Trap classification

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Traps, trap types, and the petroleum system
Chapter	Classification of exploration traps
Author	Richard R. Vincelette, Edward A. Beaumont, Norman H. Foster
Link	Web page
Store	AAPG Store

Traps are the product of the interaction of many geologic elements and processes. The outcome of all the possible combinations of geologic elements makes each trap unique. Yet each trap generally shares enough similarities with other traps in the same basin or in other basins that traps may be classified. The classification chosen depends on one’s purpose. The ultimate purpose of the trap classification presented in this chapter is to facilitate the discovery of oil and gas accumulations. This chapter discusses the philosophy of classification, shows how to classify traps in a scientifically rigorous and systematic way, and presents a classification scheme for traps found to date. The classification scheme is designed to be flexible and therefore will evolve as new trap types are found and trapping concepts change.

Classification philosophy

Most petroleum geologists classify traps according to the scheme proposed by Levorsen. Levorsen’s scheme breaks traps into three basic types: structural, stratigraphic, and combination. The trap classification scheme proposed here uses Levorsen’s scheme as a foundation and adds new trap types discovered since 1954. The proposed scheme attempts to formalize the schemes of Levorsen and others (Rittenhouse, North, Melton and Bertram, and Biddle and Weilchowsky) by developing a more systematic and rigorous approach. It uses elements critical to petroleum exploration to group traps into levels. The method is similar to the one used by biologists to classify plants and animals.

Classification flexibility

The common occurrence of combination traps, which involve many different types and varieties of trapping elements, requires a scheme that allows for such variations. Consequently, a classification scheme such as that used to organize a stamp or coin collection might be more useful, especially one in which a variety of flexible methods of organization can be used.

For example, stamps can be organized or classified in many different ways: stamps from one country, stamps from all countries for a certain period, stamps from different countries with similar themes or colors, etc. Depending upon the needs of the stamp collector, the ability to search through a stamp classification scheme and pick out whatever combination of stamps is desirable would be a powerful research tool. In a similar manner, a flexible trap classification scheme should allow for different methods of classifying and cataloging hydrocarbon traps, depending upon the needs of the investigator. The intent is that the proposed classification scheme allows for such flexibility.

Classification levels

The proposed classification scheme places traps into four ranked levels, from general to specific:

1. System

2. Regime

3. Class (Superclass if necessary)

a. Subclass

b. Style (if necessary)

4. Family (Superfamily if necessary)

a. Subfamily

b. Variety (if necessary)

Basis for each level

Most of the levels and sublevels (outlined above) are necessary to adequately describe, in the classification scheme, all of the different elements that characterize a trap. Each level has its own unique basis for classification. Trap systems are based on the controlling geologic elements that created the traps, trap regimes are based on the geologic processes that caused the traps in each system, traps classes are based on the geometry and composition of the traps within the trap regimes, and trap families are based on the genesis or origin of the traps within the trap classes.

Workflow

Classifying traps is interpretive. As more data become available, the trap classification can change or be modified. Different explorationists may classify a particular trap in completely different categories, depending on their particular viewpoints.

Traps can be classified formally or informally. An informal classification is descriptive; little knowledge is needed to classify a trap beyond learning how to describe it. A formal classification is more rigorous and requires knowing the structure of the scheme proposed in this chapter.

Informal classification

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  Figure 1 Map and cross section of the East Anschutz Ranch field. From White et al.; courtesy AAPG.

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  Figure 2 Formal classification for East Anschutz Ranch.

An informal classification is a description that conveys a general or specific impression of a trap. For example, East Anschutz Ranch field, shown in the map and cross section in Figure 1, could be informally classified as an anticlinal trap. This informal classification conveys a very general impression of the trap. Informal classification can also be more specific. For example, we might classify East Anschutz as an elongated asymmetric anticline with a gently dipping back limb and a steeply to overturned forelimb.

Formal classification

A formal classification conveys more information than an informal classification. It is also more rigorous. The diagram in Figure 2 shows the formal classification for East Anschutz Ranch.

Procedure

The list below lists the procedure for classifying a trap.

1. Determine the trap system: structural, stratigraphic, or fluidic.
   • If More than one element controls the trap Go to step 2
   • If Only one element controls the trap Go to step 3
2. Determine the primary, secondary, and (if necessary) tertiary trap system.
3. Determine the trap regime. What process formed trap closure?
4. Determine the trap class. Which class best describes trap geometry, or which class describes compositional makeup of the reservoir or seal or fluid that creates or defines trap boundaries?
5. Determine the trap family. What is the genesis of trap closure?
6. If necessary, use intermediate groupings (superclasses, superfamilies, sub- classes, styles, subfamilies, styles, varieties) to give fuller descriptions.

See also

• Classification philosophy
• How to use the trap classification scheme
• Combination trap
• Classification basis
• Trap classification levels
• Trap
• Trap systems: structural, stratigraphic, and fluidic

External links

find literature about Trap classification

• Structural trap — Wikipedia entry