Deepwater Reservoir Elements: Sheet Sands and Sandstones
Sheet sands and sandstones are considered to be some of the best high-rate, high-ultimate-recovery (HRHU) reservoirs in deep water (Chapter 2). This is because of their tendency toward the simplest reservoir geometries: good lateral continuity, potentially good vertical connectivity, high aspect ratio, narrow range in grain size (and thus greater porosity and permeability), and few erosional features. Because of the initial successes with these reservoirs in the northern deep Gulf of Mexico, industry has studied sheet sands and sandstones in great detail to better understand them and hence to find more of them. One problem, however, is that reservoirs initially interpreted as sheet sands were determined later to be amalgamated channel sands.
Sheet sands are deposited from decelerating flows at the termini of channels. Sheet sands and sandstones reflect the sediments that have bypassed through updip channels (confined flow) and are deposited in a primarily unconfined setting. They are characterized by high-aspect-ratio reservoir sand bodies (>500:1), which differ markedly in aspect from the updip channels that feed them (which have aspect ratios of 30:1 to 300:1). Unlike other deepwater reservoir elements, sheet sands commonly have an areal extent that exceeds the area of the trap. Sheet sands and sandstones are most prevalent in mixed-mudsand to mud-dominated systems (Richards and Bowman, 1998). Sheet sands and sandstones are less common in sand-rich to gravel-rich systems (Chapter 1 of this book).
Sheet sands and sandstones are characterized by their tabular external form and their excellent continuity
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.