MINERALOGICAL VARIATIONS IN SHALES DUE TO VARIABLE THERMAL MATURITY

The sedimentary sequences in the Ouachita Mountains provide a natural laboratory to study diagenesis because of the increase in thermal maturity from Oklahoma through Arkansas. Although trends in clay-mineral diagenesis have been well documented, associated variations in the non-clay fractions are not well known. Sample sites were chosen based upon a previously published map of thermal maturity (Houseknecht and Matthews, 1985). Each sample site was separated into the component mineral fractions and analyzed using X-ray diffraction, Scanning Electron Microscopy, petrographic microscope, and X-ray fluorescence. Preliminary results show that illite crystalinity data from our samples differed from the previously published thermal maturity map. Based upon this new data and previous studies a new thermal maturity map was created. Quartz grains increase in polycrystallinity as maximum temperature increased. Whole rock chemistry however, does not vary systematically with changes in thermal maturity. No correlation was determined to exist between illite crystalinity and total heavy mineral percentages or assemblages. The clay and quartz fraction in shale are dependant on diagenetic processes, while other mineral fractions are not. The low permeability of shale limits the diagenetic reactions.