Geometry and mechanics of normal faults with emphasis on three-dimensional seismic data, conjugate faults, and the effects of sedimentary layering
The geometry and mechanics of normal faults have implications for seismic hazards and the exploration and production of hydrocarbons. In this study, data from a 3D seismic and field observations are combined with numerical models to gain insight on: (1) the relationships between fault geometry and displacement fields, (2) the effect of sedimentary layering on fault slip, (3) the development of conjugate normal faults, and (4) the formation of opening mode fractures due to overburden stress. In the first part of this study, a methodology is developed for using structure contour maps to constrain fault geometries below the limits of seismic resolution. The method incorporates data from well-resolved horizons and a priori information about slip distributions of faults. Displacements computed with a 3D boundary element method program are used to restore a faulted horizon mapped in a 3D seismic survey from the Prudhoe Bay oil field, on the north slope of Alaska. Patterns of structure contours on the restored horizons are assessed to test interpreted fault geometries. Numerous studies indicate that sedimentary layering can affect the geometry and slip distributions associated with normal faults. Often, faults exhibit less slip or terminate in layers described as soft or less competent. Results from numerical models employing the finite element method (FEM) indicate that non-uniform stresses associated with uniform extension of layered rocks having varying stiffness may explain such observations. Field observations from exposures of layered sedimentary rocks in Hanksville, UT suggest that the formation of conjugate fault geometries is dependent on lithology. Certain geometries form preferentially in units that contain more silt and mud. Numerical models (FEM) of conjugate normal faults indicate that more compressible layers may facilitate the formation of such structures, which are not a preferred geometry. Lastly, results from FEM models are used to propose a new mechanism for the formation of fractures in sedimentary rocks due to overburden weight. The results indicate that heterogeneous material properties can induce tension parallel to layer boundaries and thereby create conditions favorable for jointing.
Minerals Data and Information Rescue in Alaska (MDIRA)