CCS potential of the Triassic alluvial succession in the eastern Horda Platform

The Triassic continental succession located in the eastern side of the Horda Platform (northern North Sea) represents a potential supplementary CO₂ storage formation to the principal Jurassic sandstones above. Several 1-2.5 kilometer-thick packages of Triassic sediment lie within a series of large eastward-dipping half-grabens east of the Viking Graben. The deeply buried Triassic deposits (1.8-3 km) within the prospective area are confined between the Øygarden Fault Zone to the east and the Vette Fault Zone to the west, the latter separating the prospective area from the Troll hydrocarbon fields. Despite extensive petroleum exploration within the Horda Platform, the entire Triassic interval remains largely untested and its storage potential poorly understood. The lack of wellbore penetrations and 3D seismic coverage means that reservoir quality can only be assessed using conceptual predictions and analogue studies.

A seismic stratigraphic model is built using available 2D and 3D seismic and integrated well log data to discern the Triassic basin fill history and structural development of the area. The stratigraphic succession is subdivided into seismic facies, where reflection patterns infer depositional characteristics. A shift in log facies trend between mud- and sand-rich intervals indicates a variance in subsidence rate and sedimentation supply related to tectonic displacement rate and climate. Visual analyses of the seismic data along key horizons also reveal depositional features such as channels, hanging wall fans, and footwall fans. The location and distribution of the channels are mapped in order to assess the connectivity of possible storage bodies.

Analogous Triassic sandstone reservoirs of the Snorre field (Tampen Spur) roughly 125 km away are similar in terms of stratigraphic facies and mineralogy. The Snorre field reservoirs are dominantly subarkosic and arkosic sandstones with illite-smectite and chlorite-smectite and lesser amounts of kaolinite and chlorite clay minerals within the matrix and pore spaces. Furthermore, the Triassic succession is composed of sandstones and mudstones deposited in fluvial systems with subordinate alluvial plain environments. The sandstones possess highly variable poro-perm values closely related to the depositional facies. As the Horda Platform sediments were deposited more proximal to the source than those of the Tampen Spur, it is expected that coarser grain sizes persist within the prospective interval, but perhaps at a lower degree of maturity and higher grain size variability. Overall, this preliminary assessment suggests that thick Triassic channel sandstones present in the eastern Horda Platform have promising CCS potential.

 

Keywords. Triassic, Horda Platform, CCS, basin fill history, seismic facies, tectonic displacement, climate, reservoir quality