Deepwater Reservoir Elements: Levee-overbank Sediments and their Thin Beds
Although levee-overbank areas of deepwater systems consist primarily of muds, thinly bedded sands and sandstones (hereafter termed “thin beds”) also are deposited on those 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 interbed-ding of sand and mud. Many potential reservoirs in the northern deep Gulf of Mexico were discovered in such deposits. As a result, several studies have evaluated whether those 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 eventual result is a wedge-shaped body, with a thick proximal levee and a thinner distal-overbank portion (Figure 5-1). Thin-bedded reservoirs associated with levee-overbank sediments are most prevalent in mixed-mud-sand to mud-dominated systems (Richards and Bowman, 1998; Chapter 1 of this book).
Block diagram of a channel-levee system, illustrating the key subenvironments in