Formation Factor Relationships Of Western Canada Available to Purchase

Archie's formation resistivity factor and saturation exponent equations are the basis of quantitative log analysis. Canadian practitioners of formation evaluation in Alberta are fortunate that formation factor measurements are catalogued and made available to the public under Section 11.040 of the Alberta Oil and Gas Conservation Regulations. Access to this information presented the opportunity to evaluate the "a", "m" and "n" terms with respect to overburden stress dependency, the variability of the values, lithology dependence and the possible interdependence of these three parameters. Over 140 special core studies were collected from the list provided in the December 1991 Energy Conservation Board PVT and Core Studies Index and from a list of special core analyses provided by the Saskatchewan Energy and Mines. Noticeable increases in cementation exponent occurred when core was subjected to overburden stress. Stressed formation factor values changed log-derived water saturations to the degree that the use of values obtained at surface pressure is invalid. An empirical correlation between "a" and "m" is derived and discussed. This correlation brings into question the assumption of "a" equalling one in the Archie equation F = a/?m, The use of stressed formation factor values, the wide variability in "a" and "m" and the fact that "n" frequently is not equal to two (2) are shown to have large effects on water saturation and oil and gas reserve calculations.