TRANSMISSION ELECTRON MICROSCOPY AND POWDER X-RAY DIFFRACTION INVESTIGATIONS OF THE ORDOVICIAN KOPE FORMATION: IMPLICATIONS FOR DEVELOPING A GEOTHERMOMETER AND ENVIRONMENTAL STUDIES FOR UNCONVENTIONAL GAS RESERVOIRS

Detailed mineralogical studies of the Ordovician Kope Formation are comparatively sparse and whether or not shales of the unit are suitable for studies of thermal maturation, diagenesis and environmental pollution are not known. Several samples were acquired from the Brent Submember of the lower Kope Formation on Rt. 445 in northern Kentucky, a key stratigraphic zone for comparative purposes with unconventional hydrocarbon reservoirs in eastern Ohio and Pennsylvania. Feldspar minerals are dominated by ordered albite and K-feldspar is not detected. Trace amounts of cassiterite, and minor amounts of apatite, rare earth phosphate and pyrite are observed. Illite and ripidolite are the only clay sized phyllosilicates observed in numerous samples and replicate analyses. Two textures of illite occur in the Kope formation, one being a more crystalline platy coherent texture and another being a less crystalline mossy aggregate texture. K distribution in mossy illite particles is uneven and adsorbed phosphate is common on illite.

Initial investigations of thermal response at 135°C under atmospheric conditions indicate that FWHM values for the 001 illite reflection do decrease but vary significantly (0.034 to 0.127° 2θ) and require firing times of 10+ days. Experiments conducted at 100°C show responses having no change to 0.099° 2θ decreases. Variation in thermal response is attributed to textural variation of illite. These simple experiments under atmospheric conditions indicate that thermal response occurs, however the variation currently observed prevents utility for a geothermometer. Results indicate that experiments conducted over longer times that control for appropriate pressure and oxygen fugacity are warranted and development of an experimental geothermometer for Appalachian basin rocks may be possible.

Results provide constraints for understanding evolution of chemistry of shales in a geologic context and interactions with hydraulic-fracturing fluids and produced waters. Phosphate minerals strongly control distribution and mobility of U-series radionuclides and adsorbed phosphate on phyllosilicates significantly impacts water-mineral interface behavior. The Kope Formation can provide useful samples for a range of experiments and serves and an important environmental reference material.