The Fault Trajectory Method: A New Technique for Estimating the Location and Dip of Controlling Faults Below Forced Folds

Abstract

Abstract

Many folded structures 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. Excess-area or area-depth-strain (ADS) analysis can directly determine detachment depth without restrictive kinematic assumptions. However, the standard ADS method is limited to structures with horizontal detachments where regional elevations are the same on both sides of the fault. We present a new ADS method that directly determines fault depth, dip, displacement, and layer-parallel strain for structures characterized by differing regional elevations in the footwall and hangingwall. Referred to as the fault trajectory method, the new technique relates structural relief within a fold to the dip of the controlling fault. By varying the analysis aperture, the method can also locate fault path variations such as ramps and flats.

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 fault trajectory method also provides results that agree with interpretations constrained by seismic data and well logs. In the Uinta Basin of the western U.S., well and seismic data constrain the shallow geometry of a basement-involved fold. ADS analysis of the well-imaged interval indicates the controlling fault likely steepens to ∼50° in the basement. Finally, we apply the fault trajectory method to a series of case studies including a basin-bounding normal fault in the North Sea and fold-thrust belts in the Caribbean Ranges of Venezuela, the Bermejo basin in Argentina, and the deep-water Mexican Ridges in the western Gulf of Mexico. In these locations, only the shallow fold geometry is seismically imaged while the fault locations and shapes are ambiguous. Area-depth analysis of these structures is only possible using the new fault trajectory method. Each case study shows how area-depth analysis can quickly provide interpreters with the guidance and structural parameters necessary to reduce uncertainty in complex structural settings.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016