Deepwater-Reservoir Elements: Levee-Overbank Sediments and their Thin Beds
Published:January 01, 2006
2006. "Deepwater-Reservoir Elements: Levee-Overbank Sediments and their Thin Beds", Introduction to the Petroleum Geology of Deepwater Setting, Paul Weimer, Roger M. Slatt, Renaud Bouroullec, Richard Fillon, Henry Pettingill, Matthew Pranter, Gabor Tari
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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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Introduction to the Petroleum Geology of Deepwater Setting
This publication is intended to provide the working geologist, geophysicist, and petroleum engineer with a broad overview of the petroleum systems of deepwater settings. 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 siliciclastic and carbonate reservoir systems. The study of deepwater systems requires many different remote observation techniques, each of which can only provide information on one part of the entire depositional system. As a consequence, the study and understanding of deepwater depositional systems as reservoirs has lagged behind that 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. The types of data used in the study of deepwater deposits include: outcrop studies, 2D and 3D seismic-reflection data (both for shallow and deep resolution), cores, conventional and specialized log suites, biostratigraphy, and well test and production information. These data sets are routinely incorporated into computer reservoir modeling programs for production performance simulation and forecasting. Technologies for deepwater exploration and development are improving rapidly. The intent of this publication is to provide information that will be usable even as the technologies advance beyond what we present here.