Dakota Home Report Archive

1991 Symposium: The Dakota Aquifer in Kansas


Geophysical Log Analysis of the Dakota Aquifer System

John H. Doveton
Kansas Geological Survey, Lawrence, Kansas 66047

Geophysical logs are continuous records of physical properties of rocks in boreholes drilled for hydrocarbons, minerals or water. They are made by electrical, acoustic and nuclear tools suspended on a wireline and winched upwards through the formations penetrated by the borehole. Most logging tools are run by the oil industry in both exploration holes and producing wells. Consequently, log analysis of aquifers is a stepchild of a methodology geared to the location of oil and gas. Fortunately, many of the rock properties that are used to characterize hydrocarbon reservoirs are also key parameters of aquifers. However, there are also some significant differences, so that adaptations of traditional methods of data analysis must be made with caution.

The long history of oil exploration in Kansas has resulted in the recording of hundreds of thousands of logs across the state. These are archived at the Kansas Geological Survey and provide a crucial reference framework for the correlation of sands and shales within the Dakota aquifer. The Dakota Aquifer Program has developed a large regional data-base of digital gamma-ray logs drawn from this library which are being used for detailed mapping of inferred aquifer and aquitard units.

The observation wells of the Dakota Aquifer Program have been logged by tools recently developed in the logging industry. These logs give new insights into the geological and aquifer properties of the Dakota and also provide useful keys to the interpretation of the more widely available, but older logs. The recording of multiple resistivity logs was successful in the differentiation of fresher water (higher resistivity) from more saline water (lower resistivity) units. The resistivity tools record different depths of investigation into the borehole wall. Separation of the resistivity curves appears to be an excellent indication of zones where mud filtrate has invaded the formation. This, in turn, implicitly differentiates potential aquifer and aquitard zones as evidence of fluid transmissibility. The distinction appears to be keyed to shale content and so this criterion can be used to calibrate gamma-ray logs for regional mapping of this aquifer/aquitard discrimination.

The calculation of pore volume is readily made from either of three "porosity logs" (density, neutron or sonic), using the same analysis procedures applied in the oil industry. Because the sonic log measures the speed of sound through the formations, it has also been used in Dakota aquifer studies as a means to identify reflection events seen on seismic surveys made at the surface. No wireline log measures permeability directly. However, it is hoped that useable values can be developed from predictive equations based on core measurements of permeability and porosity. If so, then porosity logs from the region can be used to estimate permeabilities for incorporation as parameters in Dakota flow models.

The modern nuclear logs run on the Dakota Program observation wells give direct measures of rock geochemistry. The spectral gamma ray logs record formation contents of potassium, thorium and uranium. The photoelectric absorption curve logged by the newer density tool is a direct function of aggregate atomic number. In the Dakota, these logs collectively respond to changes in clay mineralogy in the shales and the relative oxidation/reduction potential of the depositional environment. This information is useful in the recognition of different lithofacies, which is itself important in the mapping of these units, many of which are lensoid in shape and laterally discontinuous.

Some of the log analysis techniques that are currently being applied in the Dakota Aquifer Program are relatively novel, with broader implications for future hydrogeologic studies. In particular, as applied to the newer logs they can give a more detailed synthesis of both geologic framework and aquifer properties. However, a principal goal of the program is to extend these findings as keys to the recognition and exploitation of aquifer features on older logs. By this means, interpretative maps and cross-sections can be developed from a much larger Dakota regional data-base.

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Kansas Geological Survey, Dakota Aquifer Program
Original symposium held October 1991
Electronic version placed online April 1996
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu
URL=http://www.kgs.ku.edu/Dakota/vol3/symp/doveton.htm