Frances J. Hein

Seismic Modeling of Fluvial-Estuarine Deposits in the Athabasca Oil Sands Using Ray-Tracing Techniques, Steepbank River Area, Northeastern Alberta

ABSTRACT Most of the bitumen ressources in the Athabasca oil sands area are contained in fluvial and estuarine channel deposits of the Lower Cretaceous McMurray Formation. Stratigraphic sections were measured along a 5 km stretch of the Steepbank River, north of Fort McMurray, near the Suncor Steepbank Mine. We have identified at least four channel complexes based on stratal bounding surfaces, arrangement of exposed lithofacies, and consistent paleoflow patterns. The lower part of each channel complex contains medium to large-scale, trough crossbedded sandstone, exhibiting high porosity and permeability. These sandstones were deposited in channel axes and are the highest-grade bitumen deposits in the study area. Overlying inclined beds (sandy or muddy inclined heterolithic stratification) downlap on the lower sandy beds and were deposited in estuarine point bars, but the permeability and bitumen saturation of these strata are considerably lower. Nearby wells contain cored and logged intervals that are similar to exposed outcrops in the riverbank. For the outcrop sections, artificial sonic logs were constructed by comparison with sonic curves from these nearby wells, but the curves were modified to reflect differences in thickness and mud content at the exposure. Three structural-stratigraphic cross-sections and ray-tracing techniques were used to construct seismic models. These models compare favourably with four high-quality, industry-acquired seismic lines from the Clarke Creek ExxonMobil lease area, about 20 km southeast of the Steepbank River outcrops. The deposits at Steepbank are thus similar in scale and geometry to those in the Clarke Creek area. The Clarke Creek channel complexes are very well imaged on the seismic profiles, and our seismic modeling indicates that high-resolution seismic data are necessary to image these channels. 1 Present Address: Alberta Energy and Utilities Board, 640 - 5 Avenue SW, Calgary, AB T2P 3G4 End_Page 178------------------------

Seismic Modeling of Fluvial-Estuarine Deposits in the Athabasca Oil Sands Using Ray-Tracing Techniques, Steepbank River Area, Northeastern Alberta: REPLY TO DISCUSSION of C. W. Langenberg, F. J. Hein, D. Lawton and J. Cunningham

We appreciate the discussion of our paper (Langenberg et al., 2002) by Michael Ranger and Murray Gingras. Their elicited strong reaction to our work shows clearly the importance of this topic for understanding the oil sands in the Athabasca deposit, the complex nature of the stratigraphy and sedimentology of the deposit, and the wide range of views and opinions concerning interpretations of the oil sands. Ranger and Gingras (this issue) have two main critiques of our work: 1) the definition of channel complexes and the criteria used to distinguish them, and 2) the stratigraphic nomenclature and subdivision as applied to the McMurray Formation. Because this critique concerns sedimentology and stratigraphy of the oil sand deposit only and not the seismic modeling, we changed the order and number of responding authors. Ranger and Gingras (this issue) express the opinion that our definition of channel complexes is “simplistic, ill-defined, and of questionable validity;” and that “their channel complex model from the Steepbank River outcrops and surrounding wells is based on questionable criteria and is likely incorrect.” In our original paper, we stated, “individual channel complexes were identified in the heterolithic valley-fill units of the upper McMurray Formation. Channel complexes generally contain a basal scour surface overlain by Inclined Heterolithic Stratification (IHS) with consistent paleoflow directional indicators” (Langenberg et al., 2002, p. 188). This text was accompanied by further written description and measured stratigraphic sections from outcrops 3, 4, 7 and 9 on the Steepbank River (our Figs. 6⇓–9) and two core descriptions with wireline logs (our Figs. 10, 11). As written, with the accompanying diagrams, we thought it was apparent that a channel complex is a lithofacies package, defined by major bounding surfaces (generally a basal scour surface), with genetically related facies (in this case channel, point bar lateral …