Strain in Extensional Monoclines: Insights from Numerical Models

Kevin J. Smart, Alan P. Morris, and David A. Ferrill
Southwest Research Institute®, San Antonio, TX

Displacement on a fault that does not reach the ground surface or “blind faulting” may be accommodated by formation of folds in overlying rocks. Inhibited upward fault propagation is often the result of a mechanically weak or ductile buffer zone (e.g., thick shale or evaporite interval) overlying the stronger faulted rocks. In the case of blind normal faults, the overall extension can be transferred to the cover rocks as an extensional monocline, although a range of meso-and micro-scale structures are typically present (e.g., subsidiary faults, fractures, veins). Structural evolution is influenced by parameters such as orientation and displacement magnitude of the master fault, and thickness and strength of the buffer zone. We develop a series of finite element models to analyze the roles of these parameters on the formation of extensional monoclines.

Our simulation geometry includes a deformable cover sequence overlying mechanically strong blocks. We tested several variables, including: (i) strength and thickness of buffer material; (ii) fault dip and displacement; and (iii) mechanical layering in the cover sequence. To help understand formation of deformation features such as faults, fractures, bedding-slip surfaces, and distributed strain in extensional monoclines, we focused on capturing the evolution of displacement and strain (elastic and inelastic) in the deforming cover. Specific measurements included monocline limb dip, and spatial and temporal variations in elastic and inelastic strain throughout the cover sequence. Initial results indicate that steeper, more extended monocline limbs form in models with stronger buffer zone material, shallow fault dip, and greater fault displacement. A thinner buffer zone increases monocline dip, but decreases extensional strain. This approach provides the basis for predictions of reservoir-scale deformation zones and structural heterogeneity resulting from fault-related folding and associated deformation.