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ABSTRACT: Shapes and Sizes of Reservoir Sand Bodies--A Bird's Eye View of Modern Mississippi Delta

James M. Coleman, H. H. Roberts

Lateral migration of delta channels, overbanking, and dispersal processes at river mouths result in deposition of a wide variety of sand bodies in the Mississippi River delta. The major sand bodies include fluvial channel-fill, bay-fill, distributary channel-fill, distributary mouth bar, and delta-front environments.

As a result of continued delta outbuilding, the lower part of the fluvial system progrades over delta-plain deposits, and lateral channel migration concentrates the reworked coarser clastics. The resulting vertical sequence shows a generally fining- and thinning-upward sequence. Thicknesses of these fluvial deposits vary from 7 to 10 m to a maximum of 100 m. In a strike direction, sand body dimensions range from 15 km to less than 2 km; in a dip direction, the sand body is highly elongated ("shoestring"), but porosity and permeability are highly variable, resulting in multiple reservoir units within the overall sand body.

Distributary channel fills are overwhelmingly fine grained in nature, but the lower part often contains coarse clastics that are of reservoir quality. These sand bodies vary in width from 1 km to less than a few hundred meters, and thicknesses are highly variable, ranging from 10 m to only a few meters.

Overbanking, a common characteristic of the Mississippi River, varies in scale from small overbank splays (a few square kilometers) to large bay fills covering in excess of 100 km2. The larger bay fills contain an abundance of coarse clastics, and the vertical sequence displays a coarsening- and thickening-upward trend. The bay-fill sand bodies commonly form a blanket sand that ranges in thickness from 30 m to only a few meters. Bay fills tend to stack vertically, forming multiple stacked reservoirs.

Discharge of buoyant effluent plumes at river mouths results in building the classical vertical sequence composed of the prodelta, deltafront, and distributary mouth bar deposits. Delta-front deposits form blanketlike sequences and range in thickness from 30 to 70 m. The major sand unit, the distributary mouth bar, varies in thickness from in excess of 80 m to 10 m. The deposits are generally wedge shaped in a strike direction, with the thicker portion being found near the channel. The deposits thin laterally and generally extend for distances ranging from 4 km to less than 1 km.

Although all of these deposits represent potential reservoirs, bay-fill and the thin-bedded delta-front deposits appear to be the most prolific hydrocarbon reservoirs in the Gulf Coast sedimentary sequences.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990