Characterization of Opening Mode Fracture Systems in the Austin Chalk

J. F. W. Stowell
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

The Upper Cretaceous Austin Chalk is a low permeability, fractured reservoir that has been a target of numerous horizontal wells in Texas. For wells to be successful, parameters such as wellbore azimuth, vertical depth and length must be optimized. Furthermore, information on fracture height and the storage volume potentially connected to the wellbore is required. Two case studies are presented where the height of opening mode fractures in the Austin Chalk has been examined, together with fracture population attributes of orientation, aperture, effective aperture, spacing, aspect ratio, fracture fill and fracture permeability. The studies are of an outcrop of Upper Austin Chalk nearWaxahatchie, north Central Texas (Grove Creek), and of two laterals of a horizontal core in the Atco Member in Frio County, Pearsall Field (Kinlaw Core). Large, potentially open, fractures are commonly clustered, the distance between clusters ranging from  ~1m to  ~50m. Aperture size distributions follow power laws and spacing size distributions are negative-logarithmic or lognormal. The aperture size at which fractures are open to fluids is variable (0.14-11mm). Fracture permeability, which is scale-dependent, has been determined at 7.1D (for 18m of Lower Austin Chalk core) and 286D (for 300m of Upper Austin Chalk outcrop). Fractures may terminate at chalk/marl contacts, tip out within the marl, or pass through the marl layers. Fracture height is governed by aspect ratio and truncation at marl horizons, which in turn is partly, but not wholly, dependent on fracture aperture.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana