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Abstract

Although levee-overbank areas of deepwater systems consist primarily of clays, silts, thinly bedded sands and sandstones (hereafter termed “thin beds”) are also deposited on these areas. Such deposits are composed of thin-bedded, laminated (Bouma Tb) to rippled (Bouma Tc) sands that sometimes have excellent porosity and darcy-range permeability. Thin beds are ideal stratigraphic traps because of their lateral wedging and thin interbedding of sand and mud; in fact, many “low resistivity, low contrast pay” reservoirs in the northern deep Gulf of Mexico were discovered in such deposits. As a result, several studies have evaluated whether these reservoirs are sufficiently large to warrant economic development. In such systems, production rates can be quite high initially, then decline rapidly, and finally persist at lower levels. As stand-alone reservoirs, thin beds may not be sufficiently economic in the northern Gulf of Mexico or worldwide; however, as secondary reservoirs, they can be quite important.

Levee-overbank deposits form as the finer-grained portion of individual sediment gravity flows overtop their banks and spread laterally beyond the channel margin. Through time, the proximal levee receives more sediment than the distal levee because of the rapid reduction in flow velocity as the flow overtops its banks. The end result, after a period of time, is a wedge-shaped body, with a thick proximal levee and a thinner distal overbank portion (Figure 7-1). Thin-bedded reservoirs associated with levee-overbank sediments are most prevalent in mixed-mud-sand to mud-dominated systems (Richards and Bowman, 1998; and Chapter 1).

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