Fracture and Fault Prediction in Basement-Involved Fault-Related Folds: Comparisons Between Physical Models and Trishear Calculations

Fischer, Mark P.¹, David Keating², Nestor Cardozo³, Christina Majerowicz⁴ (1) Northern Illinois University, De Kalb, IL (2) Northern Illinois University, DeKalb, IL (3) Centre for Integrated Petroleum Research, Bergen, Norway (4) Lehigh University, Bethlehem, PA

Joints and small faults commonly form during fault-related folding, and their orientation, spatial distribution, and frequency can vary greatly both within individual folds, and between different folds. Where stratigraphy and environmental variables like burial depth can be ruled out as controlling parameters, variations in fold-related mesoscopic deformation are thought to be related to the mechanics and kinematics of folding. Understanding the evolution of fold-related strain consequently allows us to better predict the characteristics of joints and small faults that can cause reservoir compartmentalization and permeability anisotropy. To facilitate such predictions, we investigate progressive strain in scaled physical models of monoclinal, basement-involved fault-related folds. The models consist of a 3 cm thick layer of homogeneous wet clay overlying a rigid basement cut by a homoclinally dipping reverse fault. Variables we examine include fault dip and fault displacement profile. We use close-range photogrammetry to determine the 3-D geometry and strain in the surface of the model, and compare the strain patterns in the physical model with strain patterns predicted by trishear modeling of similar structures. Although the trishear predictions provide a good first-order fit to the geometry and strain patterns in the model monoclines, they differ in detail. Extensional strains are higher in the trishear models and concentrated into narrower bands than in the physical models. The locus of maximum strain in the trishear models may be shifted toward the hinterland compared to the physical models. We interpret that significant differences in fold shape and strain distribution most likely result from our use of trishear parameters that do not vary over time.