Assessing the effect of thermal maturation on the mechanical properties of organic matter in Bakken Shale using atomic force microscopy

Organic-rich shale contains varying quantities of organic matter. Additionally, the presence of clay and quartz minerals increases its degree of heterogeneity and anisotropy. Organic matter degrades over time and its properties can become altered due to heat, burial, and chemical processes. The amount of accumulated organic matter and in-situ transformations cited earlier will determine the organic richness, quality, and microstructure. However, the heterogeneous nature of organic-rich shale in addition to its maturity can significantly affect the overall mechanical properties. Furthermore, the contribution of organic matter to the mechanical properties of organic-rich shales is still not yet understood. Indeed, due to low density and soft nature of such rocks may be significantly affected. Thus, a better understanding of the controls of the mechanical properties of shale rocks will improve rock assessment techniques and applications. Here, we investigated the geochemical and geomechanical properties of artificially-matured Bakken Shale samples. The experimental analyses carried out consisted of the: (1.) X-Ray Diffraction analysis, from which we obtained the dominant minerals, (2.) scanning electron microscopy (SEM) imaging, where we analyzed the microstructure and distribution, and (3.) we used a scanning probe atomic force microscopy (AFM) technique to better understand the mechanical properties changes at immature, early mature, and late mature stages. Finally, we combined the three analyses, and applied a micro-mechanical model, and calculated the elastic properties. The results were interpreted using previous studies conducted on Bakken Shale as a baseline. The Quartz and clay minerals were found to be the dominant minerals followed by pyrite. Overall, the results indicated that thermal maturation has a non-negligible influence on Youngs Modulus. From immature to mature stages, we observe a significant change in Youngs Modulus due to changes in volume and stiffening of organic matter, less presence of clay, higher quartz that becomes more concentrated, and most certainly due to chemo-mechanical transformations. Also, the values of Youngs Modulus for organic matter are variable and were found in agreement with measurements from previous work ranging between 5-25 GPa.