DIAGENETIC CONTROLS ON FRACTURE SIZE DISTRIBUTIONS

Natural fractures in sedimentary rocks commonly open amid precipitating cements. Crack-seal opening increments within individual fractures have narrow ranges of widths, which are usually micrometer-scale. Fractures in sets have a wide range of cumulative widths and are commonly well-fit by power-law aperture-size equations. The wide range in fracture widths likely arises from grouping of these increments (localization) to form larger fractures (millimeter- to centimeter-scale widths). Such localization can be explained by the strengthening of fractures by cement. Larger, less cemented fractures break preferentially as fracture strain accumulates. Partial filling of fractures by cement thus provides a positive feedback mechanism whereby large fractures grow faster than small fractures. Using equal-sized, micron-scale growth increments, a fracture growth simulation replicates power-law fracture-width distributions. Simulation parameters can be altered to produce characteristic-width fracture size distributions. Results have implications for how fracture porosity and permeability evolve in the subsurface.