Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Vertical and Lateral Scales of Heterogeneity in a Fractured Reservoir Analog, Middle Triassic to Lower Jurassic La Boca Formation, NE Mexico
Outcrops of Middle Triassic to Lower Jurassic La Boca Formation sandstones, northeastern Mexico are an unusually well-preserved example of fracture size distributions and patterns and fracture internal textures—particularly fracture porosity structure—matching those found in cores from many deep basins. Here we show that many fracture attributes that are challenging to discover in deeply buried rocks are preserved in these outcrops, which are unique in that they preserve clear examples of effects of natural cement precipitation (diagenesis) on fracture attributes. In this fractured reservoir analog, abrupt differences in patterns of fracture sizes, clustering, and porosity preservation are evident over vertical distances of a few meters and lateral distances of 10s of meters. This heterogeneity results from interplay of stratigraphic, structural, and diagenetic effects during fracture growth and aging. Fracture sizes and shapes, fracture size scaling and internal textures match what is found in cores from many deep basins. Thus these rocks provide a rare opportunity to compare what can be sampled using core with attributes that cannot be measured using core-based methods.