Cyclicity in Lower Cretaceous point bar deposits with implications for reservoir characterization, Athabasca Oil Sands, Alberta, Canada

Abstract

The Lower Cretaceous McMurray Formation comprises the majority of the Athabasca Oil Sands deposit, which is one of the largest heavy oil accumulations in the world. Point bar deposits account for a significant proportion of the subsurface reservoir, and are characterized by bedded, sandstone-dominated strata. In-situ development of heavy oil is sensitive to lateral and vertical lithological heterogeneity in the form of interbedded siltstone beds, which are locally common in the formation and must be taken into account in order to understand permeability distribution within the reservoir. The objectives of this research are to integrate qualitative sedimentological observations with quantitative analyses of wireline log data in order to constrain the distribution of siltstone interbeds within the point bar deposit, to use this information to understand the deposition history of the fluvial system, and to provide insight into reservoir modeling. Quantitative statistical analyses of wireline log data, including autocorrelation analysis, Fourier transform analysis, and continuous wavelet transform analysis, reveal lithological cyclicity within the heterogeneous point bar sediments examined. Lithological cyclicity is evident at wavelengths ranging from 2 to 10 m, and generally increases in strength toward the downstream direction of the ancient point bar deposit where more heterogeneous sediments are prevalent. Cyclicity also increases in amplitude in the youngest layers of the laterally accreted point bar deposit; the overall finer-grained sediments correspond to decreasing energy levels and waning flow over the evolution of the fluvial system. Overall, the highest amplitude cycles were calculated at a wavelength of 3.5 m, and, when correlated to core observations, are represented as interbedded packages of thick, massive to cross-bedded sandstone that fine upward to siltstone. The control on cyclic sedimentation is difficult to discern from Cretaceous-aged point bar deposits. Through consideration of sedimentation rates from analogous modern fluvial systems, the smallest cycles observed might be attributed to seasonal variations in climate and associated flow fluctuation. However, it is more likely that the cyclicity recorded is linked to longer scale climatic signals, such as El-Niño Southern Oscillation (ENSO-like) fluctuations or variations in solar output (decadal cyclicity). The mappable variations in siltstone bed frequency and thickness across the ancient point bar deposit have implications for heavy oil development. Low permeability siltstone beds associated with the shortest wavelength cycles (i.e., < 2 m cycles) have limited impact on fluid flow within the reservoir; however, those associated with cycles longer than 4 m in wavelength may serve to compartmentalize portions of the reservoir, leading to reduced oil recovery.