PREDICTING THE DIP AND LOCATION OF MASTER FAULTS BENEATH FORCED FOLDS

Many folded structures, such as forced folds, form above faults that dip to considerable depth. In petroleum exploration and development, locating the controlling fault is a matter of practical importance. Often, seismic and well data constrain the upper fold geometry but the dip and location of the controlling fault are unknown. Standard area-depth-strain (ADS) analysis can directly determine detachment depth without restrictive kinematic assumptions. However, the standard ADS method is limited to horizontal detachments where regional elevations are the same on both sides of the fault. We present a generalized ADS method that directly determines fault depth, dip, displacement, and layer-parallel strain for folds with different regional elevations in the footwall and hangingwall. By narrowing the analysis aperture, the method can also locate the upper parts of faults that flatten at depth.

We validate the method using extensional and contractional models with known fault positions. ADS analysis of area-balanced forward models provides exact matches to the fault trajectory, displacement and layer-parallel strain distribution. In physical models, the method reliably locates the position and dip of the controlling fault regardless of model rheology. The new ADS method also provides results that agree with interpretations constrained by seismic data, well logs, and earthquake focal mechanisms. Analysis of the Gilbertown graben in Alabama reproduces the established location and dip of a major fault and correctly identifies the growth strata. For the active White Wolf fault in the San Joaquin Basin, the ADS-estimated fault dip is consistent with the focal mechanism for the 1952 Kern County earthquake that occurred on the same structure. In the Uinta Basin, well and seismic data constrain the shallow fold geometry of a Laramide uplift. ADS analysis of that interval indicates the controlling fault steepens to ~50° in the basement. Finally, we perform ADS analysis on structures from the North Sea and Argentina where only seismic data are available. The ADS-estimated fault orientations are consistent with isolated seismic reflectors that may indicate the deep fault location. In these situations, the new ADS method instantaneously provides structural parameters that refine the interpretation during development.