Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
New Techniques for Characterizing Fracture Spacing: Examples from Northeastern Mexico and Central Texas
Randall Marrett, Julia Gale, and Leonel Gomez
Previously published analytical techniques (average, coefficient of variation, cumulative distributions) used to characterize the spacing between fractures fail to quantify fracture clustering or to provide a bias-free measurement of distance between clusters. This is because these techniques ignore the sequence of fracture spacings. In addition, their results are heavily dependent on the aperture-size range of measured fractures, which are commonly restricted to only those fractures that span mechanical layers.
Using two recently developed one-dimensional techniques that take into account the sequence of fracture spacings (normalized correlogram and normalized correlation count) we were able to distinguish four types of spatial organization of fractures: 1) harmonically distributed fractures (regularly spaced), 2) fractal spatial distribution, 3) undistinguishable from random and 4) a combination of the previous three with different spatial organizations at different scales. If fracture clusters were regularly spaced, it was possible to quantify cluster spacing. Fractures of all aperture sizes that can be measured systematically at the scale of observation are included in the analysis, but a lower size limit is selected to avoid sampling truncation artifacts.
The examples presented here come from one-dimensional scanline data acquired in carbonate rocks of the lower Cretaceous Cupido Fm. in northeastern Mexico and the Pennsylvanian Marble Falls Fm. in central Texas. Comparison of the spatial distribution of layers with different values of fracture strain also provides insight into changes in the spatial organization of fractures during the evolution of a fracture network.