Thermal Conductivity of Organic Shales and Coals – How Their Presence and Persistence Effect Thermal Maturity

Abstract

Mature source-rock intervals commonly act as thermal insulators to heat flow and can be identified by the first derivative of a wireline temperature log. When displayed in a cross section, the first derivative curve readily identifies intervals where the temperature curve flattens (insulators) and steepens (conductors). The lithologic control of thermal conductivity is so strong that stratigraphy can be easily correlated using only the first derivative curve. The first derivate correlates strongly to the sonic curve in mature source rock intervals; the sonic curve shows slow velocities and the first derivative indicates flattening of the temperature curve. Because thermal gradient increases at capillary seals, they can be readily identified on first derivative curves. While it is important to identify the insulating lithologies, it is equally important to identify their persistence through time as this can profoundly affect the thermal maturity of underlying source rocks. The burial and exhumation history of the Front Range from southeastern Wyoming to the Monument Hill area of Colorado provides an example of how the pre-Oligocene erosion of nearly 2500 to 3000 feet of Paleocene section including the Arapahoe, Denver, and Dawson formations can affect the thermal maturity of source rocks such as the Niobrara or other lower Cretaceous organic shales. While it appears that the Wattenberg geothermal anomaly controls where the Niobrara is highly thermally mature, the areas where the Niobrara is anomalously immature may be a function of the erosion of the Paleocene thermal insulator prior to the Oligocene.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015