Refinement of the stratigraphic framework for the Regional C depositional unit of the McMurray Formation and implications for the early transgression of the Alberta Foreland Basin, Canada

ABSTRACT The Lower Cretaceous McMurray Formation in Alberta and Saskatchewan, Canada, comprises a series of depositional units (DUs) consisting of stacked parasequences bounded by flooding surfaces and incised by fluvio-estuarine channel belts. The fluvio-estuarine channel belts of the McMurray Fm have been the focus of numerous studies whereas the regional DUs have received substantially less attention. Of the regional DUs, Regional C (equivalent to the middle McMurray) is the most understudied, yet this interval records the history of the McMurray Formation between deposition of fluvial strata in the lower McMurray and marine facies in the upper McMurray and overlying Clearwater Formation. Determining the history of the Regional C DU is fundamental for accurately reconstructing the stratigraphic evolution of the McMurray Fm and, by extension, the early evolution of the Alberta Foreland Basin. The Regional C is divided into two DUs separated by a regionally mappable flooding surface. This surface occurs 11 to 15 m below the top of the Regional C and is traceable over a 2,550 km2 area. This flooding surface divides the thick interval of undifferentiated Regional C into a lower C2 DU and an upper C1 DU, each with a maximum thickness of < 15 m. The thickness of the C2 and C1 DUs indicates that deposition at this time also occurred in a setting of low to moderate accommodation creation, which is consistent with the rest of the McMurray Formation. The limited available accommodation space was easily surpassed by sediment supplied by the paleo-distributive channel system, leading to a basinward progradation of the shoreline. The C2 and C1 DUs are retrogradationally stacked, with the maximum regressive paleo-shoreline of C1 lying landward of that of C2. This stacking arrangement indicates that the shoreline backstepped during the early stages of transgression of the Boreal Sea. The backstepping of the paleo-shoreline from C2 to C1 time is consistent with previous studies that show continued and stepwise retrogradation and/or transgression of the paleo-shoreline from the onset of deposition in the lower McMurray Formation through to maximum transgression in the Clearwater Formation. Together, these studies demonstrate that the early drowning of the Alberta Foreland Basin was persistent and slow.

Dietary palaeoecology of an Early Cretaceous armoured dinosaur (Ornithischia; Nodosauridae) based on floral analysis of stomach contents

The exceptionally well-preserved holotype of the armoured dinosaur Borealopelta markmitchelli (Ornithischia; Nodosauridae) from the Early Cretaceous (Clearwater Formation) of northern Alberta preserves a distinct mass within the abdominal cavity. Fourteen independent criteria (including: co-allochthony, anatomical position, gastroliths) support the interpretation of this mass as ingested stomach contents—a cololite. Palynomorphs in the cololite are a subset of the more diverse external sample. Analysis of the cololite documents well-preserved plant material dominated by leaf tissue (88%), including intact sporangia, leaf cross-sections and cuticle, but also including stems, wood and charcoal. The leaf fraction is dominated (85%) by leptosporangiate ferns (subclass Polypodiidae), with low cycad–cycadophyte (3%) and trace conifer foliage. These data represent the most well-supported and detailed direct evidence of diet in an herbivorous dinosaur. Details of the dietary palaeoecology of this nodosaur are revealed, including: selective feeding on ferns; preferential ingestion of leptosporangiate ferns to the exclusion of Osmundaceae and eusporangiate ferns such as Marattiaceae; and incidental consumption of cycad–cycadophyte and conifer leaves. The presence of significant (6%) charcoal may represent the dietary use of recently burned conifer forest undergoing fern succession, early evidence of a fire succession ecology, as is associated with many modern large herbivores.

Seismic reconstruction of the in situ anisotropic stress field for caprock integrity in the Athabasca oil-sands basin, Canada

In the Athabasca Basin of Alberta, Canada, the Lower Cretaceous McMurray Formation reservoir is overlain by the Clearwater Formation, a regionally continuous layer composed predominantly of shales with interbedded mudstones. The shales form the reservoir caprock and have the role of blocking the vertical migration of the steam from thermal oil production by confining the stresses and the deformations. We have developed a new method for the 3D seismic reconstruction of the anisotropic stress field, which accounts for the formation pore pressure and the effective stress. We integrated anisotropy estimated from dipole sonic logs with formation pore pressure data from piezometers and elastic properties obtained from multicomponent seismic inversion. The method combines the Terzaghi effective stress with the Schoenberg and Sayers elastic stiffness matrix for horizontal transversely isotropic (HTI) fractured materials. The key points of this method are the estimation of the formation pore pressure in the abnormal regime of the Clearwater Formation, the normal fracture weakness parameter (based on constraints on the compressional velocity of the intact rock under the HTI assumption), and the 3D seismic anisotropic stress field. We expressed the total vertical stress as the weight of the overlying formations, and the total minimum and maximum horizontal stresses as a combination of the total vertical stress, normal fracture weakness, formation pore pressure, Biot-Willis coefficient, and Lamé elastic constants. The effective principal stresses are estimated from the equivalent total principal stresses and the formation pore pressure multiplied by the Biot-Willis coefficient. We observed excellent consistency between the calculated total minimum horizontal stress and mini-frac values. This new method for the 3D seismic reconstruction of the anisotropic stress field allows for the assessment of the caprock integrity and for operational savings based on a reduced number of mini-frac measurements, and it can be used for time-lapse stress estimation within the thermal production reservoir.

Paleokarst in the Grosmont Formation and reservoir implications, Saleski, Alberta, Canada

We delineated a bitumen-rich paleokarsted carbonate reservoir of the Upper Devonian (Frasnian) Grosmont Formation with a high-resolution 3D seismic survey tied to core and petrophysical log data from 35 wells within a [Formula: see text] study area in northern Alberta, Canada. There were two laterally continuous karst facies: a solution-enhanced vuggy dolostone that resulted from the carbonate dissolution of body fossils and a stratiform breccia that resulted from the dissolution of interbedded evaporites. Three laterally discontinuous karst facies were identified: sinkhole fills, collapsed paleocaves, and solution valley fills. We measured 368 subcircular features (sinkholes and collapsed paleocaves) having a median circle-equivalent diameter of 69 m and representing 5.5% of the total study area. Sinkhole fills include Cretaceous-aged sandstone, mudstone, and coal. Collapsed paleocaves were filled with matrix-supported breccia that had clasts of disoriented blocks of dolomite and a matrix of disaggregated dolomite and Cretaceous-aged mudstone. The paleocaves and sinkholes formed in the solution-enhanced karst facies of the Grosmont C at the interface of an interpreted ancient vadose-phreatic mixing zone. The marine deepwater deposition of the Clearwater Formation during the Albian filled the depressions created by the mechanical collapse of the paleocaves and provided a seal for thermal operations. The fracture density inferred from seismic amplitude variation with angle and azimuth analysis and corroborated by well data showed that fractures are ubiquitous and were enhanced during meteoric karst. The high-vertical permeability resulting from solution-enhanced fractures, the laterally predictable flow units, and a competent seal make this an ideal reservoir for thermal bitumen recovery.