Sequence Stratigraphic Setting of Deepwater Systems
Sequence stratigraphy is the study of sedimentary rocks within a temporal framework. The basic premise is that marine sedimentary rocks were deposited in a series of cycles that result from the relative fall and rise of sea level. The depositional pattern can vary widely from basin to basin, depending on variations in sediment supply and basin tectonics.
The fundamental mapping unit of sequence stratigraphy is a depositional sequence. Vail et al. (1977) defined a depositional sequence as “a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities.” Depositional sequences can be mapped using seismic, wireline-log, and outcrop data.
Sequence stratigraphy is distinguished from other paradigms by its interpretation of strata within a chronostratigraphic framework. Chronostratigraphy can be determined using different types of data sets. High-resolution biostratigraphy is essential to dating sequences and determining paleoenvironments. Correlation of bedding (stratal) surfaces defines time lines on the basis of physical stratigraphy. Finally, seismic reflections are generated at stratal boundaries and tend to follow them within the limits of seismic resolution. Thus, seismic reflections, with certain exceptions, represent time lines within the limits of their resolution. This key chronostratigraphic feature makes seismic reflections ideal tools for defining the chronostratigraphy of a sedimentary basin and for correlating strata.
The most controversial aspect of sequence stratigraphy is that sequence boundaries of the same age can be found in basins worldwide; therefore, sequence boundaries are considered to be caused
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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.