Fractured Sandstone Outcrops in Northeast
Mexico: Guides to the Attributes of Fractures in Tight Gas
Sandstones
Meghan E. Ward and Stephen E. Laubach. John A. and
Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin,
TX 78731,
phone: 512 471 6303, [email protected]
Opening-mode fractures are potential fluid conduits in
deeply buried sedimentary rocks yet critical attributes of fracture patterns,
such as spacing, size distribution, and porosity are challenging to measure in
the subsurface and are rarely clearly preserved in outcrop. An exception is the
fracture patterns in Triassic La Boca Formation, exposed near Galeana, northeastern Mexico. Diagenetic
features and intensity patterns of these fractures is identical to those found
in many tight gas sandstones in the Rocky
Mountain region. At least
three fracture sets are well exposed in road cuts and canyons. Synkinematic quartz bridges preserved in these fractures
match those found in fractures sampled in deep cores in the Gulf of Mexico Basin,
demonstrating that these fractures are representative of fluid conduits in
deeply buried sedimentary rocks. We measured clustered fracture spacing, size
distributions having power-law size scaling, and highly heterogeneous porosity
preservation. We used SEM-based cathodoluminescence
to analyze fracture opening histories. Quartz cement along fracture walls is
pervasive as both euhedral crystals and bridging
cement. Imaging reveals crack-seal textures in quartz bridges. Such texture
demonstrates incremental fracture opening of fracture sets in the outcrop.
Heterogeneous sealing of some parts of the fracture system occurs by late (postkinematic) carbonate and iron oxide cements. In some
outcrops, postkinematic calcite in the rock matrix
corresponds to calcite-sealed fractures. Elsewhere postkinematic
calcite in the rock matrix is rare or absent, and this corresponds to open,
quartz-lined fractures.