Alaska's North Slope is a world-class petroleum basin, with some of the largest producing fields in North America. What is not widely known is that the North Slope also holds a vast resource of heavy oil that is mostly undeveloped. The Ugnu sands, with 12 to 18 billion bbl original oil in place, account for about one half of the heavy-oil resource and are the focus of appraisal activity to determine development potential.

The Ugnu resource occurs at depths between 610 and 1524 m (2000 and 5000 ft) true vertical depth subsea along a regional normal-faulted monocline. Oil quality in the Ugnu ranges from heavy to extra heavy, with API gravities that range from 14 and 8 API, with corresponding live oil viscosities ranging from 300 to more than 50,000 cp. The gross interval ranges in thickness from 107 to 152 m (350–500 ft) and consists of unconsolidated sands separated by thinner silty mudstones units. Sandstone thickness ranges from 1 to as much as 30 m (3–100 ft). Sand quality is excellent, with porosities ranging between 30 and 36% and permeabilities between 400 and 6000+ md.

Core description of the Ugnu identified 10 facies associations that represent fluvial-deltaic, floodplain, delta-front, and offshore environments. The primary reservoir facies are fluvial-deltaic channel fills and sandsheets. The fluvial-deltaic sandstones were deposited by meandering channel systems based on paleoflow indicators from image logs and sinuous amplitudes from three-dimensional (3-D) seismic data. Palynological analysis defined five regional stratal surfaces (sequence boundaries and flooding surfaces) and revealed that lower delta-plain environments persisted during most of Ugnu deposition.

Careful integration of log, core, and seismic data provided the basis for defining the reservoir architecture and depositional model. The primary reservoir zones are the Ugnu M sands that are subdivided into the M70, lower M80, and upper M80 subzones. The M70 and lower M80 zones are characterized by regionally extensive erosional unconformities and sequence boundaries at the base that are overlain by multistory channel-fill sandstones deposited in lower and upper delta-plain settings. The upper M80 is bound by a marine flooding surface at the base (where present). The interval cleans upward and becomes progressively sandy in the east, corresponding to a transition from delta-plain to delta-front environments. The Ugnu reservoirs were deposited by meandering rivers that transected a flat coastal plain. Regionally, the Ugnu fluvial-deltaic system prograded from southwest to northeast during the Late Cretaceous to the early Tertiary and was deposited during an overall, but phased, base level rise.