Petroleum geology

Geologists seek to understand the geologic features with which oil and gas are associated in order to make exploration for these minerals less risky and more economical.

Existence of Traps

As the search for oil and gas begins, the exploration geologist identifies promising areas called prospects within a geologic region. A prospect is a potential oil or gas field. A prospect is not merely the extension of an existing field, as such work is classified as development geology. Rather, a prospect would be a potential new field some distance away from preexisting production. In order to locate prospects, a model of what such potential fields will look like needs to be developed. This model will be based on already discovered fields with well-known characteristics, although it is recognized that there will be differences between the two, and this knowledge will serve as a starting point for exploration. Despite differences between prospects and models, all prospects have certain criteria that must be met to demonstrate that a prospect is viable or, in other words, drillable. If one of the criteria cannot be met, the prospect may be regarded as too risky to drill.

The first criterion that an exploration geologist must satisfy is proving the existence of a trap. Proving that a trap is present involves the use of several tools. First, the exploration geologist must refer to the model. Once the geologist knows which type of trap to look for, the specific tool is selected. The tool used to locate a trap is a map constructed on the basis of subsurface data. The type of map constructed will depend on the type of trap present in the model. If the trap is a structural trap—that is, created by the deformation of rocks by earth movements—a subsurface structure map showing folds and faults will be constructed, similar to a topographic map. The contours on this map connect points of equal elevation relative to sea level, except that they are in the subsurface rather than on the surface. When studied in the subsurface, hills on a structural contour map represent anticlines or domes, valleys represent synclines or depressions, and sharp cliffs usually indicate faults. If the trap in the model is of a stratigraphic type—that is, formed by features intrinsic to the rock layers themselves—a stratigraphic map showing thickness of a rock unit or bed, differing rock types (facies), or ancient depositional environments is developed. The data to construct such maps are generally derived from subsurface information about the rock units in question from previously drilled wells in the vicinity. In addition to well information, data from seismic, gravity, and magnetic surveys may add support to a prospect. Surface geologic data are considered but may not accurately reflect the geologic conditions of the subsurface.

Qualities of a Reservoir

Once all the subsurface data have been analyzed and a potential trap located, a detailed examination of the rocks present in the prospect area is needed. Specifically, it must be demonstrated that a rock unit or bed within the trap can function as a reservoir for the oil or gas. The qualities that enable a rock to be a reservoir are porosity; pore spaces between grains, crystals, or open fractures; and permeability—the interconnection of pore spaces or fractures that will allow the flow of fluid (in this case, oil or gas). Demonstrating that these properties exist is usually done through a detailed study of the rocks by using special cylinder-shaped rock samples called cores collected from previously drilled wells. This core analysis is a very important aspect of petroleum geology and is the best way to describe and define reservoirs. Geophysical logs that measure certain physical properties of the rocks are also very valuable in defining a reservoir. These are collected by lowering instruments into the well and recording the rock properties along the way.

Presence of Source Bed and Seal

The next issue is demonstrating that a source bed for oil or gas exists in the prospect area. This feature is perhaps the most critical, as oil and gas are generated only under very specific chemical and physical conditions. A rock unit or bed must be located in the rocks of the prospect that have the proper chemical consistency and have been subjected to the proper temperatures needed to generate oil or gas. There also must be a pathway or mechanism to allow the oil or gas to migrate from the source bed to the reservoir, as the two will only rarely be the same unit. What is generally needed is a total organic carbon content of a bed greater than 1 percent and a temperature history that allows the organic matter to mature into hydrocarbons such as oil or natural gas. The rock type generally involved will be shale, but some fine-grained limestones and dolomites, as well as some cherts, will function as source beds. A pathway of migration for the generated oil and gas must be available. This is not a critical problem if the source bed occurs beneath the reservoir bed because of the tendency toward upward migration of oil and gas; their low density allows them to float upward on groundwater. If the source bed occurs above the reservoir, it can still function for the prospect if it has been dropped down by a fault below the reservoir unit. Direct downward migration of oil and gas is known to occur but is uncommon, and this process would have to be demonstrated as functioning in the prospect area if it were to explain the source for a prospect.

If these criteria are met, the next factor to be considered is the presence of a seal. As the tendency is for oil and gas to migrate upward, an impermeable rock unit or bed must be present above the reservoir bed to prevent the oil and gas from flowing through the reservoir unit rather than accumulating in the trap. If oil and gas cannot migrate upward, they will migrate laterally to the highest point (trap) in the last permeable bed (reservoir) they can enter. Seals can be of any impermeable rock type, but common examples are evaporites (rock salt and gypsum) and shales. Fine-grained limestones and dolomites, as well as some igneous and metamorphic rocks, can also function as seals.

Final Criterion

The final geological criterion that must be satisfied is the time of oil and gas generation and migration, and its relationship to the time of trap formation. In order to determine when these events occurred, a detailed understanding of the geologic history must be attained. In general, the trap must have been formed before the generation and migration of oil and gas in the region. If this is not the case, the trap in question will not likely contain oil and gas. Many excellent traps exist that have been drilled and do not produce because of the timing difference.

If any dry holes (previously drilled wells without production) are near a prospect, the reason for their lack of production must be explained. Generally, this will be done by showing that the wells were drilled away from the prospect in question or that technical or economic problems were encountered. Geologists do not want a prospect with a dry hole in the middle unless that well was dry for a reason that the present model can explain.

Obtaining a Lease

If all of the preceding criteria have been met, the geologist may have a prospect, provided that a lease can be obtained. An oil and gas lease gives the holder the right to explore and produce oil and gas on the land in question. These are the only rights granted to the lease holder. This lease is usually for a specified term (five or ten years) and pays the landowner an annual rental fee plus a royalty interest in any production. If production is achieved on the lease, the term will not expire until production stops. Production cannot be halted to wait for better economic conditions, such as higher prices, as this would endanger the lease. The availability of acreage is a nongeologic factor in the exploration for oil and gas but a critical one nonetheless. A prospect without the potential of acquiring a lease is of little use, as it cannot be drilled and developed.

A Process of Ideas

The process of oil and gas exploration is a difficult one because of the large degree of uncertainty involved. Advances such as high-resolution seismic profiling, computer analysis and management of well data, and the use of satellite photographs of the land surface have all improved the way geologists explore oil and gas deposits. Despite the benefits of improved technology, the exploration for oil and gas remains a process of ideas rather than equipment. This does not mean that old, established ideas will dominate the search for oil and gas. In fact, quite the opposite is true. Ideas can and do develop at a much faster rate than technology. Because of this, the potential for change in an approach is always present. Thus, a region that might be considered to have no oil and gas potential may appear differently to geologists who have a different or new idea about the geology of the area. Some new ideas affect all regions of the earth and tend to form a revolution of approaches in the oil and gas industry. Ideas over the last three decades have tended to view geology from a global perspective, and this has changed the exploration for oil and gas. The most important idea of this type is the theory of plate tectonics. This proposed mechanism resulted in a widespread reevaluation of the petroleum potentials of regions, resulting in increased discoveries of new fields.

The effect that the exploration of oil and gas has on society is a very clear one. Oil and gas are needed by almost everyone as sources of relatively inexpensive, clean, portable, and efficient energy. The present structure of the oil industry, including oil exploration and production, is sometimes criticized as being wasteful and inefficient. Even with the best methods, a large proportion of exploratory holes are dry, a fact of life inherent in the petroleum industry. In some nations, oil and gas exploration has been nationalized and is handled by a team of geologists in order to promote efficiency by eliminating competition for the same prospective new fields. This has been suggested as a model for the oil industry in the United States as a way to increase the known petroleum reserves.

Because the competitive system, in which many geologists develop varying working models of an area, is eliminated by a nationalized approach, only one model is present, and the chances of ever finding oil are greatly reduced. While many nationalized companies are very successful, their success stories often are in areas that simply had not been explored before, and the oil accumulations were very obvious. As these regions mature and oil and gas become harder to find, multiple ideas are needed to locate new fields. A competitive system ensures that as long as a profit can be made, exploration for oil and gas will continue.

Principal Terms

dry hole: a well drilled for oil or gas that had no production

lease: a permit to explore for oil and gas on specified land

permeability: a property of rocks where porosity is interconnected, permitting fluid flow through the rocks

porosity: a property of rocks where empty or void spaces are contained within the rock between grains or crystals or within fractures

prospect: a limited geographic area identified as having all the characteristics of an oil or gas field but without a history of production

regional geology: a study of the geologic characteristics of a geographic area

reservoir: a specific rock unit or bed that has porosity and permeability

seal: a rock unit or bed that is impermeable and inhibits upward movement of oil or gas from the reservoir

source rock: a rock unit or bed that contains sufficient organic carbon and has the proper thermal history to generate oil or gas

trap: a structure in the rocks that will allow petroleum or gas to accumulate rather than flow through the area

Bibliography

American Petroleum Institute. Primer of Oil and Gas Production. 3d ed. Washington, D.C.: Author, 1976. A basic introduction to the procedures and techniques of oil and gas production. Includes chapters on the origin and accumulation of oil and gas, and the properties of reservoirs. Written as a beginner’s text in nontechnical language with helpful illustrations.

Baker, Ron. A Primer of Oil Well Drilling. 6th ed. Austin: University of Texas at Austin, 2000. An introduction to the procedures involved in oil and gas well drilling. Includes chapters on exploration and on oil and gas accumulations. Well illustrated with many on-site photographs. Suitable for high school or college students.

Gluyas, Jon, and Richard Swarbrick. Petroleum Geoscience. Malden, Mass.: Blackwell Science, 2004. Describes the tools and methods used in petroleum exploration. Includes limited images, drawings, and tables, as well as extensive references and further reading lists and an index. Appropriate for graduate students, academics, and professionals.

Hyne, Norman J. Dictionary of Petroleum Exploration, Drilling, and Production. Tulsa, Okla.: PennWell, 1991. Covers all terms associated with petroleum and the petroleum industry. Appropriate for beginners in the field. Illustrations and maps.

‗‗‗‗‗‗‗‗‗‗. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production. 2d ed. Tulsa, Okla.: PennWell, Corporation, 2001. Provides a well-rounded overview of the processes and principles of gas and oil drilling. Covers foundational material. Discusses exploration, refining, and distribution. Appropriate for professionals in the oil-drilling field, geologists, or students of similar fields. Illustrations, bibliography, and index.

LeRoy, L. W., and D. O. LeRoy, eds. Subsurface Geology. 4th ed. Golden: Colorado School of Mines, 1977. An important reference for all who are interested in the geology of the subsurface. Includes chapters on petroleum geology, but exploration for oil and gas is not the only focus of this book. Chapters on oil and gas are written by experts in the field, very complete, and well illustrated. A basic reference for the geology of oil and gas by oil-business professionals.

Levorsen, A. I. Geology of Petroleum. 2d ed. Tulsa, Okla.: American Associations of Petroleum Geologists, 2001. A full textbook on petroleum geology designed for undergraduates who have taken some basic geology courses. A reference for people with an interest in some of the more detailed aspects of petroleum exploration. The training textbook for many present-day exploration geologists. Includes a bibliography.

Li, Guoyu. World Atlas of Oil and Gas Basins. Hoboken, N.J.: John Wiley & Sons, 2011. Includes an overview section followed by information on oil and gas distribution throughout the countries of the world organized by continent or region. Covers each area briefly. Appropriate for undergraduates and oil professionals.

Owen, E. W. Trek of the Oil Finders: A History of Exploration for Petroleum. Tulsa, Okla.: American Association of Petroleum Geologists, 1975. A detailed history of the development of the petroleum industry throughout the world. Written from a historical perspective, in technical language only when necessary. Well indexed by subject, geographic location, and proper names. Chapters are organized by region.

Raymond, Martin S., and William L Leffler. Oil and Gas Production in Nontechnical Language. Tulsa, Okla.: PennWell Corporation, 2005. Provides a good overview of the industry. Includes pictures, charts, graphs, and drawings. Describes oil exploration, and how oil and gas are found and extracted. Written in a manner accessible to the layperson.

Selley, Richard C. Elements of Petroleum Geology. 2d ed. San Diego: Academic Press, 1998. Covers the specifics of oil and gas and their relationship to geology. Designed as a college textbook for students near the end of their coursework in geology and for geologists beginning careers in the petroleum industry. Requires a basic understanding of geological concepts. Contains subject and name indexes, useful illustrations, and appendices that include a well classification table, a glossary of oil terms and abbreviations, and a table of conversion factors.

West Texas Geological Society. Geological Examples in West Texas and South-eastern New Mexico (the Permian Basin) Basic to the Proposed National Energy Act. Midland: West Texas Geological Society, 1979. Designed to inform state and federal government members of the procedures and costs of exploration for oil and gas. Based on case histories and maps; diagrams and actual costs are included. A valuable reference, written in clear, nontechnical language.