The Clearwater Formation, Cold Lake, Alberta: a worldclass hydrocarbon reservoir hosted in a complex succession of tide-dominated deltaic deposits
The Lower Cretaceous (Lower Albian) Clearwater Formation at Cold Lake, Alberta is a complex assemblage of nonmarine, marginal marine and shallow marine strata. Within the study area, these strata form four unconformity-bounded depositional sequences that in most cases consists of a lower progradational unit overlain by a retrogradational unit. The base of all sequences, except the first, is marked locally by a deeply incised, northwest–south-trending paleovalley. Although eroded during a fall of relative sea level, each paleovalley was filled mostly with sediment deposited by a northwest-prograding tide-dominated delta during each subsequent rise of relative sea level. Reservoir strata occur in the lower, deltaic part of the three youngest sequences, and consist primarily of nonmarine fluvial and high-energy tidal sand flats deposits. Deltaic progradation was halted, however, by a possible decrease in sediment flux and a return to retrogressive conditions. This formed an areally extensive flooding surface, which then was overlain by retrogradational shallow-marine deposits (embayment-mouth deposits). Accordingly, each sequence represents a short-term (4th order?) fluctuation of relative sea level that was superimposed on a long-term (3rd order?) rise of relative sea level. In addition, each successive sequence indicates a progressively more basinward migration of (tide-dominated) deltaic facies (prograding sequence set) suggestive of temporal changes in some number of local and/or regional sedimentary controls, such as sediment flux, rates of relative sea level, or depositional gradient.