Abstract: Spectral gamma-ray and lithodensity-neutron log characterization of sedimentary facies in the Dakota Aquifer of central Kansas

Peer Hoth, John H. Doveton, Howard Feldman

The Cretaceous Dakota Aquifer in Kansas is comprised of the Cheyenne, Kiowa, and Dakota formations. The sedimentary record is a basal incised-valley fill (Cheyenne) that is succeeded by marine and coastal deltaic deposits (Kiowa-Longford), and fluvial sandstones and alluvial plain clays, terminated by estuarine and deltaic units (Dakota) that underlie the marine Graneros Shale.

Spectral gamma-ray and lithodensity-neutron logs of the Dakota Aquifer show striking patterns of variability that can be related directly to both compositional and geochemical aspects of depositional environments. These relationships are demonstrated by the systematic analysis of core and logs taken in the KGS Jones #1 water observation well in Lincoln County. Information

from detailed core descriptions and petrographic analyses (x-ray diffraction, thin-section and scanning electron microscopy) were integrated with wireline log data. Kaolinitic paleosols are readily differentiated on logs from illitic marine shales by relative thorium enrichment and potassium depletion. Uranium concentration appears to be closely linked with organic matter content, while the photoelectric absorption curve is a sensitive indicator of iron-rich zones. Core data were used to calibrate log measurements in numerical classification and prediction models for sedimentary facies analysis in other Dakota Aquifer wells that have been logged but not cored. These facies predictions are important aids in the tracing the complex lateral geometry of Dakota Aquifer and aquitard units f r use as a framework for both deterministic and stochastic flow models.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994