Leslie B. Magoon
U.S. Geological Survey
Menlo Park, Calif.
The reasons for carrying out oil-and-gas-related investigations in a petroleum province are to find where to look for undiscovered commercial quantities of petroleum and to determine the related risk.
To objectively determine risk, what is known should be clearly separated from what is unknown. When the play concept includes both the known and the unknown, however, the explorationist may not always separate these two types of information clearly when discussing risk with management. In contrast, if the petroleum-system concept is used to present only the known, then the play concept can be used to present the unknown. Thus, the play concept used in this manner complements the petroleum-system concept.
The term "risk" has many definitions in exploration, 1-2 but the main objective of its determination is to quantify and specify all the factors that affect the chances for discovering a commercial petroleum accumulation. The term "play" also has many definitions, 2-11 but its underlying objective is to help find undiscovered petroleum accumulations at a profit. The petroleum-system concept of Magoon and Dow12 genetically relates an occurrence of oil and gas to its provenance.
Play and prospect
The play and the prospect are concepts used by the explorationist to present a geologic argument or idea so as to justify drilling for undiscovered commercial petroleum accumulations. Because geologists use these two concepts in many different ways when exploring for petroleum, numerous definitions of the terms "play" and "prospect" exist in the literature. In this report, I define a "play" as one or more geologically related prospects, and a "prospect" as a potential trap that must be evaluated by drilling as to whether it contains commercial quantities of petroleum. A prospect disappears when drilled because it becomes either a dry hole or a producing field.
The way in which a prospect is geologically related to a play, which can include oil and gas fields, is defined by the explorationist. For example, if a prospect is identified near three fields whose oil is trapped in anticlines, it could be argued, using geology, geophysics, and geochemistry, that the prospect is an anticlinal trap charged with oil. In this example, the play would include the three oil fields and the prospect. Because this play is based on trap type, other fields in the same province whose oil is stratigraphically trapped would be exempt from this designation. Other types of plays might require the same stratigraphic interval or a depositional setting, such as a carbonate reef. Depending on the objective of the explorationist, the play concept defined in this manner can have any degree of geologic similarity.
Play concepts defined in this manner use discovered accumulations as a basis for determining the exploration risk for undiscovered accumulations. In some combination, this risk evaluation involves three independent variables: the presence of a petroleum charge, the presence of a suitable trap, and the formation of the trap before the charge was available. These variables are all independent because each has different attributes and can occur in nature without the other two. A petroleum charge is a liquid or gaseous fluid that occurs at any time and in many places other than in traps, for example, as seeps. A trap involves sedimentary rocks that occur anywhere in the geologic column and commonly lack petroleum. Timing determines whether a trap formed before the petroleum charge arrived; these two events can occur in any order or simultaneously.
Each independent variable has equal weight because if any one of them is absent (0), the prospect in the play is a failure; if all three variables are present (1.0), the prospect is a commercial success. Therefore, each independent variable can be evaluated on a scale of 0 to 1.0; for example, the probability of a trap being present may be 0.5. Exploration risk is determined by multiplying together the three independent variables. This exercise can be carried out for the play, as well as for each prospect.
For a more detailed risk evaluation, each independent variable can be subdivided. Thus, "petroleum charge" is subdivided into volume and characteristics of the oil and gas available to the trap, if it exists. "Volume of petroleum" can be further subdivided into richness, quality, and thermal maturity of the source rock, as well as volume of mature or active source rock. Each of these subdivisions is evaluated on a scale of 0 to 1.0 and multiplied together to compute the risk for a petroleum charge. "Trap" can be subdivided into reservoir rock, seal, trapping geometry, and volume formed by the reservoir-seal interface, and further into porosity and permeability of the reservoir rock, and lithology and thickness of the seal. As long as these subdivisions are consistently applied for all three independent variables, the precision and degree of these subdivisions are at the discretion of the explorationist.
Petroleum system
The petroleum system is the naturally occurring hydrocarbon-fluid system in the geosphere. It encompasses a pod of active source rock (provenance) and all related oil and gas and includes all the essential elements and processes needed for oil and gas accumulations to exist.12
The petroleum-system concept infers that by reason of the provenance of genetically related oil and gas accumulations, migration pathways must exist, either now or in the past, connecting the provenance with the accumulations. Using the principles of petroleum geochemistry and geology, this fluid system can be mapped as it evolves over time in the geosphere. In contrast to the play and prospect concepts, economics is unimportant to the petroleum-system concept.
A petroleum-system map includes an outline of the pod of active source rock, subsurface shows, seeps, and discovered accumulations, circumscribed by a line, called the geographic extent, that represents the maximum extent of petroleum migration. A source rock is active when it is generating petroleum, either in the past or in the present. A petroleum-system case study is more involved and includes this map, as well as a cross section, burial-history chart, events chart, and table of accumulations.
One goal of the petroleum geologist is to map the evolution of this natural fluid system, or petroleum system, over time to locate undiscovered hydrocarbons. As our ability to characterize and map a petroleum system improves, exploration risk decreases, and the probability of success increases. A petroleum-system case study provides an objective basis on which to determine exploration risk for a related or complementary play.
Complementary play
The relation of the complementary play to the petroleum system is that in the play the petroleum accumulations are undiscovered and expected to be commercial, whereas in the petroleum system the petroleum occurrences are already discovered but not necessarily commercial.
Traditionally, a play includes commercial accumulations and is predicated without any particular petroleum system in mind; however, when a play is based on a particular petroleum system, it is a "complementary play," as defined here.
Using the petroleum-system events chart, the link between the petroleum system and the complementary play is the three independent variables: trap formation to trap, generation-migration-accumulation to petroleum charge, and time of occurrence of trap formation to generation-migration-accumulation to timing, respectively (Fig. 1). As discussed in the section above entitled "Play and Prospect," these three independent variables are derived for both the petroleum system (discovered accumulations) and the complementary play (undiscovered accumulations). In the same way that a complementary angle is additive, a complementary play is used to add petroleum to a petroleum system.
Operationally, separating the complementary play from the petroleum system in this manner is similar to building an addition onto your house. You conceive the idea of adding two bedrooms, draw up the plans, estimate the cost, and then find the financing and builder. At this point, regardless of how real these bedrooms are in your mind, they are not a reality in fact because they still must be built, similar to the complementary play before it is drilled. When the rooms are actually built, however, they cease to be plans and become part of your house, albeit a larger house.
Similarly, after a prospect in a complementary play is drilled and a new oil field is discovered, it ceases to be a prospect in the complementary play and becomes part of the petroleum system.
By mapping a petroleum system, an explorationist learns new play concepts to add new oil or gas fields to the petroleum system, as expressed in the equation
PStotal=PSpartial+CP1
+CP2+CP3...
where PStotal is a petroleum system with all accumulations discovered, PSpartial is a petroleum system with only some of the accumulations discovered, and CP1 and so on are the complementary plays used to find the remaining undiscovered accumulations in the total petroleum system.
This equation states that the total petroleum system equals the sum of the partial petroleum system plus all complementary plays. Because explorationists will seldom say that all the commercial petroleum has been found, the total petroleum system is only conceptual. This concept has a practical side in that explorationists would likely agree on the geographic, stratigraphic, and temporal extent of a partial petroleum system, but never on those of a total petroleum system.
Exploration risk
Defining a play so that it complements the petroleum-system concept allows statistics to be developed for the petroleum system that characterize the chance of success for a complementary play.
The volume of recoverable oil and gas reported by province or basin in which a complementary play occurs now needs to be reported by petroleum system. Dry-hole ratios determined for petroleum systems, rather than for provinces, are used to define risk, especially if placed within an historical context of wells drilled to find a cumulative volume of petroleum. Field-size distributions, once reported by play, are now reported by petroleum system so that the field size can be interpreted within the geologic context of the complementary play(s).
These statistics provide a yardstick for measuring exploration risk. Even though this yardstick cannot necessarily be applied equally to all complementary plays for a given petroleum system, the statistics are, by definition, genetically related.
A petroleum-system map can be used to evaluate the timing and volume of petroleum charge or to assign risk to a complementary play or prospect by using its position relative to the geographic extent of the system (Fig. 2). Using this figure and stipulating that the complementary play is on the migration path for this petroleum system and that traps are present, a play located within or outside the geographic extent of the system may have, for example, the following levels of risk:
- this complementary play or prospect has the least risk because accumulations surround this trap-chance of success, 75-100%;
- this complementary play has some risk because accumulations are located on three sides -- chance of success, 50-75%;
- this complementary play has more risk because accumulations are located on only one side -- chance of success, 25-50%; or
- this complementary play has the most risk because accumulations are located at some distance from the prospect -- chance of success, 0-25%.
Summary
The complementary play can be the basis for evaluating the exploration risk of finding undiscovered hydrocarbons associated with a particular petroleum system.
First, the petroleum-system case study is completed. As the case study develops, an idea(s) or play(s) that involves this petroleum system will occur to the investigator. This play complements this petroleum system because it could add hydrocarbons, if discovered, to the system. The events chart shows how the complementary play is related to the petroleum system vis-a'-vis three independent variables: trap, petroleum charge, and timing.
By using the petroleum-system concept to present known information and the complementary-play concept to present unknown information, risk can be more objectively determined.
Acknowledgment
The author appreciates the comments and suggestions of the reviewers, K.J. Bird and J.W. Schmoker.
References
- Rose, P.R., Risk behavior in petroleum exploration, in Steinmetz, R., ed., The business of petroleum exploration, AAPG Treatise of Petroleum Geology Handbook of Petroleum Geology, 1992, pp. 95-104.
- White, D.A., Selecting and assessing plays, in Steinmetz, R., ed., The business of petroleum exploration, AAPG Treatise of Petroleum Geology Handbook of Petroleum Geology, 1992, pp. 87-94.
- Bois, C., Petroleum-zone concept and the similarity analysis contribution to resource appraisal, in Haun, J.D., ed., Methods of estimating the volume of undiscovered oil and gas resources, AAPG Studies in Geology 1, 1975, pp. 87-89.
- White, D.A., 1980, Assessing oil and gas plays in facies-cycle wedges, AAPG Bull., Vol. 64, 1980, pp. 1,158-78.
- White, D.A., Oil and gas play maps in exploration and assessment: AAPG Bull., Vol. 72, 1988, pp. 944-949.
- Kingston, D.R., Dishroon, C.P., and Williams, P.A., Hydrocarbon plays and global basin classification, AAPG Bull., Vol. 67, 1983, pp. 2,194-98.
- Dolton, G.L., Bird, K.J., and Crovelli, R.A., Assessment of in-place oil and gas resources, in Bird, K.J., and Magoon, L.B., eds., Petroleum geology of the northern part of the Arctic National Wildlife Refuge, northeastern Alaska, USGS Bull. 1778, 1987, pp. 277-298.
- Bird, K.J., The geologic basis for appraising undiscovered hydrocarbon resources in the National Petroleum Reserve of Alaska by the play-appraisal method, in Gryc, George, ed., Geology of the National Petroleum Reserve in Alaska, USGS Professional Paper 1399, 1988, pp. 81-116.
- Podruski, J.A., Barclay, J.E., Hamblin, A.P., Lee, P.J., Osadetz, K.G., Procter, R.M., and Taylor, G.C., Resource endowment, in Conventional oil resources of western Canada, GSC Paper 87-26, part I, 1988, pp. 7-125.
- Mast, R.F., Dolton, G.L., Crovelli, R.A., Root, D.H., Attanasi, E.D., Martin, P.E., Cooke, L.W., Carpenter, G.B.., Pecora, W.C., and Rose, M.B., Estimates of undiscovered conventional oil and gas resources in the U.S. -- a part of the nation's energy endowment, USGS and U.S. Minerals Management Service, 1989, 44 p.
- USGS National Oil and Gas Resource Assessment Team, 1995 National Assessment of U.S. Oil and Gas Resources, USGS Circular 1118, 1995, 20 p.
- Magoon, L.B., and Dow, W.G. Dow, The Petroleum System, in Magoon, L.B., and Dow, W.G., eds., The petroleum system -- from source to trap, AAPG Memoir 60, 1994, pp. 3-24.
Bibliography
Palmer, A.R., The decade of North American geology 1983 geologic time scale, Geology, Vol. 11, 1983, pp. 503-504.
The Author
Leslie B. Magoon has been a research geologist with the U.S. Geological Survey since 1974. Before that he spent 8 years with Shell Oil Co. as an exploration geologist working the southern California offshore, Rocky Mountains, and Sabah, Malaysia, offshore. At USGS he conducted research on the origin of the petroleum system in Cook Inlet and Alaska North Slope. He was involved in the petroleum evaluation of National Petroleum Reserve in Alaska and Arctic National Wildlife Refuge. His primary research topic the last 10 years has been the petroleum system concept and its application. He has a BS in biology and an MS in geology from the University of Oregon.