Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Predicting Open Natural Fractures in Unconventional Sandstone Reservoirs: Spatial Distribution, Diagenesis, Timing, and Opening Rates
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.
Natural fractures are important for storage and transport in tight-gas sandstone reservoirs. We constrain the evolution of fracture opening in such reservoirs and in analogous surface rocks in order to predict the distribution of fracture porosity. Scanning electron microscope-based cathodoluminescence (SEM-CL) imaging allows for the detailed mapping of parallel fractures with widely ranging aperture sizes (~0.001 to ~10 mm of total displacement), as well as internal fracture cements deposited during fracture opening. Such cements typically seal all fractures below a size range characteristic to each fracture set. Larger fractures preserve porosity in between cement deposits that bridge across fractures. The crack-seal texture of the cement bridges indicates repeated stages of fracture opening and concurrent cementation. Fluid inclusions are trapped during cementation and record the overall range in temperature, pressure, and fluid composition history during fracture opening. Opening durations of individual macrofractures are obtained through detailed microthermometry of the fluid inclusion, combined with textural reconstructions of the fracture opening history. Temperature data are then correlated with known burial history models to obtain the timing and rate of fracture opening. In the Mesaverde Group of the Piceance Basin, Colorado, fracture opening lasted over a time period of ~33 m.y. This duration of fracture opening corresponds to fracture opening strain rates of 10-18 to 10-19 s-1. Similar opening strain rates are obtained for fractures in the East Texas basin. These ultraslow strain rates are comparable to intraplate tectonic strain rates.