structural geology, propagation mechanics and - Stanford School of ...

Commonly from ~1 mm to ~1.5 cm in thickness and ~1 to ~100+ m in extent, DBs
generally exhibit displacements, both closing-mode (compaction) and shear, on the order
of millimeters (Antonellini et al., 1994; Aydin and Johnson, 1978; Mollema and
Antonellini, 1996). All types of DBs are characterized by the absence of any distinct
plane of displacement discontinuity. The petrophysical changes accommodated within
DBs—compaction, grain size reduction, chemical diagenesis and clay infiltration—
typically produce internal porosity reductions of about an order of magnitude—often
down to a residual of only 1 or 2% (Ahlgren, 2001; Antonellini and Aydin, 1994). The
porosity loss, in combination with related reductions in mean pore throat diameter and
connectivity, can produce drastic drops in permeability within DBs, and significant
efforts have been made to quantify this phenomenon.
Freeman (1990) used gas permeameter readings on plugs of Aztec sandstone to
determine that the presence of DBs can cause 2-order-of-magnitude reductions in
effective permeability relative to DB-free rock. Antonellini and Aydin (1994) used a gas
injection mini-permeameter to measure individual DB permeability in a variety of
sandstones, finding DBs to be 2 to 4 orders of magnitude less permeable than the host
rock, with an average permeability drop of 3 orders of magnitude. They also found that
the relative permeability drop is greater for DBs in higher porosity sandstones (Entrada
and Navajo at >20%) than in lower porosity sandstones (Morrison and Chinle at