Fracture sets and sequencing

Fractures in a network are commonly divided into “sets” to facilitate their description and analysis. Sets can be based on many different criteria that include the type, geometry, size, spatial distribution, relative age and the kinematics of the fractures. Orientation is the most widely used criterion, but alone may be inadequate to define a fracture set, since fractures of different type, origin and age may share similar orientations. The criteria used should be clearly stated, with quantitative measures where possible, to allow unambiguous and reproducible allocation of fractures to sets, with assessment of the variability or uncertainty involved. Identifying a consistent sequence of development is a key aspect of fracture set determination. This can be quantified by considering the abutting and cross-cutting relationships between different fracture sets using a sequence matrix.

Examples of networks of joints and faults are presented to illustrate different aspects of set definition and network characterization, emphasising the need for criteria that are appropriate for the type of fracture network, available data and the hypotheses to be tested. We discuss how the “deconstruction” of a fracture network into sets is important for fracture network characterization, and how these sets may then be used to “reconstruct” a fracture network to produce models suitable for studies of tectonics, mechanics and fluid flow.