Deepwater Reservoir Elements: Channels and their Sedimentary Fill
Deepwater channels have received considerable attention in the petroleum industry during the past decade because of (1) the important discoveries that have been made in several deepwater basins in which reservoir performance was critical to development decisions and strategies (e.g., Campos Basin, Brazil; offshore Angola; the Nile; the Mahakam Delta; the northern Gulf of Mexico; West of Shetland Islands; and offshore mid-Norway); (2) the ability of 3D seismic to increasingly image the complex internal geometries of channel systems (especially those that are sinuous); and (3) the need to avoid shallow flow problems while drilling channel-fill sediments.
Many slope systems in passive margins are very muddy. The importance of channels as sand conduits for bypass to the basin floor probably was not fully appreciated until about 15 years ago, when large volumes of sand were recognized to occur downdip of muddy slope systems (e.g., Angola and the northern Gulf of Mexico). Many slope channels are marked by evidence of sediment bypass (coarse-grained lags, traction deposits, hetero-lithic deposits of fine-grained tails, and fine-grained levees, in some instances).
Channels and their fills have been studied for many years, from different perspectives and using multiple data sets, including data from the modern seafloor, from the shallow subsurface (shallow seismic for shallow-hazards drilling surveys), from deeper-exploration seismic, from reservoirs, and from outcrops. In this introductory section, we present a few general, qualitative concepts that are widely accepted concerning channels and their fills.
Because of the large number of published studies, and because most channel fills
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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.