Bill Drennen

This story is expanded from the print version.

Fifty years ago an offshore development in 15 meters of water was considered deep and a grand challenge. Explorers looking to go offshore simply extended land plays into "deepwater settings." Twenty years later, industry had started producing in 100 meters of water and explorers were drilling in nearly 600 meters. Industry now is pursuing opportunities in the Gulf of Mexico in water depths reaching 3,000 meters and extending more than 400 kilometers from shore.

Advancements like these in our industry would not have been possible without profound developments in our understanding of the physical processes of sand transport and deposition, drastic improvements in the quality of seismic data, and major leaps in technologies for deepwater drilling and production.
The keys to success have included concomitant advances in geoscience concepts and technologies and the integration of these to investigate and accurately model Earth's crust from the global to the molecular scale.

Linking concepts and technology

Sequence stratigraphy, one of the most powerful geoscience concepts, provides a framework for defining the source, burial history and reservoir components of a hydrocarbon system from the basin scale down to the reservoir scale. The chronostratigraphy concepts of the 1950s evolved through seismic stratigraphy into sequence stratigraphy, in large part from the contributions of ExxonMobil geoscientists Peter Vail, Robert Mitchum, John VanWagoner and others. These concepts are the fundamental framework for characterizing reservoirs today.

The great strides achieved in sequence stratigraphy were rivaled by advancements in technologies for acquiring and processing seismic data. Since the 1960s, dynamite was succeeded by the sleeve exploder, the open-bottom gas gun, air guns and, most recently, marine vibrators as the source for seismic energy. Parallel increases from four-fold to 96-fold seismic data acquisition yielded tremendous improvements in telling the signal from the to noise. The 3-D seismic survey, invented by ExxonMobil in the 1960s, is now the most widely used tool for subsurface interpretation.

The corresponding growth in the volumes of seismic data has been triggered, in part, because high-performance computers, faster algorithms and streamlined software have raised the practical limitations of processing these large volumes. Powerful graphics computers and software adapted from the medical industry are helping geoscientists to quickly and accurately visualize, model and interpret the 3-D subsurface with resolutions that would have been unbelievable just a decade ago. State-of-the-art visualization technology (Kay and Russell, ((Computerworld)), April 1, 2002) offers geoscientists a powerful tool for understanding a resource and optimizing plans for developing a field.

Start global

Success, particularly offshore, requires a holistic approach to synthesizing information and applying technology from the global scale down to the regional, basin, prospect and molecular scales. Satellite imaging capabilities have given geoscientists a truly global view of the tectonic features and basins of the offshore. Coupling technology with the concept of plate tectonics, the geoscientist can unravel the tectonostratigraphic evolution of a region and understand the interplay of tectonics, structure, the space available for sediment accumulation and the sediment itself as this interplay determines an area's petroleum potential. Plate tectonics provides the conceptual framework for extracting the knowledge gained in one setting, and applying it to similar tectonic settings. What we learn from offshore Brazil can be applied to exploring the conjugate margins of West Africa and contribute to our understanding of the other passive margin settings.

The hydrocarbon system

One of the most important developments in our understanding of petroleum geoscience is the concept of the hydrocarbon system. Accurate prediction of hydrocarbon distribution, type and quality is critical to success in the offshore (Isaksen and Ledje, AAPG Bulletin v. 85, n. 5). Geoscientists have greatly improved our understanding of hydrocarbon systems, including source rock facies, kinetics, migration mechanisms and pathways, the nature of traps, and the evolution of hydrocarbon composition in response to the changing physical and chemical conditions over geologic time (Symington et al, Journal of the Geological Society, Geological Society of London, v. 141, p.169; Schneider and Wolf, Marine Petroleum Geology, v.17, n. 841).

Technical advances along a variety of fronts made this possible. These include, remote sensing, seismic imaging, analytical chemistry, computer modeling, surveying marine petroleum seeps, and developing technologies for direct detection of hydrocarbons. Analytical techniques have improved to the point that geochemistry of individual organic fossil molecules can be used for oil-oil and oil-source correlations can you provide an explanation of these correlations?. And we can use this geochemical information to determine the degree of thermal maturity for oil and natural gas (Peters and Fowler, Organic Geochem. v. 33, p. 5). In addition, recent advances in analyzing fluid inclusions (Parnell et al, Marine Petroleum Geology, v. 18, p. 535) are helping to identify migration pathways and predict oil quality.

But predicting the presence of hydrocarbon accumulations or even locating a giant field in the deepwater does not ensure its economic viability. The challenge is to accurately predict hydrocarbon type and quality as well as quantity ahead of the drill. Such foresight This requires the using geochemical information, modeling source and reservoir facies, and applying sophisticated basin modeling and visualization software to develop quantitative, 3-D models of the hydrocarbon system (Clendenen et al., GeoArabia v. 7, p. 220; Abu-ali et al., GeoArabia v. 7, p 190).

Reservoir prediction

The pursuit of deepwater resources has increased the demand on geoscientists to provide extremely precise, quantitative predictions about reservoir occurrence, distribution, and continuity and performance potential. Successful acquisition -- exploration and development of deepwater acreage, where wells are costly and the number of wells required determines a field's economic viability -- requires a solid understanding of the controls on reservoir size, continuity and quality, not to mention petroleum composition and mobility. New models of diagenetic processes that explain porosity preservation -- even under deep burial or high pressure and high temperature conditions (Bloch et al., AAPG Bulletin, V. 86, p. 301) -- expand our view of reservoir potential.

New concepts based on integrating outcrop, log and core studies; physical models; and detailed, seismic-based sequence stratigraphic interpretations are rapidly transforming our understanding of deepwater depositional processes and reservoirs (Beaubouef et al., AAPG Cont. Ed. Notes, v.40). Continuous, high-quality, 3-D seismic coverage and high-resolution side-scan sonar images (Gee et al., Sedimentology v. 48, p. 1389) allow us to see a deepwater system from the shelf to the basin floor. The simple submarine fan models developed during the 1970s and 1980s, based on outcrop studies and 2-D seismic data, are giving way to a spectrum of depositional models that capture the range of reservoir styles and provide a basis for predicting reservoir quality (Lopez, Marine Petroleum Geology, v. 18, p. 479). In addition, physical and numerical modeling of sediment-gravity flows -- sediment and water moving under gravity's influence -- are enhancing our understanding of transport and depositional processes. Two types of sediment-gravity flows, debris flows and turbidity currents, are now recognized to be the extreme forms of a continuum of gravity flow types that rely on differing mechanisms, to move sediment downhill. (Sprague et al., AAPG annual meeting, 2002; Johnson et al., Sedimentology v. 48, p. 987). These differences control the patterns of erosion and deposition associated within each flow type as well as the porosity, permeability and reservoir style of the resulting deposits. Armed with this new understanding of marine depositional systems, geoscientists can predict the reservoir distribution patterns within frontier basins. They can also better estimate a reservoir's potential; more precisely predict net sand, reservoir thickness and facies for placing exploration wells; and provide detailed input to geologic models.

Geologic modeling is the primary integrator of all geologic, geophysical, and interpreted or conceptual information about the distribution of a resource and the performance properties of a reservoir. It is used to provide detailed numerical characterizations of reservoir-scale attributes that govern fluid flow: such as reservoir geometry, continuity and connectivity, and rock and fluid properties.

A similar model today can be completed in days on sophisticated workstations that have multiple systems for interpolation and 3-D graphics capabilities. Can you provide a transition sentence here that explains what the model today is similar to? The industry continues to push the limits of this technology. Increasingly more realistic models of more complex reservoirs are being generated because the capabilities to integrate seismically derived information, including predictions about a rock's properties, into object-based geologic models continue to improve. Geologic modeling is now used in all stages of a field's life cycle from pre-discovery for evaluating the impact of geologic uncertainty on a reservoir's performance, to designing a flexible plan for developing a new field, to quantifying depletion for updating strategies and surveillance.

Big promise, big challenges

The prize in the offshore is large. Scientists predict that more than 100 billion barrels of oil remain to be discovered in the deepwater. However, the high costs of exploration wells and deepwater development projects present new challenges for the geoscientist and reservoir engineer.To further reduce uncertainty to acceptable levels for making billion-dollar investment decisions -- often based on one or possibly two exploration and appraisal wells in a field -- geoscientists and engineers will need to work together, from exploration through production, using fully integrated information and 3-D models. This integration will require highly competent, creative people generating new concepts and breakthrough ideas, and applying new and innovative technologies. For companies to be selective, successful and profitable in the deepwater, the accelerated pace and magnitude of the technological revolution observed over the past 10 years must continue.

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William T. Drennen III is vice president for the North America region of the ExxonMobil Exploration Company. Email

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