Analogs across the sea – Using detailed outcrop architectural models of the fluvialCastlegate Sandstone of Utah as an analog for hydrocarbon reservoirs of the Norwegian Sea

Building high resolution analog models based on outcrop exposures is a powerful tool for describing complex geologic systems. By creating detailed architectural models, we further our understanding of these sedimentological systems and increase our ability to predict heterogeneity in the subsurface. These same methods can be applied to the overall understanding of the evolution and architecture of fluvial systems worldwide, and reservoirs across the geologic timeline. This study examines the outcrop architecture and connectivity of fluvial depofacies within the Cretaceous lower Castlegate Sandstone of central Utah, USA, a proposed analog for the Mesozoic fluvial sandstones such as the Grey Beds and lower Åre Formation of the Norwegian Sea and Eiriksson Formation of the North Sea. 

The Castlegate Sandstone is a well exposed fluvial system that transported sediment eastward from the Sevier Orogeny toward the Cretaceous Interior Seaway. It is a highly progradational package of strata that overlies the proximal and marine deposits of the Blackhawk Formation. It was selected for study due to its similarity to the Late Triassic - Early Jurassic Grey Beds and lower Åre Formation, offshore Norway – a complex system of floodplain, coal-bearing swamps and channel sand deposits. The lower Castlegate Sandstone and underlying Blackhawk Formations exhibit some coal bearing beds, carbonaceous mudstones, and channel sand deposits, indicating proximity to the seaway and a depositional environment like that of the Åre Formation. Preliminary geochemical analyses using Rock-Eval pyrolysis reveal a mix of Type II and III kerogens within the upper Blackhawk, indicating algal marine and terrestrial carbon sources, with terrestrial influences increasing upward. Samples taken within the Castlegate Sandstone do not 
show algal influence. 

Detailed outcrop descriptions and measured sections are integrated into a multi-kilometer photogrammetric model. From this model, important temporal and spatial trends are defined. At the transition between the Blackhawk Formation and the Castlegate Sandstone there is an abrupt change in the proportion of mudstone versus sandstone while maintaining internal channel organization. This indicates a measurable basinward shift of the depositional system, likely due to increased sediment supply related to tectonism or other allogenic forces. Initial observations of the lower Castlegate Sandstone show fine- to coarse-grained sandstones ranging from <1 to 8 meters thickness, generally thickening upward. Fluvial bedforms exhibit important trends as well. Lower, more isolated channels are dominated by finer grains, frequent soft sediment deformation, and are mostly single-story, while stratigraphically higher sections are dominated by medium- to coarse-grained sandstones with trough cross stratification and are commonly multi-storied. Amalgamation surfaces are present throughout, evident in frequent mud rip ups, pebble lags, and abrupt changes in bedforms/grain size, but are more frequent and dramatic higher in the section. Architectural elements are defined from the photogrammetric model and exported to subsurface modeling software to be further analyzed. Through the integration of traditional field methods, photogrammetry, and predictive three-dimensional modeling, the lower Castlegate Sandstone serves as an important analog for subsurface exploration of fluvial systems in the Norwegian Sea and elsewhere.