New insights into the hydrostratigraphy of the High Plains aquifer from three-dimensional visualizations based on well records

Abstract

Regional aquifers in thick sequences of continentally derived heterolithic deposits, such as the High Plains of the North American Great Plains, are diffi cult to characterize hydrostratigraphically because of their framework complexity and the lack of highquality subsurface information from drill cores and geophysical logs. However, using a database of carefully evaluated drillers’ and sample logs and commercially available visualization software, it is possible to qualitatively characterize these complex frameworks based on the concept of relative permeability. Relative permeability is the permeable fraction of a deposit expressed as a percentage of its total thickness. In this methodology, uncemented coarse and fi ne sediments are arbitrarily set at relative permeabilities of 100% and 0%, respectively, with allowances made for log entries containing descriptions of mixed lithologies, heterolithic strata, and cementation. To better understand the arrangement of high- and low-permeability domains within the High Plains aquifer, a pilot study was undertaken in southwest Kansas to create three-dimensional visualizations of relative permeability using a database of >3000 logs. Aggregate relative permeability ranges up to 99% with a mean of 51%. Laterally traceable, thick domains of >80% relative permeability embedded within a lower relative permeability matrix strongly suggest that preferred pathways for lateral and vertical water transmission exist within the aquifer. Similarly, domains with relative permeabilities of <45% are traceable laterally over appreciable distances in the subsurface and probably act as leaky confi ning layers. This study shows that the aquifer does not consist solely of local, randomly distributed, hydrostratigraphic units, as suggested by previous studies.