Chap02

Each trap is unique. Enough similarities exist, however, for traps to be classified.Different classifications serve different purposes. The purpose of the classification pre-sented 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 wouldexplore for it. A trap exists in a context of geologic elements and processes that constitutea petroleum system. The probability that a trap will be filled depends on the right combi-nation of source, maturation, migration, trap location, and timing. Studying the petrole-um systems within a basin can lead to the creation of new plays and prospects.

Introduction

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

In this part

Chapter 2: Classification of Exploration Traps

Chapter 3: Petroleum Systems

Traps, Trap Types, and the Petroleum System

Part I

Chapter 2

Classification of Exploration Traps

by

Richard R. Vincelette,

Edward A. Beaumont,

and

Norman H. Foster

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

Edward A. BeaumontEdward 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 fromthe University of Kansas. Currently, he is generating drilling prospects in Texas,Oklahoma, and the Rocky Mountains. His previous professional experience was as a sedi-mentologist in basin analysis with Cities Service Oil Company and as Science Directorfor AAPG. Ted is coeditor of the Treatise of Petroleum Geology. He has lectured on cre-ative exploration techniques in the U.S., China, and Australia and has received theDistinguished Service Award and Award of Special Recognition from AAPG.

Norman H. FosterNorman H. Foster received a bachelor’s degree (1957) and a master’s degree (1960) ingeology at the University of Iowa. In 1963 he completed his Ph.D. in geology at theUniversity 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 oppor-tunities to participate in a number of important discoveries, including the giant IrianJaya field in Indonesia. In 1979, he became an independent geologist and continued toprospect both in the United States and abroad. In addition to winning AAPG’s SidneyPowers Memorial Medal for 1999, the former AAPG president received the LevorsenAward (1980), two Certificate of Merit awards (1987 and 1992), and the DistinguishedService Award (1985). He was a member of the AAPG Foundation and an AAPG TrusteeAssociate since 1979. His professional activities included GSA, SEG, SPE, SIPES, SEPM,and the National Academy of Sciences.

Overview • 2-3

Traps are the product of the interaction of many geologic elements and processes. Theoutcome of all the possible combinations of geologic elements makes each trap unique. Yeteach trap generally shares enough similarities with other traps in the same basin or inother basins that traps may be classified. The classification chosen depends on one’s pur-pose. The ultimate purpose of the trap classification presented in this chapter is to facili-tate the discovery of oil and gas accumulations. This chapter discusses the philosophy ofclassification, 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 isdesigned to be flexible and therefore will evolve as new trap types are found and trappingconcepts change.

Introduction

Overview

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

In this chapter

2-4 • Classification of Exploration Traps

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 asa foundation and adds new trap types discovered since 1954. The proposed scheme attemptsto formalize the schemes of Levorsen and others (Rittenhouse, 1972; North, 1985; Meltonand Bertram, 1992; and Biddle and Weilchowsky, 1994) by developing a more systematicand rigorous approach. It uses elements critical to petroleum exploration to group trapsinto levels. The method is similar to the one used by biologists to classify plants andanimals.

Introduction

Section A

Classification Philosophy

This section discusses the following topics

Topic 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

In this section

Classification Philosophy • 2-5

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

Early concepts of traps

What is a Trap?

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 subsur-face fluids, can allow hydrocarbons to accumulate.

Trap definition

Three main trapping elements comprise every subsurface hydrocarbon accumulation:1. Trap reservoir—storage for accumulating hydrocarbons and can transmit hydro-

carbons.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 sealedreservoir.

Trappingelements

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

Trapboundaries

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

Traps vs.accumulations

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

Trappingconditions

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

Capillarycontrasts


Trap closure is a measure of the potential storage capacity or size of the trap defined bythe trap boundaries. Vertical closure is a measure of the maximum potential hydrocarboncolumn of the trap. Areal closure is a measure of the maximum area of the potentialhydrocarbon accumulation within the trap boundaries. Volumetric closure integrates ver-tical and areal closure with pay thickness, porosity, and hydrocarbon saturation to pro-vide the volume of the potential hydrocarbon accumulation within the trap boundaries.

Trap closure

What is a Trap? continued

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

Trapclassification


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

Introduction

Classification Basis

The main purpose of the proposed scheme is to help explorationists find more oil or gastraps. 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 man-

ner. Once properly organized, the various trap types can be analyzed and compared toone another to provide valuable information for the exploration and development ofsimilar features.

• A good classification scheme provides standardized terminology that can be used incommunicating information to others about oil and gas accumulations.

Purpose

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

Consideringother schemes

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

Basis forbiologicalclassification


The biological classification scheme places organisms in seven ranked levels, going fromgeneral to specific:

1. Kingdom2. Phylum

3. Class4. Order

5. Family6. Genus

7. Species

The species represents one certain type of organism, which, by definition, cannot inter-breed with an organism of a different species. The largest or most general grouping is atthe kingdom level, which Aristotle originally used to separate plants (plant kingdom) andanimals (animal kingdom). Interestingly, Aristotle also identified a third kingdom—themineral kingdom—in which hydrocarbon traps presumably belong.

Ranking classesof organisms

Classification Basis, continued

The use of shape and evolutionary history in the biological classification scheme providesa basis for the use of similar attributes in a trap classification scheme. We propose to uti-lize 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 trapclassification, primarily because our trap “species” are prolific interbreeders. Trying toclassify most traps would be like trying to classify an organism that is a cross between anelephant and a bee with an apple tree growing out of its head. The “treephantbee” mightbe difficult to classify in the standard biological classification. However, if a proper classi-fication would enable us to locate a herd of treephantbees, and if honey were oil, we mightbecome rich indeed!

Similarities ofclassifyingtraps andorganisms


The common occurrence of combination traps, which involve many different types andvarieties 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 collec-tion might be more useful, especially one in which a variety of flexible methods of organi-zation can be used.

For example, stamps can be organized or classified in many different ways: stamps fromone country, stamps from all countries for a certain period, stamps from different coun-tries 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 combi-nation of stamps is desirable would be a powerful research tool. In a similar manner, aflexible trap classification scheme should allow for different methods of classifying andcataloging hydrocarbon traps, depending upon the needs of the investigator. The intent isthat the proposed classification scheme allows for such flexibility.

Classificationflexibility

Classifying Traps

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

1. System2. Regime

3. Class (Superclass if necessary)a. Subclassb. Style (if necessary)

4. Family (Superfamily if necessary)a. Subfamilyb. Variety (if necessary)

Classificationlevels

Most of the levels and sublevels (outlined above) are necessary to adequately describe, inthe classification scheme, all of the different elements that characterize a trap. Each levelhas its own unique basis for classification. Trap systems are based on the controlling geo-logic elements that created the traps, trap regimes are based on the geologic processesthat caused the traps in each system, traps classes are based on the geometry and compo-sition of the traps within the trap regimes, and trap families are based on the genesis ororigin of the traps within the trap classes.

Basis for eachlevel


The proposed classification scheme divides traps into three main groups or systems,based on the controlling geologic element that created the trap:1. Structural2. Stratigraphic3. Fluidic

Introduction

Trap Systems: Structural, Stratigraphic, and Fluidic

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

Systemdefinition

Structural and stratigraphic traps are well established in geological literature. The fluidicsystem 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.

Discussion

One of the more difficult tasks in categorizing an oil or gas accumulation into a specifictrap type is determining the dominant element that creates the trap boundaries sinceoften more than one element is involved, giving rise to the combination trap. We recognizethis problem but hope that, by determining the primary, secondary, and, in some cases,tertiary trapping elements, explorationists can give combination traps useful labels. Thelabel 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.

Classifyingcombinations


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

ClassificationLevel Basis Function/Definition

System Controlling geologic Dominant control of the trap—structural, stratigraphic, or element fluidic.

Regime Process The dominant way of forming part or all of the trap closure. If thetrap is structural, was the closure formed by folding, faulting, orfracturing? If the trap is stratigraphic, was the closure formed bydepositional, erosional, or diagenetic processes? If the trap is flu-idic, was closure formed by pressure, temperature, or chemicalprocesses?

Class Geometry and/or Geometry—the external shape and size of the trap; may also composition include geometry of internal trap elements.

Composition—the makeup of the reservoir, seal, or fluid that cre-ates or defines the trap boundaries.

Family Genesis The way all or part of the trap closure came into being. For exam-ple, for an isolated marine carbonate trap, was the closure formedbecause of the reservoir/seal facies relationships of a reef, an oolitebar, or a tidal channel?

Basis for eachlevel

Trap Classification Levels

Using one term to classify traps geometrically, compositionally, or genetically is notalways adequate. To solve this problem, intermediate groupings such as superfamilies orsuperclasses can be added where necessary. Trap classes can be subdivided into subclass-es and styles if needed, and trap families can be subdivided into subfamilies and varieties.These extra levels allow a fuller description of traps.

Sublevels

Classification requires different degrees of interpretation at different levels of the pro-posed scheme. The highest levels require presumably less interpretation than do thelower levels. The higher levels are broader generalizations and reveal broader relation-ships. Classification at lower levels is more interpretive and therefore more open to dis-agreement. For example, trap geometries, which establish trap classes, are typically wellunderstood; 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 ele-ment fully understood.

Degree ofinterpretationat differentlevels


The proposed classification scheme divides traps into the following systems and their cor-responding regimes.

Structural Trap SystemFold RegimeFault RegimeFracture Regime

Stratigraphic Trap SystemDepositional RegimeErosional RegimeDiagenetic Regime

Fluidic Trap SystemPressure RegimeTemperature RegimeFluid-Composition Regime

Trap systems &correspondingregimes

Trap Classification Levels, continued

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

Trap classesand families

How to Use the Classification Scheme • 2-13

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 ele-ments. In these traps, basic trapping elements occur in combination to provide the ulti-mate trapping mechanism. Classifying combination traps is a matter of deciding whichare the primary, secondary, and, in some cases, tertiary controlling elements. This sectiondiscusses the classification of simple and combination traps and shows some examples ofhow to classify them.

Introduction

Section B

How to Use the Classification Scheme

This section contains the following topics.

Topic Page

Classifying Traps 2–14

Classifying Combination Traps 2–16

In this section


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

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

Introduction

Classifying Traps

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

Informalclassification

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


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

Formalclassification

Classifying Traps, continued

Figure 2–2.

The table below lists the procedure for classifying a trap.

Step Action

1 Determine the trap system: structural, stratigraphic, or fluidic.

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, orwhich class describes compositional makeup of the reservoir or seal or fluidthat 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.

Procedure

If... Then...

More than one element controls the trap Go to step 2

Only one element controls the trap Go to step 3


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

Introduction

Classifying Combination Traps

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

Primarytrappingelement

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

Classifyingcombinationtraps informally

Figure 2–3. Courtesy RMAG.


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 theformal classification for Upper Valley field.

Classifyingcombinationtraps formally

Classifying Combination Traps, continued

Figure 2–4.

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 structuraltrap.

Combinationstructural traps

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


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

Buck Peakformalclassification

Classifying Combination Traps, continued

Figure 2–6.

Details of the Trap Classification Scheme • 2-19

Section C

Details of the Trap Classification Scheme

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

Introduction

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


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 externalgeometry of the trap and can be further subdivided into subclasses based on other geo-metric parameters such as areal extent, vertical relief, structural dip, and internal geome-try. The structural classes listed below are not intended to be all inclusive but merelyillustrate how the classification scheme is intended to operate.

Introduction

Subsection C1

Structural Trap System

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

In thissubsection


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 followingoutline shows the classes, some of the subclasses, style, and their definitions. TheSubclasses listed are not inclusive but represent the more common types of fold traps andshow 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 surfacesinto a curved or nonplanar arrangement (Biddle and Weilchowsky, 1995).

Class: MonoclineA fold that occurs where strata dip or flex from the horizontal in one direction onlyand are not a part of a anticline or syncline (AGI Glossary of Geology, 1972). A mono-cline can form only part of the trap closure and must combine with other elements forclosure.Class: Regional noseA short plunging anticline without closure. A trap where a nose is a trapping elementmust combine with other elements to have closure.Class: Local noseClass: Regional domeCircular or elliptical anticline many miles (km) in diameter.Class: Local domeCircular or elliptical anticline < 20 mi (32 km) in diameter.Class: Regional anticlineElongate convex upward fold.Class: Local anticline

Subclass: Simple Subclass: Fault bounded or cross faulted

Styles: Relief, symmetry, internal faulting, internal geometryClass: SynclineElongate concave upward fold.

Classes anddefinitions

Fold Trap Regime

Based on their genesis, folds can be divided into two superfamily categories: tectonic andnontectonic. The outline below presents the superfamilies, families, and subfamilies andtheir definitions for anticlinal fold traps. As with fold classes, the fold families listed beloware not inclusive but merely represent the more common fold families.Regime: Fold

Class: Local anticlineSuperfamily: TectonicFolds resulting from deformation by tectonic processes such as compression orextension.

Family: Compressional foldFormed by compressive tectonic deformation.

Subfamily: Thrust-belt foldSubfamily: Foreland foldSubfamily: Cratonic-basin fold

Variety: PaleostructureVariety: Inverted

Families


Family: Transpressional foldSubfamily: Single phaseSubfamily: PaleostructureSubfamily: Inverted structure

Family: Structural drape foldForm over a deeper structural feature such as a fault block.Family: Extensional foldForm by extensional tectonic deformation.

Subfamilies: Rollover anticlineForm as a result of rollover into a listric normal fault.

Family: Basement upliftSuperfamily: NontectonicFolds result from deformation by nontectonic processes such as uplift by intrusionor diapirism, differential compaction, salt withdrawal, salt solution, or meteoricimpact.

Family: Intrusive coreDome or anticline form as a result of intrusion.

Subfamilies: Salt dome or anticlineForm as a result of uplift by salt movement.Subfamilies: Shale dome or anticlineForm as a result of a shale diapir.Subfamilies: Igneous domeForm as a result of igneous intrusion.

Family: Differential-compaction (drape) anticlineForm by the differential compaction of sediments over a buried structure suchas a reef or horst.Family: Salt-solution anticlineForm as a result of salt dissolution.Family: Salt-withdrawal anticlineForm as a result of salt movement out of an area.Family: AstroblemeForm as result of the impact of a meteor.

Families(continued)

Fold Trap Regime, continued


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 andsubclasses of these traps or trapping elements.

Regime: Fault trapFault(s) forms part or all of the closure of the trap by sealing the reservoir either laterallyand/or from the top (after Biddle and Wielchowsky, 1995).

Class: Normal fault trapOne or more form all or part of lateral closure by sealing the reservoir.

Subclass: Tilted fault blockBlock of rock bounded on one or more sides by normal faults. Rotation trapshydrocarbons along edges or in corners.Subclass: HorstBlock of rock bounded on all sides by normal faults.Subclass: Listric fault trapAll or part of closure formed by a fault whose plane curves downward and is con-cave upward.

Class: Reverse fault trapOne or more faults form all or part of the closure by sealing the reservoir. Class: Thrust fault trapForms all or part of the closure by sealing the reservoir either laterally or from the topor bottom.

Subclass: OverthrustForms all or part of the closure by sealing the reservoir laterally.Subclass: SubthrustForms all or part of the closure by sealing the reservoir laterally or from the top.

Class: Wrench fault trapForms all or part of the closure by sealing the reservoir laterally or from the top.

Subclass: Flower structureOpposing reverse faults diverge upward, forming a fan or flower cross section pat-tern. Develop along wrench fault zones.

Classes anddefinitions

Fault Trap Regime

Based on the genesis of the bounding faults, traps classified as fault traps can be dividedinto tectonic and nontectonic superfamilies. The outline below presents examples of trapfamilies for several common fault traps. Similar trap families and subfamilies can bedefined for most fault traps when the genesis of the trap is understood fully.

Regime: Fault trapsClass: Normal faults

Subclass: Tilted fault blockSuperfamily: TectonicNormal fault, resulting from deformation by tectonic processes, forms all orpart of the closure.

Family: ExtensionalFault, resulting from extensional deformation, forms all or part of the clo-sure.

Subfamily: Rift basinSubfamily: Basin and rangeSubfamily: Growth fault

Families


Class: Reverse faultsSubclass: Thrust faults

Superfamily: TectonicThrust fault, resulting from tectonic deformation by tectonic processes, formsall or part of the closure.

Family: CompressionalFault, resulting from tectonic compressional deformation, forms all or partof the closure.

Subfamily: Regional thrust beltSubfamily: Foreland fold faultSubfamily: Forearc basin

Class: Wrench faultsSubclass: Flower structures

Superfamily: TectonicNormal fault, resulting from tectonic deformation by tectonic processes, formsall or part of the closure.

Family: TranspressionalFault, resulting from tectonic transpressional deformation, forms all orpart of the closure.

Subfamily: Regional wrench systemVariety: Inverted

Class: Normal faultsSuperfamily: NontectonicResult from deformation by nontectonic processes such as uplift by intrusion ordiapirism, differential compaction, salt withdrawal, salt solution, or meteoricimpact.

Family: ExtensionalFault, resulting from nontectonic extensional deformation, forms all or part ofthe closure.

Family: Vertical upliftFamily: Vertical subsidence

Subfamily: Salt solutionSubfamily: Salt withdrawal

Families(continued)

Fault Trap Regime, continued


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

Introduction

Fracture Traps

The following definitions for fracture, fracture reservoir, and fracture trap are used in theclassification scheme.

Fracture—Approximately planar surface along which originally contiguous rocks havebroken and separated and along which the relative displacement of originally adjacentpoints across the fracture is small compared with fracture length (from Pollard andSegall, 1987).

Fracture Reservoir—Reservoir in which most of the permeability and some of theporosity is provided by open fractures.

Fracture Trap—Trap in which lateral boundaries of the trap are provided by changefrom fractured reservoir to unfractured or less fractured rock or by change from open, per-meable fractures to cement-filled or narrow-aperture, low-permeability fractures.

Definitions

Fractures can be divided into three major groups: extension, shear, and complex.

Extension Fracture—displacement of originally adjacent points across the fracture isperpendicular to the fracture surface. The term “extension” refers only to the opening orextension of the space between the fracture walls, not to the stress system that caused thefracture.

Shear Fracture—displacement of originally adjacent points across the fracture is paral-lel 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 extensionfracture or vice versa.

Fracture types


Fracture traps are divided into three classes: extension, shear, and complex, based on theinternal characteristics and geometry of the fractures that make up the reservoir. Theoutline below shows the classes, some subclasses, and possible styles into which thesetraps may be subdivided.Regime: Fracture trapLateral boundaries of the trap are provided by change from fractured reservoir to unfrac-tured or less fractured rock or by change from open, permeable fractures to cement-filledor narrow-aperture, low-permeability fractures.

Class: Extension fracture trapDominant reservoir fractures are extension fractures.

Subclass: Parallel fracturesOpen fractures in a fractured reservoir, predominantly unidirectional in bothstrike and dip.

Style: Mineralized fracturePartially or totally mineralized by postfracture cements, typically calcite, gyp-sum, or silica.Style: Nonmineralized fractureContains no postfracture cements or minerals.

Subclass: Intersecting fracturesOpen fractures in fractured reservoir of several intersecting sets, either alongfracture strike or fracture dip.

Style: Mineralized fractureStyle: Nonmineralized fracture

Class: Shear fracture trapThe dominant reservoir fractures are shear fractures.

Subclass: Parallel fracturesStyle: Mineralized fractureStyle: Nonmineralized fracture

Subclass: Intersecting fracturesStyle: Mineralized fractureStyle: Nonmineralized fracture

Class: Complex fracture trapDominant reservoir fractures are complex fractures.

Subclass: Parallel fracturesStyle: Mineralized fractureStyle: Nonmineralized fracture

Subclass: Intersecting fracturesStyle: Mineralized fractureStyle: Nonmineralized fracture

Definitions andexamples

Fracture Trap Regime

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

Families


The outline below shows the order of the families of the tectonic fracture trap superfamilyand defines some of them.

Regime: FractureSuperclass: ExtensionSuperclass: ShearSuperclass: Complex

Class: ParallelClass: Intersecting

Superfamily: Tectonic fracture trapFractures were generated by crustal tectonic stresses, whether compressional,extensional, or transpressional.

Family: Fold-related fracture trapFractures are intimately associated with and controlled by tectonic folds.

Subfamily: Related to zone of maximum curvature.Subfamily: Hydrofractures

Family: Fault-related fracture trapsFractures are intimately associated with or controlled by tectonic faults.

Subfamily: Fractures related to normal faultsSubfamily: Fractures related to wrench faults

Family: Regional fracture trapFractures occur over a broad area unrelated to specific folds or faults andin which fractures are thought to have been created by regional tectonicstresses.

Tectonic fracturetrap families

Fracture Trap Regime, continued

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

Fracturesrelated tofolding

Numerous examples exist in which fracture intensity increases with proximity to faults. Anumber 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 areassociated, e.g., normal-fault fracture trap or wrench-fault fracture trap.

Fracturesrelated tofaulting

Within regionally fractured areas, local fracture swarms with enhanced permeability andfracture frequency often occur. The transition from these high-permeability fractureswarms to areas of lower fracture frequency or fracture permeability often provides locallateral trap boundaries within the regional system. These fracture swarms may or maynot be related to or associated with local secondary folds or faults.

Fracturesrelated toregionalstresses


A wide variety of nontectonic elements have been interpreted to cause fractures and frac-ture traps. The more common ones include salt solution, piercement by mobile cores,meteorite impact, compaction drape, shrinkage due to cooling or diagenesis, pore-fluidoverpressuring, erosional uplift and unloading, and hydrothermal fracturing. Each ofthese can be used as nontectonic fracture trap families. Where necessary, subfamilies andvarieties can be created for any of these families. The outline below shows the order andsome definitions for these traps.

Superclass: Extension fracturesClass: ParallelClass: Intersecting

Superfamily: Nontectonic fracture trapFractures generated by nontectonic stresses, e.g., salt solution, piercement,shrinkage, and overpressuring.

Family: Solution collapseFamily: PiercementFamily: Impact Family: Compaction drapeFamily: Shrinkage

Subfamily: Chert diagenesis Subfamily: Cooling joints

Family: OverpressuringSubfamily: Source rock maturationSubfamily: Geothermal pressuringSubfamily: Clay dewatering

Family: UnloadingFamily: Hydrothermal

Nontectonicfracture families

Fracture Trap Regime, continued


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

Introduction

Subsection C2

Stratigraphic Trap System

This section contains the following topics.

Topic Page

Depositional Trap Regime 2–30

Erosional Trap Regime 2–34

Diagenetic Trap Regime 2–36

In this section


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

Introduction

Depositional Trap Regime

The basis for the three classes of the depositional regime is the geometric arrangement ofthe facies resulting from depositional processes. The subclasses describe reservoir compo-sition or lithology. Where described, trap styles can be listed based on the lithology orcomposition of the sealing rocks. The outline below shows the classes and subclasses ofthe depositional regime.

System: StratigraphicRegime: DepositionalTrap boundaries are created primarily by depositional processes and can involveigneous rocks as well as sedimentary rocks. Three classes are recognized, based onwhether the trap involves an isolated reservoir, an updip pinch-out, or depositionalrelief on top of the reservoir.

Class: Isolated (local) depositional reservoirsReservoir rock is partially or completely isolated by sealing rocks, which providetop, side, and often bottom seals. These traps are often of limited areal extent,with trap closure defined largely by reservoir distribution.

Subclass: Sandstone reservoirsPartially or completely isolated by seal.Subclass: Carbonate reservoirsPartially or completely isolated by seal.Subclass: Igneous reservoirsPartially isolated by seal.

Class: Depositional pinch-outsDepositional processes form an updip pinch-out of permeable rock into imperme-able rock. Trap closure is usually created by an updip re-entrant of the pinch-outboundary or by a combination of the pinch-out with other trap elements, such astectonic nosing or hydrodynamics. Pinch-out boundaries typically involve top,side, and bottom seals.

Subclass: Regional sandstone pinch-outsRegional updip pinch-outs of sandstone into an impermeable facies such asshale or anhydrite.Subclass: Local sandstone pinch-outsLocal updip pinch-outs of sandstone into an impermeable facies such as shaleor anhydrite.Subclass: Regional carbonate pinch-outsRegional updip pinch-outs of carbonate into an impermeable facies such asshale or anhydrite.Subclass: Local carbonate pinch-outsLocal updip pinch-outs of carbonate into an impermeable facies such as shaleor anhydrite.

Classes


Depositional Trap Regime, continued

Genetic families for the various depositional trap classes and subclasses have been estab-lished based primarily on the genesis of the reservoir. Where desired, trap varieties canbe added based on the genesis or origin of the sealing units. Larger superfamilies havebeen 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 depositionalclasses (geometry). This lets cross-correlations be made between different geometric trapclasses within similar genetic settings, e.g., isolated reservoirs or pinch-outs within theshallow marine environment. Thus, if desired, trap classes can be combined under similartrap families. Examples of the more common depositional trap superfamilies, families,and subfamilies are given below.

Regime: Depositional reservoirsClass: Isolated depositional reservoirs

Subclass: Isolated carbonate reservoirsSuperfamily: Marine carbonate reservoirs

Family: Open-shelf (high-energy) carbonatesSubfamily: Shoal

Variety: OoliteVariety: Skeletal

Family: Tidal-zone carbonatesSubfamily: Tidal channel

Subclass: Isolated sandstone reservoirsSuperfamily: Marine sandstone traps

Family: Shallow-water sandstone reservoirsSubfamily: BeachSubfamily: Barrier islandSubfamily: Offshore bar

Family: Deepwater sandstone reservoirsSubfamily: TurbiditesSubfamily: Turbidite channelSubfamily: Submarine fans

Superfamily: Alluvial sandstone reservoirsFamily: Fluvial

Subfamily: ChannelVariety: Deltaic

Families

Class: Depositional relief trapsProcess forms positive relief on top of the reservoir; this topographic reliefbetween top seal and reservoir creates the trap closure.

Subclass: Sandstone depositional relief trapsSubclass: Carbonate depositional relief traps

Style: Carbonate reservoirs sealed by shaleStyle: Carbonate reservoirs sealed by tight carbonateStyle: Carbonate reservoirs sealed by evaporites

Classes(continued)


Family: DeltaicSubfamily: Distributary channel

Subclass: Isolated igneous reservoirsSuperfamily: Intrusive igneous bodies

Family: Intrusive sillsClass: Depositional pinch-outs

Subclass: Sandstone pinch-outsSuperfamily: Marine sandstone pinch-outs

Family: Shallow marineUpdip pinch-out of shallow marine sands into lagoonal or basinal shales andsilts.

Subfamily: Barrier barVariety: Pinch-out into lagoonal shaleVariety: Pinch-out into marine shale

Family: Deep marineSubfamily: TurbiditeUpdip pinch-out of marine turbidite sandstone into marine shale.

Superfamily: Lacustrine sandstone pinch-outsFamily: Lacustrine delta pinch-out

Subclass: Carbonate pinch-outsSuperfamily: Marine carbonate pinch-outs

Family: Tidal zoneSubfamily: Tidal-flat carbonate pinch-out

Variety: Pinch-out into silts and shalesVariety: Pinch-out into tight dolomites and anhydrite

Family: Open shelf (high energy)Subfamily: Carbonate bank pinch-out

Reservoir variety: Rudistid limestone bankSeal variety:

Top: Marine shaleSide: Tight shelf limestone

Class: Depositional reliefSubclass: Sandstone

Superfamily: Eolian sandstone reservoirsFamily: Dune

Superfamily: Marine sandstone reservoirsFamily: Deep water

Subfamily: Turbidite fanSubclass: Carbonate

Superfamily: Marine carbonate reservoirs

Families(continued)



Family: BiohermsTrap results from depositional relief created by porous organic carbonatebuildup sealed by overlying and adjacent tight lithologies. These buildups arecommonly referred to as reefs. A wide variety of reef traps have been describedand classified based upon both the environment of deposition and geometry ofthe carbonate reservoir. Oil and gas have been trapped in barrier reefs, fring-ing reefs, platform reefs, atoll reefs, patch reefs, pinnacle reefs, reef mounds(or mud mounts), and carbonate banks (James and Gelsetzer, 1989). Theseterms can be used as subfamilies, as noted below. If a more detailed classifica-tion is needed, varieties can be established based upon the facies and genesisof the sealing units surrounding the buildups.

Subfamily: Pinnacle reefsHigh-relief, circular or ovoid mounds created by upward grown of car-bonate frame-building organisms in basinal setting. Reef typically con-tains a significant amount of high-energy carbonate detritus (grain-stones, wackestones) as well as boundstones and framestones. Reefwidth is less than 10% of height (James and Geldsetzer, 1989).Subfamily: Platform reefsLarger reefal carbonate buildup in which lateral dimensions are mea-sured in kilometers and in which reef width is more than 10 times reefheight (James and Geldsetzer, 1989).Subfamily: Patch reefsSmall, low-relief carbonate mounds developed by frame-buildingorganisms on top of a shelf.Subfamily: Mud moundsDepositional carbonate mounds consisting largely of clean lime mud-stone with relatively little macro-fossil debris.

Class: Supraunconformity trapsSubclass: Onlap pinch-out

Superfamily: Nonmarine erosion surfaceFamily: Sequence-boundary unconformity onlap

Subfamily: Depositional re-entrantReservoir variety: Deltaic sandstonepinch-outSeal variety:

Top: Deltaic shaleBottom: Subunconformity units

Subclass: Buttress pinch-outsSuperfamily: Nonmarine erosional surface

Family: Sequence-boundary unconformity buttressSubfamily: Pinch-out against erosional ridge

Reservoir variety: Alluvial sandstoneSeal variety:

Top: Nonmarine shaleSide: Marine shale of subcrop ridgeBottom: Marine shale beneath unconformity

Families(continued)



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

Definition

Erosional Trap Regime

Three trap classes have been identified, based on the geometry of the reservoir beds incontact with the unconformity surface. The hierarchical structure and definitions areshown below.

System: Stratigraphic trapsRegime: Erosional trapsForm as a result of erosional processes

Class: Truncation trapsUp-dip reservoir boundary created by truncation of a reservoir beneath an uncon-formity, followed by deposition of sealing unit on top of an unconformity. Theunconformity surface provides top seal, but closure usually requires the presenceof depositional or tectonic side and bottom seals beneath the unconformity.

Subclass: Regional subcropUniformly or gently dipping reservoir bed beneath an unconformity. Top sealis provided by postunconformity beds; bottom seal is provided by sealing unitsbeneath reservoir.Subclass: Paleostructural subcropFolded and/or faulted reservoir beds beneath postdeformation unconformity.Reservoir distribution and trap boundaries are controlled by preunconformitydeformation and subsequent erosion.

Class: Erosional relief trapsClosure provided by topographic relief beneath an unconformity.

Subclass: Buried hillTop-seal closure created by positive erosional relief of reservoir beneath a seal-ing unconformity surface.Subclass: Truncation edgeSide 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.

Classes andsubclasses

Families of the erosional trap regime have been established based upon the genesis of theunconformity 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 canbe utilized to provide more details of reservoir and seal genesis. The outline below showsthe superfamilies, families, and subfamilies of the subclass gorge-edge traps.

Regime: Erosional trapsClass: Truncation traps

Subclass: Regional subcropSuperfamily: Nonmarine erosional surface

Family: Sequence-boundary unconformity

Families


Reservoir variety: Marine sandstone subcropSeal variety:

Top: Marine transgressive shale above unconformity and marineshale above sandstone.Bottom: Marine shale beneath sandstone

Subclass: Paleostructural subcropStyle: Complex fold-fault subcrop

Superfamily: Nonmarine erosional surfaceFamily: Sequence-boundary unconformity

Subfamily: Postcompressional fold-fault unconformityReservoir variety: Multiple truncated marine sandstonesSeal variety:

Top: Postunconformity marine shaleBottom: Underlying marine shale

Class: Erosional relief trapSubclass: Buried hill

Superfamily: Nonmarine erosion surfaceFamily: Sequence-boundary unconformity

Subfamily: Local differential erosionReservoir variety: Basement igneous complexSeal variety: Marine shales above unconformity

Subclass: Truncation-edge trapsSuperfamily: Marine erosion surface

Family: Submarine canyon or gorge trapSuperfamily: Nonmarine erosional surface

Family: Channel-relief or valley-edge relief traps

Families(continued)

Erosional Trap Regime, continued


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

Introduction

Diagenetic Trap Regime

The hierarchical structure and definitions for diagenetic traps are shown below.

System: Stratigraphic trapsRegime: Diagenetic traps

Class: Diagenetic reservoirsForm where porosity is created as a result of local diagenetic processes such asdissolution or dolomitization.

Subclass: Secondary dolomite reservoirsForm as a result of localized secondary dolomitization.Subclass: Leached (secondary) porosityForm as a result of localized leaching of nonreservoir-quality rock to createenhanced porosity and permeability. Common beneath unconformity surfaces.

Class: Diagenetic sealsTrap boundaries created by plugging original reservoir porosity and permeabilityby diagenetic cements or minerals.

Subclass: Secondary anhydrite barriersForm as a result of secondary anhydrite precipitating in original pore spaces.Subclass: Secondary clay barriersForm as a result of secondary clay, commonly kaolinite, precipitating in origi-nal pore space.

Style: Secondary pore-throat trapCaused by a reduction in the size of pore-throat radii of reservoir rocks bydepositional or diagenetic processes. These trap types typically have rela-tively porous units in the sealing facies that are impermeable to oil migra-tion due to capillary restraints but contain producible water.

Classes

Families of diagenetic traps based on genesis of the diagenetic processes that created ormodified either the reservoir or the seal are somewhat difficult to establish because notall diagenetic processes are well understood and disagreements often exist about thecause of specific diagenetic processes that trapped hydrocarbons. Furthermore, almost allreservoirs and seals have undergone some degree of diagenesis since deposition, makingdiagenesis at least a secondary trapping element in many fields.

The common occurrence of dolomitized reservoirs is a classic case in point. Dolomitizedreefs and dolomitized tidal-flat carbonates are common oil and gas reservoirs. Althoughdolomitization was a critical element in creating commercial porosity and permeability inthese reservoirs, exploration efforts usually focus on defining the original depositional fair-way of these reservoirs, i.e., reefs or tidal flats. As a consequence, the primary trap classifi-cation of these reservoirs occurs under the depositional trap regime. For our purposes, dia-genetic traps are those in which trap boundaries are created by diagenetic processes large-ly independent of the original depositional environment of reservoir or seal.

Discussion ofdiagenetic trapfamilies


Below is a suggested classification outline for some of the superfamilies, families, andsubfamilies of the Diagenetic Reservoirs and Diagenetic Seals classes.Regime: Diagenetic traps

Class: Diagenetic reservoirsSubclass: Secondary dolomite reservoirs

Family: Hydrothermal diagenesisSubfamily: Ascending hot brines

Subclass: Leached (secondary) porosityFamily: Subunconformity diagenesis

Subfamily: DissolvingVariety: Karst and cavern formation

Class: Diagenetic sealsSubclass: Secondary anhydrite barrier

Family: Subunconformity diagenesisSubfamily: Cementation

Families

Diagenetic Trap Regime, continued


Fluidic traps are oil and gas accumulations in which the trapping element is the physicalor chemical condition of the reservoir fluids. There are three regimes in the fluidic system: 1. Pressure traps2. Temperature traps3. Fluid-composition traps

This section discusses the regimes, classes, and families of the fluidic trap system.

Introduction

Subsection C3

Fluidic Trap System

This subsection discusses the following topics.

Topic Page

Pressure Trap Regime 2–39

Temperature Trap Regime 2–40

Fluid-Composition Trap Regime 2–41

In thissubsection


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

Introduction

Pressure Trap Regime

The outline below presents the hierarchy and definitions for pressure traps.

System: Fluidic trapsRegime: Pressure traps

Class: Overpressure trapsLimits controlled by change from overpressured reservoir to normally pressuredreservoir. Common in some fractured reservoirs in which overpressures wererequired to generate and support open fractures in the subsurface.

Family: High-pressure trapsVariety: Hydrocarbon generationOverpressure due to volume increase in conversion of kerogen to oil.

Class: Tilted oil–water contactsBoundary defined by tilted oil–water or gas–water contact.

Family: Hydrodynamic trapsTilted hydrocarbon–water contact in accumulation due to hydrodynamicgradient.

Classes andfamilies


Temperature traps are those in which trap boundaries are created or controlled by sub-surface temperatures. Both low temperatures and high temperatures can create trappingconditions for hydrocarbons.

Introduction

Temperature Trap Regime

The classes and families of the temperature trap regime are outlined below.

Regime: Temperature trapsClass: Gas hydrate traps

Family: Low-temperature trapsAccumulations in which hydrocarbon gases occur as a solid, ice-like compound ofgas and water, formed under conditions of extreme low temperature and highpressure.

Class: Basin-center gasAccumulations typically found in deep, hot basin centers in which all available porespace is saturated with gas.

Family: High-temperature trapsSubfamily: Generative trapsHigh temperatures result in maturation of source rock, which generates andexpels sufficient volume of hydrocarbons to saturate pore space of all nearbyreservoirs.

Classes andfamilies


Fluid-composition traps are controlled by the physical or chemical properties of thetrapped fluids themselves. The insolubility, immiscibility, and density contrasts betweenhydrocarbons and water are major factors that allow hydrocarbons to concentrate intosubsurface accumulations. Therefore, in its purest sense, all hydrocarbon accumulationshave fluid-composition trapping elements. However, for classification purposes, fluid-com-position traps are those in which unique properties of the hydrocarbons provide trappingmechanisms.

Introduction

Fluid-Composition Trap Regime

The outline below presents the hierarchy and definitions for fluid-composition traps.Regime: Fluid-composition trapsResult from a chemical property of the trapped oil or gas.

Class: Viscosity trapsPetroleum can be trapped by the presence of barriers created by highly viscous oil(asphalt, tar) or solid hydrocarbons (albertite, gilsonite, or grahamite).

Subclass: Tar sealsFamily: Bacterial degradationUpdip tar seal created by bacteria degradation of the hydrocarbons, preferen-tially removing the lighter fractions.

Subclass: DisseminatedTar or asphalt fills the pores of shales, sands, or carbonates. These traps are oftencalled tar sands, oil shales, bituminous sandstones, or bituminous limestones.Subclass: VeinsThe 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 deadseepage.

Class: Coal-bed methaneCoal beds trap large volumes of methane, where most is adsorbed onto internal sur-faces of micropores or along cleat faces. Coalification generates methane along withwater, carbon dioxide, and carbon monoxide.

Classes andfamilies


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 petrophysicalproperties of the Permian Kaibab Formation, south-central Utah, in S.M. Goolsby andW.M. Longman, eds., Occurrence and Petrophysical Properties of Carbonate Reservoirs inthe 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 ofPetroleum 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 sealingsurfaces: 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 fractureswith 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., StratigraphicOil and Gas Fields: AAPG Memoir 16, p. 14–28.

Vincelette, R.R., and N.H. Foster, 1992, Fractured Niobrara of northwestern Colorado, inJ.W. Schmoker, E.B. Coalson, and C.A. Brown, eds., Geologic Studies Relevant toHorizontal 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: AAPGTreatise of Petroleum Geology, p. 31–56.

White, I.C., 1855, The geology of natural gas: Science, vol. 5, p. 521–522.

Section D

References

Chap02

Education

trap unique

trap location

new trap types

philosophy of classification

petroleum exploration

group traps

conceptualizing traps

elusive hydrocarbon