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A Forced Regressive Asymmetric Delta, Lower Cretaceous Viking Formation, Kaybob - Fox Creek Fields, Alberta, Canada


The Lower Cretaceous (Upper Albian) Viking Formation at Kaybob-Fox Creek, Alberta, Canada, is characterized by a hydrocarbon-bearing, NW-SE trending, sharp-based coarsening-upward succession, interpreted to reflect a forced regressive shoreline of the falling stage systems tract (FFST). In proximal positions, the FSST overlies a regressive surface of marine erosion (RSME) incised into underlying marine parasequences, and is locally demarcated by firmground omission suites of the Glossifungites Ichnofacies. In basinward localities, the FSST overlies a non-erosional basal surface of forced regression (BSFR). Continued base level fall led to top truncation of the FSST by a subaerial unconformity (SU), with a concomitant shift of the shoreline eastward. The SU was transgressively modified during later base level rise.

The FSST shows marked variability along paleodepositional strike. In the Kaybob Field to the northwest, successions display a HCS- and micro-HCS-bearing coarsening-upward succession. Trace fossil suites are diverse, with pervasive to sporadically distributed bioturbation (BI 0-5). Ichnogenera are robust and show little evidence of physico-chemical stress, consistent with progradation of a storm-influenced shoreface. By contrast, successions in the Fox Creek Field lying to the southeast are markedly heterolithic. HCS-, and micro-HCS-bearing sandstones are also common, but with associated combined flow ripples, current ripples, syneresis cracks, and abundant, largely unburrowed carbonaceous mudstone drapes. Trace fossil suites are less diverse, sporadically distributed, and show BI 0-3. Facies display a strong wave influence but with persistent evidence of fluvial sediment influx, recorded by mud drapes attributed to fluid mud and/or mud flocculation from buoyant plumes. These successions are interpreted as a mixed river- and wave-influenced delta.

The spatial distribution of facies associations indicates that these forced regressive deposits record progradation of an asymmetric delta. Updrift positions (NW) show little fluvial influence and persistent marine conditions. By contrast, downdrift areas (SE) received the bulk of river discharge, leading to heterolithic deposition in a physico-chemically stressed setting. Wells lying between these two areas record avulsion of distributary channels. Reservoir properties decline towards the SE, suggesting that the increase in fluvial influence was detrimental to reservoir properties.