Thermal Evolutions and Resulting Microstructural Changes in Kerogen-Rich Marcellus Shale

Shale rock is a complex geochemical system, which contains inorganic minerals and organic matter (e.g., kerogen), of which the latter possesses porous, high-molecular-weight carbon structures. The pores within organic matter hold the majority of recoverable unconventional oil and natural gas. The organic matter also provides a possible source of hydrocarbon fuel upon pyrolysis. To promote engineering developments in hydrocarbon recovery using heating methods, it is essential to have a fundamental understanding of the nature of the thermal behavior of shale. Consequently, we have investigated the thermal evolution of a shale sample from the Marcellus Formation, Pennsylvania, using a multi-faceted materials science approach, including in situ X-ray diffraction, in situ diffuse reflectance spectroscopy, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry, and transmission electron microscopy. Our aim was to link the naturally heterogeneous and complex chemistry of the shale with its mineralogy and thermal stability up to 900 °C. The thermally induced decomposition of organic and inorganic phases resulted in systematic changes in the shale characteristics. More specifically, kerogen underwent complex decomposition reactions between 200 and 600 °C, depending upon the heating rate and atmosphere (oxidative or inert); pyrite decomposed from 300 to 400 °C; and above 600 °C, inorganic minerals, such as carbonate and clay, broke down. These decompositions created microscopic cracks and left empty pores within the rock. Our results provide insight into the pyrolysis process of shale for hydrocarbon recovery.