western flank by the westward-dipping Vette Fault Zone. The proposed CO2 storage formation and seal intervals are comprised of Late Jurassic Viking Group sandstones and overlying shaley units, respectively. The maximum structural closure is ~80 m high at the footwall cutoff, however, no hydrocarbons were encountered in the solitary well that tested it. Furthermore, mixed Cretaceous siliciclastic and carbonate overburden lithologies in the hanging wall are juxtaposed against the Jurassic footwall sandstones. Given that these overburden rocks are poorly characterized locally, and that Vette Fault Zone seal quality remains unproven, a critical concern is whether the Fault Zone can seal CO2 injected into Alpha. Analogous to the Vette Fault Zone is the Tusse Fault Zone, which bounds the western side of the Troll East field 17 km west of Alpha. Unlike Vette, the Tusse Fault Zone originally sealed a ~250 m fill-to-spill gas column in equivalent Jurassic footwall sandstones. Despite this difference, both fault zones have similar throw magnitudes (>500 m) and juxtapose comparable footwall and hanging wall stratigraphies, although minor juxtapositions with Early Paleogene-aged units are also observed along the Tusse fault. As fault seal is imperative for the success of the Alpha CO2 prospect, we have utilized the analogous Tusse Fault Zone sealing Troll East to assess fault seal potential of the unproven Vette Fault Zone by employing a fault juxtaposition approach. We have determined lithologic properties of juxtaposed stratigraphy and constructed a 3D framework based on seismic data, well logs, and cuttings to generate detailed Allan diagrams. A key component of this analysis is the identification and qualification of potential leakage points along each fault zone. We find that several areas of seemingly higher juxtaposition leakage risk are present in both cases, but that juxtaposition leak may not have a significant impact for CO2 storage at Alpha assuming negligible leakage and recharge is occurring at Troll East.