Fault Displacement Gradients and Associated Deformation on Normal Faults

Morris, Alan P.; McGinnis, Ronald N.; Ferrill, David A.

Faults are important components of hydrocarbon and other reservoirs; they can affect trapping of fluids, flow pathways, compartmentalization, production rates, and through these, production strategies, and economic outcomes. Displacement gradients on faults are associated with off-fault deformation, which can be manifest as faulting, extension fracturing, or folding. In this work, displacement gradients - both in the slip direction and laterally - on a well-exposed large-displacement ("seismic-scale") normal fault within the Balcones Fault System of south-central Texas are correlated with "anomalous" deformation patterns adjacent to the fault. This anomalous deformation is manifest by two superposed small-displacement fault systems, including (i) an earlier set that formed in response to a displacement gradient in the slip direction, and (ii) a later set of oblique faults that formed in a perturbed stress and strain field in response to a lateral displacement gradient on the fault. Use of bed-dip, fault cutoff relationships, and small-displacement fault patterns in the adjacent rock volume to inform strain and paleo-stress estimates indicates that displacement gradients on the seismic-scale fault provide a means by which the smaller (subseismic-scale and off-fault) deformation features can be predicted both in terms of orientation and intensity. Specifically, lateral displacement gradients on a normal fault with dip-slip displacement will generate fault-strike-parallel extension, causing anomalously oriented (in the far-field stress context) deformation features adjacent to the fault. Displacement gradient analysis can be used to help predict the characteristics of sub-seismic scale deformation within a reservoir adjacent to a seismic-scale normal fault.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013