Thermal Conductivity of Sedimentary Rocks: Measurement and Significance

Abstract

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.