CLAY MINERALOGY OF THE KOPE AND MARTINSBURG FORMATIONS: EVALUATING THE POTENTIAL USE OF ORDOVICIAN SHALES AS GEOTHERMOMETERS AND ENVIRONMENTAL REFERENCE MATERIALS IN THE APPALACHIAN BASIN

Ordovician shales located throughout the eastern and central regions of the North America are currently being evaluated and developed as unconventional oil and natural gas reservoirs. An issue with the evaluation of these rocks lies in developing reliable tools to quickly and efficiently evaluate thermal maturation, a key indicator of oil and or gas potential. While methods already exist to evaluate thermal maturation for rocks of Devonian age and younger such as the vitrinite reflectance, rocks of Ordovician age and older lack vitrinite - an organic plant fossil material. A possible tool to evaluate Ordovician and older rocks in an efficient manner involves observing the alteration of common clay minerals such as illite. Previous studies have shown that the crystallinity of illite increases with increasing burial temperatures in numerous settings. However, studies of the crystallinity of illite in the Kope formation and in many Ordovician rocks to the east have not been investigated in detail.

A preliminary comparison of powder X-ray diffraction (XRD) patterns of stratigraphically equivalent samples from the Upper Ordovician Kope (northern Kentucky) and Martinsburg Formation (eastern PA) indicates a difference in crystallinity in illite measured at FWHM values consistent with existing thermal history data. For the < 2 µm size fraction, the Kope Formation has FWHM values of 0.347 and 0.445. FWHM values increase with decreasing particle size above Scherrer equation boundary conditions (300 nm). FWHM values for Martinsburg samples vary from approximately 0.170 to 0.210. XRD analyses indicate both Kope and Martinsburg shales have ripidolite of similar chemical compositions. The similarity of the mineralogy suggests experiments and analytical work comparing the two formations in the context of thermal maturation and environmental evaluation is appropriate. Scanning electron microscoscopy and transmission electron microscopy indicate that minor mineralogical diversity differences between the two units that likely reflect sediment source variations.