Fracture spacing in layered rocks: a new explanation based on the stress transition

Opening-mode fractures (joints and veins) in layered sedimentary rocks ofte n are periodically distributed with spacings linearly related to the thickn ess of the fractured layer. To better understand this linear relation, we h ave investigated the stress distribution between two adjacent opening-mode fractures as a function of the fracture spacing to layer thickness ratio us ing a three-layer elastic model with a fractured central layer. The results show that when the fracture spacing to layer thickness ratio changes from greater than to less than a critical value (approximately 1.0) the normal s tress acting perpendicular to the fractures changes from tensile to compres sive. This stress state transition precludes further infilling of fractures unless there are existing flaws and/or the fractures are driven by an inte rnal fluid pressure or other mechanisms. Hence, for fractures driven by tec tonic extension, the critical fracture spacing to layer thickness ratio def ines a lower limit, which also defines the condition of fracture saturation . The critical value of the fracture spacing to layer thickness ratio is in dependent of the average strain of the fractured layer, and it increases wi th increasing ratio of Young's modulus of the fractured layer to that of th e neighboring layers. The critical value increases with increasing Poisson' s ratio of the fractured layer, and with increasing overburden stress (dept h), but it decreases with increasing Poisson's ratio of the neighboring lay ers. For representative variation of the elastic constants of the fractured layer and the neighboring layers, and overburden stress, the critical frac ture spacing to layer thickness ratio varies between 0.8 and 1.2. This rang e encompasses the often cited spacing to layer thickness ratios in the lite rature for well-developed fractures sets. (C) 1999 Elsevier Science Ltd. Al l rights reserved.