Structurally-Enhanced Accommodation and Preservation of Shallow Water Reservoir Units: Brookian Clinoform Succession, North Slope Alaska

Abstract

Recent topset (Nanushuk Fm.) discoveries (e.g., Willow and Pikka) in the Brookian Succession on the Alaskan North Slope total significant recoverable oil and gas volumes. The Brookian is characterized by high rates of clastic input from the ancestral Brooks Range and eastern Siberian uplands and subsidence of the Colville Trough from Cretaceous to Neogene. Sediment is organized into numerous eastward propagating axial high-relief (400-800 m) slope clinoform packages, which combined, are ca. 3.5 km thick and extend across the North Slope. In the west-central North Slope, the Nanushuk (topset)-Torok (bottomset) Fms. comprise the slope clinoforms. This study is focused on the shallow to marginal marine deposits of the Albian-Cenomanian Nanushuk Fm., which has reported net pay thicknesses of ca. 20-70 m.

Integration of high-resolution stratigraphic and structural analyses suggests accommodation of sand-rich topset reservoirs was influenced by an interplay of stratigraphic architecture and normal faulting. We suggest that high sedimentation rates throughout Nanushuk-Torok deposition resulted in unstable clinothem packages that were subjected to enhanced compaction/dewatering, slumping, and subsequent normal faulting. Myriad small-scale normal faults are present in each clinothem package, which are isolated to the slope system and are unrelated to regional stress. In addition, we interpret fine-grained bounding clinoform surfaces to act as local detachment surfaces. The enhanced accommodation created by compaction/dewatering, slumping, and normal faulting of the clinothem packages is infilled by successively younger topset deposits of the sandy, shallow to marginal marine Nanushuk Fm. These zones of structurally-enhanced accommodation effectively trap sand-rich reservoir facies, which are preferentially preserved during subsequent transgressive events, and sealed by overlying transgressive muds. Seismic reflection terminations within each clinothem package indicate extreme onlap, yielding apparent forced regressive geometries.

Similar mechanisms have been identified in numerous subsurface and outcrop locations globally (e.g., Niger Delta, Gulf of Mexico Shelf). Further documentation of this concept and integration of high resolution structural and stratigraphic analyses provides a solid foundation for future Brookian-aged discoveries on the North Slope and a set of criteria for assisting in the evaluation of high-relief clinoform topset reservoirs worldwide.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018