Changes in microstructure and mineralogy of organic-rich shales caused by heating

Abstract

Understanding of microstructural changes in gas shales caused by their thermal maturation is of practical importance for evaluation of extractability of hydrocarbons from these low permeability reservoirs through methods such as sweet spot mapping from surface seismic. Two organic-rich shales (ORS), one with extremely high total organic carbon (TOC) and the other extremely low TOC are chosen for this study. The Upper Jurassic Kimmeridge Shale from and the Upper Cretaceous Mancos Shale contain around 23% and 1% TOC, respectively. Samples are subjected to temperatures in the range of 300 to 510°C. Changes in their mineralogical composition, TOC, weight and microstructure with temperature increase are monitored. The Kimmeridge Shale shows rapid decomposition of the organic matter at the temperatures of 370-390°C. This process is accompanied by fracture development and propagation. The Mancos Shale exhibits shrinkage of the solid organic matter with mobile bitumen expulsion and relocation. No fracture development is directly observed in microtomograms. Further work has to be done to understand whether the ability of shale to develop a fracture network depends on its TOC content, the mineralogical composition of its inorganic matrix or on other parameters.