Rock compositional control on geomechanical properties of the Montney Formation, Western Canadian Basin

We examine geological and geomechanical properties of the Montney Formation of Western Canada, a reservoir dominated by silt-rich mudstone (siltstone) that hosts one of the largest hydrocarbon resources in Western Canada. Geomechanical properties are critical to assessment and exploitation of this reservoir, affecting both the development of natural fractures and the formation’s response to hydraulic stimulation. We evaluate relationships between rock composition (mineralogy and organic matter), lithology and rock fabric, and dynamic Young’s modulus, dynamic Poisson’s ratio, hardness and brittleness in five wells that represent a range of burial depths. Our study highlights similarities and differences between the silt-rich mudstone of the Montney Formation and more commonly studied clay-rich mudstones (shales). Statistical analysis established that no one of the parameters investigated strongly correlates with their dynamic rock mechanical properties. Nonetheless, clay content is the most significant component affecting the strength and brittleness of the Montney siltstone, an observation consistent with several other fine-grained reservoirs. We also showed that water saturation has a pronounced effect on dynamic elastic moduli and concluded that hardness measurements are a poor proxy for rock brittleness in the Montney siltstone, in contrast to observations of several shale formations. No relationship was found between the geomechanical properties and sedimentary facies in the Montney Formation, in contrast to the behavior of many shale formations, which we attribute to the size and compositional homogeneity of the detrital material comprising the siltstone, and to pervasive cementation at shallow burial depth that further homogenized the rock. Rock fabric, encompassing the orientation, shape, and size of the grains, grain contacts, porosity, organic matter distribution and authigenic cements (mineralogy and distribution) is what distinguishes silt-rich or silt-dominated mudstones from clay-rich or clay-dominated mudstones. A comparison of fabric controls on geomechanical properties of silt-rich mudstones, mud-rich mudstones, and sandstones revealed that: 1) intergranular porosity in a sandstone may be a proper analog for the influence of porosity, organic matter distribution, and grain boundaries on the geomechanical properties of the Montney; and 2) shales may be a proper analog for the influence of clay content and total cement volume on the geomechanical properties of the Montney siltstone.