HETEROGENEITIES IN STRATA OF THE CRETACEOUS PRINCE CREEK FORMATION DRIVEN BY PALEOENVIRONMENTAL CONDITIONS ON THE COASTAL PLAIN OF ARCTIC ALASKA: EFFECTS ON RESERVOIR QUALITY AND GEOMETRIES

The Upper Cretaceous Prince Creek Formation (PCF) outcrops along the Colville River on the North Slope of Alaska. We believe the PCF to be an ideal outcrop analogue for low net/gross, highly heterogeneous fluvial-deltaic reservoirs, particularly those on the North Slope containing viscous to heavy oil (e.g. Ugnu). Stratigraphic heterogeneities in the PCF appear to result primarily from fluctuations in fluvial discharge and variations in the water table and/or base level.

Inclined heterolithic stratification (IHS) is ubiquitous in PCF suspended-load meandering river deposits. IHS preserves recurring mud drapes on point bars, and is thought to record tidal effects on flow combined with seasonality fluctuating discharge. Deposits of meandering trunk channels are scarce. Instead, lower-order meandering and anastomosing distributaries with abundant IHS make up the bulk of strata, forming crevasse splay complexes. Along with smaller-scale splays, splay complexes repeatedly interfinger with rooted carbonaceous siltstone and mudstone on organic rich floodplains, expanding the heterogeneity of the system. Weakly developed compound to cumulative floodplain soils between splays exhibit evidence of frequent sediment input from nearby distributaries. Soil-rich floodplains typically consist of gleyed, clay-rich paleosols interbedded with thin sands, likely delivered as alluvium.

Bonebeds found on PCF floodplains are unique in that they exhibit a recurring facies pairing and bipartite division of flow consistent with deposition by fine-grained, viscous hyperconcentrated flows. We interpret these deposits to result from exceptional discharge events that entrained mud and ash stored on point bars and floodplains, increasing suspended-sediment concentrations in rivers and generating hyperconcentrated flows that extended onto floodplains adjacent to distributary channels. Size and paleodischarge estimates for PCF rivers are consistent with rivers that regularly produce hyperpycnal plumes. In fact, heterolithic delta-front hyperpycnites are found on deltas fed by PCF rivers.

When taken as a whole, our observations suggest that heterogeneities occur on many different scales in the PCF. These heterogeneities will most likely affect reservoir quality and control hydrocarbon recovery.