Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240, part 1
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Introduction

Heat-flow studies in the Great Plains, and Nebraska in particular (Gosnold et al., 1981; Gosnold and Eversoll, 1981, 1982; Gosnold and Fischer, 1986; Gosnold, 1990), have shown that high heat flow and high geothermal gradients occur over extensive areas of ground-water discharge on the eastern margin of the Denver-Julesberg Basin, a major north-south Laramide feature, developed along the eastern front of the Rocky Mountains. The basin lies on the west-central margin of the Great Plains, occupying all of eastern Colorado and extending into Wyoming, Nebraska, and Kansas (Jorgensen et al., 1993). The basal Cretaceous sandstones, which include the Dakota sandstones, make up an important aquifer system within the basin.

In the past, published data on heat-flow density for the Kansas part of the Great Plains was limited: 63 mW/m² for central Kansas (Sass et al., 1971); 59 mW/m² for southwestern Kansas (Birch, 1947); 52-62 mW/m² for eastern Kansas (Blackwell and Steele, 1989). None of these data was from the area of the Dakota aquifer.

Thermal logs, which were measured for a preliminary study of the subsurface temperature conditions in Kansas and used to estimate geothermal resources (Steeples and Stavnes, 1982), show that the mean geothermal gradients range between 25°C/km and 55°C/km. The highest values were observed in the northwestern and north-central portion of the state. Because these mean temperature gradients give only a rough estimate of the thermal conditions of an area, there is room for speculation about the cause of such variations. In sedimentary basins where vertically different lithologic (stratigraphic) units occur, the lithology and petrophysical properties might also vary laterally to a great extent, affecting the subsurface temperature conditions. The separation of the different effects on the geothermal field requires the detailed interpretation of thermal logs. The study of geothermal gradients for lithologically homogenous units (formations) allows, then, in conjunction with the estimation of formation thermal conductivity, an estimate of heat-flow density along the profile, as shown by Blackwell and Steele (1989) for relatively deep boreholes in central and southeastern Kansas.

The temperature profiles in northwestern Kansas are from relatively shallow boreholes that penetrate rocks of Tertiary and Cretaceous age. Because of the structure of the Dakota aquifer, it is likely that the subsurface temperatures and geothermal gradients are affected by heat transfered by fluid flow out of the Denver-Julesberg Basin in the west. Therefore, the focus of this study is to investigate the thermal structure of the units overlying the Kansas portion of the Dakota aquifer to determine if additional heat input by fluids exists and to clarify whether the overall conductive heat flow from the basement through the sequence might be overprinted by heat advection.

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