A large-scale study of more that 125 wells has provided excellent fluid saturation data for two complex Devonian Reef reservoirs in Western Canada. These data, presented as plots of elevation vs water saturation profiles for specified ranges of porosity, show fluid saturations to vary with elevation to a height more than 225 feet above the field-established oil/water contacts. Quantitative interpretations were made possible by an excellent combination of logs, special techniques for reading and correcting the logging curves, and special core analyses. Induction logs were found applicable to low-conductivity, oil-bearing beds by using dense beds to correct for the conductivity zero errors and borehole signals. Calculations were obtained through a special digital computer program using an IBM 7044 computer; a Cal Comp plotter provided the water saturation profiles for eight different ranges of porosity.
Quantitative log interpretation is generally not considered applicable to the complex Devonian reef limestones of western Canada. Some of the factors which may disturb interpretations are:
large mud cake effect on pad resistivity logs,
large mud filtrate invasion effect on deep investigation resistivity and induction logs,
inaccuracies of induction logs in the low-conductivity, oil-bearing beds,
unusual formation factor vs porosity relationships,
unusual porosity vs transit-time relationships, and
variability from well to well of the porosity vs transit time relationships.
To obtain adequate information for reservoir description, operators have resorted to extensive coring and core analysis. Oil-base cores have provided water saturation data. Some time ago, with the assistance of Imperial Oil Limited (Alberta, Canada), a special study was made to determine whether or not the Devonian reef log interpretation problems could be solved. It was hoped that log analyses could be used to help reduce the amount of core data needed.
