--> Abstract: Variable Formation Water Resistivity (Rw) and Bitumen "Flushing" in the Athabasca Oil Sands; Effect on Bitumen Assessment, by Michael J. Ranger and Brian Rottenfusser; #90039 (2005)

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Variable Formation Water Resistivity (Rw) and Bitumen "Flushing" in the Athabasca Oil Sands; Effect on Bitumen Assessment

Michael J. Ranger1 and Brian Rottenfusser2
1 Consultant, Calgary, AB
2 Redfoot Enterprises, Calgary, AB

Reservoir sands of the Lower Cretaceous McMurray Formation in the Athabasca Oil Sands Deposit lie at a relatively shallow depth compared to most conventional oil fields. Phreatic water therefore appears to a component of the formation fluids in some areas. Because bitumen does not flow at reservoir conditions in much of Athabasca, fluid sample recovery and analysis is not normal exploration practice. However, core recovery and analysis of core fluids is common. A rough estimate of bitumen saturation from logs alone relies upon the simple technique of using a resistivity-porosity cutoff as a general indicator of grade. This is effective where such a cutoff has been calibrated to bitumen analysis from nearby core, but is not always reliable. In some areas, resistivity response is observed to fluctuate over an interval with otherwise uniform porosity and lithology, yet core analyses indicate no change in bitumen saturation. Vertical variation in formation water resistivity (Rw) accounts for these observations. Cores and accompanying logs from the OPTI Nexen Long Lake and ConocoPhilips Surmont projects demonstrate this effect.

Typical bitumen saturation in the Athabasca reservoir is thought to be at a physical maximum, containing only irreducible residual water. But at shallow depths, the reservoir may have been partially "flushed" such that bitumen saturations are lower than maximum, i.e. contains more than just irreducible water. These flushed zones degrade the value of the resource, but may also have an effect, positive or negative, on the production process. On resistivity logs such flushed zones may be confused with zones of varying Rw, and can only be resolved with confidence from core observations and analysis.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005