CHAPTER 1 THE GEOLOGICAL AND GEOPHYSICAL ENVIRONMENT OF THE OUTER CONTINENTAL SHELF
| Jurisdiction | United States |
(Oct 1998)
THE GEOLOGICAL AND GEOPHYSICAL ENVIRONMENT OF THE OUTER CONTINENTAL SHELF
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Introduction
The last three to four years have seen an increase in industry interest in the Deepwater Gulf of Mexico Basin. Both record leasing activity and rig utilization suggest growing industry confidence in the Deepwater Gulf of Mexico Plays. There are many reasons for this resurgence of growth. Foremost is the belief in the enhanced commerciality of exploration within this high cost environment. Evidence includes the discovery of large volume fields, predicted and established high reservoir performance (Holman and Robertson, 1994; Pfeiffer et al., 1995; Haines, 1996) and the expansion of infrastructure into progressively deeper water depths. In addition, technological gains have been made in both drilling and development. State of the art drillships are now capable of drilling in water depths up to 10,000 feet and can reach total drilling depths in excess of 25,000 feet. New production depth records have been established within the Gulf of Mexico OCS area. In July 1997, Mensa, Shell's subsea development in Mississippi Canyon, set a world water depth record at 5,300 feet. The Gulf's deepest permanent production platform is Ram Powell located in 3,214 feet of water. Moreover, industry has plans to develop several other fields in water depths greater than 3,000 feet within the next two years. There are many different types of development systems in use in the Deepwater Gulf of Mexico. An example of a new platform technology is Oryx/CNG's Neptune SPAR platform in 1,930 feet of water in Viosca Knoll Block 826. These technological gains have not only allowed new frontiers to be considered, but have played a key role in reducing the cost of development as the industry pushes into these deeper water depths. The introduction of the Deepwater Royalty Relief regulations has provided further incentive to Deepwater operators.
In addition to economic attractiveness and technological advances, the shared perception that significant exploration opportunity remains in the Deepwater Gulf of Mexico has driven activity levels. Opportunity has two components; the first being the belief that the proper geologic conditions are present to postulate the existence of significant undrilled hydrocarbon accumulations. Recent evolution of geologic concepts and techniques integral to exploration in the Deepwater province has influenced industry to drill for deeper targets in ever-deeper water depths. Key among these are recent changes in the understanding of salt tectonics and its impact on sediment distribution (see reviews in Jackson et al., 1995; Travis et al., 1995 and Alsop et al., 1996) as well as enhanced geophysical acquisition, processing and interpretation techniques. Opportunity also requires the ability to participate in the leasing and drilling of these targets. Many 10year leases purchased in the mid to late 1980's are approaching expiration. Industry has
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responded with a surge in wildcat drilling as companies attempt to evaluate properties before expiration. Those leases which remain unevaluated and leases in progressively deeper waters have been the focus of the record Deepwater OCS Lease sales from 1996 forward. Moreover, high levels of leasing have prompted the acquisition of large, affordable commercial 3D seismic data surveys. Interpretation of these new seismic surveys has led to even more leasing.
All of these factors have played a role in refocusing major petroleum company domestic exploration on the Deepwater Gulf of Mexico. The purpose of this paper is to provide a non-specialist insight into the exploration process in this challenging environment. The goal is to investigate the collective mind of an exploration staff and understand how basin understanding leads to a play concept and evolves into a drillable prospect. We will discuss the tools and techniques of the exploration process, a process that ultimately results in the development of a drill site. In particular, this paper will emphasize the importance of information and how an explorationist turns information into knowledge that is used to make better decisions for their company.
Figure 1.
The Life Cycle of an Oil and Gas Asset — Scope of Discussion
The overall life cycle of an asset in the upstream oil and gas business has five major stages (Figure 1). These are (1) Acquisition — The leasing and/or purchasing of an opportunity. (2) Exploration — The evaluation and drilling of an untested opportunity. (3) Development — The drilling of additional wells and construction of facilities necessary to produce the discovered hydrocarbons. (4) Production — The production and sale of the hydrocarbons. (5) Disposal — The abandonment, sale or trade of the asset. It is important to note that companies can both acquire and dispose of assets during any phase of the cycle. The focus of this paper will be on the exploration phase. Emphasis will be placed
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on where to target an exploration program, and within this framework, defining the elements that make up a drillable prospect.
Techniques of Analysis — Science, Creativity, Business
Geoscientists, like all scientists, employ a version of the scientific method in their work. The scientific method in its purest form consists of four basic steps. (1) State the Problem — A problem cannot be solved unless it is understood. (2) Form a working hypothesis — This is a possible solution to the problem formed after gathering information about the problem. (3) Test the hypothesis — Something (an experiment) is done to determine if the hypothesis solves the problem. (4) Draw conclusions — After examining the data from the experiment, conclusions can be drawn. In an ideal case, the conclusion will be "yes" the hypothesis was correct or "no" the hypothesis was not correct. If the hypothesis is proven false, then the information gained from analysis is used to develop a new working hypothesis. It is an iterative process of making hypotheses to explain observations, gathering data, and based on these data, drawing conclusions that confirm or deny the original hypothesis.
To the uninitiated it may appear that in oil and gas exploration the problem is "Where is the next commercial accumulation of hydrocarbons going to be found?"; the hypothesis is a drillable prospect and the experiment is the drilling of an exploratory test. In reality, however, the true application of the scientific method in exploration is in defining a series of interim problems that the geoscientist must answer in order to develop a prospect to test. These problems concern the presence of trap, reservoir, source rock and seal, as well as processes such as the generation, migration, accumulation and preservation of hydrocarbons. In other words, the exploration geoscientist devotes the majority of time to determining the probability that key elements and processes necessary to the accumulation of hydrocarbons are present. The ability to answer or gain predictive understanding into these questions is possible because the scientific method relies on "cause and effect" relationships in nature.
In practice, much frontier wildcat exploration is done at a time in the exploration process when there is limited well data (hard results) to be integrated into the analysis. The geoscientist has to rely on analogs from other geologically similar places, personal experience and intuition to develop concepts for testing. In other words, they take the principles of what makes a commercial hydrocarbon accumulation possible and, with limited data, extend those principles into a new area. There is an art involved; a creativity that extends the probable into the possible. This is where the geoscientist earns the name explorationist.
An oil and gas explorationist also needs to combine a well-refined sense of business with creativity and science. The ability to understand and quantify the uncertainty associated with the cost of finding, developing and producing hydrocarbons is the financial key to economically successful oil and gas exploration.
The Exploration Process — Petroleum System Methodology
Integrating the techniques of science, art and business into a usable methodology has long been the goal of exploration departments. Although variations of this methodology have been in use since the early days of oil and gas exploration, perhaps the best documentation of this methodology is provided in American Association of
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Petroleum Geologists Memoir 60 — The Petroleum System — From Source to Trap (Magoon and Dow, 1994a). In addition to reviewing the published record of exploration processes, this volume proposes a systematic methodology for exploration. Many companies currently employ a version of this published process. Understanding this methodology yields insights into geologic and geophysical attributes of prospect definition.
The Petroleum System analysis, as proposed in Memoir 60, is a hierarchical series of logic steps that begin at the mega-regional (basin scale) and proceed into greater and greater detail toward the final goal of delimiting a drill site location. This is schematically illustrated in Figure 2. Each level of the process has defined objectives. Progression from one level to the next is contingent upon meeting the minimum criteria prescribed in the previous level. To describe this further requires defining the levels of investigation.
Figure 2.
Sedimentary Basin Analyses
All investigations begin at the Sedimentary Basin Level. At this level, the scientist analyzes the structural and stratigraphic development of a given basin. The term "basin" can have different meanings for different groups. Even among geologists...
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