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Abstract: Timing of Fracturing of Sedimentary Beds Around Evolving Salt Shapes: Self-Consistent Behaviors and Mohr's Criterion

K. Petersen, I. Lerche

For the intertwined evolution of both salt and sediments, a self-consistent quantitative model has been constructed that can be used to provide automatically information on the changing

bed geometries with time in a manner guaranteed to be consistent with present-day observations of salt bodies and sediment geometries. Instead of trying to specify a large number of mechanical properties to calculate the stress in the evolving salt-sediment system, and then attempting to calculate the corresponding strain, the procedure simplifies this process by reversing the order: (1) determine the strain; (2) define the response of the sediments to stress (for instance, elastic); and (3) calculate the corresponding stress. (A simple analogy is Newton's second law, F = m·a, which says that a unit of force F equals the force needed to accelerate a mass m, of one kilogram, one meter per second per second. If the mass m and acceleration a are given, the force F can be calculated. Conversely, if the force F and mass m are known, the acceleration can be calculated.)

Two cases exemplify how the strain/stress patterns in the sediments depend on the evolving geometry of the salt through time. A large salt overhang results in massive strain next to, and immediately below, the overhang due to the horizontal spreading of the salt and the associated void space created near the salt stem. Because no void can exist, the sediments must deform to fill the void. Therefore, the sediments below the overhang "slide" into the space created as the salt structure feeds the overhang development from deeper salt. When the salt shape shows only a small overhang development through time, the sediments will move dominantly vertically past the evolving structure and sustain smaller deformations.

From the calculated strain behavior of the sediments, the stress associated with the strain can be evaluated once the spatial variations of the sedimentary Lame constants are specified. When the Mohr-Coulomb criterion for failure is included, the timing and development of sedimentary fracturing can be calculated. Knowledge of the spatial fracture pattern and its temporal evolution is crucial to the evaluation of hydrocarbon migration pathways and trap development near salt structures. The timing of oil generation, which depends on the thermal history of the sediments, can then be compared and contrasted to the timing of migration through the fractured regime. The examples illuminate different cases that are possible depending on the evolution of salt and sediment juxtapositioning and haping with time.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994