STEFFENS, GARY S., Shell Offshore, New Orleans, LA

ABSTRACT: Gulf of Mexico Deepwater Seismic Stratigraphy

Shell Offshore Inc. is actively exploring and exploiting turbidite reservoirs on the present-day continental slope and abyssal plain of Northern Gulf of Mexico. The turbidite reservoirs are part of a large, Neogene deepwater section that is composed of repetitive successions of stacked, deepwater sequences that correspond to fourth -and fifth order glacio-eustatic cycles.

Two turbidite complexes are recognized: the unconfined, large river complex, and the confined, ponded complex. The Mississippi Fan Complex was deposited on the featureless, unconfined, abyssal plain of Northeastern Gulf of Mexico. It was produced by the Ancestral Mississippi River; nine individual fan sequences have been identified, ranging in age from late Miocene through late Pleistocene. The confined, ponded complex is situated in a partitioned network of salt withdrawal minibasins along the present-day continental slope. These minibasins display pronounced bathymetric relief during development. This relief controlled the distribution and type of turbidite facies deposited. Deepwater sequences passively onlapped and ponded around actively deforming salt, progressively filling and s illing in a basinward direction.

Regardless of the type of turbidite complex, seismic facies analysis of turbidite sequences indicate that there are several depositional elements or "building blocks" that are common to all systems. The elements developed as depositional and erosional features during various stages of fan growth. These elements include slumps/debris flows, leveed channel complexes, and basin-floor fans that consist of sheet sands and amalgamated channel sands. The stacking pattern of these depositional elements has been found to be similar for any one turbidite system. This repetitive, vertical succession of depositional elements usually consists of basin-floor sands at the base of each sequence (sometimes starting with a basal shale or debris flow section), passing upwards into leveed channel complex s and capped by marine shales and marls. These depositional elements and their stacking patterns areprobably the products of the gradual reduction in coarse-grained sediment associated with the progressive rise of sea level. Variations are seen in this vertical sequence, and are probably due to any number of factors (i.e., position on the fan, type of sediment supply, etc.).

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.