Interpreting Reservoir Geometries and Assessing Reservoir Qualities from Oil Sands Cores – Challenges during Exploration and Development

S. M. Hubbard¹, M. Fustic ^2,1, R. Spencer¹, I. Gates¹, and S. Larter^1
1Alberta Ingenuity Centre for InSitu Energy, University of Calgary, Alberta
²NEXEN Inc., Calgary, Alberta

The Cretaceous McMurray Formation, which contains a significant portion of the bitumen in-place in northeast Alberta, is characterized by rapid vertical and lateral facies changes. Reservoir complexities are inherited from a complex estuarine depositional environment that developed within restricted accommodation space and under the influence of multiple transgressive-regressive cycles in an overall transgressive system.

Since post-depositional diagenetic changes are minimal (due to minimal burial history and early emplacement of petroleum), reservoir quality and distribution is directly controlled by the original depositional framework of sedimentary units. Hence, interpreting depositional environments is critical for finding potentially good prospects when acquiring land, as well as later, for positioning well pairs in InSitu development projects. On selected cores we demonstrate challenges faced in both exploration and development phases of oil sands projects.

An inherent difficulty faced during the investigation of the McMurray Formation is the presence of lithologies and sedimentary structures in cores that could potentially lead to the assignment of individual units to several possible estuarine depositional environment interpretations. Upon correlating core data with logs, particularly dipmeter information, the number of options can be reduced. Implications of different interpretations are that sedimentary bodies would be associated with different lateral extent and overall reservoir geometries. For the core shown, possible interpretations include abandoned channel-fill and progradational delta. If the first interpretation is correct, the presence of an excellent reservoir in the vicinity associated with sandy point-bar deposits may be expected, while if the second is right, sandstone will potentially be distributed in a different manner.

An additional core is shown from a developmental area where the depositional facies framework is well established and interpretation of reservoir geometries is supported by high-quality 3D seismic information. Challenges in this scenario are associated with the up-scaling of lithofacies into reservoir flow units, and correlating sub-seismic-scale heterogeneities between wells for the purpose of reservoir simulation. Here, slight variation in the geological interpretation can lead to significant changes in the assessment of recoverable oil sands reserves. It is also at this critical step that positioning well pairs for optimizing production is considered.

Cores have long been known to provide invaluable information about the distribution of reservoir-quality rocks in the McMurray Formation. Their careful analysis and interpretation, coupled with the integration of other pertinent information (e.g., seismic and dipmeter data), is key for narrowing down possible depositional model interpretations and thus optimizing exploration and development strategies.

AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada