An investigation on the effect of thermal maturity and rock composition on the mechanical behavior of carbonaceous shale formations

An accurate assessment of the geomechanical behavior of shale reservoirs is extremely significant in screening the potential intervals for hydraulic stimulation and sweet spot mapping. While most of the studies were carried out on siliciclastic shales, we here examine the mechanical behavior of a carbonaceous organic-rich shale in the Lurestan Basin, Iran, and the factors controlling them. Direct laboratory measurements at reservoir stress conditions (static method) and well-logging interpretations (dynamic approach) were employed to assess the main geomechanical properties. Besides, various experimental techniques have been employed to evaluate the dominant shale mineralogy, geochemical properties and rock fabric. It is observed that Young's modulus and Poisson's ratio, obtained from either static or dynamic approaches, are capable of reflecting rock brittleness. Conducted sensitivity analyses also reveal that abundances of organic matter, clay and carbonate minerals are the most significant factors controlling rock brittleness. An increase in the brittleness is envisioned by increasing the carbonate content and inversely by decreasing the concentrations of clay minerals and organic matter. Moreover, week or no clear relationship regarding the possible effects of thermal maturity and quartz content could be distinguished. Incorporating the distinguished controlling factors into a compositional index, a similar relative brittleness sequence with the elasticity-based approach was detected. This indicates that both independent methods are suitable for the identification of brittle layers. Inspected shale samples illustrate notably higher strengths in comparison with well-characterized organic-rich shales in the world, translated as a direct contribution of firm carbonate textures to the rock framework. Taking these textures together with the main mineral contents into account, the reservoir can be subdivided into three lithofacies including limestone, argillaceous limestone and micritic limestone. Micritic limestones show relatively higher strength and brittleness, whereas limestone and argillaceous limestone facies, in spite of high variability, reflect in average lower strength and brittle characteristics.