Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Geological Society, Tulsa, Oklahoma, February 24, 2004
Macrofractures, and Fracture-Cement Relations in Sandstones: Examples
from Oklahoma and Texas
Robert M. Reed
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
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.
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.