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Predicting Reservoir Quality Distributions in Storm-Dominated Shoreface and Delta Environments of a Highstand Systems Tract, Lower Cretaceous Viking Formation, Crossfield Area, Alberta, Canada


The Lower Cretaceous Viking Formation is a prolific hydrocarbon producer in the Crossfield area, Alberta, although there are marked variations in reservoir quality along paleodepositional strike. Crossfield and surrounding area encompasses roughly 5000 km2, with cumulative production of 1.29 MMbbl of oil and 26.41 BCF of gas. In this study, sedimentological and ichnological observations from 56 cored wells are coupled with analysis of 1000 geophysical well logs, and together these data permit high-resolution, facies-driven mapping of reservoir distributions in a sequence stratigraphic framework. The Viking Formation comprises parts of four discrete depositional sequences, but the main reservoir units at Crossfield are associated with stacked shallow-marine parasequences in the highstand systems tract of Sequence 3. These parasequences downlap onto a maximum flooding surface and are erosionally overlain by the regionally mappable subaerial unconformity (SU) of Sequence 4. Relief on the overlying SU locally removes reservoir facies at the tops of some parasequences complicating prediction of reservoir distributions. Stratigraphic correlation of the highstand system tract permits the high-resolution paleogeographic mapping of the reservoir interval. Detailed facies analysis demonstrates that the interval records deposition in stacked sanding-upward successions locally recording storm-dominated strandplain shorefaces, wave-dominated deltas and distributary channels. Mapping shows an increasing abundance of carbonaceous mudstone interbeds in both distal and proximal facies as successions transition along depositional strike from strandplain shorefaces to mixed process deltas. While deltaic sandstones are slightly coarser grained than shoreface sandstones, the presence of markedly heterolithic, mud-dominated prodelta to distal delta front successions and the persistence of mudstone interbeds into proximal delta-front sandstone intervals lead to overall lower-quality reservoirs in deltaic parasequences. Further, the strong wave energy reflected in the facies favors development of delta asymmetry, impacting predictions of reservoir distribution along depositional strike. This study demonstrates that by carefully mapping paleoenvironments within discrete systems tracts, it is possible to greatly improve the discrimination and characterization of contemporaneous deltaic versus shoreface reservoir units.