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Fluvial Architecture and Reservoir Modeling Along Strike Direction of the Trail Member of the Ericson Sandstone, Mesaverde Group in Wyoming


The Trail Member of the Upper Cretaceous Ericson Sandstone, part of the Mesaverde Group, is exposed along hundreds of square kilometers through Wyoming along the flanks of several Laramide structural uplifts. This presents a unique opportunity to study the detailed architecture based on bed-scale heterogeneity and better assess the reservoir potential of these strata in outcrop exposure on a regional-scale, and to then relate these observations to producing fields nearby. The fluvial-dominated Trail Member formed as sediments traveled from the active Sevier thrust belt to the Cretaceous interior seaway, forming a basinward progradational clastic wedge along a relatively high gradient. Though there is an abundance of sand-rich strata in the Trail Member, production from this interval has been unpredictable in current and past fields such as the Trail Unit of southwest Wyoming.

Twelve detailed stratigraphic columns were described at three sites along the eastern flank of the Rock Springs Uplift to show facies heterogeneity beyond what is often available through wells, and 75 samples were collected for determination of permeability and porosity values. This, along with photogrammetric characterization at the three sites where sections were measured, shows that spatial variability in the net-to-gross of the Trail Member, as well as temporal variability in the amount and character of reservoir sands, play an important role in the unpredictability of this reservoir. Moving nearly 40 km along the Rock Springs Uplift in the direction of depositional strike, from Point of Rocks down to Heulgy Canyon, significant regional trends can be seen. Average permeability increases from north to south, while net-to-gross decreases. The vertical changes in fluvial architecture within the Trail Member reflects changes in available accommodation, where we determined a relationship between low net-to-gross and a cleaner, high accommodation outcrop found more distal to the orogeny. While thickness of the Trail Member is highly variable, ranging from 63 to 104 meters across the study area, there is an overall trend of thickening to the south. It is significant that although the character of the Trail strata changes appreciably along strike, this interval is consistently very rich in sand, and grain size does not change drastically along the length of observed outcrops. Overall, this research allowed the cumulation of a large quantitative and qualitative dataset from bed to regional-scale to enable reservoir modeling and aid in future exploration projects within the Trail Member.