CONDUCTIVE THERMAL TRANSPORT PROPERTIES OF CARBONATE MINERALS AND ROCKS ACROSS A RANGE OF CRUSTAL TEMPERATURES

Carbonate rocks, dominated by the minerals calcite and dolomite, compose a significant proportion of sedimentary basins. These regions feature high potential for energy-related resources, from hydrocarbon reservoirs, to areas of high heat flow with the potential for geothermal energy production. Thus the ability of carbonate minerals and rocks to conduct heat, and the resulting influence on the thermal structure of basins is of strong interest for geologic energy research. To understand these conductive thermal transport properties, this project directly measured thermal diffusivity (D) of a suite of carbonate rocks and minerals with changing temperature.

Laser Flash Analysis (LFA) was used to measure D of the minerals calcite (cal), dolomite (dol), magnesite (mgs), and rhodochrosite (rds) across a range of relevant basin temperatures. At near-surface temperatures, mgs (MgCO₃) had the highest D (4.36 mm²/s @ 296 K). Although dol is composed of alternating layers of CaCO₃ and MgCO₃, and has the same rhombohedral structure, its D (2.71 mm²/s at 300 K) is ~15% lower than the average of cal (1.61 mm²/s at 300 K) and mgs at room-T. This effect remains at elevated temperatures, as dol D continued to be ~15% lower than the average of cal and dol to 600 K (dol – 1.02 mm²/s, cal – 0.659 mm²/s, mgs – 1.68 mm²/s). All minerals measured produced a marked decrease in D as temperature increased, with the strongest drop occurring in the interval ~300-500 K. Crystal orientation had no significant effect on results.

For rocks, mineralogy, temperature and porosity were the strongest controls on rock D. For example, cal limestones showed proportionally lower D than the mineral, scaling roughly with pore fraction. For rock mineralogy, dolostone produced higher D than all cal limestones across the interval 300-600 K. This effect is more pronounced in dolomitic marble, as D was 40-60% higher than dolostone in the same interval, and 30-50% higher than cal-dominated marbles in the interval 300-900 K.

Using literature values for temperature-dependent, isobaric heat capacity, we calculated the thermal conductivity (k) of the materials measured with the LFA method. These results show a stronger T-dependence for k of carbonate minerals and rocks than previous studies, with low-T results higher, and high-T results lower than previously published values.