Characterization of North Slope Viscous Oil Reservoirs at Orion Field: A Sequence-Stratigraphic Approach for Predicting and Modeling Reservoir Heterogeneity

Sandra Phillips¹, Anthony B. DePrang², David N. Greet¹, Jonathan D. Williams¹, and Greg E. Bernaski^1
1 BP Exploration Alaska Inc, Anchorage, AK
² ExxonMobil Production Company,

Alaska's viscous oil developments offer a strategic opportunity to tap the North Slope's 25+ billion barrel viscous resource and unlock future heavy oil potential. Development of the Greater Prudhoe Bay area Orion Field is targeting compartmentalized, stacked shallow marine sandstone reservoirs in the Schrader Bluff Fm., that exhibit marked variability in reservoir quality. An integrated geological model has been developed for the field to define 3D reservoir architecture using high-resolution sequence stratigraphy for reservoir zonation, facies analysis and definition of reservoir heterogeneity.

Regionally, the Schrader Bluff O-sand interval contains a series of, multistory, thin (<10-40 ft thick), laterally continuous reservoir sandstone packages, vertically separated by laterally continuous mudstones. These packages comprise a series of stacked, shallow marine shoreface parasequences, punctuated by flooding surfaces and composite sequence boundaries. Integration of core-based reservoir description and high-resolution biostratigraphy, with log-derived detailed stratigraphic correlations at the field-scale and facies scale, allows delineation of cyclical stratigraphic partitioning and facies–related heterogeneity that control reservoir quality distribution within the field.

Internally, reservoir packages consist of two primary components; 1) regressive highstand (HST) shoreface deposits and 2) incised transgressive (TST) shoreface deposits, each with distinct facies architecture and reservoir quality characteristics. Reservoir architecture within individual packages typically consists of thin, clean, areally restricted, high-permeability (100 md-800+ md) transgressive (TST) sands superposed above thicker, lower permeability, (<100 md) shaly, highstand (HST) sands. This sequence-keyed architecture provides important geologic constraints for building static reservoir models including stratigraphic reservoir layering, distribution of rock properties and definition of flow units.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005