Reports: UR855610-UR8: Megaregional Correlation and Sequence Architecture of the Fox Hills Sandstone: Implications for Unconventional and Conventional Reservoir Prediction

Jennifer Aschoff, PhD, University of Alaska, Anchorage


C.B. Rust and J.L. Aschoff

The Campanian to Maastrichtian Fox Hills Sandstone records the terminal retreat of the Western Interior Seaway (WIS) in which shoreline sandstones regressed >1300 km from Colorado to Canada (Figure 1). This regressive sandstone is important for: 1) constraining paleogeographic interpretations of the WIS that help predict the distribution of clay in the Mancos and Pierre shale plays, and 2) understanding the source-to-sink linkages and deltaic stratigraphies associated with long-distance shoreline progradation. Despite numerous local studies of the Fox Hills Sandstone, the transregional “source-to-sink” connection of the fluvial-to-marginal marine depositional system over such large area is not well known. This study 1) defined 17 depositional facies from the outcrop to delineate geographic/stratigraphic variation in sedimentary process and identify critical sequence-stratigraphic surfaces, and 2) constructed a transregional, sequence-stratigraphic correlation based on three main unconformities and maximum flooding surfaces using a combination of outcrop, subsurface, biostratigraphic and geochronologic data.

Seventeen lithofacies define eleven depositional environments within the Fox Hills S.s. including: offshore marine, lower shoreface, middle shoreface, upper shoreface, pro-delta, delta front, delta plain, distributary channel, fluvial channel, estuary, and flood plain. Sharp stratigraphic shifts in these facies (i.e., non-Waltherian shifts) highlight two transregional sequence boundaries that define two distinct incised valley systems responsible for providing sediment from the mountains to the two main regressive shorelines constituting the Fox Hills S.s.. The oldest incised valley system in Casper, WY feeds three parasequences that form a SE-stepping, progradational shoreline with flat-to-falling trajectory (Figure 1). The older shoreline consists of mixed-energy to wave-dominated deltaic deposits. It contained incised valley systems that fed shorelines from a topographic dividing point in Central Wyoming toward the southeast. These shoreline sands pinch out in northern Colorado and southern Montana. The incised valley systems that fed this shoreline is well-exposed in the Muddy Guard Reservoir and Casper, Wyoming areas. The older incised valley systems are oriented to the south-west.

By contrast, the youngest incised valley system feeds five parasequences that form a NE-stepping, progradational shoreline with a more vertically-stacked trajectory near Glendive, MT. The number of parasequences increases from five to eight in the area NE to E of Glendive, MT, suggesting that there was more accommodation in that area, possibly due to tectonics. The younger shoreline consists of fluvial-dominated deltaic sandstones that prograded to the Northeast from a topographic divide in Central Wyoming. The younger shoreline sands are more extensive, owing to a greater progradation and preservation, with progradation distance about 700km and progradion in a more eastward direction in Montana. The incised valley system that fed this younger fluvial dominated deltaic shoreline is well exposed in Casper, WY, Buffalo, WY, and Lavina, MT.

In general, parasequencess are more numerous and more vertically stacked from in the NE-E part of the study area, indicating a NE-E increase in the ratio of accommodation to sediment supply. Accommodation in the Fox Hills system could be driven by eustasy, Sevier and Laramide tectonic subsidence, and/or dynamic subsidence migration (Liu et al., 2014). We infer a main control of migrating dynamic subsidence because: 1) the Fox Hills S.s. is preserved and progrades beyond the profile for Sevier- or Laramide-related flexural subsidence during a time of sea-level fall in the WIS, and 2) Fox Hills shorelines generally parallel the positions of previously-proposed dynamic basins. The high-resolution ammonite zonation in the WIS was used to determine the rates at which these shorelines prograded. Preliminary results indicate progradation rates >200 km per 2 ma from collected ammonites Bacullites grandis and Hoploscaphites nicollett in Buffalo, WY, Marmarth, ND, and Jordan, MT (Figure 1).