Mechanical controls on the spatial density of opening-mode fracture networks

Abstract

Understanding the controls on fracture density is critical to interpreting the geologic evolution, permeability, and mechanical properties of jointed rock. Analyses of fracture trace maps and seismic inversions reveal that near-surface fracture densities are generally less than ∼1–2 over a range in scale of 14 orders of magnitude. The observed densities are approximately an order of magnitude greater than the range typical of deeper (>500 m) formations and may indicate that surficial controls on fracture growth are different than those at depth. The development of greater densities at the surface may be limited by the significant decrease in effective elastic stiffness associated with fracture densities near unity. As fracture densities approach 1.0, additional strain accommodated by the fractures decreases the driving tension, limiting further fracture growth.