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

Tulsa Geological Society, Tulsa, Oklahoma, February 24, 2004

Microfractures, Macrofractures, and Fracture-Cement Relations in Sandstones: Examples from Oklahoma and Texas

Robert M. Reed

Abstract:

Microfractures in sandstone samples can be used as surrogates to infer characteristics of macrofractures, which can be difficult to sample in the subsurface. Naturally occurring macrofractures and microfractures are common in many sandstone units. Quartz-lined microfractures in the sandstones are visible through the use of SEM-based cathodoluminescence (CL) systems. Cement-fracture relations visible in CL images allow fracture timing to be better constrained. All samples examined contain postdepositional microfractures interpreted to be compactional or compressional in origin on the basis of their distinctive morphologies. Sandstones also contain one or more microfracture sets interpreted to have formed in response to regional extension. Where comparison is possible, orientation maxima of these microfracture sets typically correlate with macrofracture orientations. Transgranular microfractures provide the best correlation with regional macrofractures.

Quartz-lined microfractures are ubiquitous, but they are found in different degrees in different siliciclastic samples. Postdepositional microfracture densities measured thus far range from 50 to 200 microfractures per square millimeter. Microfracture walls provide fresh quartz surfaces that make excellent nucleation sites for quartz cement. In many sandstones with early nonquartz cement or grain coatings, microfracture fill makes up a significant percentage of the quartz cement present.

Macrofractures are typically wholly or partially filled by one or more of the following: quartz, calcite, ferroan calcite, ankerite, barite, or rarely clay. Microfractures are dominantly filled by quartz and only very rarely contain other minerals. Quartz cement formed synkinematic to the macrofractures. Carbonate fill in fractures is generally postkinematic. Ankerite is the most common postkinematic cement and is the primary cause of loss of fracture porosity. Open fractures are propped open by quartz bridges. These quartz bridges typically show crack-seal texture indicating that the quartz cement was synkinematic and also that the macrofractures opened in small increments.