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
Figures & Tables
This course provides the working geophysicist with a broad overview of the petroleum systems of deepwater settings. The six main elements of petroleum systems will be covered: reservoirs, traps, seals, source rocks, generation, migration, and timing. The course is designed to teach students approximately 80% of what is important. For those interested in further study of a specific topic, each chapter has extensive references for the current literature. About 10% of the current cutting-edge information remains proprietary and cannot be included.
Deepwater depositional systems are the one type of reservoir system that cannot be easily reached, observed, and studied in the modern environment, in contrast to other sili-ciclastic and carbonate reservoir systems. The study of deepwater systems requires many remote-observation systems, each of which can provide only one view of the entire depositional system. As a consequence, the study and understanding of deepwater depositional systems as reservoirs have lagged behind those of the other reservoir systems, whose modern processes are more easily observed and documented.
For this reason, geoscientists use an integrated approach, working in interdisciplinary teams with multiple data types (Figure 1-1). The types of data used in the study of deep-water deposits include detailed outcrop studies, 2D and 3D seismic-reflection data (both for shallow and deep resolution), cores, log suites, and biostratigraphy. These data sets are routinely incorporated into computer reservoir modeling and simulation (Figure 1-1).
The following chapters integrate all of these data types and disciplines to characterize the many facets of deepwater systems. Technologies for deepwater exploration and development are improving rapidly. The intent of the course is to provide information that will be usable even as the technologies advance beyond what we present here.
With that in mind, this chapter introduces basic deepwater terminology and concepts for deepwater systems that will be used throughout this book.
Geoscientists routinely use several terms to describe the sedimentary processes and characteristics of deepwater settings and deposits. For the sake of consistency in this book, we define these terms as follows.
The term deep water is used informally in industry in two ways. First, deep water refers to sediments deposited in water depths considered to be “deep,” i.e., those under gravity-flow processes and located somewhere in the upper- to middle-slope region of a basin. Sediment gravity-flow processes are operative in lakes in relatively shallow water and in cratonic basins where water depths may be less than 300 m. Thus, unless stated otherwise, we use the term deepwater systems to refer to marine-sediment gravity-flow processes, environments, and deposits.