Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Microfractures in Sandstone
A systematic inventory of nearly 27,000 microfractures in 64 samples of 21 sandstone units shows that fossilized (mineral-filled) microfractures are ubiquitous in rocks that have experienced moderate to deep burial (1 to 6 km) even in tectonically quiescent and otherwise structureless areas. Microfractures were detected using SEM-CL, which detects fractures despite optical continuity between host grain and fracture-filling quartz that renders fractures invisible to transmitted-light observation. Microfracture abundance is highly variable, ranging from less than 35 to more than 250 microfractures per square millimeter. Sandstones contain populations of inherited fractures, which are potentially useful provenance indicators, as well as microfractures that developed in situ. Such microfracture populations provide a record of sandstone consolidation, diagenetic history, and deformation that is not accessible using geochemical or mechanical approaches alone. Crosscutting relations and other kinematic indicators show that fracture patterns evolve with increasing consolidation, from fractures associated with grain contact in porous material to sets of subparallel fractures in solid rock. This latter fracture category commonly features isolated to interconnected pores. Some microfractures have orientation, timing, and size distribution patterns that show that they are merely smaller examples of macroscopic fractures that are present in the same rock. For these fracture types apertures range over more than three orders of magnitude (0.0001 to >0.1 mm) and commonly have size populations that can be described with power laws.