From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
2005 Southwest Section AAPG Convention, April 10–13, Fredericksburg, Texas
Opening history and porosity evolution of fractures in sandstone beneath an evaporite detachment, Gulf of Mexico Basin
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 beneath a regional evaporite detachment zone. 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. Preliminary point counts suggest several increments of porosity evolution that vary in the same structure, within tens of meters laterally and vertically. In a key canyon outcrop, postkinematic calcite in the rock matrix is prevalent, and this corresponds to calcite-sealed fractures. In a key road cut, postkinematic calcite in the rock matrix is rare or absent, and this corresponds to open, quartz-lined fractures. This variation of fracture porosity preservation may reflect proximity to igneous intrusions and stratigraphic position. Linking fracture opening history and diagenesis leads to a better understanding of the contribution of fractures to subsurface fluid flow.