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Implications for Lower Cretaceous Reservoir Distribution and Geologic Controls for Shelf-Edge Delta Successions; North Slope Alaska

Abstract

Shelf-edge delta deposits (SED’s) host reservoirs and form stratigraphic traps in the Gulf of Mexico and other petroleum basins. Previous studies postulated that SED’s are present on the North Slope of Alaska in the Nanushuk and Torok formations. Recent exploration drilling near lower Cretaceous paleo-shelf-edges resulted in several oil discoveries in the Nanushuk and Torok formations that are thought to be SED’s. The most notable discoveries were Pikka (2015), Willow (2016), and Horseshoe (2017). However, the distribution, internal character, variability, and external controls on SED formation are not known. Brookian Colville foreland basin sediments were sourced from both the Chukotka fold and thrust belt and Brooks Range orogen (Lease et al., 2014).Tectonic elements such as the Beaufort rift shoulder, Brooks Range orogen, and Chukotka fold and thrust belt might have influenced the distribution of SED’s by controlling along-strike variability in sedimentation and sediment routing system (Till, 2016). Using a dataset of core, well logs, 2D and 3D seismic surveys covering nearly 40,000 km2 this study provides new insight into the internal character, distribution, variability, and regional controls on SED’s that formed near the terminal Brookian shelf margin (Houseknecht et al., 2009). Eight seismic facies were defined based on a combination of the following: 1) their position along the shelf margin profile [proximal, medial, or distal], 2) external geometry [sigmoidal, discontinuous truncated, or horizontally continuous], 3) internal geometry [shingled, discontinuous, or horizontally continuous], and 4) trajectory of shingled reflectors when present [flat, falling, or aggradational]. Preliminary seismic interpretation confirms the presence of 10 SED’s along the 2 youngest shelf margins, including a combination of Type A and Type C deltas (Steel and Porebeski, 2003). Preliminary seismic facies mapping indicates SED’s occur within 2400 to 6700 m of defined shelf margin inflections in the dip direction, but are highly variable along strike. Along strike, sedimentation is postulated to be controlled by sediment entry point locations, and structural/tectonic influences. This has not yet been determined. In conclusion, 10 SED’s were mapped on the 2 youngest shelf margins; these can be classified as Type A and Type C deltas. Type A form 60% and Type C forms 40% of the SED’s present on these margins. SED’s occur within 7 km of the defined shelf margin inflection, and range in thickness from 50 m to 130 m. Regional distribution is potentially controlled by interplay of two feeder systems, one originating from the Chukotka fold and thrust belt and the other from the Brooks Range orogen.