E-Book, Englisch, Band Volume 9, 544 Seiten
Origin, Recognition, Initiation, and Reservoir Quality
E-Book, Englisch, Band Volume 9, 544 Seiten
Reihe: Handbook of Petroleum Exploration and Production
ISBN: 978-0-444-56355-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark
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Chapter 1 Introduction Dr. G. Shanmugam Deep-water sandstone reservoirs have continued to be a major economic asset to the petroleum industry in the twenty-first century. In the deep-water Gulf of Mexico, for example, exploration efforts in 2008 alone have resulted in 15 new deep-water discoveries (Nixon et al., 2009). Exploratory drilling has found more than 6.6 billion barrels of oil equivalent (BBOE) since 2002, more than double the amount reported in 2005. On the production side, several large deep-water projects have transitioned from an exploratory and appraisal phase into production phase. At present, there are nearly 4,000 active oil and gas platforms (Figure 1.1). Since 1996, 17 deep-water discoveries have been made in the Lower Tertiary Trend in the northern Gulf of Mexico (Table 1.1). The significance of these 17 discoveries is that they not only occur in the present-day water depths of greater than 5,000 feet (1,524 m), but also comprise major reservoirs that were deposited in ancient deep-water settings, such as the BAHA discovery (Meyer et al., 2007). FIGURE 1.1 Map showing locations of nearly 4,000 active oil and gas platforms in the northern Gulf of Mexico. Image credit: National Oceanic and Atmospheric Administration, U.S. Department of Commerce. http://oceanexplorer.noaa.gov/explorations/06mexico/background/oil/media/platform_600.html. Accessed March 27, 2011. TABLE 1.1 List of 17 Discoveries in Present-Day Water Depths Greater than 5,000 ft (1,524 m) in the Northern Gulf of Mexico. Petroleum Reservoirs in these Discoveries are Associated with the Lower Tertiary Trend. Despite the enormous economic importance of deep-water reservoir sandstones worldwide, their origin is still mangled in controversies and confusion. I have made an attempt to untangle this sedimentological mess in an earlier volume (Shanmugam, 2006a). In continuing that effort, the primary purpose of this volume is to provide the much-needed clarity by explaining the inherent problems with the prevailing practice of interpreting deep-water sands as “turbidites” and by providing alternative options through systematic documentation of processes in modern oceans and their deposits in the geologic record using underwater photographs, side-scan sonar images, velocity measurements, conventional core, and outcrop data. 1.1 WHAT IS DEEP WATER?
Controversies still abound surrounding the interpretation of ancient sandstone as deep water in origin (Mulder et al., 2009a; Higgs, 2010; Mulder et al., 2010). The term “deep water” is used with different meanings by geoscientists and by drilling engineers in the petroleum industry. For example, geologists use the term to denote deepwater depositional origin of the subsurface reservoir, whereas drilling engineers use the term to denote the present-day drilling depths for the target reservoir, irrespective of its depositional origin (see “Deep water” in Appendix A). In general, the term “deep water” refers to areas in bathyal water depths (>200 m), which occur seaward of the continental shelf break, on the continental slope, continental rise, and abyssal plain environments. However, there is no consensus on the precise water depth that defines deep water. Pickering et al. (1989) use the term “deep water” to denote environments that occur exclusively below storm wave base. The depth of “storm wave base” is not a constant value, and it varies with the wind velocity of tropical cyclones. The maximum sustained wind velocity of cyclones changes from 61 km h-1 for a “tropical depression” to more than 249 km h-1 for a “Category 5 hurricane” in the Saffir–Simpson Scale (see “Tropical Cyclone” in Appendix A). Typically, storm wave base occurs in water depths ranging from 20 to 30 m on the continental shelf during low-intensity cyclones. However, the storm wave base may reach the shelf break and beyond (>200 m) during high-intensity cyclones, resulting in sediment transport beyond the shelf edge (Chapter 5). Therefore, the storm wave base, which varies between 20 m and >200 m, is not an objective criterion. Plus, the validity of hummocky cross-stratification, which is commonly used as a criterion for establishing storm wave base, is in dispute (Mulder et al., 2009a; Higgs, 2010). In the Gulf of Mexico, the threshold that separates shallow water from deep water ranges from 200 to 457 m (Richardson et al., 2004). The U.S. Department of the Interior uses the terms “deep water” and “ultra-deep water” for water depths greater than or equal to 1,000 feet (305 m) and 5,000 feet (1,524 m), respectively (Nixon et al., 2009). Gore (1992) considers continental shelf to occupy in water depths less than 180 m. On the continental margin off northwestern Africa, the shelf break is found invariably at 100–110 m (Seibold and Hinz, 1974). Hesse and Schacht (2011) prefer a water depth of 500 m for defining deep water in order to exclude deposition on the upper slope during periods of sea-level lowstand. In lacustrine basins that may not have well-developed shelf breaks, the defining criterion of deep water is problematic. In an attempt to resolve these basic issues, I suggest the following guidelines. 1. Because shelf edge plays a critical role on continental margin sedimentation (Stanley and Moore, 1983), shelf edge is used here as the defining criterion of deep-water settings in modern oceans. Examples are the Gulf of Mexico (Figure 1.2) and the U.S. Atlantic margin (Figure 1.3, see color plate), among others. However, the seafloor topography (Figure 1.3, see color plate) and the corresponding variation in water depth are extremely complex and unpredictable (Figure 1.4). Shelf edge is a useful physiographic conceptual boundary in separating shallow water from deep-water settings. This is because shallow-water shelf setting is dominated by wave and tidal processes, whereas deep-water slope setting is dominated by gravity-driven processes. Although absolute water depths are difficult to interpret in older strata, shelf facies and slope facies may be distinguished with reasonable certainty. 2. The shelf-edge criterion has its limitations. First, in areas such as the Indonesian seas, where a complex array of passages linking shallow and deep seas are complicated by the Indonesian throughflow (ITF) and dissipation of tidal energy (Gordon et al., 2010), distinguishing shelf facies from slope facies in the ancient counterpart could be a challenge (Section 4.8.5). Second, shelf-edge criterion is obsolete in submarine canyon settings because the shelf-slope break does not exist within submarine canyons. Canyons serve as a single environmental entity with increasing water depths from estuary to canyon (e.g., Zaire Canyon and West Africa, Section 4.7.1). In such cases, shelf facies and slope facies are absent. However, outside of the canyon, the shelf-slope break is an important physiographic boundary between the two major submarine provinces, namely shelf and slope (Vanney and Stanley, 1983). Most canyon-fill deposits are composed of mass-transport deposits (MTDs) and tidalites, mimicking deepwater and shallow-water facies, respectively (Shanmugam, 2003). 3. In the modern Bering Sea (Figure 1.5, see color plate), the shelf-slope break straddles between 150 m and 175 m (Carlson and Karl, 1988). The importance of the Beringian margin in this discussion is that it contains the world's largest submarine canyon with a measured volume of 5,800 km3 of material removed from the shelf and slope (Carlson and Karl, 1988). The 1,400-km-long Beringian continental slope extends from the Aleutian Islands in the south to the Siberian margin in the north (Figure 1.5). The steep Beringian slope, with an average gradient of about 5°, separates the shallow (<150 m in water depth) Bering Shelf to the east and the deep (>3,600 m in water depth) Aleutian Basin to the west. The complicating factor here is the presence of earthquake-prone Aleutian Trench that to the south (Figure 1.6). In such a complex setting, both deep-water and shallow-water facies are likely to be severely deformed and be subjected to sand injection (Section 6.8). 4. Conventionally, deep-water facies have been distinguished using physical, biological, and chemical parameters in the rock record (Rich, 1950; Krumbein and Sloss, 1963; Benedict and Walker, 1978; Shanmugam and Benedict, 1983). Stratigraphic position (Krumbein and Sloss, 1963; Shanmugam, 1978; Shanmugam and Walker, 1978) and facies association (Reading, 2001) are commonly used for inferring ancient depositional environments. Although various isotopes are useful for reconstructing paleoclimates and paleo-ocean circulations, they do not reveal information on water depths. 5. Because modern deep-water slope environments in both the Gulf of Mexico (McAdoo et al., 2000) and in the U.S. Atlantic margin (Twichell et al., 2009), among others, are characterized by MTDs, the dominance of subaqueous MTDs in the rock record could be used as a criterion for interpreting ancient deep-water marine...
