The Ouachita Mountains contain a thick section of Carboniferous strata that display an extreme range in thermal maturity as determined by vitrinite reflectance. Clay mineralogy, illite crystallinity, and organic geochemistry of shales from those strata are systematically related to thermal maturity.

Shales of the Stanley, Jackfork, and Atoka formations are predominantly composed of illite and chlorite with minor occurrences of mixed-layer clays (restricted to samples characterized by mean vitrinite reflectance less than 1.5%) and pyrophyllite (restricted to samples characterized by mean vitrinite reflectance greater than 2.7%). Illite crystallinity is significantly related to vitrinite reflectance (R_o). Weaver's illite sharpness ratio (SR) increases with increasing R_o: log (SR) = 0.28 + 0.08 (R_o); whereas Kubler's illite crystallinity index (CI) decreases with increasing R_o: log (CI) = 1.01 - 0.07 (R_o).

Plots of bitumen ratio (bitumen/total organic carbon) vs. vitrinite reflectance, Weaver's illite sharpness ratio, and Kubler's crystallinity index all reveal hydrocarbon generation-preservation curves that define submature, mature, and supermature zones with regard to a liquid hydrocarbon window.

These results suggest that, in the absence of vitrinite, illite crystallinity can be used quantitatively to estimate levels of thermal maturity and cautiously to approximate hydrocarbon generation-preservation stages of potential source rocks.