GEOMECHANICAL ANALYSIS OF SEDIMENTARY LAYERING AS A STRUCTURAL CONTROL ON FAULT PROPAGATION

Deformation associated with normal fault propagation and displacement places controls on the distribution and flow of sub-surface fluids. With a better understanding of how sedimentary units deform in response to a propagating fault, we can better predict how fluids might flow through the system at different stages of displacement. To elucidate the role of sedimentary layering on fault tip propagation, we use ABAQUS/CAE finite element analysis of a propagating normal fault to identify patterns of stress distribution and accumulation. While holding material properties constant (e.g., Young’s Modulus, Poisson’s Ratio, dilation angle, and the internal angle of friction), we simulate the initial stages of plastic failure in front of a normal fault tip propagating at 60° through bedded sandstone. All models terminated at 0.125 m displacement. We test the effects of incrementally increasing the number of mechanical layers from a single 20-m thick layer to five 4-m thick layers. We find that the presence of layering allows for simultaneous, but discontinuous, plastic failure in multiple locations ahead of a propagating fault tip. Additionally, although inter-layer stress accumulation is hindered by an increased number of layers, elevated regions of maximum stress occur further ahead of the propagating fault tip with an increased number of layers. Mechanical layering influences the stress field ahead of a propagating fault, but we show that the coefficient of friction of the contacts between those layers do not have a strong influence on how the fault tip propagates.

Our results show that mechanical layering re-distributes stress ahead of a propagating fault tip so that a bedded section of sandstone will begin to fracture differently than a single coherent section. Such behavior is likely to influence the development of subsurface fluid conduits. We extrapolate our results to predict that the presence of bedding planes may increase the development of conduits during early stages of normal fault propagation.