--> ABSTRACT: Predicting Sub-Salt Normal Faults from Shallow Grabens in the Overburden: Geomechanical Analysis and Application, by B. Kilsdonk, L. A. Reinen, and W. F. Dula, Jr; #91021 (2010)

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Predicting Sub-Salt Normal Faults from Shallow Grabens in the Overburden: Geomechanical Analysis and Application

KILSDONK, BILL, LINDA A. REINEN, and WILLIAM F. DULA JR

We combine seismic observation with mechanical analyses to explain the genetic and spatial links between shallow grabens and sub-salt faults. The result is a simple, but realistic, model of the processes that control structural and geometric evolution. Consequently, the analyses provide a technique to predict or interpret the location, trend, and timing of a seismically-cryptic sub-salt normal fault using observations of well-imaged grabens in the shallow overburden. Displacement on a sub-salt fault produces a sharp discontinuity at the base of the salt layer. The salt transmits strain from the fault upward into its overburden, but salt flow both dampens the strain intensity and distributes the strain over a wider region. The overlying section deforms to remain in contact with the salt. This locally enhances near surface extension in a region over the footwall block of the sub-salt fault. A graben, possibly accompanied by an underlying diapir, forms in this region. The horizontal distance, D, from the fault to the incipient graben depends on 4 factors: (1) combined thickness of the salt layer and the overlying sedimentary rocks; (2) strength ratio of sedimentary rock to salt; (3) sub-salt fault dip; (4) relative thicknesses of sedimentary rock and salt layers. Data from the Southern North Sea constrains strength ratios to be on the order of 10. The analysis is relatively insensitive to variations in strength ratio that are less than an order of magnitude.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.