--> ABSTRACT: Three-Dimensional Forward Modeling of Closure on Normal Faults, by John H. Spang; #91020 (1995).

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Three-Dimensional Forward Modeling of Closure on Normal Faults

John H. Spang

A series of balanced, geologic cross sections of uniformly varying displacement are arranged vertically in arrays to simulate, for example, gain or loss of displacement on one or more faults. These models show the sequential development and resulting compartmentalization of the structures and the development and migration of depocenters with increasing displacement. Rheological models used to draw the cross sections include both vertical simple shear and inclined simple shear with a 60° dip. The cross sections are stitched together to map the structures using isopachs of growth sediments aid structure contours. Growth sediments have been modeled as level-filled basins but underfilled or overfilled normal fault-bounded basins are easily done. Regional dip can be incor orated, but scale problems (e.g. line widths) dictate a relatively high dip of 1 in 10 (or 5.7°).

For the simplest case of a doubly terminated, single, straight normal fault, no closure develops with or without a regional dip. When two straight normal faults propagate toward each other and merge, closure develops in the region where the faults merge due to reduced stratigraphic throw in that area, and the amount of closure is enhanced by a regional dip. Closure can develop on a single isolated normal fault when there is a regional dip and the fault is curved so that the ends of the fault curve toward the dip direction. As displacement on a normal fault decreases toward the ends of the fault, fold crests on hanging wall folds are inclined upward relative to the layering. Closure can develop on a single curved normal fault, if the curvature is large enough so that the rate of upward inclination of the fold crests is less than the downward rate of the regional dip.

For an ideal normal fault with constant fault geometry along strike, there would be no closure since the crests of all folds would be horizontal. However, when some of the displacement on the main fault is transferred to either a synthetic or antithetic fault in the hanging wall, the addition of the second fault has the effect of depressing the crest which creates closure due to a structural low. Similarly, a local steepening of fault dip which also creates a structural low as has been demonstrated by others. When there is closure on a rollover anticline, the addition of a small synthetic fault can create closure in both the hanging wall and footwall of the small fault, which results in a compartmentalized reservoir.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995