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Impact of Carbonate Stratigraphy on Fracture Development in Fault Damage Zones, Central Texas

Chris Zahm1, Jerome Bellian2, and Peter Hennings1
1 ConocoPhillips Subsurface Technology, Houston, TX
2 The Univeristy of Texas at Austin, Austin, TX

A combination of LIDAR surveying, stratigraphic facies mapping, and fracture characterization was conducted at an extensional fault relay north of San Antonio, Texas. LIDAR surveying greatly improved the detailed spatial characterization of facies and fracture variations. This outcrop displays a hierarchy of fracture development, including: (1) faults that have several meters of offset that are regionally extensive and apparently oblivious to local facies; (2) small faults with centimeters of offset that extend through, but are mechanically altered by, weaker rock facies; and (3) shear fractures which have millimeters of offset and are strongly controlled by the rock facies. Fracture intensity does vary by stratigraphic facies, but the more critical factor for intensity is the geometry of the small, vertically-extensive faults. The presence of regionally correlative third-order cycle boundaries (<20 cm argillaceous mudstones) caused significant change in the fault dip and orientation. These changes create discontinuities in the fault surface which spawn additional fracture development as the fault continues to slip.

This study illustrates the important role that mechanical stratigraphy plays on fracture development in carbonate fault damage zones. The characterization of carbonate facies that are prone to fracture development represents a critical link between 3D reservoir characterization and flow simulation. Reservoir models that incorporate both facies and fracture heterogeneities are critical if flow behavior is to be predicted.