Natural Hydrofracturing and Ductile Faulting in Austin Chalk, Texas

Kevin P. Corbett

The Austin Chalk possesses two well-developed orthogonal joint sets in both the outcrop and subsurface. Mutual crosscutting relationships indicate that the joint sets formed simultaneously. The joints exhibit a strong preferred orientation with one set parallel to the bedding strike and the other parallel to the dip direction; both sets are vertical. Commonly, joints are filled with sparry calcite, but open fractures occur with equal abundance. Characteristically, the Austin Chalk is undercompacted for depths of burial determined by vitrinite reflectance. Undercompaction can be attributed to the evolution of abnormally high pore-fluid pressure during burial. Evidence for abnormally high pore pressure and the lack of evidence for tectonic stresses, which could produce frac uring, suggest a hydrofracture origin for the Austin Chalk joint system.

Additional support for the hydrofracture interpretation is gained from the development of S-C fabrics in normal faults within the chalk. S-C fabrics form when a shear zone crosscuts a previously developed planar fabric, usually cleavage. However, for the Austin Chalk the previously developed planar fabric consists of originally horizontal, bedding-parallel extension veins. This orientation indicates that, at least at the time of faulting, pore-fluid pressure exceeded overburden pressure, which was the maximum compressive stress. Extension veins forming the fault fabric contain as many as three generations of sparry calcite fill, indicating cyclic pore pressure buildup and fracturing. The lack of twinning in the vein fill calcite suggests displacement in the fault zones occurred before crystallization of vein material, while the fractures were still fluid filled. Apparently, fault movement contributed to the pressure cycling by faults serving as conduits for fluid transport.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.