THE INFLUENCE OF HIERARCHICAL SEDIMENTARY STRUCTURE ON PERMEABILITY HETEROGENEITY: A LITHOFACIES-BASED STUDY OF THE BORDEN AQUIFER

The aquifer at Canadian Forces Base Borden, Ontario, Canada has been considered relatively homogenous. However, the conservative tracer experiment conducted by Sudicky (1983, Water Resources Research) resulted in a split tracer plume – a result that is not consistent with a simplified (nearly homogenous) subsurface. Results such as these point out the importance of knowledge of the spatial distribution of permeability (k). Recently, we collected core samples and mapped lithofacies in the Borden aquifer as part of a collaborative research project to determine spatial cross-correlation between the properties of the aquifer that control reactive transport. A horizontal transect of seventy-two densely-spaced (0.25-1.0m) cores allowed us to determine the spatial distribution and lateral dimensions of lithofacies units, which were not clearly discernible in previous studies. Our lithofacies mapping shows the following sedimentary features: dipping strata, channel-like structures, gradational transition among facies units, erosional discontinuities in the low-k materials, and isolated occurrences of silt materials. Permeability was measured for a representative set (~350) of core subsamples. Geostatistical analysis of the horizontal cross-transition probabilities for lithofacies at two hierarchical levels reveals the contribution of stratal architecture to the k-sample semivariogram. This information suggests that the characterization of spatial variability based solely on statistical abstractions without consideration of geologic structure may neglect important and useful information.