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Impacts of Mechanical Stratigraphy in Geomechanical Models of Reservoir-Scale Extensional Fault-Propagation Folds

Smart, Kevin J.1; Ferrill, David A.1; Morris, Alan P.1
1 Geosciences & Engineering Division, Southwest Research Institute, San Antonio, TX.

Mechanical stratigraphy is an important control on the temporal and spatial evolution of deformation in most geologic structures in the upper crust. Mechanical stratigraphy can be manifest as contrasts in mechanical properties of adjacent rock layers and/or the strength of layer interfaces. The appropriate representation of mechanical stratigraphy is necessary to accurate numerical models of geomechanics. Geomechanical models of deformation in mechanically layered systems that do not include observed variations in material properties or do not allow for the possibility of interlayer slip may not accurately represent the natural system. Current numerical geomechanical modeling techniques are sufficiently robust so that complex mechanical stratigraphic variations can be captured in a computationally tractable manner. Here, we use finite element geomechanical models of a reservoir-scale extensional fault-propagation fold to explore the deformation response for several different configurations of mechanical stratigraphy. The models consist of a cover sequence of layered rock above a steeply dipping normal fault. Fault displacement is accommodated in the overlying cover sequence by macroscale folding of layers and distributed smaller-scale inelastic (i.e., permanent) strain. The cover sequence configurations in the finite element models include both contrasts in mechanical properties (alternating stronger and weaker intervals) as well as contrasts in the frictional strength of the layer interfaces, which controls the extent to which layers can slide with respect to each other. Gravitational body forces are included in all models and deformation is driven by a displacement boundary condition on the base of the model. Finite element model results demonstrate that the inclusion or exclusion of mechanical stratigraphy produces fundamental differences in the monocline geometry as well as the stress and strain distributions (orientations and intensity). Models that include variations in material properties and allow interlayer slip produce monoclines with steeper limbs than models without any mechanical stratigraphy. Maximum extension directions in the monocline limb are approximately layer-parallel in models that allow for interlayer slip, but oblique to layering when interlayer slip is not allowed. Maximum differential stress distributions show generally greater concentrations in the hinge regions for models without variations in material properties.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009