MULTI-SCALE HETEROGENEITY IN THE SPIRITWOOD BURIED-VALLEY AQUIFER REGION MODELED USING TRANSITION PROBABILITY GEOSTATISTICS

The Spiritwood aquifer region has been previously interpreted as having multiple buried-valley aquifers with trends along azimuths of approximately 160° and having heterogeneity both within aquifers and within the intervening (background) material. For this study, the region was conceptualized as having three facies assemblages: broad aquifers (6%), narrow aquifers (2%), and background (92%). Each assemblage is composed of the same two facies (m, mud and diamicton; s, sand and gravel). Transition probabilities for the assemblages were modeled with a Markov chain model and the assemblages simulated with sequential indicator simulation and quenching. The model indicates greater aquifer correlation along an azimuth of 70° than along 160°, and therefore the results are not consistent with previous interpretations. Thus, the data alone do not support the prior interpretations. However the geologic insight that prior interpretation was based upon can be incorporated into the Markov model.

Greater aquifer continuity along the azimuth 160° was imposed through the modification of the transition rate matrix in the Markov chain model. This leads to a simulation where the aquifer assemblages show greater continuity along an azimuth of 160°, as previously interpreted. Conditioning data still limits continuity due to the occurrences of background material in wells located between aquifer segments. Thus, two alternative models at the assemblage scale were created, one developed from the original rate matrix and based on data alone, and one from the modified rate matrix.

Within facies assemblages the proportion, geometry, and pattern of the two facies were then characterized and modeled by auto-transition probabilities and variography. The sequential indicator simulations with these models were then combined with the simulations at the assemblage scale.

The simulations were analyzed to find the largest group of cells with continuous connections (face-to-face) among them. In each simulation a single group of about 480,000 cells were so connected. This number represents 87% of all cells containing facies s, suggesting a high degree of interconnection, even within the background assemblage facies, where facies s represents 17% of the total volume.