Whole Core versus Plugs: Scale Dependence of Porosity-Permeability Measurements in Platform Carbonates

Ehrenberg, S. N.¹ (1) Statoil, N-4035 Stavanger, Norway

There is widespread agreement that plug data do not adequately describe heterogeneous carbonate fabrics containing vugs, but whole-core measurements are commonly recommended based on common sense rather than data. This study compares porosity and permeability measurements between 23 whole-core samples and 54 one-inch plugs drilled from the centers of the same samples. The samples are coarsely bioclastic carbonates from Miocene carbonate platforms of the Marion Plateau, offshore northeast Australia (ODP Leg 194). The differences observed may be significant for some applications but not for others. After averaging multiple plug values taken from the same whole-core sample, whole-core/plug porosity differences are "major" (>2% absolute) in 7 of 23 cases, and permeability differences are "major" (>0.5 log units) in 9 of 23 cases. Nevertheless, whole-core and plug data show overall agreement in terms of trends defined on a permeability-porosity plot. Comparisons with thin sections made from the ends of the whole cores show that neither the apparent degree of heterogeneity observed in thin section nor textural category are useful predictors for similarity between whole-core and plug values. In 8 of the 9 cases showing "major" differences between whole-core and plug permeability, the whole-core value is higher than the average of the plug values from the same rock volume. This tendency could reflect the presence of optimal flow paths through the whole-core volumes that preferentially exploit connections between vugs. The smaller plug volumes would have lower probability of including portions of these optimal flow paths, resulting in lower permeability in most cases.