Depositional Environment and Reservoir Characteristics of the Upper Mesaverde Group, Upper Philadelphia Creek, Douglas Creek Arch, Colorado
Exhumed strata of the upper Mesaverde Group (Late Cretaceous) on the Douglas Creek Arch, northwestern Colorado are representative of a low- to moderate-sinuosity fluvial system deposited during the regression of the Western Interior Seaway. Fluvial channel sandstones (N=202) and floodplain mudstones present in these strata serve as outcrop analogs to laterally equivalent reservoir elements in the adjacent Piceance and Uinta basins. Seven general sandstone body forms can be recognized based on their external geometry and internal architecture. Of these, the dominant channel-body types are interpreted as the deposits of sinuous-channel fluvial systems. Values for paleomorphological characteristics for the interval were estimated using empirically-based equations from various authors. Using sandstone-body dimensions (N=17) and paleocurrent data (N=163) as input parameters, a sinuosity range of 1.3 to 1.9 was calculated (median=1.6). The results of these calculations, in addition to the range and relative abundance of sandstone-body types observed across the outcrop interval, reveal a fluvial system deposited by both low- and high-sinuosity rivers, but dominated by low- to moderately-sinuous, meandering rivers.
Nine outcrop photomosaics were interpreted to create "homogeneous" and "heterogeneous" sandstone-body polygon maps. These maps, which display in two dimensions the abundance and distribution of sandstone bodies and their associated internal surfaces, were assessed via pixel analysis to determine the net-to-gross, connected-reservoir area, and reservoir-area connectivity of intervals between pseudowells at 10- and 40-acre pseudowell spacing. The average net-to-gross ratio of the upper Mesarverde interval is 45.4%. For comparison, the average net-to-gross ratio observed in six outcrop measured sections (N=1,505 ft) is 40.9%.
Reservoir-area connectivity ranges from 22.7% to 91.4% (average=60.0%) at 10-acre pseudowell spacing and 37.3% to 57.4% (average=41.0%) at 40-acre pseudowell spacing assuming homogeneous sandstone bodies. If internal heterogeneity of sandstone bodies is considered, the average reservoir-area connectivity ranges from 0.0% to 47.9% (average=13.2%) at 10-acre pseudowell spacing. Intervals between pseudowells at 40-acre pseudowell spacing have no connectivity. Comparing local changes in net-to-gross ratio to reservoir-area connectivity reveals that while connectivity is, in general, directly dependent on net-to-gross ratio, it is more reliant on the dimensions, spatial placement, and internal heterogeneity of sandstone bodies.