Analysis of Subseismic Faults and Extension Fractures in a Laramide-Age Anticline in Cretaceous Carbonates at Persimmon Gap, Big Bend National Park
Small-displacement faults and extension fractures are critical to porosity and permeability, and reservoir performance in anticlinal oil fields. To understand the relationship between small-scale brittle deformation and structural position in a fault related anticline, an analog study was performed at Persimmon Gap anticline in west Texas. Persimmon Gap is an erosional gap within the Santiago Mountains at the northeastern entrance to Big Bend National Park. At the northwestern end of the Sierra del Carmen trend, this gap exposes Paleozoic rocks (Maravillas Formation, Caballos Novaculite, and Tesnus Formation) that are unconformably overlain by Cretaceous rocks (basal conglomerates and carbonates of the Glen Rose Formation). Two contractional deformation events are recorded in the rocks: (i) a Late Paleozoic Ouachita event that overturned and faulted the Paleozoic sedimentary rocks, and (ii) a subsequent Late Cretaceous Laramide event that re-folded the Paleozoic rocks and folded and thrust-faulted the Cretaceous rocks. The Laramide anticline exposed at Persimmon Gap is southwestward verging with a wavelength on the order of 1 km and amplitude of 200 to 300 m. This is a chevron fold with an interlimb angle of 40° that plunges toward 140° at approximately 8°. The overturned forelimb contains numerous extensional features that indicate the fold is not a simple single-step fault propagation fold, but one that has experienced forelimb extension and thinning as a result of limb-locking during folding. Joints are ubiquitous around the anticline, however calcite extension veins and bed-extending faults are common in the forelimb, but are absent from the backlimb and regions of high curvature. Local vein-accommodated bed-parallel extensions range from 7 to 18%. Small-displacement (< 5 m) faults account for approximately 45% extension bed-parallel extension. Small-displacement-fault orientations, displacements, and slip directions from the forelimb were used to estimate the stress state at the time of forelimb deformation. Results show a concentration of faulting and extensional veins in the anticlinal forelimb rather than hinge region. Although structural position is a key predictor of brittle deformation, curvature is not likely to be an adequate predictor of fracturing in Cretaceous carbonate reservoirs.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California