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Carboniferous shales from the Ouachita Mountains have been studied to determine mineralogy and thermal maturities, the latter ascertained by means of vitrinite reflectance and bitumen/organic carbon ratios.
The less than 2 µm fractions of these shales indicate 2 major clay-mineral components, illite and chlorite, and 2 minor varieties, expandable clays and pyrophyllite. Expandable clays are found at low thermal maturities and pyrophyllite at high maturity. Scanning electron micrographs show differences in clay morphology and texture, which are influenced by the degree of thermal maturity.
Weaver's sharpness ratio for illite and Kubler's crystallinity index are both significantly related to mean vitrinite reflectance. The log of the sharpness ratio increases while the log of the crystallinity index decreases with increasing mean vitrinite reflectance. These relationships suggest that illite crystallinity is controlled by the same geologic agents that control vitrinite reflectance, namely temperature and time.
A plot of vitrinite reflectance and/or crystallinity index versus bitumen/organic carbon ratio shows a maximum analogous to a hydrocarbon window.
These statistically significant correlations provide useful means of estimating the thermal maturity of these strata where they contain insufficient amounts of vitrinite for thermal maturity evaluation.
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