Comparison Of Log And Core Analysis Results For An Extremely Heterogeneous Carbonate Reservoir

Accurate evaluation of Williston Basin carbonate reservoirs is difficult due to the extreme heterogeneity of the carbonate rock. The results presented in this paper represent a part of a U. S. Bureau of Mines research effort to increase the available knowledge of the physical characteristics and flow properties of carbonate reservoirs. Log and core analysis results for the Ratcliffe interval of the Madison formation in Montana's Flat Lake oil field were compared. Data for 79 wells were included in the study. In contrast to most published log-core analyses comparisons, which force porosity log response to match the corresponding core porosities, the log analyses in this study were relatively independent of the core analyses. The only core analysis data used in the log analyses were the matrix densities and modified residual oil saturations. Sonic and resistivity log porosities averaged ten percent greater and eleven percent less, respectively, than the corresponding core porosities. Water saturations derived from both Sonic and resistivity logs were consistently lower than the corresponding core residual water saturations.