Petroleum Traps in Deepwater Settings
A trap is any geometric arrangement of rock—regardless of its origin—that permits a significant subsurface accumulation of oil or gas or both (Biddle and Wielchowsky, 1994). Deepwater reservoirs produce petroleum from a wide variety of traps. As we noted in Chapter 2, about 25% of the giant deepwater fields produce from structural traps that have four-way closure, approximately 9% produce from purely stratigraphic traps, and most deepwater fields (66%) produce from combined structural-stratigraphic traps (Figure 9-1).
Many deepwater settings are characterized by syndepositional tectonics, such that the creation of structural traps is linked inextricably with the evolution of the other elements of petroleum systems. The lapout or truncation of the reservoir elements against the flank of contemporaneously active structures helps create many of the combined structural-strati-graphic traps (Figure 9-2). In addition, the continued deformation in many settings constantly changes other local elements of the petroleum systems, including the local and regional pressure systems and migration pathways.
The subject of traps in deepwater settings is extremely broad, because most deep-water sedimentary basins have multiple trapping styles. In this chapter, we begin by looking at four types of deepwater settings: (1) basins with mobile substrates (salt or shale), (2) basins with nonmobile substrates (i.e., basement blocks, wrench tectonics), (3) unconfined basins, and (4) shallow to continental reservoirs that now rest in deep water (Figure 9-3; see also Chapter 2 of this book) (Worrall et al., 2001). Within each
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.