From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, April 21, 2006

SEM-CL and Fracture Scaling, or, Too Many Microfractures!

John Hooker

Abstract:

Fracture characterization is one of the greatest challenges facing exploration and development geologists because it is nearly impossible to collect meaningful samples. We study natural microfracture populations in order to predict attributes of associated macrofracture populations. Microfractures are fractures so small they require a microscope to be seen. Microfractures in sandstone are usually filled with quartz cement that is in optical continuity with the grains and cements the fractures cut, making them difficult or impossible to detect with conventional microscopy. SEM-CL microscopy distinguishes diagenetic quartz from igneous and metamorphic quartz, so microfractures that are virtually invisible under transmitted light are delineated in sharp contrast.

Fracture intensity is an important fracture attribute, potentially affecting, among other things, fluid flow and seismic response. UT research has pioneered a technique called fracture scaling, which is done by measuring the relative abundances of microfractures of a range of sizes in order to predict the intensity of macrofractures in the same rock. A key step in this type of analysis is correctly identifying scalable microfractures from the large number of inherited and compaction-related fractures that have no relation to large fractures. This is illustrated by a study of horizontal core from the Piceance basin, Colorado, where we sampled macrofractures to test a microfracture-based scaling prediction. Omitting microfractures at high angle (>25°) to the dominant macrofracture strike and those that do not cross grain boundaries decreased the error in predicted abundance of 1mm-wide macrofractures from a factor of over 100 to a factor of less than two. This is probably because opening-mode fractures of differing strike may have developed at different times and not in response to the same loading conditions, and therefore scale independently. Moreover, many intragranular fractures are likely inherited (introduced with the grain at deposition) and do not reflect fracture strain in the sampled sandstone. Observations of abundant microfractures in natural sand support this hypothesis. These microfractures do reflect the structural history inherited in the grain, and are potentially useful provenance indicators.