Closely spaced fractures in layered rocks: initiation mechanism and propagation kinematics

Spacings of opening-mode fractures (joints and veins) in layered sedimentary rocks often scale with the layer thickness. Field observations reveal that the ratio of fracture spacing to the thickness of the fractured layer, S/T-f, ranges from less than 0.1 to greater than 10. There is a critical spacing to layer thickness ratio that defines the condition of fracture saturation, and explains the observed spacing ratios between 0.8 and 1.2. Values of S/T-f > 1.2 are explained as the results of the fracturing process having not reached the saturation level. To explain ratios of S/T-f < 0.8, we study the possibility for further fracture infilling, by considering flaw distributions. between adjacent fractures loaded by extension of the layer. Results show that infilling fractures grow more easily from flaws located near the interface than from those ill the middle of the fractured layer. The propagation of a flaw located in the middle of the fractured layer is unstable, but for the flaw to propagate toward the interfaces. its height has to be greater than a critical size. This critical size decreases with increasing S/Tf. The propagation behavior of a flaw with one of its tips at the interface depends on S/T-f. The propagation is unstable when S/T-f is greater than a critical value. When S/T-f is less than this critical value, the propagation is first unstable. then stable, and then unstable again. An infilling fracture can cut through the fractured layer only if S/T-f is greater than another critical value, otherwise the infilling fracture can only partially cut the fractured layer. For models with the same elastic constants for the fractured layer and the neighboring layers, this critical value is 0.546, and the minimum spacing to layer thickness ratio of fractures formed by the in filling process under extension is 0.273. (C) 2000 Elsevier Science Ltd. All rights reserved.