Sequence Stratigraphy and Sedimentology of Beaufortian Strata (Jurassic – Lower Cretaceous) In The National Petroleum Reserve – Alaska (Npra)

Abstract

Abstract Beaufortian strata (Jurassic - earliest Cretaceous) in NPRA have become a focus of exploration since the 1994 discovery of the nearby Alpine oil field. Interpretation of sequence stratigraphy and depositional facies from a regional 2-D seismic grid and well data has resulted in the definition of four major depositional sequences that each display unique stratal geometries and thickness trends across NPRA. Sequence K1 (Lower-Middle Jurassic) accumulated in a depocenter in northcenteral NPRA. Constituent depositional sequences include lowstand (LST), transgressive (TST), and highstand (HST) systems tracts. Sequence K2 (Oxfordian-Kimmeridgian) is a composite of higher-order depositional sequences that each reflects a forced regression, which caused widespread erosion across northcentral NPRA and accumulation of the derived sediment at the shelf margin in lowstand systems tracts. Subsequent flooding of the shelf resulted in deposition of transgressive systems tracts that commonly include well-winnowed, upper shoreface sandstones capped by a condensed section of fissile shale, forming good stratigraphic trapping potential. Sequence K3 (Valanginian) displays relatively distal facies character, suggesting high relative sea level during most of the depositional history. An important exception is the presence of a transgressive systems tract at the base of K3 that contains winnowed, shoreface sandstones. Sequence K4 (Hauterivian) is a shelf-margin wedge that developed as the result of tectonic uplift along the Barrow Arch. It comprises lowstand and transgressive systems tracts that display stratal geometries suggesting dynamic incision and synsedimentary collapse of the shelf margin. Cores of Beaufortian strata from NPRA exploration wells display a variety of key stratigraphic surfaces and depositional facies. When integrated with the regional grid of public domain seismic data, these provide a unique opportunity for interpreting the sequence stratigraphy and depositional systems oil-prospective strata in a frontier province.