THE EFFECTS OF TEMPERATURE DEPENDENT THERMAL PROPERTIES IN MODELING GEOTHERMAL GRADIENTS OF THE ILLINOIS BASIN

Past and present geothermal gradients of the Illinois Basin have played a fundamental role in many geologic and anthropic processes including the maturation of fossil fuels, deposition of ore deposits, and sequestration of atmospheric carbon dioxide. Modeling past and present geothermal gradients requires experimental measurements of physical and thermal transport properties of sedimentary rocks within the Illinois Basin, such as thermal conductivity (k), which is the product of thermal diffusivity (D), isobaric heat capacity (C_P), and density (ρ). These properties vary as a function of temperature, composition, porosity, and grain size. However, few measurements have been made above room temperature, and most experimental measurements of kunderestimate the true value by ~10-20% due to contact imperfections between the sample and the instrument and/or thermocouple.

We measured D using the contact-free laser flash analysis (LFA) method, and C_P using differential scanning calorimetry (DSC) from 25°C to 300°C, for a series of well-characterized mudstones collected from drill core in the Illinois Basin. We found that: (1) mineralogy affects D more than texture or elemental composition, (2) mudstones have a lower D than sandstones, limestones, or dolostones, (3) the k of mudstones decreases between 4% and 18% from 25 to 300˚C, which is a smaller decrease than for other sedimentary rocks, and (4) our measured k values are not reproduced well by current models that estimate kbased on composition and texture.

The new values of temperature dependent thermal properties were used in seven one-dimensional, conductive heat flow models of past and present geothermal gradients in the Illinois Basin. There is good agreement between the models and present-day borehole temperature (BHT) measurements. The temperature effect on k is greatest at low temperatures, demonstrating the importance of using temperature dependent thermal properties when modeling geothermal gradients of sedimentary basins. The variation between measured and predicted k values suggests more high temperature measurements of thermal transport properties are needed.