ANALYSIS OF MACRO-FRACTURES IN THE STILLWELL ANTICLINE, WEST TEXAS: IMPLICATIONS FOR COUPLED FOLD-FRACTURE EVOLUTION

Because fracture networks control the flow rate and direction of subsurface fluids, the relationship between fracture orientations and fold evolution is vital for predicting flow pathways. Our research sheds light on fold-related fracture formation in the Stillwell anticline, west Texas. The Stillwell anticline is a well-exposed Laramide age structure that is best defined by the well-bedded Cretaceous Santa Elena limestone. The anticline is an 8 km long, 500 m wide, fault-propagation fold with three NW trending, NE-vergent, left-stepping en echelon segments. At most locations, the fold system displays a gently-dipping backlimb, a shallowly foreland-dipping mid-limb, and a steeply-dipping forelimb.

We measured the orientation of more than 800 macro-fractures from all parts of the fold system as well as from several locations unaffected by folding. These data reveal 4 major, steeply-dipping fracture sets trending N (set F1), NE (F2), ENE (F3), and NW (F4). F1 is 30° from the fold axis, is present throughout the area in both folded and unfolded strata, and is interpreted as a tensional set possibly associated with the opening of the Gulf of Mexico. Sets F2 and F3 are cross-fold sets consistent with mixed mode formation as a conjugate pair early in fold formation. F4 is subparallel to the fold axis and is likely dilatational, associated with bed flexure during folding. We created detailed, outcrop-scale fracture maps that reveal a significant range of fracture intensities for each fracture set. Fracture intensities within the Santa Elena limestone appear to be inversely related to bed thickness.

The chronology of fracture formation within the anticline system suggests changing fluid flow directions through time, with pre-Laramide flow constrained to N-S and later fracture sets within the anticline providing the potential for more isotropic flow within the fold system. Flow rates would likely vary stratigraphically, with greater flow rates in thinner beds, which have higher fracture intensities. In addition, the orientation of the F2-F3 conjugate set indicates a sigma-1 direction oblique to the anticline’s axis, supporting the accommodation of left-lateral shear during fold formation, an idea proposed by previous researchers.