Testing the Use of Microfractures to Predict Fracture Orientation and Intensity, La Boca Formation, Northeast Mexico

Meghan Ward, Stephen E. Laubach, and Randall Marrett
The University of Texas at Ausitn, Austin, TX

Triassic-Jurassic La Boca Formation sandstone outcrops in NE Mexico are close matches to some moderately to deeply buried tight gas sandstones in terms of rock type, diagenesis, and fracture morphology. Because characteristics of large fractures are known, we were able to test the reliability of microfractures to predict macrofracture attributes. Using specially designed comparators and scanning electron microscope-based cathodoluminescence, we measured fracture sizes and orientations over a wide range of sizes. The largest microfractures, having lengths of 1 millimeter or more and apertures greater than 0.01 mm, have aperture size scaling that can be described by power laws over approximately three orders of magnitude. The largest microfractures also have strikes that match those of macrofractures. Owing to cement, microfractures are sealed, but their population statistics provide the most reliable guide to intensity of larger fractures, which are in some cases open. Although variable, the abundance of La Boca microfractures that give reliable information on large fractures is such that in 4-inch-diameter core populations are likely to be sufficient to specify average fracture spacing for large fractures. Our study shows how these methods applied to high-resolution core analysis can provide otherwise inaccessible information on fracture orientation and intensity.