Insulating effect of coals and black shales in the Appalachian Basin, Western Pennsylvania

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Sedimentary rocks rich in organic matter often have lower thermal conductivities than other rock types. As a result of this natural insulating effect, temperature gradients in carbonaceous strata may become elevated even under conditions of low to average heat flow, causing underlying rocks to reach higher temperatures and higher thermal maturities than would otherwise occur. This hypothesis was tested by determining the weighted-average thermal conductivity of a Carboniferous coal-bearing section from the Conemaugh Group of the Appalachian Basin. Coals from the sampled section yielded low thermal conductivity values of 0.2 ± 0.15 Wm −1 K −1 when measured with a needle-probe apparatus. Measurements of other carbonaceous units also showed low thermal conductivity values: 0.9 ± 0.05 Wm −1 K −1 for black shales and 1.2 ± 0.20 Wm −1 K −1 for dark gray shales. Relative abundances of all measured lithologies were estimated from a 288-well database. These abundances yielded an average conductivity of 1.6 Wm −1 K −1 for the entire Conemaugh Group. Geothermal gradients for the Conemaugh Group estimated from thermal conductivity and heat flow data range from 38 to 47 mK m −1 , in good agreement with gradients estimated from the actual thermal maturity of coals. The additional temperature increment contributed to underlying rocks by this insulating effect may help explain the anomalous thermal maturity of the Appalachian Basin, reducing the need to call upon excessive burial or higher values of heat flow in the past.