Crack-Seal Deformation and the Development of Power-Law Size Distributions of Fractures in Sedimentary Rocks

Hooker, John N.¹; Laubach, Stephen E.¹; Gale, Julia ¹; Gomez, Leonel ²
(1)Bureau of Economic Geology, The University of Texas at Austin, Austin, TX. (2) ExxonMobil Upstream Research Company, Houston, TX.

Patterns of fracture-filling cement, revealed using light and scanning-electron microscopy, indicate that geologic fractures sometimes grow incrementally, by repeated cracking and sealing, during diagenesis. The degree of cement filling of fractures by these crack-seal deposits varies with fracture aperture size, opening rate, thermal history, and host-rock mineralogy. The process can leave some fractures open even where local cement deposits are present that record opening history. Fracture aperture size and opening rate effects lead to fracture arrays that include microfractures that are mostly sealed and large fractures that are mostly open. Larger fractures may contain isolated cement deposits that span between fracture walls, separated by fracture pore space.

Cement patterns vary systematically with overall fracture size distribution patterns. Microscopic imaging allows the number and sizes of crack-seal increments to be identified. Fracture sets that grow by repeated cracking and sealing of individual fractures typically show power law scaling. Such is the case for fractures in sandstone from the Piceance basin, from the Sierra Madre Oriental of northeastern Mexico, and from Cambrian quartzarenites in Argentina and Scotland; and for fractures in carbonate rocks from northeastern Mexico and from central Texas. For sets of fractures that follow power-law size distributions, the variation in aperture size generally depends on the number of crack-seal increments, which show relatively little variation in width, present within each fracture. In contrast, fracture sets that do not follow power-law size distributions usually do not show crack-seal cement deposit patterns. We find examples of such fractures in Argentinian limestones and quartzites.

Fractures that “scale” (follow power-law size distributions) likely have different fracture permeability characteristics from fractures that feature characteristic size distributions. Fracture permeability in power-law size distributions is dominated by the largest fractures whereas in non power-law distributions the permeability is likely shared more equally among the fractures. When low numbers of fractures are directly sampled, inspection of fracture-filling cement patterns provides evidence for the overall type of fracture size distribution present, and can aid in selecting an appropriate scaling equation to extrapolate abundance of larger fractures.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.