Quantifying Microstrain of Quartz and Illite During Thermal Maturation of Organic Matter

Strain is an important mechanical parameter and mechanical response to stress perturbation in geomaterials that should be studied in detail since it decides the outcome of hydraulic fracturing in shale oil reservoir, in particular. Despite the importance of the strain phenomena, its detailed analysis, particularly in organic-rich, liquid-rich specimens where thermal maturity would directly control pore structure causing various degrees of strain is still unknown. In this study, we collected an immature sample from the Qingshankou Formation and applied two separate pyrolysis methods (in the presence and absence of water) to artificially alter the sample thermal maturity from immature to over mature. Next, XRD analysis combined with the Williamson–Hall method was employed to study the microstrain of two mechanically distinct minerals (illite and quartz) that were abundant in the aliquots as thermal maturation progressed. The results showed that the microstrain value of both quartz and illite would alter during the maturation. Moreover, under the same pyrolysis temperature, the microstrain of illite was found much larger than that of the quartz which was related to its crystal form. Furthermore, surface area and pore volume decreased as the compression microstrain of illite increased, demonstrating that the microstrain of illite would contribute to the pore structure changes more notably. This study is the first in its nature to quantify the microstrain of quartz and illite during thermal progression, which can be important to explain variation in mechanical properties and microstructures of unconventional shale plays during thermal maturation.