ABSTRACT: Deep-Water Bottom-Current Reworked Sands: Their Recognition and Reservoir Potential, Northern Gulf of Mexico

G. Shanmugam, T. D. Spalding, R. A. Kolb, T. M. Lockrem

Some Pliocene and Pleistocene reservoir sands in intraslope basins of the northern Gulf of Mexico exhibit features that are interpreted to be indicative of reworking by deep-water (bathyal) bottom currents (contour currents). These fine sands have previously been interpreted as turbidites associated with levee overbanks and lobes of submarine fan complexes; however, sedimentological features characteristic of turbidites are rare in these "laminated" sands. Common features observed are (1) numerous (up to 75 layers/m) thin (<2 cm) sand layers, (2) sharp upper contacts, (3) inverse grading, (4) current ripples, (5) lenticular bedding, (6) flaser bedding, (7) bidirectional cross-lamination, and (8) "sigmoidal" bed forms with mud drapes (i.e., mud offshoots).

These features, dominated by traction structures, can be explained by reworking of overbank turbidite sands by deep-bottom currents. In the present Gulf of Mexico, the surface Loop Current is considered to be a major cause of deep circulation. We propose that similar bottom currents and perhaps minor deep tidal currents existed during Pliocene and Pleistocene times. The entire sediment package may be thick and continuous, but individual sand layers within the package are thin and discontinuous. Porosity values of these sands range from 27 to 40%, and permeability ranges from 100 to 2000 md at 200 psi. In seismic profiles or seismic facies maps, it is difficult to distinguish the reworked sands from turbidites. Therefore, geologic models based on core and process sedimentology are the ey to a better understanding of these often misinterpreted complex reservoir facies.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990