High Resolution Sequence Stratigraphy of the NPRA, North Slope, Alaska

Abstract

The deposition of sediment in the Ikpikpuk-Umiat Basin of the NPRA has been governed by four distinct tectonic phases: 1) Franklinian crustal shortening, 2) Ellesmerian continental sag, 3) Beaufortian continental rifting, and 4) Brookian crustal shortening and tectonic inversion. These tectonic phases and changes in global sea-level resulted in identifiable depositional packages (megasequences). Utilization of the Vail seismic method and corresponding Galloway petrophysical motif analysis identify four parasequence sets: Highstand Systems Tract (HST), Regressive Systems Tract (RST), Lowstand Systems Tract (LST), and Transgressive Systems Tract (TST). These parasequence sets are separated and divided by 25 operational sequence boundaries from the Mississippian to the Cretaceous. Comparison of local and global sea-level curves indicate that the Ikpikpuk-Umiat basin was controlled by allocyclic processes until Brookian time, in which depositional loading of sediment forced the basin to subside. Chronostratigraphic Wheeler diagrams illustrate the results of the complex relationship between auto- and allocyclic accommodation changes. An understanding of the relationship between these depositional phases will substantially increase hydrocarbon exploration strategies. Recent studies suggest the Nanushuk in particular has promising potential for hydrocarbon production, and the relative locations of the underlying systems tracts provide information on the most likely locations of traps, seals, reservoirs, and source rocks.