Abstract It is generally accepted that the clay mineralogy of the shale formation is a primary causative factor of shale instability. This review considers a scenario of shale instability relating to illitic minerals. From the literature the thickness of the double electric layer (DEL) of the aqueous solutions associated with the charged external surfaces of clay minerals is of the same order or even thicker than the sizes of a significant proportion of the pores found in shales. In these circumstances, overlap of the DELs associated with the exposed outer surfaces of clay minerals on opposing sides of slit-like micropores (<2 nm in diameter) and mesopores (2–50 nm in diameter) in a lithostatically compressed shale would bring about electrostatic repulsion and lead to increased pore/hydration pressure in illitic shales. In shales and sandstones, illitic material is usually described in terms of two different phases, namely illite per se and mixed-layer illite–smectite (I/S). Evidence is presented to show that it is often the case that only one illite phase exists and that in reality the mixed-layer I/S is simply very thin illite (<5 nm in thickness) in the early stages of its growth. Such material is of common occurrence in the unconventional hydrocarbon reservoirs of the USA.

Abstract Soil smectites differ from standard bentonite-type smectites in several respects. First, soil smectites generally have a composition varying between montmorillonite and beidellite. They commonly contain >20% octahedral Fe 3+ (excluding magnesium) and can be described as iron-rich beidellites. Second, interstratified smectites formed in soils differ from interstratified smectites formed during diagenesis. Thus, interstratified soil smectites commonly form by means of transformation reactions involving relatively large crystals of mica or chlorite, whereas, according to a recently developed conceptual model, diagenetic interstratified clays consist of exceedingly fine particles some tens of Ångstrom units in thickness which are characterized by the phenomenon of interparticle diffraction. Third, soil smectites may be interlayered with non-exchangeable aluminous, organic, or other material, a phenomenon that may produce anomalously high basal spacings on X-ray powder diffraction patterns. Many soil smectites are, therefore, significantly different from bentonite-type smectites; such differences must be fully characterized if the properties and behavior of smectitic soils are to be understood.