Progressive Fault and Fracture Response to Neogene Extension in the Western Delaware Basin, Texas

Session: Faults, Fractures, and Geomechanics for the Energy Transition

Presenting Author:

David Ferrill

Authors:

Ferrill, David A.¹, Cawood, Adam J.², Smart, Kevin J.³, Lehrmann, Daniel James⁴, Bryce, Bethany Rysak⁵, Evans, Mark A.⁶, Stockli, Lisa Danielle⁷, Stockli, Daniel F.⁸

(1) Southwest Research Institute, San Antonio, TX, USA, (2) Southwest Research Institute, Space Science & Engineering, San Antonio, TX, USA, (3) Southwest Research Institute, Space Science Division, San Antonio, TX, USA, (4) Trinity University, San Antonio, TX, USA, (5) Southwest Research Institute, San Antonio, Texas, USA; Ovintiv Inc.,, Denver, Colorado, USA, (6) Central Connecticut State Unviersity, New Britain, CT, USA, (7) University of Texas at Austin - Jackson School of Geosciences, Austin, TX, USA, (8) Dept. of Earth and Planetary Sciences, University of Texas at Austin, Austin, TX, USA,

Abstract:

Normal faults are commonly thought to originate as conjugate 60°-dipping shear fractures, although recent work shows that normal faults may initiate with near-vertical to near-horizontal dips via tensile, hybrid, shear, compactive shear, and compactive failure.  Some normal faults develop by shear reactivation of opening-mode (tensile) fractures or by down-dip shear along tilted bedding.  We investigated normal faults in the Brushy Canyon Formation along the eastern edge of Cenozoic Basin and Range/Rio Grande Rift deformation in the western Delaware Basin.  Our field-based analysis integrates outcrop data collection and sampling, digital photogrammetry, cross-section construction and restoration, mineralogic and lithostratigraphic analysis of mechanical layers, vein fluid inclusion analysis, and U-Pb dating of calcite veins to understand the origin, timing, and fluid history of normal faulting in the western Delaware Basin. 

Two roadcut exposures in the southern Guadalupe Mountains along the eastern edge of the Salt Flat Graben reveal (i) steep normal faults in subhorizontal layers, and (ii) tilted layering and antithetic normal faults in a hanging-wall rollover of the asymmetrical graben.  Normal fault orientations in both exposures closely match orientations of abundant opening-mode fractures in well-cemented turbidite sandstone beds.  In horizontal layers, opening-mode fractures and faults are near-vertical, and where bedding is tilted 20-30° in the rollover, fractures and faults remain perpendicular to bedding.  We interpret that progressive rotation of initially vertical opening-mode fractures during layer tilting increased slip tendency leading to shear displacement. Faults with displacements of 1-50 cm within competent sandstone layers abruptly terminate in claystone beds (~60-95 wt. % clay) due to ductile flowage, effectively compartmentalizing deformation. Minor down-dip normal displacement locally occurred along weak bedding laminae, offsetting bed-perpendicular veins.

U-Pb dating of calcite from normal faults and opening-mode fractures yielded ages of (i) 16.5-10.0 Ma for the high-angle faults exposure, and (ii) 20.5-4.0 Ma from the rollover exposure, consistent with regional timing for Basin and Range extension. Oil inclusion fluorescence of vein calcites indicate ~29 to 32 API gravity oil. Depleted δ¹³C and similar δ¹⁸O stable isotope values relative to other veins in the area are consistent with hydrocarbon oxidation.  These results provide constraints on progressive structural evolution and fluid movement in the Delaware Basin, providing context for hydrocarbon migration and fault reactivation by hydraulic fracturing, production, and wastewater injection.