John L. Steele

Thermal Conductivity of Sedimentary Rocks: Measurement and Significance

The thermal histories of sedimentary basins and their effect on organic maturation are topics of active study. The focus of these studies is on large-scale thermal events, such as an initial rifting event, that affect temperatures in a basin. Events of less global significance, however, are more important to the internal temperatures of a sedimentary basin. Such effects as internal thermal events (magma intrusion, diaparism), contrasts in heat production of U, Th, and K in the sediments and underlying basement, large- and small-scale flow of fluid, and thermal conductivity variations, both vertical and horizontal, can raise or lower temperatures much more than lithospheric-scale events. The nature and effect of such thermal effects are briefly discussed in this chapter. The most basic effect, but one of the least well known, is the thermal conductivity of the rocks in the basin. If the mean thermal conductivity cannot be accurately predicted, even the most sophisticated and appropriate modeling techniques for analyzing thermal histories and organic maturation levels may fail when applied to real basins. Temperature variations related to thermal conductivity variations are illustrated using precision temperature-gradient logs from various sedimentary basin settings. Different ways of determining the thermal conductivity of sedimentary rocks are discussed, including laboratory measurements on cuttings and core samples, in situ direct measurements, inference from well log measurements of travel time, gamma-ray activity and so forth, conversion of seismic reflection travel time to thermal resistance, and inversion of detailed temperature logs. Laboratory measurements are in some cases unreliable, especially for shales, one of the most abundant sedimentary lithologies. Actual shale thermal conductivities appear to be 25 to 50% lower than the literature values and do not appear to vary as a function of compaction in the expected way. Thus, some sort of in situ technique of thermal conductivity determination is needed. The use of precision temperature logs with spot sampling for laboratory comparison is favored and several examples of this technique from the Midcontinent, Gulf Coast, and Rocky Mountains are illustrated. The detailed temperature log from the Gulf Coast demonstrates high gradients in shale sections at 2 km depth because of the low thermal conductivity. The thermal properties of shale have implications for interpretation of the thermal effects of geopressuring.

Heat flow patterns of the North American continent: A discussion of the DNAG Geothermal Map of North America

The large and small-scale geothermal features of the North American continent and surrounding ocean areas illustrated on the new 1:5,000,000 DNAG Geothermal Map of North America are summarized. Sources for the data included on the map are given. The types of data included are heat flow sites coded by value, contours of heat flow with a color fill, areas of major groundwater effects on regional heat flow, the top-of-geopressure in the Gulf Coast region, temperature on the Dakota aquifer in the midcontinent, location of major hot springs and geothermal systems, and major center of Quaternary and Holocene volcanism. The large scale heat flow pattern that is well known for the conterminous United States and Canada of normal heat flow east of the Cordillera and generally high heat flow west of the front of the Cordillera dominates the continental portion of the map. However, details of the heat flow variations are also seen and are discussed briefly in this and the accompanying papers.

Use of Precise Temperature Logs in Determination of Thermal Properties of Sedimentary Rocks and Investigation of Thermal Evolution of Sedimentary Basins: ABSTRACT

The temperatures within a sedimentary basin during its evolution are controlled more by the variation in thermal properties of the contained rocks with space and time and with hydrodynamic effects such as dewatering and regional ground-water flow than with transient heat flow variations at the base of a thick sedimentary pile. Although the effect of basal heat flow has been extensively discussed, the effects of thermal property variations and the evolution of fluid flow systems in basins have rarely been addressed. The thermal properties of many sedimentary rocks are not well known and the depth and time variable changes associated with compaction, diagenesis, etc are difficult to evaluate. In one of the dominant rock components, clays, anisotropy, and sampling difficulti s make laboratory measurements difficult if not impossible. Hydrodynamic systems associated with basins are just beginning to be understood and much remains to be learned. Heat flow and temperature studies are techniques for investigating these effects. Accurate temperature logs associated with measurements, where possible and suitable, of the thermal conductivities of the rock result in two quantities which can be used to unravel some of the unknown temperature controls in a sedimentary basin. These accurate data can be sued for correlation of geothermal gradient and thermal conductivity with well log properties such as seismic velocity (travel time), density, and gamma ray activity. These resulting correlations can then be used to infer the spacial variations in heat flow within the sedimentary basin and to accurately evaluate the effects of present fluid motions in the basin. The data can also be used to develop a catalog of thermal property variations as a function of the many variable parameters. Examples are presented from the Mid-Continent showing the correlation between geothermal gradient, natural gamma ray activity, and seismic travel time for the suite of rocks occurring there. A major result of this study is that the thermal properties of shale have been misestimated in the literature and that the thermal properties of Paleozoic shales appear to be 50 to 100% lower than those assumed in most thermal modeling, leading to a consequent error of 50 to 100% in temperature calculations of basin thermal history. Temperature and heat flow data are used to evaluate regional fluid circulation, with possible associated petroleum migration, in units such as the Madison Limestone and the Dakota Group. In addition, heat flow studies may outline areas where conditions are locally favorable for maturation. An example of large-scale basement heat flow variation in Nebraska is used to illustrate an area of unusually radioactive basement rocks producing local areas of higher temperature. End_of_Article - Last_Page 424------------