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.