Timing, Diagenesis and the Spatial Arrangement of Fractures: An Example from the Cupido Formation, Sierra Madre Oriental, Mexico

Leonel A. Gomez and Randall Marrett
The University of Texas at Austin, Austin, TX

We measured aperture and spacing of opening-mode fractures along 1D scanlines in selected dolostone layers of Cretaceous Cupido Fm. In those fracture data sets we used Normalized Correlation Count (NCC), a recently developed analytical technique, to distinguish between five possible types of spatial arrangements: random, harmonically arranged clusters, fractal, more abundant than random (plateau) or a combination of the previous four arrangements.

Recent studies have generated a detailed paragenetic sequence of the Cupido Fm. Petrographic study of the characteristics of fractures and their relationships to other diagenetic episodes allowed the differentiation of two fracturing events, one before and one after dolomite cement (D2). D2 is the predominant diagenetic event affecting these rocks that replaces rock matrix. Although both pre- and post-D2 fracturing events occurred before exhumation and folding, post-D2 fractures are only present in layers where D2 replacement is equal or close to 100% of the rock matrix.

NCC analyses indicate that pre-D2 fractures display a weak signal of harmonically arranged clusters at rock sample scale while showing random arrangement at outcrop scale, even when fracture strain reaches unusually high values of as much as 10%. In contrast, post-D2 fracture arrays have harmonically arranged clusters at both outcrop and rock sample scales with fracture strain that in some layers is bellow 1%. This likely indicates that the mechanical conditions under which these two distinct fracturing episodes took place were different and that those conditions were controlled mainly by the diagenetic history of individual layers.