Mechanical Modeling of Slip Along Regional Faults in the Arbuckle Anticline to Estimate Fracture Orientation and Density Constrained by Geohistory
Russell K. Davies1, Michael Welch2, Colin Daly3, and Rob Knipe4
1 Rock Deformation Research, USA Inc, McKinney, TX
2 Rock Deformation Research Ltd, Leeds, United Kingdom
3 Roxar, Stavanger, Norway
4 Rock Deformation Research
The Arbuckle anticline in south central Oklahoma has been interpreted as a fold that formed above the tip of a propagating thrust fault. In outcrop, limestone beds in the fold limbs show fractures confined to individual beds and through-going fracture sets that cut across several layers. The through-going fractures are oriented NE-SW subparallel to a more regional trend on the shallow dipping limb of the fold south of the major bounding Washita Valley Fault (WVF). A second set of regional fractures oriented nearly north south lie north of the WVF and west of the tip of the Reagan fault to the north. To estimate fracture orientations as analogs to similar fractures sets in the subsurface, we modeled shear on the large bounding faults using finite element and boundary element models. Discrete fracture sets can be produced from a boundary element code in which the regional faults slip under the applied load to a zero residual stress state on the fault surface. By assuming a failure criterion in the model material, discrete fracture sets can be predicted. The models show that a single stress boundary condition is insufficient to produce the two regional fracture orientations and a multi-stage stress history is required suggesting independent development of the fractures observed. In this structure the ENE-WSW shortening explains the southern fractures, and NNW-SSE shortening the northern fractures, which suggests a multiphase deformation history for fracture development in the Arbuckles. This geomechanical method demonstrates the importance of structural history and regional fault orientation and shape in prediction of fracture orientation and in turn the limits in fracture productivity.