IMPLICATIONS OF FRACTURE CHARACTERISTICS FOR FLUID FLOW WITHIN FOLD SYSTEMS: A CASE STUDY FROM THE STILLWELL ANTICLINE, WEST TEXAS

We performed detailed analysis of fracture orientations and characteristics across the Stillwell anticline, west Texas, a NW-trending contractional fold system that formed during the Laramide orogeny. By documenting fracture characteristics including morphology, calcite fill texture, number of generations of precipitation, fill composition, isotopic signature, and host rock alteration, we are able to place constraints on the evolution of the fluid flow network.

Previous research established four fracture sets in the fold system. These sets trend N (set F1), NE (set F2), ENE (set F3), and NW (set F4). Analysis of these orientations in the context of fold formation reveal that set F1, present in both folded and unfolded strata, formed prior to folding, conjugate sets F2 and F3 are consistent with shear formation associated with compression and folding, and set F4 is subparallel to the fold axis and is likely related to bed flexure during fold formation.

We used field data and rectified field photos to create fracture maps, and we collected core samples of fractures and host rock from the 4 fracture sets using a portable drill. We analyzed high resolution optical scans and documented features observed in petrographic thin sections. We also utilized a portable XRF spectrometer to determine the compositions of host rocks, alteration halos, and fracture fills, and we interpreted both C- and O-isotope values collected from a large aperture fracture with more than 15 fill events.

Our results and analysis of fracture characteristics reveal stronger correlation by sample location than by fracture set. Our work suggests that: 1) fractures at every location were open for some time prior to sealing by calcite precipitation; 2) all fractures at a given location were simultaneously filled by locally-sourced fluids with little isotopic variation; and 3) fracture fill characteristics vary greatly by sample location. Thus, we hypothesize that fluid flow rates and connectivity was maximized, with relatively isotropic flow, during and immediately after fold formation, and that flow-inhibiting precipitation was likely unpredictable, rapid, and localized, making prediction of subsurface flow rates and pathways difficult.