A Multifaceted Approach To The Analysis Of Aquifer/Aquitard Unit Distribution In The Dakota Aquifer Of Kansas

P. Allen MacFarlane, J. H. Doveton, R. Olea, Howard Feldman, J. M. Holbrook, and T. Ray

The Dakota aquifer of Kansas consists of Cretaceous continental and marine discontinuous sandstone bodies encased in mudstone that comprises one partial and two entire unconformity-bounded sequences. The proportion of sandstone ranges from less than 20% to more than 80% of the total thickness. Colorized images of the subsurface in cross section from closely-spaced gamma-ray logs reveal the complex arrangement of sandstone bodies. A major objective in the development of local- to regional-scale flow and solute-transport mathematical models is to characterize the aquifer/ aquitard architecture with limited subsurface data. Sequence stratigraphic and geostatistical analysis of gamma-ray logs reveal complex, three-dimensional genetic and spatial relationships between lithofac es in the subsurface across all scales. Geologic process modeling is not limited by the assumption of continuity and produces subregional simulations of the stratigraphic architecture conditioned on the distribution of geometric building blocks and their statistical stacking patterns and behavior from field and subsurface data. Pumping-test results at the well-field scale are also sensitive to boundaries between aquifer/aquitard units and indicate "flow channeling" through the sandstone aquifers. At the core scale, hydraulic conductivity increases with grain size and degree of sorting and is strongly influenced by bedding. The results from these investigations suggest that a hierarchical analytical approach is the most appropriate for the characterization of complex aquifer/ aquitard sys ems.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California