Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240, part 1
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Methodology of Heat-flow Determination (continued)

Thermal conductivity measurements were made in the laboratory of the Bundesanstalt für Geowissenschaften und Rohstoffe in Hannover (Germany) on cores from two boreholes, the Guy F. Atkinson No. 1 Beaumeister well in Cheyenne County and the Stanton County well (table 2). The measurements were carried out on both dry and saturated samples using a divided bar apparatus. The thermal-conductivity values determined have an accuracy of 3%.

Table 2. Thermal-conductivity data from two Dakota cores measured at different temperatures. Borehole 1 = Guy F. Atkinson No. 1 Beaumeister well, SE SE NE sec. 31, T. 2 S., R. 39 W. in Cheyenne County; borehole 2 = Stanton Co. well, SE SW SE sec. 21, T. 29 S., R. 43 W.

In the remaining boreholes with thermal logging data (table 1), however, thermal-conductivity data were lacking. Therefore, the interpretation of interval thermal gradients from the sequence overlying the Dakota Formation was limited to such lithology for which thermal conductivity measured elsewhere could be extrapolated into the study area. We assumed that, because of the relatively small variability in the thermal conductivity of shale in eastern Kansas and in the northern Great Plains (Gosnold et al., 1981; Sass and Galanis, 1983; Blackwell and Steele, 1989; Gosnold, 1990), shale (preferably dark, noncalcareous shale) could be used to evaluate thermal gradients and estimate heat-flow density. As those studies showed, although the Paleozoic shales are more indurated than the younger and softer Mesozoic shales, the thermal conductivity does not differ to a great extent.

For example, for Paleozoic shales showing porosities between 6% and 12% from four boreholes in central and eastern Kansas, the in situ thermal conductivity inferred from the heat-flow density of the underlying carbonates was in the range of 1.2 ± 0.1 W/mK (Blackwell et al., 1981; Blackwell and Steele, 1989). This value is consistent with data from seven wells penetrating the Paleozoic shales in Nebraska (Gosnold and Eversoll, 1981). For the Upper Cretaceous Pierre Shale, a thermal conductivity between 1.19 ± 0.05 W/mK (vertical component) and 1.38 ± 0.04 W/mK (horizontal component) was measured by Sass and Galanis (1983) on a preserved core sample from a well near Hayes, South Dakota, using the needle-probe technique. These results suggest the shales have only a modest compaction effect on thermal conductivity because dense lower Paleozoic shale have a thermal conductivity similar to Upper Cretaceous shale (see Blackwell and Steele, 1988). Gosnold's later study (1990) showed shale conductivity in the same range as obtained previously. He obtained facies-specific conductivity values for the Pierre Shale in Nebraska from differences in temperature gradients according to lithology. For the eastern facies (marine shales) of the Pierre Shale (Tourtelot, 1962), which extends from Nebraska into northwestern Kansas, a general increase in thermal conductivity from about 0.9-1.0 W/mK in the lower members (i.e., the Sharon Spring Member) to the higher members of the section was reported. Gosnold also observed that those low conductivity values in the lower members of the Pierre Shale are about equal to the apparently equivalent thermal conductivity of the Carlile and Graneros Shales. The effective conductivity of the Pierre Shale can be accounted for by about 1.2 W/mK (eastern facies), whereas values of 1.1 W/mK for the Pierre Shale were reported to be typical in the Williston Basin and in southern South Dakota (Gosnold, 1990). Because the Pierre Shale is homogeneous and the shales in our area generally have a lithology similar to those with thermal-conductivity estimates outside the area, it is assumed that a thermal-conductivity value of 1.1-1.2 W/mK is reliable for the shales investigated here.

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