Terminology for structural discontinuities

Abstract

Strain localization structures such as fractures, stylolites, and deformation bands have important effects on reservoir performance but lack a consistent terminology. Advances in the recognition and interpretation of such structures now motivate a comprehensive framework that stresses their similarities instead of their differences. We review and assess the classical terms for localized geologic structures, followed by a comprehensive nomenclature that accounts for joints, faults, fractures, anticracks, shear zones, and deformation bands in compact and high-porosity rocks. Geologic structural discontinuities are defined by their lengths and by the sense and rate of displacement change across them. For example, structural discontinuities having negligible thickness, and consequently, a discontinuous displacement across them, are called sharp discontinuities. Depending on the sense of displacement (opening, shearing, or closing), these structures are called cracks, faults, or anticracks. However, structural discontinuities having a measurable thickness in outcrop or hand specimen and a continuous change in displacement across them are called tabular discontinuities. Correspondingly, these types of deformation bands are called dilation bands, shear bands, or compaction bands. The class of structural discontinuity, i.e., sharp or tabular, depends on the properties of the deforming rock. Consistent characteristics and patterns of these structural discontinuities, and their displacement-length scaling relations, demonstrate the rich yet consistent varieties of strain localization that are manifested in crustal rocks in general, and reservoir rocks in particular.