T369 Structural Geology and Geomechanics in the Petroleum Industry (Posters)

Wednesday, 30 October 2013
9:00 AM - 6:30 PM
Hall D

2D COMPUTER KINEMATIC FORWARD MODELING OF THE STILLWELL ANTICLINE FOLD SYSTEM, WEST TEXAS: TESTING MODELS OF FOLD EVOLUTION

Paper number: T369-1
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

We performed 2D forward kinematic modeling to constrain the fault system beneath the Stillwell anticline, west Texas. The fold is defined by Cretaceous marine units, with the present-day surface approximately 600 m above Paleozoic basement. The anticline is a NW-trending, NE-vergent, 8 km long, 500 m wide system that displays a left-stepping en echelon pattern with flat-ramp fault-propagation fold geometries, including a gently dipping back-limb, a middle-limb, and a steeply dipping forelimb. However, the 8-12° NE dip of the mid-limb, the 50-100 m vertical offset between hinterland and foreland, and the linear, localized deformation are not consistent with a simple, shallow, flat-ramp fault propagation origin. We hypothesize that reactivation of high-angle faults in the basement were necessary to produce these features. We tested fault-related fold formation using FaultFoldForward (FFF) to constrain relative timing and a possible link between basement and flat-ramp faults.

FFF uses algorithms governed by trishear kinematics that permit folds to develop in a triangular zone of distributed shear that expands ahead of a propagating fault tip. FFF permits manipulation of 5 values: ramp angle, trishear angle, fault slip, propagation-to-slip ratio (P/S), and fault tip location. We first tested the reactivation of a high angle basement fault (Stage 1) that might produce the mid-limb dip, foreland-hinterland offset, and narrow region of deformation as constrained by field data. Our best-fit model supports a reactivated basement fault system initiated approximately 600 meters below the present-day surface, with a vertical offset of approximately 80 m, a 60° ramp angle, 92 m of fault slip, a 35° trishear angle, and a P/S of 1.5.

The best-fit Stage 1 model became the starting point to model a shallow flat-ramp fault-propagation fold (Stage 2) that matches field observations. Systematic testing yielded a range of variable values that successfully produced fold geometries consistent with those documented in the field. Significant discrepancies between Stages 1 and 2 fault slip values and spatial constraints on fault geometries support an asynchronous, 2-stage model of fold formation, with an early-forming monocline acting as a nucleation point for later folding generated by a shallow flat-ramp fault system.