Finding generic trends in mechanical and physical rock properties will help to make predictions of the rock-mechanical behaviour of shales. Understanding the rock-mechanical behaviour of shales is important for the successful development of unconventional hydrocarbon reservoirs. This paper presents the effect of heterogeneities in mineralogy and petrophysical properties on the validity of generic trends on multiple scales in rock-mechanical and rock-physical properties of the Whitby Mudstone. Rock-mechanical laboratory experiments have been performed on Whitby Mudstone samples from multiple outcrops within five kilometres laterally in order to investigate the heterogeneity and possible trends on an outcrop scale. Unconfined compression tests and acoustic measurements have been conducted to obtain the rock-mechanical properties, including rock strength, Young's modulus, Poisson's ratio, and velocity anisotropy. The rock-physical properties, including mineralogy, porosity, and matrix density, were measured using X-ray fluorescence and helium pycnometry. Various methodologies have been applied to the resultant data in order to derive different brittleness indices. Significant heterogeneity in rock-mechanical and rock-physical properties is present on an outcrop scale. There is no obvious correlation between mineral content and rock-mechanical properties on an outcrop scale in the Whitby Mudstone. Comparison with shales from different basins show, however, correlations between composition and elastic properties. The presence of significant heterogeneities on an outcrop scale and between shales from different basins make it difficult to find generic trends in rock-physical and rock-mechanical properties.@en

We studied the rock mechanical behaviour of the outcropping Whitby Mudstone Formation shales (Toarcian Age) under varying temperature and confining pressure conditions, focusing on strength and elastic moduli. We compared the rock mechanical properties of the Whitby shale to published data of time and depositional equivalent shales from Northern Europe and producing shales from the US. We performed uniaxial and triaxial tests in order to constrain the effect of temperature and pressure on the mechanical behaviour of the shale. We loaded the samples normal to the bedding plane under room humidity conditions. We performed uniaxial tests at room temperature and zero confining pressure, whereas we applied confining pressure in several steps up to 50 MPa and considered temperature values between 20-150°C during triaxial tests. Ultimate strength and elastic moduli are strongly influenced by mineralogy, temperature and pressure. An increase in temperature enhances the deformability, whereby the Young’s Modulus and ultimate strength decreases. With an increase in confining pressure, Young’s Modulus and ultimate strength increases, whereas the Poisson’s ratio decreases.@en