Decoding subsurface fluvial architecture: an outcrop-based case study of the Iron Springs Formation as an analog for fluvial systems in the North Sea

Ancient sand-rich fluvial systems form critical subsurface reservoirs for hydrocarbon 
exploration, carbon storage and a variety of other uses; however, their complex architecture often 
hinders the generation of accurate reservoir models. Detailed study of analogous outcrop 
exposures is critical for managing exploration risk and developing efficient production strategies, 
providing a detailed look at the architecture and potential connectivity of subsurface reservoir 
targets. 

The Late Triassic – Early Jurassic Grey Beds and lower Åre Formation are fluvial formations 
which are being increasingly targeted as potential hydrocarbon reservoirs within the Norwegian 
Sea. Reservoirs such as this have the potential to prolong energy production from the Northern 
North Sea Basin for decades to come and are the focus of ongoing regional exploration. One of 
the challenges this play concept faces is that much of the strata is entirely preserved within the 
subsurface, has few wells, and is poorly understood due to limited data. 

Here we present an analysis of the Late Cretaceous Iron Springs Formation in Southwest Utah. 
These exposures represent an upward-coarsening, high net-to-gross fluvial system deposited 
proximal to the Sevier orogenic belt into the Cretaceous Interior Seaway. This depositional 
system provides an excellent analog for the Grey Beds and lower Åre Formation in the 
Norwegian Sea and other high net-to-gross sandstones in the geologic record. 

This study utilizes a combination of traditional and digital field methods to create detailed 
outcrop descriptions. Our preliminary field study reveals important details not often apparent in 
subsurface datasets. Exposures of the upper Iron Springs near Parowan, Utah, range from 100m 
to 150m in thickness. Based on detailed measured stratigraphic sections described as part of this 
study, amalgamation surfaces are common and grain size varies appreciably. Lateral and vertical 
connectivity of architectural elements is complex, but excellent three-dimensional exposures in 
the field area allow for spatial description of these relationships. Often, high net-to-gross systems 
are assumed to be braided; however, our initial findings from the Iron Springs Formation show 
variability between braided and meandering elements, with suggestion of more distal 
environments suggested in the lower part of the formation. 

Photogrammetric models built in Agisoft Metashape software using high resolution drone 
imagery allow for identification of high-resolution architectural elements in three dimensions 
along two kilometers of continuous outcrop. Channel elements are identified from this dataset, 
and fine-scale details observed from geolocated measured sections add details regarding the 
facies present within these elements. Geobodies delineated within photogrammetric models are 
exported into Schlumberger’s Petrel software, and extrapolated models based on outcrop control 
suggest a high degree of channel connectivity in this system even in zones of the Iron Springs 
Formation interpreted to be dominated by meandering channel morphologies. 

This model will be used to build geospatial descriptive and predictive models for subsurface 
fluvial systems that may be underrepresented due to limited sampling and poor seismic 
resolution of fine-scale reservoir elements.