The Albian-Cenomanian Nanushuk Formation located on the North Slope of Alaska is the result of fluvial, deltaic, and shoreface processes and has been the focus of recent petroleum exploration activity in the Colville Basin. The Nanushuk and underlying Torok formations together contain an estimated 8.7 billion barrels of oil and 25 trillion cubic feet of natural gas reserves within the resource-rich North Slope. The Nanushuk Formation is composed of sediment sourced from the Chukotka Peninsula to the west and deposited axially within the Colville Basin. High sedimentation rates led to rapid progradation of shoreface and deltaic systems, which effectively filled the entire basin during the Cretaceous. Resulting in a thick stratigraphic succession of marginal to deep marine sandstones and mudstones. However, considerable facies variation within the basin has led to a previous lack of understanding of the spatial distribution of depositional elements. Integration of outcrop photogrammetry, detailed measured sections, core interpretations, hand samples, and thin section microscopy from the Colville Basin in this study reveal the distribution of litho- and depo-facies within the Nanushuk Formation. Three key outcrop locales along the northern flank of the Brooks Range expose ~200 to ~1000 feet of mudstone, silty sandstone, and sandstone that record the transition from distal shoreface sedimentation to deltaic and fluvial processes over time. Progradationally-stacked parasequences are clearly identifiable in both outcrop and core, recording rapid axial progradation to the east. While shoreface processes do exist in the Nanushuk, outcrop observations show a dominance of deltaic processes, consistent with the highly progradational nature of the strata. Further, there is a trend towards more fluvially-dominated deltaic processes in the more axial part of the Colville syncline, as shown by core, compared to more wave-dominated deltaic facies associations along the margins of the axial basin where outcrops were described. These trends, both vertically and spatially, have important implications for understanding the geologic evolution of the formation and for targeting areas for further exploration within this evolving hydrocarbon play.

The Cretaceous Nanushuk Formation of northern Alaska records extremely rapid shoreline progradation across the Colville Basin during the Albian-Cenomanian epochs. Additionally, it records a period of tectonic realignment in the region, as the main sediment source began to transition from the Chukotkan orogeny to the west to a more local sediment source of the proximal Brooks Range to the south. Through detailed outcrop characterization and 3-D modeling of three exposed outcrops along the axial trend of the Colville Basin on the North Slope and the Upper section of the Nanushuk, this study clarifies the relative timing of the early Brookian orogenic uplift. Outcrop characterization also allows for a detailed description of spatial and temporal distribution of deltaic facies associations within the Nanushuk Formation. Previously these outcrops were hypothesized to represent dominantly shoreface depositional processes, but field studies indicate the Nanushuk Formation has a complex facies architecture consistent with deltaic environments. This work has important significance for geometry predictions for hydrocarbon exploration and development targeting the Nanushuk sands as a reservoir unit. This new interpretation of both dominant sedimentary process and provenance has implications for 1) reservoir architecture and connectivity, and 2) textural and compositional maturity of Nanushuk sandstones, an emerging hydrocarbon play along the North Slope of Alaska. A detailed mineralogical analysis of 50 samples, coming from the three key outcrops and two additional outcrops from previous studies, using TESCAN Integrated Mineral Analyzer (TIMA), an automated system that utilizes Scanning Electron Microscopy (SEM), shows increased amounts of chromite and biotite associated with key outcrops proximal to the Brooks Range. Chromite mass weight percent ranges from 0.00 % to 0.15 % across five outcrops, with Tuktu Bluff, which is proximal to the Brookian orogenic front having weight percentages that range from 0.01 % to 0.15 % with an average of 0.06 %, in contrast to the other outcrops, which range from 0.00 % to 0.03 % with an average of 0.01 %. In contrast, non-diagnostic elements such as quartz do not show meaningful variance, ranging from 42.66 % to 93.78 % across all outcrops, with proximal outcrop showing an average of 60.64 % while the rest of the outcrops have an average of 64.36 %. Substantial localized enrichment in both chromite and biotite found within outcrops of the Nanushuk Formation at the Tuktu Bluff locality support subaerial exposure of the Brooks Range within the Albian-Cenomanian epochs. The non-uniform distribution of these minerals on both a strike and dip direction show that uplift was non-uniform and suggest specific point sources dominated Brookian input. A better understanding of these patterns will help to focus exploration in areas of best potential reservoir quality. Measured sections and 3-d models were created to better understand the depositional architecture and how the deltaic influence in this highly progradational system affects the reservoir geometries. In all three outcrops of interest along the axial trend and higher up in the Nanushuk section, we observed current ripples, lensoidal geometries, low angle trough cross bedding, scoured bases, and wood chunks and pebble lags at channel basses, all of which, are consistent with fluvial dominated deltas. Deltaic deposition complicates predicting the geometry of the reservoir away from outcrop control with lateral avulsions of the delta system and laterally changing distributary channels. At Ninuluk, Kanayut, and Rooftop Ridge, distributary and interdistributary channels comprise approximately 19.5 %, 23.5 % and 20.36 % of the outcrop, respectively. Overall, net to gross sand ratios of 38.69 %, 66.54 % and 55.47 % were calculates at Ninuluk, Kanayut, and Rooftop Ridge, respectively. If assuming the sands in the net to gross are the reservoir rock, then approximately half of the reservoir rocks at each outcrop are in distributary channel settings which adds to the complexity of facies prediction away from the control outcrops.