2 Classification of Exploration Traps

Part I Traps, Trap Types, and the Petroleum System Introduction

Each trap is unique. Enough similarities exist, however, for traps to be classified. Different classifications serve different purposes. The purpose of the classification presented in Part I is to aid in conceptualizing traps and knowing how to explore for them. The classification of each trap says something about the manner in which one would explore for it. A trap exists in a context of geologic elements and processes that constitute a petroleum system. The probability that a trap will be filled depends on the right combination of source, maturation, migration, trap location, and timing. Studying the petroleum systems within a basin can lead to the creation of new plays and prospects.

In this part

The two chapters of Part I discuss the concept of a trap, how traps are classified, and the context that a trap has within a petroleum system. Chapter 2:

Classification of Exploration Traps

Chapter 3:

Petroleum Systems

Chapter 2

Classification of Exploration Traps by Richard R. Vincelette, Edward A. Beaumont, and Norman H. Foster

Richard R. Vincelette Richard Vincelette graduated with a B.S. degree in geological engineering from Montana Tech. in 1960 and received a Ph.D. in geology from Stanford in 1964. He has spent the last 35 years searching for, and occasionally finding, the elusive hydrocarbon trap. At present he is a geologist and chief curmudgeon with JOG Corporation in Healdsburg, California.

Edward A. Beaumont Edward A. (Ted) Beaumont is an independent petroleum geologist from Tulsa, Oklahoma. He holds a BS in geology from the University of New Mexico and an MS in geology from the University of Kansas. Currently, he is generating drilling prospects in Texas, Oklahoma, and the Rocky Mountains. His previous professional experience was as a sedimentologist in basin analysis with Cities Service Oil Company and as Science Director for AAPG. Ted is coeditor of the Treatise of Petroleum Geology. He has lectured on creative exploration techniques in the U.S., China, and Australia and has received the Distinguished Service Award and Award of Special Recognition from AAPG.

Norman H. Foster Norman H. Foster received a bachelor’s degree (1957) and a master’s degree (1960) in geology at the University of Iowa. In 1963 he completed his Ph.D. in geology at the University of Kansas. His geological career began in 1962, with Sinclair Oil in Casper, Wyoming. When Sinclair merged with Arco in 1969, Foster was offered expanding opportunities to participate in a number of important discoveries, including the giant Irian Jaya field in Indonesia. In 1979, he became an independent geologist and continued to prospect both in the United States and abroad. In addition to winning AAPG’s Sidney Powers Memorial Medal for 1999, the former AAPG president received the Levorsen Award (1980), two Certificate of Merit awards (1987 and 1992), and the Distinguished Service Award (1985). He was a member of the AAPG Foundation and an AAPG Trustee Associate since 1979. His professional activities included GSA, SEG, SPE, SIPES, SEPM, and the National Academy of Sciences.

Overview Introduction

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.

In this chapter

This chapter contains four sections. Section

Topic

Page

A

Classification Philosophy

2–4

B

How to Use the Classification Scheme

2–13

C

Details of the Trap Classification Scheme

2–19

D

References

2–42

Overview • 2-3

Section A

Classification Philosophy Introduction

Most petroleum geologists classify traps according to the scheme proposed by Levorsen (1954). 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, 1972; North, 1985; Melton and Bertram, 1992; and Biddle and Weilchowsky, 1994) 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.

In this section

This section discusses the following topics Topic

2-4

Page

What is a Trap?

2–5

Classification Basis

2–7

Classifying Traps

2–9

Traps Systems: Structural, Stratigraphic, and Fluidic

2–10

Trap Classification Levels

2–11

• Classification of Exploration Traps

What is a Trap? Early concepts of traps

The earliest concept of a trap was made by William Logan in 1844 when he noted the occurrence of oil on anticlines. I.C. White took Logan’s anticlinal trap concept and applied it to search for oil and gas in 1855. Since then, models and applications of trap concepts have evolved as new trap types have been discovered.

Trap definition

A trap consists of a geometric arrangement of permeable (reservoir) and less-permeable (seal) rocks which, when combined with the physical and chemical properties of subsurface fluids, can allow hydrocarbons to accumulate.

Trapping elements

Three main trapping elements comprise every subsurface hydrocarbon accumulation: 1. Trap reservoir—storage for accumulating hydrocarbons and can transmit hydrocarbons. 2. Trap seal—an impediment or barrier that interferes with hydrocarbon migration from the reservoir. 3. Trap fluids—physical and chemical contrasts—especially differences in miscibility, solubility, and density—between the common reservoir fluids (primarily water, gas, and oil) that allow hydrocarbons to migrate, segregate, and concentrate in the sealed reservoir.

Trap boundaries

Trap boundaries define the limits of the trap and usually consist of (1) boundaries between solids, such as the contact between reservoir and seal, or (2) boundaries between fluids, such as oil–water or gas–water contacts. Temperature can also control a trap boundary as displayed by gas hydrate traps.

Traps vs. accumulations

A trap may or may not contain oil or gas. Accumulations, or pools, are traps that contain oil or gas.

Trapping conditions

Subsurface conditions that impede oil or gas migration include the following: • Capillary contrasts in pore throats in the seal vs. the reservoir • Contrasts in physical/chemical properties of subsurface fluids (primarily oil, gas, and water) • Rock/fluid chemical and physical interactions

Capillary contrasts

Capillary contrasts are differences in the capillary properties of the pore-throat apertures of seal and reservoir rocks, generally caused by a difference in pore-throat aperture sizes. These capillarity contrasts commonly create the trap boundaries between reservoir and seal.

Classification Philosophy

• 2-5

What is a Trap? continued Trap closure

Trap closure is a measure of the potential storage capacity or size of the trap defined by the trap boundaries. Vertical closure is a measure of the maximum potential hydrocarbon column of the trap. Areal closure is a measure of the maximum area of the potential hydrocarbon accumulation within the trap boundaries. Volumetric closure integrates vertical and areal closure with pay thickness, porosity, and hydrocarbon saturation to provide the volume of the potential hydrocarbon accumulation within the trap boundaries.

Trap classification

A meaningful trap classification scheme must consider reservoir, seal, and fluid properties and how these properties relate to one another to form closure. Since closure is defined by trap boundaries, the proposed classification scheme is based on the geometry, composition, and genesis of trap boundaries.

2-6

• Classification of Exploration Traps

Classification Basis Introduction

All classification schemes have a basis from which they are organized. Some are more organized and systematic than others. The basis used for classification depends on the purpose of the classification. The bases for this scheme are the geologic elements critical to finding similar traps.

Purpose

The main purpose of the proposed scheme is to help explorationists find more oil or gas traps. A well-constructed classification scheme for traps can serve other useful purposes: • It provides a means whereby traps can be organized and cataloged in an orderly manner. Once properly organized, the various trap types can be analyzed and compared to one another to provide valuable information for the exploration and development of similar features. • A good classification scheme provides standardized terminology that can be used in communicating information to others about oil and gas accumulations.

Considering other schemes

Several classification schemes were analyzed and, where appropriate, were used as a basis for setting up the proposed trap classification scheme. Of particular interest was the biological classification scheme used to catalog and describe plants and animals. First proposed by Aristotle and then expanded and improved by Linnaeus, this system has stood the test of time. Although competing schemes have been proposed and modifications to the scheme are the basis of ongoing debate, the scheme has provided a valuable method of organizing and studying organisms.

Basis for biological classification

The basis for the biological classification scheme is similarity of morphology (shape) and phylogeny (evolutionary history) (Curtis, 1983). In addition, the processes that led to these similarities are also used in biological classification. For example, one of the major differences between plants and animals is that animals are mobile and can search for food, whereas plants are fixed or rooted and rely on food to be brought to them (Curtis, 1983).

Classification Philosophy

• 2-7

Classification Basis, continued Ranking classes of organisms

The biological classification scheme places organisms in seven ranked levels, going from general to specific: 1. Kingdom 2. Phylum 3. Class 4. Order 5. Family 6. Genus 7. Species The species represents one certain type of organism, which, by definition, cannot interbreed with an organism of a different species. The largest or most general grouping is at the kingdom level, which Aristotle originally used to separate plants (plant kingdom) and animals (animal kingdom). Interestingly, Aristotle also identified a third kingdom—the mineral kingdom—in which hydrocarbon traps presumably belong.

Similarities of classifying traps and organisms

2-8

The use of shape and evolutionary history in the biological classification scheme provides a basis for the use of similar attributes in a trap classification scheme. We propose to utilize similarities in geometry, composition, and genesis as the basis for classifying traps. These similarities are really critical geological elements that can guide exploration. Unfortunately, using a scheme identical to that used in biology has limitations in trap classification, primarily because our trap “species” are prolific interbreeders. Trying to classify most traps would be like trying to classify an organism that is a cross between an elephant and a bee with an apple tree growing out of its head. The “treephantbee” might be difficult to classify in the standard biological classification. However, if a proper classification would enable us to locate a herd of treephantbees, and if honey were oil, we might become rich indeed!

• Classification of Exploration Traps

Classifying Traps 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.

Classification Philosophy

• 2-9

Trap Systems: Structural, Stratigraphic, and Fluidic Introduction

The proposed classification scheme divides traps into three main groups or systems, based on the controlling geologic element that created the trap: 1. Structural 2. Stratigraphic 3. Fluidic

System definition

Following are definitions for the three systems. Trap Type

Trapping element is ...

Structural

Post- or syndepositional deformation or displacement of reservoir and/or sealing units

Stratigraphic

Depositional, erosional, or diagenetic configuration of reservoir and/or sealing units

Fluidic

Physical and/or chemical property or condition of reservoir fluids

Discussion

Structural and stratigraphic traps are well established in geological literature. The fluidic system is new; often, traps of this nature are referred to as “miscellaneous” or “other” or “unconventional” but seem to fit nicely into a distinct system of their own.

Classifying combinations

One of the more difficult tasks in categorizing an oil or gas accumulation into a specific trap type is determining the dominant element that creates the trap boundaries since often more than one element is involved, giving rise to the combination trap. We recognize this problem but hope that, by determining the primary, secondary, and, in some cases, tertiary trapping elements, explorationists can give combination traps useful labels. The label would combine classification labels such as structural/fluidic trap, stratigraphic/ structural trap, anticline/fault trap, and tilted fault-block/unconformity truncation trap, with the primary element listed first.

2-10 • Classification of Exploration Traps

Trap Classification Levels Basis for each level

A fairly flexible classification scheme has been devised in which each of the three trap systems is divided into three trap regimes, based primarily on the processes that control trap formation in the systems. The trap regimes can be divided into geometric classes and the classes into genetic families. Classifying traps using levels and the same basis at each level brings more consistency and value to classification. The table below shows the basis for each classification level and its definition. Classification Level

Basis

Function/Definition

System

Controlling geologic element

Dominant control of the trap—structural, stratigraphic, or fluidic.

Regime

Process

The dominant way of forming part or all of the trap closure. If the trap is structural, was the closure formed by folding, faulting, or fracturing? If the trap is stratigraphic, was the closure formed by depositional, erosional, or diagenetic processes? If the trap is fluidic, was closure formed by pressure, temperature, or chemical processes?

Class

Geometry and/or composition

Geometry—the external shape and size of the trap; may also include geometry of internal trap elements. Composition—the makeup of the reservoir, seal, or fluid that creates or defines the trap boundaries.

Family

Genesis

The way all or part of the trap closure came into being. For example, for an isolated marine carbonate trap, was the closure formed because of the reservoir/seal facies relationships of a reef, an oolite bar, or a tidal channel?

Sublevels

Using one term to classify traps geometrically, compositionally, or genetically is not always adequate. To solve this problem, intermediate groupings such as superfamilies or superclasses can be added where necessary. Trap classes can be subdivided into subclasses and styles if needed, and trap families can be subdivided into subfamilies and varieties. These extra levels allow a fuller description of traps.

Degree of interpretation at different levels

Classification requires different degrees of interpretation at different levels of the proposed scheme. The highest levels require presumably less interpretation than do the lower levels. The higher levels are broader generalizations and reveal broader relationships. Classification at lower levels is more interpretive and therefore more open to disagreement. For example, trap geometries, which establish trap classes, are typically well understood; whereas trap genesis, used to establish trap families, relies on interpretation, is often the subject of considerable disagreement and debate, and may be the last trap element fully understood.

Classification Philosophy

• 2-11

Trap Classification Levels, continued Trap systems & corresponding regimes

The proposed classification scheme divides traps into the following systems and their corresponding regimes.

Trap classes and families

As mentioned previously, the various trap regimes can be subdivided into trap classes and trap families, based on geometry, composition, and genesis. Ultimately, every oil accumulation can hopefully be classified correctly at the class and family levels.

Structural Trap System Fold Regime Fault Regime Fracture Regime Stratigraphic Trap System Depositional Regime Erosional Regime Diagenetic Regime Fluidic Trap System Pressure Regime Temperature Regime Fluid-Composition Regime

2-12 • Classification of Exploration Traps

Section B

How to Use the Classification Scheme Introduction

One geologic element may control the existence of a trap. These are pure stratigraphic, structural, or fluidic traps. But many traps are a combination of two or three geologic elements. In these traps, basic trapping elements occur in combination to provide the ultimate trapping mechanism. Classifying combination traps is a matter of deciding which are the primary, secondary, and, in some cases, tertiary controlling elements. This section discusses the classification of simple and combination traps and shows some examples of how to classify them.

In this section

This section contains the following topics. Topic

Page

Classifying Traps

2–14

Classifying Combination Traps

2–16

How to Use the Classification Scheme

• 2-13

Classifying Traps Introduction

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

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 below, 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.

Figure 2–1. From White et al., 1990; courtesy AAPG.

2-14 • Classification of Exploration Traps

Classifying Traps, continued Formal classification

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

Figure 2–2.

Procedure

The table below lists the procedure for classifying a trap. Step 1

Action Determine the trap system: structural, stratigraphic, or fluidic. If...

Then...

More than one element controls the trap

Go to step 2

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, subclasses, styles, subfamilies, styles, varieties) to give fuller descriptions.

How to Use the Classification Scheme

• 2-15

Classifying Combination Traps Introduction

Secondary or even tertiary trapping elements commonly modify the primary trapping agent. Structural traps may have a stratigraphic component or vice versa. Sometimes the distinction whether the trap belongs to one system or another is quite blurred. Traps with two or more trapping elements are called combination traps.

Primary trapping element

To determine what the primary trapping element is, consider each element of the trap and ask, “Would the trap exist if that element were not part of the trap?” We could also ask, “Which element would I look for first if I were exploring for this trap?”

Classifying combination traps informally

To classify a combination trap informally, list the primary trap element first, followed by secondary and tertiary trap elements. You can classify a combination trap informally in at least two different formats. For example, Upper Valley field, Utah, shown in the map and cross section below, could be classified informally as (1) a hydrodynamically modified anticlinal trap or as (2) a hydrodynamic/anticlinal trap.

Figure 2–3. Courtesy RMAG.

2-16 • Classification of Exploration Traps

Classifying Combination Traps, continued Classifying combination traps formally

We classify a trap formally by listing the regimes, classes, and families for the primary, secondary, and (if necessary) tertiary systems. For example, the diagram below shows the formal classification for Upper Valley field.

Figure 2–4.

Combination structural traps

Some structural traps are combinations of the three structural trap regimes: fold, fault, and fracture. The Buck Peak field shown below is an example of a combination structural trap.

Figure 2–5. Modified from Vincelette and Foster, 1992; courtesy RMAG.

How to Use the Classification Scheme

• 2-17

Classifying Combination Traps, continued Buck Peak formal classification

The diagram below shows the formal classification for the Buck Peak field, which is a combination structural trap.

Figure 2–6.

2-18 • Classification of Exploration Traps

Section C

Details of the Trap Classification Scheme Introduction

The details of the proposed classification scheme are presented in this section. The scheme should be considered as flexible and subject to further refinement. The authors hope future discussion resulting from analyzing this classification scheme will result in more rigorous and geologically acceptable terminology used to describe trap geometry and similar modification of the proposed families, subfamilies, and varieties used to describe trap genesis. The three subsections of this chapter contain the classification scheme for the three trap systems: structural, stratigraphic, and fluidic.

In this section

The following topics are discussed in this section. Subsection

Topic

Page

C1

Structural Trap System

2–20

C2

Stratigraphic Trap System

2–29

C3

Fluidic Trap System

2–38

Details of the Trap Classification Scheme • 2-19

Subsection C1

Structural Trap System Introduction

The structural system contains three regimes: fold traps, fault traps, and fracture traps. The classification of structural traps at the class level depends primarily on the external geometry of the trap and can be further subdivided into subclasses based on other geometric parameters such as areal extent, vertical relief, structural dip, and internal geometry. The structural classes listed below are not intended to be all inclusive but merely illustrate how the classification scheme is intended to operate.

In this subsection

This subsection discusses the following topics. Topic

Page

Fold Trap Regime

2–21

Fault Trap Regime

2–23

Fracture Traps

2–25

Fracture Trap Regime

2–26

2-20 • Classification of Exploration Traps

Fold Trap Regime Classes and definitions

The proposed classes in the fold trap regime are monocline, local dome, regional dome, local nose, regional nose, local anticline, regional anticline, and syncline. The following outline shows the classes, some of the subclasses, style, and their definitions. The Subclasses listed are not inclusive but represent the more common types of fold traps and show how the proposed classification scheme can be developed and defined. Regime: Fold A fold trap is formed by syn- or postdepositional processes that deform geological surfaces into a curved or nonplanar arrangement (Biddle and Weilchowsky, 1995). Class: Monocline A fold that occurs where strata dip or flex from the horizontal in one direction only and are not a part of a anticline or syncline (AGI Glossary of Geology, 1972). A monocline can form only part of the trap closure and must combine with other elements for closure. Class: Regional nose A short plunging anticline without closure. A trap where a nose is a trapping element must combine with other elements to have closure. Class: Local nose Class: Regional dome Circular or elliptical anticline many miles (km) in diameter. Class: Local dome Circular or elliptical anticline < 20 mi (32 km) in diameter. Class: Regional anticline Elongate convex upward fold. Class: Local anticline Subclass: Simple Subclass: Fault bounded or cross faulted Styles: Relief, symmetry, internal faulting, internal geometry Class: Syncline Elongate concave upward fold.

Families

Based on their genesis, folds can be divided into two superfamily categories: tectonic and nontectonic. The outline below presents the superfamilies, families, and subfamilies and their definitions for anticlinal fold traps. As with fold classes, the fold families listed below are not inclusive but merely represent the more common fold families. Regime: Fold Class: Local anticline Superfamily: Tectonic Folds resulting from deformation by tectonic processes such as compression or extension. Family: Compressional fold Formed by compressive tectonic deformation. Subfamily: Thrust-belt fold Subfamily: Foreland fold Subfamily: Cratonic-basin fold Variety: Paleostructure Variety: Inverted

Details of the Trap Classification Scheme • 2-21

Fold Trap Regime, continued Families (continued)

Family: Transpressional fold Subfamily: Single phase Subfamily: Paleostructure Subfamily: Inverted structure Family: Structural drape fold Form over a deeper structural feature such as a fault block. Family: Extensional fold Form by extensional tectonic deformation. Subfamilies: Rollover anticline Form as a result of rollover into a listric normal fault. Family: Basement uplift Superfamily: Nontectonic Folds result from deformation by nontectonic processes such as uplift by intrusion or diapirism, differential compaction, salt withdrawal, salt solution, or meteoric impact. Family: Intrusive core Dome or anticline form as a result of intrusion. Subfamilies: Salt dome or anticline Form as a result of uplift by salt movement. Subfamilies: Shale dome or anticline Form as a result of a shale diapir. Subfamilies: Igneous dome Form as a result of igneous intrusion. Family: Differential-compaction (drape) anticline Form by the differential compaction of sediments over a buried structure such as a reef or horst. Family: Salt-solution anticline Form as a result of salt dissolution. Family: Salt-withdrawal anticline Form as a result of salt movement out of an area. Family: Astrobleme Form as result of the impact of a meteor.

2-22 • Classification of Exploration Traps

Fault Trap Regime Classes and definitions

There are four classes in the fault trap regime: normal fault, reverse fault, thrust fault, and wrench fault. The outline below shows definitions and examples for the classes and subclasses of these traps or trapping elements. Regime: Fault trap Fault(s) forms part or all of the closure of the trap by sealing the reservoir either laterally and/or from the top (after Biddle and Wielchowsky, 1995). Class: Normal fault trap One or more form all or part of lateral closure by sealing the reservoir. Subclass: Tilted fault block Block of rock bounded on one or more sides by normal faults. Rotation traps hydrocarbons along edges or in corners. Subclass: Horst Block of rock bounded on all sides by normal faults. Subclass: Listric fault trap All or part of closure formed by a fault whose plane curves downward and is concave upward. Class: Reverse fault trap One or more faults form all or part of the closure by sealing the reservoir. Class: Thrust fault trap Forms all or part of the closure by sealing the reservoir either laterally or from the top or bottom. Subclass: Overthrust Forms all or part of the closure by sealing the reservoir laterally. Subclass: Subthrust Forms all or part of the closure by sealing the reservoir laterally or from the top. Class: Wrench fault trap Forms all or part of the closure by sealing the reservoir laterally or from the top. Subclass: Flower structure Opposing reverse faults diverge upward, forming a fan or flower cross section pattern. Develop along wrench fault zones.

Families

Based on the genesis of the bounding faults, traps classified as fault traps can be divided into tectonic and nontectonic superfamilies. The outline below presents examples of trap families for several common fault traps. Similar trap families and subfamilies can be defined for most fault traps when the genesis of the trap is understood fully. Regime: Fault traps Class: Normal faults Subclass: Tilted fault block Superfamily: Tectonic Normal fault, resulting from deformation by tectonic processes, forms all or part of the closure. Family: Extensional Fault, resulting from extensional deformation, forms all or part of the closure. Subfamily: Rift basin Subfamily: Basin and range Subfamily: Growth fault Details of the Trap Classification Scheme • 2-23

Fault Trap Regime, continued Families (continued)

Class: Reverse faults Subclass: Thrust faults Superfamily: Tectonic Thrust fault, resulting from tectonic deformation by tectonic processes, forms all or part of the closure. Family: Compressional Fault, resulting from tectonic compressional deformation, forms all or part of the closure. Subfamily: Regional thrust belt Subfamily: Foreland fold fault Subfamily: Forearc basin Class: Wrench faults Subclass: Flower structures Superfamily: Tectonic Normal fault, resulting from tectonic deformation by tectonic processes, forms all or part of the closure. Family: Transpressional Fault, resulting from tectonic transpressional deformation, forms all or part of the closure. Subfamily: Regional wrench system Variety: Inverted Class: Normal faults Superfamily: Nontectonic Result from deformation by nontectonic processes such as uplift by intrusion or diapirism, differential compaction, salt withdrawal, salt solution, or meteoric impact. Family: Extensional Fault, resulting from nontectonic extensional deformation, forms all or part of the closure. Family: Vertical uplift Family: Vertical subsidence Subfamily: Salt solution Subfamily: Salt withdrawal

2-24 • Classification of Exploration Traps

Fracture Traps Introduction

The reason for including fold traps and fault traps in the structural system may be obvious, but the reason for including fracture traps may not. Some might argue that fractures are just another porosity type and should be thought of as part of the reservoir, not as a trap regime. This classification scheme includes fracture traps in the structural system because fractures are a result of deformation and/or displacement and therefore are structural in nature.

Definitions

The following definitions for fracture, fracture reservoir, and fracture trap are used in the classification scheme. Fracture—Approximately planar surface along which originally contiguous rocks have broken and separated and along which the relative displacement of originally adjacent points across the fracture is small compared with fracture length (from Pollard and Segall, 1987). Fracture Reservoir—Reservoir in which most of the permeability and some of the porosity is provided by open fractures. Fracture Trap—Trap in which lateral boundaries of the trap are provided by change from fractured reservoir to unfractured or less fractured rock or by change from open, permeable fractures to cement-filled or narrow-aperture, low-permeability fractures.

Fracture types

Fractures can be divided into three major groups: extension, shear, and complex. Extension Fracture—displacement of originally adjacent points across the fracture is perpendicular to the fracture surface. The term “extension” refers only to the opening or extension of the space between the fracture walls, not to the stress system that caused the fracture. Shear Fracture—displacement of originally adjacent points across the fracture is parallel to the fracture surface. Complex Fracture—fracture with a complex history of displacement and rejuvenation, and which may have originated as a shear fracture and been rejuvenated as an extension fracture or vice versa.

Details of the Trap Classification Scheme • 2-25

Fracture Trap Regime Definitions and examples

Fracture traps are divided into three classes: extension, shear, and complex, based on the internal characteristics and geometry of the fractures that make up the reservoir. The outline below shows the classes, some subclasses, and possible styles into which these traps may be subdivided. Regime: Fracture trap Lateral boundaries of the trap are provided by change from fractured reservoir to unfractured or less fractured rock or by change from open, permeable fractures to cement-filled or narrow-aperture, low-permeability fractures. Class: Extension fracture trap Dominant reservoir fractures are extension fractures. Subclass: Parallel fractures Open fractures in a fractured reservoir, predominantly unidirectional in both strike and dip. Style: Mineralized fracture Partially or totally mineralized by postfracture cements, typically calcite, gypsum, or silica. Style: Nonmineralized fracture Contains no postfracture cements or minerals. Subclass: Intersecting fractures Open fractures in fractured reservoir of several intersecting sets, either along fracture strike or fracture dip. Style: Mineralized fracture Style: Nonmineralized fracture Class: Shear fracture trap The dominant reservoir fractures are shear fractures. Subclass: Parallel fractures Style: Mineralized fracture Style: Nonmineralized fracture Subclass: Intersecting fractures Style: Mineralized fracture Style: Nonmineralized fracture Class: Complex fracture trap Dominant reservoir fractures are complex fractures. Subclass: Parallel fractures Style: Mineralized fracture Style: Nonmineralized fracture Subclass: Intersecting fractures Style: Mineralized fracture Style: Nonmineralized fracture

Families

Based on the interpretation of the genetic causes of fracture traps, two fracture trap families are recognized: tectonic and nontectonic.

2-26 • Classification of Exploration Traps

Fracture Trap Regime, continued Tectonic fracture The outline below shows the order of the families of the tectonic fracture trap superfamily and defines some of them. trap families Regime: Fracture Superclass: Extension Superclass: Shear Superclass: Complex Class: Parallel Class: Intersecting Superfamily: Tectonic fracture trap Fractures were generated by crustal tectonic stresses, whether compressional, extensional, or transpressional. Family: Fold-related fracture trap Fractures are intimately associated with and controlled by tectonic folds. Subfamily: Related to zone of maximum curvature. Subfamily: Hydrofractures Family: Fault-related fracture traps Fractures are intimately associated with or controlled by tectonic faults. Subfamily: Fractures related to normal faults Subfamily: Fractures related to wrench faults Family: Regional fracture trap Fractures occur over a broad area unrelated to specific folds or faults and in which fractures are thought to have been created by regional tectonic stresses. Fractures related to folding

Several mechanisms have been proposed to explain the common occurrence of highly fractured zones in various positions along tectonic folds, including bending-induced fractures along zones of maximum curvature, typically along the flanks of monoclines, anticlines, or synclines, as well as along the plunge axis of anticlines. Another cause of fracturing along compressive anticlines could be due to hydrofracturing during compression and squeezing of the folded rocks. Where appropriate, subfamilies can be established, such as maximumcurvature fracture traps and hydrofracture traps.

Fractures related to faulting

Numerous examples exist in which fracture intensity increases with proximity to faults. A number of fracture traps have been attributed to fault-induced or associated fracturing. Subfamilies can be established based on the type of fault with which the fractures are associated, e.g., normal-fault fracture trap or wrench-fault fracture trap.

Fractures related to regional stresses

Within regionally fractured areas, local fracture swarms with enhanced permeability and fracture frequency often occur. The transition from these high-permeability fracture swarms to areas of lower fracture frequency or fracture permeability often provides local lateral trap boundaries within the regional system. These fracture swarms may or may not be related to or associated with local secondary folds or faults.

Details of the Trap Classification Scheme • 2-27

Fracture Trap Regime, continued A wide variety of nontectonic elements have been interpreted to cause fractures and fracNontectonic fracture families ture traps. The more common ones include salt solution, piercement by mobile cores, meteorite impact, compaction drape, shrinkage due to cooling or diagenesis, pore-fluid overpressuring, erosional uplift and unloading, and hydrothermal fracturing. Each of these can be used as nontectonic fracture trap families. Where necessary, subfamilies and varieties can be created for any of these families. The outline below shows the order and some definitions for these traps. Superclass: Extension fractures Class: Parallel Class: Intersecting Superfamily: Nontectonic fracture trap Fractures generated by nontectonic stresses, e.g., salt solution, piercement, shrinkage, and overpressuring. Family: Solution collapse Family: Piercement Family: Impact Family: Compaction drape Family: Shrinkage Subfamily: Chert diagenesis Subfamily: Cooling joints Family: Overpressuring Subfamily: Source rock maturation Subfamily: Geothermal pressuring Subfamily: Clay dewatering Family: Unloading Family: Hydrothermal

2-28 • Classification of Exploration Traps

Subsection C2

Stratigraphic Trap System Introduction

The geological controls for stratigraphic system traps are stratigraphic in nature and formed as a result of depositional, erosional, or diagenetic processes. These processes are the basis for the three regimes of the stratigraphic system.

In this section

This section contains the following topics. Topic

Page

Depositional Trap Regime

2–30

Erosional Trap Regime

2–34

Diagenetic Trap Regime

2–36

Details of the Trap Classification Scheme • 2-29

Depositional Trap Regime Introduction

Traps in the depositional regime formed primarily by processes that created facies changes between reservoir and seal-quality rocks. Besides deposition by sedimentary processes, this regime also includes deposition by igneous processes.

Classes

The basis for the three classes of the depositional regime is the geometric arrangement of the facies resulting from depositional processes. The subclasses describe reservoir composition or lithology. Where described, trap styles can be listed based on the lithology or composition of the sealing rocks. The outline below shows the classes and subclasses of the depositional regime. System: Stratigraphic Regime: Depositional Trap boundaries are created primarily by depositional processes and can involve igneous rocks as well as sedimentary rocks. Three classes are recognized, based on whether the trap involves an isolated reservoir, an updip pinch-out, or depositional relief on top of the reservoir. Class: Isolated (local) depositional reservoirs Reservoir rock is partially or completely isolated by sealing rocks, which provide top, side, and often bottom seals. These traps are often of limited areal extent, with trap closure defined largely by reservoir distribution. Subclass: Sandstone reservoirs Partially or completely isolated by seal. Subclass: Carbonate reservoirs Partially or completely isolated by seal. Subclass: Igneous reservoirs Partially isolated by seal. Class: Depositional pinch-outs Depositional processes form an updip pinch-out of permeable rock into impermeable rock. Trap closure is usually created by an updip re-entrant of the pinch-out boundary or by a combination of the pinch-out with other trap elements, such as tectonic nosing or hydrodynamics. Pinch-out boundaries typically involve top, side, and bottom seals. Subclass: Regional sandstone pinch-outs Regional updip pinch-outs of sandstone into an impermeable facies such as shale or anhydrite. Subclass: Local sandstone pinch-outs Local updip pinch-outs of sandstone into an impermeable facies such as shale or anhydrite. Subclass: Regional carbonate pinch-outs Regional updip pinch-outs of carbonate into an impermeable facies such as shale or anhydrite. Subclass: Local carbonate pinch-outs Local updip pinch-outs of carbonate into an impermeable facies such as shale or anhydrite.

2-30 • Classification of Exploration Traps

Depositional Trap Regime, continued Classes (continued)

Families

Class: Depositional relief traps Process forms positive relief on top of the reservoir; this topographic relief between top seal and reservoir creates the trap closure. Subclass: Sandstone depositional relief traps Subclass: Carbonate depositional relief traps Style: Carbonate reservoirs sealed by shale Style: Carbonate reservoirs sealed by tight carbonate Style: Carbonate reservoirs sealed by evaporites Genetic families for the various depositional trap classes and subclasses have been established based primarily on the genesis of the reservoir. Where desired, trap varieties can be added based on the genesis or origin of the sealing units. Larger superfamilies have been created based on the general depositional environment of the reservoir, i.e., marine, continental, or lacustrine. Note that the same genetic families and subfamilies can be used for different depositional classes (geometry). This lets cross-correlations be made between different geometric trap classes within similar genetic settings, e.g., isolated reservoirs or pinch-outs within the shallow marine environment. Thus, if desired, trap classes can be combined under similar trap families. Examples of the more common depositional trap superfamilies, families, and subfamilies are given below. Regime: Depositional reservoirs Class: Isolated depositional reservoirs Subclass: Isolated carbonate reservoirs Superfamily: Marine carbonate reservoirs Family: Open-shelf (high-energy) carbonates Subfamily: Shoal Variety: Oolite Variety: Skeletal Family: Tidal-zone carbonates Subfamily: Tidal channel Subclass: Isolated sandstone reservoirs Superfamily: Marine sandstone traps Family: Shallow-water sandstone reservoirs Subfamily: Beach Subfamily: Barrier island Subfamily: Offshore bar Family: Deepwater sandstone reservoirs Subfamily: Turbidites Subfamily: Turbidite channel Subfamily: Submarine fans Superfamily: Alluvial sandstone reservoirs Family: Fluvial Subfamily: Channel Variety: Deltaic

Details of the Trap Classification Scheme • 2-31

Depositional Trap Regime, continued Families (continued)

Family: Deltaic Subfamily: Distributary channel Subclass: Isolated igneous reservoirs Superfamily: Intrusive igneous bodies Family: Intrusive sills Class: Depositional pinch-outs Subclass: Sandstone pinch-outs Superfamily: Marine sandstone pinch-outs Family: Shallow marine Updip pinch-out of shallow marine sands into lagoonal or basinal shales and silts. Subfamily: Barrier bar Variety: Pinch-out into lagoonal shale Variety: Pinch-out into marine shale Family: Deep marine Subfamily: Turbidite Updip pinch-out of marine turbidite sandstone into marine shale. Superfamily: Lacustrine sandstone pinch-outs Family: Lacustrine delta pinch-out Subclass: Carbonate pinch-outs Superfamily: Marine carbonate pinch-outs Family: Tidal zone Subfamily: Tidal-flat carbonate pinch-out Variety: Pinch-out into silts and shales Variety: Pinch-out into tight dolomites and anhydrite Family: Open shelf (high energy) Subfamily: Carbonate bank pinch-out Reservoir variety: Rudistid limestone bank Seal variety: Top: Marine shale Side: Tight shelf limestone Class: Depositional relief Subclass: Sandstone Superfamily: Eolian sandstone reservoirs Family: Dune Superfamily: Marine sandstone reservoirs Family: Deep water Subfamily: Turbidite fan Subclass: Carbonate Superfamily: Marine carbonate reservoirs

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Depositional Trap Regime, continued Families (continued)

Family: Bioherms Trap results from depositional relief created by porous organic carbonate buildup sealed by overlying and adjacent tight lithologies. These buildups are commonly referred to as reefs. A wide variety of reef traps have been described and classified based upon both the environment of deposition and geometry of the carbonate reservoir. Oil and gas have been trapped in barrier reefs, fringing reefs, platform reefs, atoll reefs, patch reefs, pinnacle reefs, reef mounds (or mud mounts), and carbonate banks (James and Gelsetzer, 1989). These terms can be used as subfamilies, as noted below. If a more detailed classification is needed, varieties can be established based upon the facies and genesis of the sealing units surrounding the buildups. Subfamily: Pinnacle reefs High-relief, circular or ovoid mounds created by upward grown of carbonate frame-building organisms in basinal setting. Reef typically contains a significant amount of high-energy carbonate detritus (grainstones, wackestones) as well as boundstones and framestones. Reef width is less than 10% of height (James and Geldsetzer, 1989). Subfamily: Platform reefs Larger reefal carbonate buildup in which lateral dimensions are measured in kilometers and in which reef width is more than 10 times reef height (James and Geldsetzer, 1989). Subfamily: Patch reefs Small, low-relief carbonate mounds developed by frame-building organisms on top of a shelf. Subfamily: Mud mounds Depositional carbonate mounds consisting largely of clean lime mudstone with relatively little macro-fossil debris. Class: Supraunconformity traps Subclass: Onlap pinch-out Superfamily: Nonmarine erosion surface Family: Sequence-boundary unconformity onlap Subfamily: Depositional re-entrant Reservoir variety: Deltaic sandstone pinch-out Seal variety: Top: Deltaic shale Bottom: Subunconformity units Subclass: Buttress pinch-outs Superfamily: Nonmarine erosional surface Family: Sequence-boundary unconformity buttress Subfamily: Pinch-out against erosional ridge Reservoir variety: Alluvial sandstone Seal variety: Top: Nonmarine shale Side: Marine shale of subcrop ridge Bottom: Marine shale beneath unconformity

Details of the Trap Classification Scheme • 2-33

Erosional Trap Regime Definition

Erosional traps are those in which trap boundaries occur along contacts between erosional surfaces and underlying or overlying reservoirs.

Classes and subclasses

Three trap classes have been identified, based on the geometry of the reservoir beds in contact with the unconformity surface. The hierarchical structure and definitions are shown below. System: Stratigraphic traps Regime: Erosional traps Form as a result of erosional processes Class: Truncation traps Up-dip reservoir boundary created by truncation of a reservoir beneath an unconformity, followed by deposition of sealing unit on top of an unconformity. The unconformity surface provides top seal, but closure usually requires the presence of depositional or tectonic side and bottom seals beneath the unconformity. Subclass: Regional subcrop Uniformly or gently dipping reservoir bed beneath an unconformity. Top seal is provided by postunconformity beds; bottom seal is provided by sealing units beneath reservoir. Subclass: Paleostructural subcrop Folded and/or faulted reservoir beds beneath postdeformation unconformity. Reservoir distribution and trap boundaries are controlled by preunconformity deformation and subsequent erosion. Class: Erosional relief traps Closure provided by topographic relief beneath an unconformity. Subclass: Buried hill Top-seal closure created by positive erosional relief of reservoir beneath a sealing unconformity surface. Subclass: Truncation edge Side seal provided by truncation of a reservoir against later erosional valley, channel, or gorge wall and subsequent deposition of sealing beds in valley, channel, or gorge.

Families

Families of the erosional trap regime have been established based upon the genesis of the unconformity surface that controls the trap boundaries. Two superfamilies are recognized: nonmarine erosion surfaces and marine erosion surfaces. Within these superfamilies, more detailed family classifications can be established. Where desired, trap varieties can be utilized to provide more details of reservoir and seal genesis. The outline below shows the superfamilies, families, and subfamilies of the subclass gorge-edge traps. Regime: Erosional traps Class: Truncation traps Subclass: Regional subcrop Superfamily: Nonmarine erosional surface Family: Sequence-boundary unconformity

2-34 • Classification of Exploration Traps

Erosional Trap Regime, continued Families (continued)

Reservoir variety: Marine sandstone subcrop Seal variety: Top: Marine transgressive shale above unconformity and marine shale above sandstone. Bottom: Marine shale beneath sandstone Subclass: Paleostructural subcrop Style: Complex fold-fault subcrop Superfamily: Nonmarine erosional surface Family: Sequence-boundary unconformity Subfamily: Postcompressional fold-fault unconformity Reservoir variety: Multiple truncated marine sandstones Seal variety: Top: Postunconformity marine shale Bottom: Underlying marine shale Class: Erosional relief trap Subclass: Buried hill Superfamily: Nonmarine erosion surface Family: Sequence-boundary unconformity Subfamily: Local differential erosion Reservoir variety: Basement igneous complex Seal variety: Marine shales above unconformity Subclass: Truncation-edge traps Superfamily: Marine erosion surface Family: Submarine canyon or gorge trap Superfamily: Nonmarine erosional surface Family: Channel-relief or valley-edge relief traps

Details of the Trap Classification Scheme • 2-35

Diagenetic Trap Regime Introduction

Diagenetic traps are those in which the trap boundaries are due to postdepositional diagenetic processes, which can create new reservoirs or new seals.

Classes

The hierarchical structure and definitions for diagenetic traps are shown below. System: Stratigraphic traps Regime: Diagenetic traps Class: Diagenetic reservoirs Form where porosity is created as a result of local diagenetic processes such as dissolution or dolomitization. Subclass: Secondary dolomite reservoirs Form as a result of localized secondary dolomitization. Subclass: Leached (secondary) porosity Form as a result of localized leaching of nonreservoir-quality rock to create enhanced porosity and permeability. Common beneath unconformity surfaces. Class: Diagenetic seals Trap boundaries created by plugging original reservoir porosity and permeability by diagenetic cements or minerals. Subclass: Secondary anhydrite barriers Form as a result of secondary anhydrite precipitating in original pore spaces. Subclass: Secondary clay barriers Form as a result of secondary clay, commonly kaolinite, precipitating in original pore space. Style: Secondary pore-throat trap Caused by a reduction in the size of pore-throat radii of reservoir rocks by depositional or diagenetic processes. These trap types typically have relatively porous units in the sealing facies that are impermeable to oil migration due to capillary restraints but contain producible water.

Discussion of diagenetic trap families

Families of diagenetic traps based on genesis of the diagenetic processes that created or modified either the reservoir or the seal are somewhat difficult to establish because not all diagenetic processes are well understood and disagreements often exist about the cause of specific diagenetic processes that trapped hydrocarbons. Furthermore, almost all reservoirs and seals have undergone some degree of diagenesis since deposition, making diagenesis at least a secondary trapping element in many fields. The common occurrence of dolomitized reservoirs is a classic case in point. Dolomitized reefs and dolomitized tidal-flat carbonates are common oil and gas reservoirs. Although dolomitization was a critical element in creating commercial porosity and permeability in these reservoirs, exploration efforts usually focus on defining the original depositional fairway of these reservoirs, i.e., reefs or tidal flats. As a consequence, the primary trap classification of these reservoirs occurs under the depositional trap regime. For our purposes, diagenetic traps are those in which trap boundaries are created by diagenetic processes largely independent of the original depositional environment of reservoir or seal.

2-36 • Classification of Exploration Traps

Diagenetic Trap Regime, continued Families

Below is a suggested classification outline for some of the superfamilies, families, and subfamilies of the Diagenetic Reservoirs and Diagenetic Seals classes. Regime: Diagenetic traps Class: Diagenetic reservoirs Subclass: Secondary dolomite reservoirs Family: Hydrothermal diagenesis Subfamily: Ascending hot brines Subclass: Leached (secondary) porosity Family: Subunconformity diagenesis Subfamily: Dissolving Variety: Karst and cavern formation Class: Diagenetic seals Subclass: Secondary anhydrite barrier Family: Subunconformity diagenesis Subfamily: Cementation

Details of the Trap Classification Scheme • 2-37

Subsection C3

Fluidic Trap System Introduction

Fluidic traps are oil and gas accumulations in which the trapping element is the physical or chemical condition of the reservoir fluids. There are three regimes in the fluidic system: 1. Pressure traps 2. Temperature traps 3. Fluid-composition traps This section discusses the regimes, classes, and families of the fluidic trap system.

In this subsection

This subsection discusses the following topics. Topic

Page

Pressure Trap Regime

2–39

Temperature Trap Regime

2–40

Fluid-Composition Trap Regime

2–41

2-38 • Classification of Exploration Traps

Pressure Trap Regime Introduction

Pressure traps are those in which changes in fluid pressures control or modify trapping elements. Changes from normal pressure gradients to overpressures can create or destroy pressure seals and create isolated pressure compartments that may define trap boundaries. In addition, the presence of a hydrodynamic gradient can cause tilted oil–water and gas–water contacts, which define trap boundaries.

Classes and families

The outline below presents the hierarchy and definitions for pressure traps. System: Fluidic traps Regime: Pressure traps Class: Overpressure traps Limits controlled by change from overpressured reservoir to normally pressured reservoir. Common in some fractured reservoirs in which overpressures were required to generate and support open fractures in the subsurface. Family: High-pressure traps Variety: Hydrocarbon generation Overpressure due to volume increase in conversion of kerogen to oil. Class: Tilted oil–water contacts Boundary defined by tilted oil–water or gas–water contact. Family: Hydrodynamic traps Tilted hydrocarbon–water contact in accumulation due to hydrodynamic gradient.

Details of the Trap Classification Scheme • 2-39

Temperature Trap Regime Introduction

Temperature traps are those in which trap boundaries are created or controlled by subsurface temperatures. Both low temperatures and high temperatures can create trapping conditions for hydrocarbons.

Classes and families

The classes and families of the temperature trap regime are outlined below. Regime: Temperature traps Class: Gas hydrate traps Family: Low-temperature traps Accumulations in which hydrocarbon gases occur as a solid, ice-like compound of gas and water, formed under conditions of extreme low temperature and high pressure. Class: Basin-center gas Accumulations typically found in deep, hot basin centers in which all available pore space is saturated with gas. Family: High-temperature traps Subfamily: Generative traps High temperatures result in maturation of source rock, which generates and expels sufficient volume of hydrocarbons to saturate pore space of all nearby reservoirs.

2-40 • Classification of Exploration Traps

Fluid-Composition Trap Regime Introduction

Fluid-composition traps are controlled by the physical or chemical properties of the trapped fluids themselves. The insolubility, immiscibility, and density contrasts between hydrocarbons and water are major factors that allow hydrocarbons to concentrate into subsurface accumulations. Therefore, in its purest sense, all hydrocarbon accumulations have fluid-composition trapping elements. However, for classification purposes, fluid-composition traps are those in which unique properties of the hydrocarbons provide trapping mechanisms.

Classes and families

The outline below presents the hierarchy and definitions for fluid-composition traps. Regime: Fluid-composition traps Result from a chemical property of the trapped oil or gas. Class: Viscosity traps Petroleum can be trapped by the presence of barriers created by highly viscous oil (asphalt, tar) or solid hydrocarbons (albertite, gilsonite, or grahamite). Subclass: Tar seals Family: Bacterial degradation Updip tar seal created by bacteria degradation of the hydrocarbons, preferentially removing the lighter fractions. Subclass: Disseminated Tar or asphalt fills the pores of shales, sands, or carbonates. These traps are often called tar sands, oil shales, bituminous sandstones, or bituminous limestones. Subclass: Veins The solid, essentially infusible form of petroleum is called pyrobitumen. It occurs in veins 1 mm to 8 m across and appears to have been injected or is a dead seepage. Class: Coal-bed methane Coal beds trap large volumes of methane, where most is adsorbed onto internal surfaces of micropores or along cleat faces. Coalification generates methane along with water, carbon dioxide, and carbon monoxide.

Details of the Trap Classification Scheme • 2-41

Section D

References Biddle, K.T., and C.C. Weilchowsky, 1994, Hydrocarbon traps, in L.B. Magoon and W.G. Dow, eds., The Petroleum System—from Source to Trap: AAPG Memoir 60, p. 219–235. Curtis, Helen, 1983, Biology, 4th Ed.: New York, Worth Publishing, Inc., 1159 p. Goolsby, S.M., L. Druyff, and M.S. Fryl, 1988, Trapping mechanisms and petrophysical properties of the Permian Kaibab Formation, south-central Utah, in S.M. Goolsby and W.M. Longman, eds., Occurrence and Petrophysical Properties of Carbonate Reservoirs in the Rocky Mountain Region: RMAG, p. 193–212. Jackson, J.A,. ed., 1997, Glossary of Geology, 4th Ed.: American Geological Institute, 769 p. James, N.P., and H.H.J. Gelsetzer, 1984, Introduction, in H.H.J. Geldsetzer, N.P. James, and G.E. Tebbutt, eds., Reefs — Canada and Adjacent Areas: Canadian Society of Petroleum Geologists Memoir 13, p. 1–8. Leverson, A.I., 1954, Geology of Petroleum: San Francisco, W.H. Freeman and Co., 703 p. Logan, W.E., 1844, Canada Geological Survey Report of Progress, p. 141. Milton, N.J., and G.T. Bertram, 1992, Trap styles: a new classification based on sealing surfaces: AAPG Bulletin, vol. 76, p. 983–999. North, F.K., 1985, Petroleum Geology: Boston, Allen and Unwin, 607 p. Pollard, D.D., and P. Segall, 1987, Theoretical displacements add stresses near fractures with applications to fault, joints, veins, dikes, and solution surfaces, in B.K. Atkinson, ed., Fracture Mechanics of Rock: London, Academic Press, p. 277–349. Rittenhouse, G., 1972, Stratigraphic-trap classification, in R.E. King, ed., Stratigraphic Oil and Gas Fields: AAPG Memoir 16, p. 14–28. Vincelette, R.R., and N.H. Foster, 1992, Fractured Niobrara of northwestern Colorado, in J.W. Schmoker, E.B. Coalson, and C.A. Brown, eds., Geologic Studies Relevant to Horizontal Drilling: Examples from Western North America: RMAG, p. 227–242. White, R.R., T.J. Alcock, and R.A. Nelson, 1990, Anschutz Ranch East Field, in E.A. Beaumont and N.H. Foster, eds., Structural Traps III, Atlas of Oil and Gas Fields: AAPG Treatise of Petroleum Geology, p. 31–56. White, I.C., 1855, The geology of natural gas: Science, vol. 5, p. 521–522.

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