The effect of carbonate cementation on permeability heterogeneity in fluvial aquifers: An outcrop analog study

Spatial variations in permeability have been widely recognized as a dominant control on solute transport and petroleum migration. Outcrop studies are one means of improving our understanding of the nature of permeability heterogeneity and the underlying geologic controls. To date, these studies have either neglected the importance of cementation or investigated only the modern cemented permeability without explicitly considering how cementation has affected permeability. Two outcrops of channel sand deposits in the Plio-Pleistocene Sierra Ladrones Formation, New Mexico are investigated to quantify (1) the effect of cementation on permeability at a given location, and (2) the patterns of cementation and their impact on permeability heterogeneity. We analyzed approximately 400 spatially located samples (∼ 10 cm 3 each) from the two outcrops. Cementation is typically weak to moderate, filling less than 60% of the original pore space. Permeability and cement content are bimodally distributed and negatively correlated. Permeability reduction is investigated by measuring modern permeability, minus-cement permeability, minus-cement porosity, and volumetric cement content. Permeability is reduced locally by up to three orders of magnitude. By combining models of permeability for uncemented sediments and cemented sediments, we derive a model of cemented permeability ( k c) that depends primarily on three factors: uncemented permeability ( k u), original porosity, and cement fraction. Our results suggest that accounting for porosity reduction only may result in an overestimation of k c. Conversely, accounting for porosity reduction, increased specific surface area, and increased tortuosity appears to result in an underestimation of k c. Accounting for porosity reduction and increased specific surface area provide the best estimate of k c. Cementation is highly variable spatially and is poorly correlated with lithofacies. Variogram analysis of cementation and permeability indicates that the spatial distribution of cementation is the dominant control on the permeability correlation structure. The spatially variable cementation also results in an order-of-magnitude increase in the variance of log-permeability. Applying stochastic models of effective flow and transport properties to the results of this study suggest that cementation has a relatively small effect on effective permeability but can potentially increase dispersivity by two orders of magnitude.