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Gomez, Leonel1
(1) The University of Texas at Austin, Austin, TX

ABSTRACT: Predicting Macrofracture Spacing from Small Rock Samples: Testing New Analytical Techniques Using Microfracture Spacing

Populations of microfractures (fractures requiring magnification to be measured) ranging several orders of magnitude in aperture have been used to predict the orientation and aperture distribution of genetically related macrofractures (fractures observable with the unaided eye). This research tests the hypothesis that the spatial distribution of fractures at a small scale (thin section) can statistically predict the spatial distribution at a larger scale (outcrop, reservoir).
Macrofracture aperture and spacing data were acquired along one-dimensional scanlines in outcrops of Lower (Cupido Fm.) and Upper Cretaceous (Agua Nueva Fm.) rocks in the mountains of the Sierra Madre Oriental. Those layers have abundant macrofractures ranging in aperture over two orders of magnitude. We collected oriented samples from those layers along or near the location of the scanlines and prepared a continuous series of thin sections from each sample. Digital mosaics of consecutive plane-light images were generated using a petrographic microscope. The same procedure was applied for cathodoluminescence images acquired in petrographic and scanning electron microscopes. Microfracture aperture and spacing data were acquired along one-dimensional scanlines in those mosaics.
Two recently developed analytical techniques (normalized correlogram and normalized correlation count) were used to quantify independently the character of the spatial arrangements of both macrofractures and microfractures from the same rock. The macrofracture arrays measured reflect four types of spatial distributions: harmonic, fractal, indistinguishable from random or a combination of the previous three organizations. Preliminary results indicate that under limited circumstances microfractures might provide some indication about how genetically related macrofractures are distributed in space.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.