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Dakota Aquifer Program--Petrophysics

Geophysical Log Analysis of the Dakota Aquifer


Geophysical logs are 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 (Figure 1). A logging tool may be about fifty feet in length and its measurements are recorded as traces on a graphic chart of depth, known as a "log". Most tools are run by logging service companies for the oil industry in both exploration holes and producing wells. Fortunately, many of the rock properties that are used to locate and describe oil and gas reservoirs are also useful in the search for aquifer beds with usable water.

Figure 1. Combination logging tool for measuring acoustic travel time (delta t), natural gamma radiation (GR), shallow-focussed (SFL) and deep-focussed electrical conductivity (IL), and spontaneous electrical potential (SP) of rock formations in a borehole, together with a typical gamma-ray log of a Dakota Aquifer section. The tool shown is 55 feet 7 inches long.


The long history of oil exploration in Kansas has resulted in the recording of hundreds of thousands of logs across the state. These are filed with the Kansas Corporation Commission and then archived at both the Kansas Geological Survey in Lawrence and the Kansas Geological Society in Wichita, where copies are available for purchase by the public. The main use of logs is for the identification of the depths of stratigraphic formation boundaries ("tops") and their correlation between wells. The maps of correlated tops are similar in style to topographic maps. However, a map of an underground surface cannot be seen, but must be estimated in between the available well control.

In addition, logs also provide valuable information on the Dakota Aquifer because they can be used for:

(1) rock type recognition
Sandstones (aquifers) can usually be distinguished easily from shales (aquitards) in the Dakota on most logs. This information can be used to give both depth and footage of sandstones at the well location, as well as for tracing them between wells.
(2) sandstone storage capacity determination
Some logging tools (the density, neutron, and acoustic velocity or sonic) make measurements that can be used to calculate the volumetric proportion of the sandstone that is water.
(3) water quality estimation
The electrical logging measurements of spontaneous potential (SP) and resistivity can be used to estimate the salinity of water within Dakota sandstones.

These three applications are explored in more detail in the following pages, where the properties and uses of the common logs are described. All the logs illustrated have been taken from a single well, so that the properties and interrelationships can be understood more easily.

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Kansas Geological Survey, Dakota Aquifer Program
Updated July 5, 1996
Scientific comments to P. Allen Macfarlane
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