Abstract: Architecture and Evolution of a Plio-Pleistocene Salt Withdrawal Mini-basin: Eugene Island, South Addition, Block 330, Offshore Louisiana

Laurel L. Alexander, Peter B. Flemings

The Plio-Pleistocene mini-basin evolved in at least three phases: deepwater, stalled-shelf, and bypass. These three phases appear to be typical of many salt withdrawal mini-basins and provide a framework for understanding the distribution and characteristics of reservoir sands.

During the deepwater phase, deep-water and prodelta shales loaded and mobilized an underlying salt sill causing the salt to flow southward where it formed into ridges and domes. An arcuate listric growth fault system developed along the flanks and northern margin of the basin. A counter-regional fault formed the southern margin of the mini-basin. Sand-rich turbidites were occasionally deposited within topographic lows and encased by the shales of the deepwater phase. If charged with hydrocarbons, these sands can be attractive reservoirs. In the stalled-shelf phase, the shelf prograded southward to the northern margin of the present structural mini-basin where it remained until salt withdrawal abated. Deposition of thick deltaic sands which thin to the south and towards the margins of he mini-basin occurred during this time. These sands have excellent reservoir potential due to their good porosity and permeability, overlying shale seals, and tendency to have structural closure. As expected, most of the exploited reservoirs in the Block 330 Field were deposited in this interval. During the bypass phase, the delta front prograded to the south and thick fluvial sands were deposited over the mini-basin. These fluvial sands lack structural closure and tend to be water wet, although stratigraphic traps may be present.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994