Abstract

The Oligocene Vicksburg Formation of South Texas was deposited as a coarse clastic shelf-deltaic system formed on the fringe of the uplifted Sierra Madre. Deposition within the Vicksburg is considered to have been in a combination of both shelfal and deltaic environments. Shelf-delta growth was by a complex series of syndepositional growth faults initially oriented NE-SW. For much of the Vicksburg the shelf-delta was detached from the underlying Eocene and crept seaward 'en-masse' with shelf and deltaic sediments to in-filling depressions formed by faults.

An alteration in tectonic control in Middle Vicksburg caused uplift, erosion and a change in structural style. The NW-SE ridges began forming in conjunction with significant N-S oriented en echelon counter-regional growth faults. Wrench fault movement in the basement is thought to form the ridges that later became the focus of hydrocarbon accumulations. The N-S oriented counter-regional growth faults formed as a result of deeper detachment within the Eocene.

The key to the successful exploitation of the Vicksburg is being able to unravel the extensive low angle discontinuous fault sets that dominate the structure in the majority of rotated fault blocks. These faults formed during the growth and rotation of the individual slump structures. Faulting occurred when the curvature on the listric fault plane was reduced as the slump rotated to a horizontal position. When rotation was complete, all the curvature from the base of the slump was transposed to the upper surface and the subsequent low angle faults dissected the crest of the slumps.