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Definitions and Concepts

Many terms are used to describe the various parts of a trap The anticlinal trap thesimplest type will be used as our reference ( Figure 1 Nomenclature of a trap using

a simple anticline as an example)

Figure 1

The highest point of the trap is the crest or culmination The lowest point is the spill

point A trap may or may not be full to the spill point The horizontal plane through

the spill point is called the spill plane The vertical distance from the high point at the

crest to the low point at the spill point is the closure The productive reservoir is the

pay Its gross vertical interval is known as the gross pay This can vary from only

one or two meters in Texas to several hundred in the North Sea and Middle East

Not all of the gross pay of a reservoir may be productive For example shale

stringers within a reservoir unit contribute to gross pay but not to net pay ( Figure 2

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Facies change in an anticlinal trap illustrating the difference between net pay and

gross pay ) Net pay refers only to the possibly productive reservoir

Figure 2

A trap may contain oil gas or a combination of the two The oil-water contac t OWC

is the deepest level of producible oil within an individual reservoir ( Figure 3a Fluid

contacts within a reservoir in an oil-water system)

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Figure 3a

It marks the interface between predominately oil-saturated rocks and water-

saturated rocks Similarly either the gas-water contact GWC ( Figure 3b Fluid

contacts within a reservoir in a gas-water system) or the gas-oil contact GOC (

Figure 3c Fluid contacts within a reservoir in a gas-oil-water system) is the lower

level of the producible gas The GWC or GOC marks the interface between

predominately gas-saturated rocks and either water-saturated rocks or oil-saturated

rocks as the case may be

Before the reserves of the field can be calculated it is essential that these surfaces

be accurately evaluated Their establishment is one of the main objectives of well-

logging and testing

Oil and gas may occur together in the same trap as separate liquid and gaseous

phases In this case gas overlies oil because of its lower density Source rock

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chemistry and level of maturation as well as the pressure and temperature of the

reservoir itself are important in determining whether a trap contains oil gas or both

In some oil fields (eg Sarir field in Libya) a mat of heavy tar is present at the oil-

water contact Degradation of the oil by bottom waters moving beneath the oil-water

contact may cause this tar to form Tar mats cause considerable production problems

because they prevent water from moving upwards and from displacing the produced

oil

Boundaries between oil gas and water may be sharp ( Figure 4a

Figure 4a

Transitional nature of fluid contacts within a reservoir-- sharp contact ) or gradational

( Figure 4b Transitional nature of fluid contacts within a reservoir-- gradational

contact ) An abrupt fluid contact usually indicates a permeable reservoir Gradational

contacts usually indicate low permeability reservoirs with high capillary pressure

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Directly beneath the hydrocarbons is the zone of bottom water ( Figure 5

Nomenclature of underlying reservoir waters) The zone of edge water is adjacent to

the reservoir

Figure 5

Fluid contacts in a trap are almost always planar but are by no means always

horizontal Should a tilted fluid contact be present its early recognition is essential

for correct evaluation of reserves and for the establishment of efficient production

procedures

One of the most common ways in which a tilted fluid contact may occur is through

hydrodynamic flow of bottom waters ( Figure 6 Tilted fluid contact caused by

hydrodynamic flow )

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Figure 6

There may be one or more separate hydrocarbon pools each with its own fluid

contact within the geographic limits of an oil or gas field ( Figure 7 Multiple poolswithin an oil and gas field ) Each individual pool may contain one or more pay zones

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Figure 7

Remember the ratio between gross pay and net effective pay is important and is

generally mapped from well data as the field is developed

Classification

There are many different types of hydrocarbon traps Several classification schemes

have been proposed (Clapp 1910 1917 Lovely 1943 and Hobson and Tiratsoo

1975) Basically traps can be classified into four major types structural

stratigraphic hydrodynamic and combination ( Table 1 below )

Table 1 Classification of Hydrocarbon Traps

TRAP TYPES CAUSES

Structural Traps

Fold Traps Compressional Folds Compactional Folds

Tectonic processes Depositional Tectonic processes Tectonic Processes

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Diapir Folds

Fault TrapsTectonic Processes

Stratigraphic Traps Depositional morphology or diagenesis

Hydrodynamic Traps Water flow

Combination Traps

Combination of two or more of the above processes

Structural traps are primarily due to post-depositional processes which modify the

spatial configuration of the reservoir rock mainly by folding and faulting

Stratigraphic traps are those whose geometry is due to changes in lithology The

lithological changes may be depositional as in channels reefs and bars or post-

depositional where strata are truncated or where rock lithologies have been altered

by diagenesis

In hydrodynamic traps the downward movement of formation waters prevents the

upward movement of oil Combination traps combine two or more of the previously-

defined generic groups A good summary of the more common trap types and their

formational environments is found in Bailey and Stoneley (1981)

Structural Traps

Fold Traps ( Compressional )

Anticlinal traps which are due to compression are most likely to be found in or near

geosynclinal troughs These troughs are usually associated with active continental

margins where there is a net shortening of the earths crust ( Figure 1 Active

continental margin with net shortening of crust- subduction zone)

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Diapir Folds

Fault TrapsTectonic Processes

Stratigraphic Traps Depositional morphology or diagenesis

Hydrodynamic Traps Water flow

Combination Traps

Combination of two or more of the above processes

Structural traps are primarily due to post-depositional processes which modify the

spatial configuration of the reservoir rock mainly by folding and faulting

Stratigraphic traps are those whose geometry is due to changes in lithology The

lithological changes may be depositional as in channels reefs and bars or post-

depositional where strata are truncated or where rock lithologies have been altered

by diagenesis

In hydrodynamic traps the downward movement of formation waters prevents the

upward movement of oil Combination traps combine two or more of the previously-

defined generic groups A good summary of the more common trap types and their

formational environments is found in Bailey and Stoneley (1981)

Structural Traps

Fold Traps ( Compressional )

Anticlinal traps which are due to compression are most likely to be found in or near

geosynclinal troughs These troughs are usually associated with active continental

margins where there is a net shortening of the earths crust ( Figure 1 Active

continental margin with net shortening of crust- subduction zone)

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Figure 1

In California the Tertiary basins form a major hydrocarbon province which contains

compressional anticlinal traps Within this province are a number of fault-bounded

troughs infilled by thick regressive sequences in which organic-rich basinal muds are

overlain by deep-sea sands and capped by younger continental beds as shown by

Figure 2 (Generalized west-southwest-east-northeast structural cross-section) a

cross- section of the Los Angeles basin

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Figure 2

These basins have been locally subjected to tight compressive folding associated with

the apparent transcurrent movement of the San Andreas fault system (Barbat 1958

Schwade at al 1958 and Simonson 1958) Anticlinal traps of a broad gentle

character may also be formed in large cratonic basins of stable shelf sediments

Many oil and gas fields in this province are also associated with faulting either

normal reverse or strike-slip

The Wilmington oil field in the Los Angeles basin ( Figure 3 Oil fields of the Los

Angeles basin) is a giant anticlinal trap with ultimate recoverable reserves of about 3

billion barrels of oil

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Figure 3

It is approximately 15 kilometers long and nearly 5 kilometers wide The overall

anticlinal shape of the field is shown by the structure contours on top of the main

pay zone ( Figure 4 Structural contours on top of Ranger zone Wilmington field

CA) Notice also the cross-cutting faults

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Figure 4

From a southwest-northeast cross section of the Wilmington field you can see the

broad arch of the anticline ( Figure 5 Southwest-northeast cross-section A-ZWilmington field ) The main reservoir occurs beneath the Pliocene unconformity in

Miocene- and Pliocene-age deep-sea sands

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Figure 5

The foothills of the Zagros mountains in Iran contain one of the best-known

hydrocarbon provinces with production from compressional anticlines ( Figure 6

Location map southwest Iran and Persian Gulf )

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Figure 6

Individual anticlines are up to 60 kilometers in length and 10-15 kilometers in width

Sixteen of these anticlinal fields are in the giant category with recoverable reserves

of over 500 million barrels of oil or 35 trillion cubic feet of gas (Halbouty et al

1970) The Asmari limestone (Oligocene-Miocene) a reservoir with extensive

fracture porosity provides the main producing reservoir Some single wells have

flowed up to 50 million barrels Figure 7 (Southwest-northeast generalized sections

through Asmari oil fields)

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Figure 7

shows two schematic cross sections through the Asmari oil fields according to two

different interpretations of deep structure one showing anticlines without thrusting

and one with thrust faulting For further detailed descriptions of these fields the

reader is also referred to Lees (1952) Falcon (1958 1969) and Colman-Sadd

(1978)

In areas of still more intense structural deformation anticlinal development may be

associated with thrust faulting Such thrust fault belts are usually found within

mountain chains throughout the world The thrust faults cause a thickening of the

sedimentary column as older rocks are thrust up over younger rocks causing

repeated sections Traps may occur in anticlines above thrust planes and in

reservoirs sealed beneath the thrust

In Wyoming the Painter Reservoir field is a fairly tight anticline ( Figure 8

Structural contours on top of Nugget sandstone Painter Reservoir field Wyoming)

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beneath a thrust plane which itself is involved in thrusting along its southeastern

border

Figure 8

In cross section the anticline is overturned and thrust faulted on its southeastern

flank ( Figure 9 Northwest-southeast cross-section through Painter Reservoir field )

The anticline occurs beneath a series of thrust slices that in turn occur beneath a

major unconformity

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Figure 9

Fold Traps ( Compactional )

Compactional fold frequently occurs where crustal tension associated with rifting

causes a sedimentary basin to form The floor is commonly split into a system of

basement horsts and grabens An initial phase of deposition fills this irregular

topography Anticlines may then occur in the sedimentary cover draped over the

structurally-high horst blocks ( Figure 1 Compactional anticlines in sediments

draped over underlying structurally high horst blocks )

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Figure 1

These anticlines develop by differential compaction of sediment At the time of

deposition thickness of a given sedimentary unit is thinner over the crest of theunderlying structural high ( Figure 2a Developmental stages of compactional

anticlines--initial stage of deposition)

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Figure 2a

Compaction then takes place over the feature ( Figure 2b Developmental stages of

compactional anticlines--compactional stage) Though the percentage of compaction

is constant for crest and trough the actual amount of compaction is greater for the

thicker sediment in the trough Deep-seated recurrent fault movement may enhance

the structural closure ( Figure 2c Developmental stages of compactional anticlines--

structural closure enhanced by recurrent fault movement )

Differential depositional rates may also enhance the closure Carbonate

sedimentation tends to be thicker in the shallower waters over underlying structural

highs Therefore shoal and reefal facies may pass off-structure into thinner

increments of basinal lime mud Sandbar or shoal sands may also develop on the

crest of structures with deep-water muds present further down the flanks For this

reason reservoir quality often diminishes down the flank of such structures

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In the North Sea there are several good examples of compactional anticline traps

where Paleocene deep-sea sands are draped over deep-seated basement horsts

These include the Forties (Hill and Wood 1980) Montrose (Fowler 1975) and East

Frigg fields (Heritier et al 1980)

The Forties field is an example of a compactional anticline on the western side of the

North Sea Regional structure is a southeasterly-plunging nose bounded to the

northeast and southwest by faults ( Figure 3 Structural contours on top of

Paleocene reservoir Forties field area North Sea)

Figure 3

A north-south cross section depicts the anticline developed at the Paleocene level

where the reservoir sands are sealed by overlying Tertiary clays ( Figure 4

Schematic north-south cross-section A-Z through Forties field North Sea)

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Figure 4

The anticline overlies a deep-seated horst of late Jurassic volcanics Source rocks of

upper Jurassic age occur around the edge of this horst structure Differential

compaction and recurrent fault movement seem to have controlled the structure

throughout the Cretaceous and into the Tertiary

Only differential compaction folds occurring over deep-seated horst blocks have been

discussed Compaction folds however may also occur over reefs and other deep-

seated rigid features

Fold Traps Comparison of Major Types

There are many differences between the fold traps caused by compression and

those caused by compaction (Selley 1982) Compressional folds form after

sedimentation so the porosity found in them is more related to primary depositional

causes than to structure These folds may also contain fracture porosity as they are

usually lithified when deformed

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With compaction folds porosity may vary between crest and flank As already

discussed there may be primary depositional control of reservoir quality

Furthermore secondary diagenetic porosity may also be developed on the crests of

compactional folds as such structures are prone to sub-areal exposure and leaching

Compressional folds are generally oriented with their long axis perpendicular to the

axis of crestal shortening whereas compactional folds are often irregularly shaped

due to the shape of underlying features

Compressional folds commonly form from one major tectonic event while

compactional folds may have a complex history due to rejuvenation of underlying

basement faults

Fault Traps

In many fields faulting plays an essential role in entrapment Of great importance is

whether a fault acts as a barrier to fluid migration thus providing a seal for a trap

The problem is that some faults seal while others do not

In general faults have more tendency to seal in plastic rocks than in brittle rocks

Faults in unlithified sands and shales tend to seal particularly where the throw

exceeds reservoir thickness Clay within a fault plane however may act as a seal

even when two permeable sands are faulted against each other - as recorded from

areas of overpressured sediments like the Niger Delta and the Gulf of Mexico (Weber

and Daukoru 1975 and Smith 1980) In the Gulf coast it has been noted that

where sands are faulted against each other the probability of the fault being a

sealing fault increases with the age difference of the two sands (Smith 1980)

Figure 1 (Schematic cross-section of Nigerian field showing traps and possible

accumulation model ) shows a complex faulted situation in the Niger Delta in which

some faults seal while others are conduits

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With compaction folds porosity may vary between crest and flank As already

discussed there may be primary depositional control of reservoir quality

Furthermore secondary diagenetic porosity may also be developed on the crests of

compactional folds as such structures are prone to sub-areal exposure and leaching

Compressional folds are generally oriented with their long axis perpendicular to the

axis of crestal shortening whereas compactional folds are often irregularly shaped

due to the shape of underlying features

Compressional folds commonly form from one major tectonic event while

compactional folds may have a complex history due to rejuvenation of underlying

basement faults

Fault Traps

In many fields faulting plays an essential role in entrapment Of great importance is

whether a fault acts as a barrier to fluid migration thus providing a seal for a trap

The problem is that some faults seal while others do not

In general faults have more tendency to seal in plastic rocks than in brittle rocks

Faults in unlithified sands and shales tend to seal particularly where the throw

exceeds reservoir thickness Clay within a fault plane however may act as a seal

even when two permeable sands are faulted against each other - as recorded from

areas of overpressured sediments like the Niger Delta and the Gulf of Mexico (Weber

and Daukoru 1975 and Smith 1980) In the Gulf coast it has been noted that

where sands are faulted against each other the probability of the fault being a

sealing fault increases with the age difference of the two sands (Smith 1980)

Figure 1 (Schematic cross-section of Nigerian field showing traps and possible

accumulation model ) shows a complex faulted situation in the Niger Delta in which

some faults seal while others are conduits

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Figure 1

In the Gaiselberg field of Austria the Steinberg fault trends northeast-southwest

and provides the trap for this field ( Figure 2 Structural contours on top of Sarmatian horizon 18 of the Gaiselberg field )

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Figure 2

The fault is downthrown to the southeast with impermeable metamorphosed Tertiary

flysch comprising the upthrown block and younger Tertiary unmetamorphosedsediment comprising the downthrown block It is these younger sediments which

contain an oil field with a small gas cap ( Figure 3 West-northwest-east-southeast

cross-section A-Z through the Gaiselberg field )

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Figure 3

There are two requirements for this trap to be valid First the rocks in the upthrown

fault block adjacent to the down-thrown reservoir rocks must be impermeableSecond the Steinberg fault which extends to the surface must be a sealing fault

otherwise oil and gas would leak up the fault plane to the surface and entrapment

would not occur

A particularly important group of traps is found associated with growth faults

Growth faults typically form as down-to-basin faults contemporaneous with

deposition in areas characterized by rapidly-prograding deltaic sedimentation

Figure 4 (Diagramatic illustration showing four stages in the development of a

growth fault ) illustrates the stages of development of a typical growth fault as

presented by Bruce (1973)

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Figure 4

In the first cross section rapid progradational deposition of a sandy sediment takes

place over an unconsolidated deep-water clay ( Figure 4a Initial rapid

progradational depositionclay ) This results in downwarping of the under-compacted

clay under the heavier sand body ( Figure 4b Downwarping of under compacted )

In the next cross section continued deposition of sand generates a growth fault with

an expanded thickness of sediment in the downthrown block The fault remains

active as long as the axis of deposition is maintained at the same location ( Figure 4c

Generation of growth fault )

The final cross section shows the fault as a mature growth fault with downthrown dip

reversal into the fault accompanied by antithetic faulting ( Figure 4d Mature growth

fault ) Figure 5 (Schematic cross-section of a mature growth fault ) illustrates the

characteristic downthrown reversal of regional dip as the beds slump into the fault

plane

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Figure 5

This creates rollover anticlines with the dip reversal enhanced by antithetic faulting

Antithetic faults are downthrown toward the major fault and also dip toward the

major fault The angle of the major fault diminishes downward and typically soles out

into high-pressure low-density shale or into a salt formation

As illustrated in Figure 1 (Schematic cross-section of Nigerian field showing traps

and possible accumulation model ) hydrocarbons can be trapped in several situations

in growth faults There may be genuine fault traps where sands are sealed updip by

the main or antithetic fault However the principal trap for oil and gas is in the

rollover anticlines downthrown to the master fault

Along the Texas Gulf coast one of the best-known areas of large-scale growth

faulting is along the Vicksburg fault zone often referred to as the Vicksburg flexure

It extends as a uniquely narrow system of growth faulting for some 500 kilometers

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around the Gulf coast of Texas ( Figure 6 Vicksburg fault zone South Texas and

adjacent hydrocarbon fields)

Figure 6

Additional parallel zones of growth faulting are present basinward from the Vicksburg

fault zone

A cross section across the Vicksburg fault zone shows how the Vicksburg

stratigraphic section of Oligocene-age thickens on the downthrown side of the fault

( Figure 7 West-east schematic stratigraphic dip-section A-Z across the Vicksburg

fault zone South Texas near Mexican border )

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Figure 7

The maximum increase in sediment thickness across the fault is approximately 1500

meters near the Mexican border Most of this thickening occurs in the Vicksburg

group but some also occurs in the overlying Frio group (Miocene)

Characteristically there is a local reversal of the easterly regional dip adjacent to the

fault plane with a series of rollover anticlines developed on its downthrown side Oil

and gas are trapped in both these anticlines as well as in sand pinch-out

stratigraphic traps These anticlines pinch-outs and the fault itself provide traps for

an estimated 3 billion barrels of recoverable oil and 20 trillion cubic feet of gas

In southern Louisianas deltaic depositional province growth faults provide traps for

considerable oil and gas reserves

An example of growth fault-related production is Vermilion Block 76 field offshore

Louisiana Gas condensate production is found in nineteen separate Pliocene- and

Miocene-age sands ranging in depth from 3000 ft to 9000 ft and trapped in a rollover

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Figure 9

Similarly the Niger Delta of West Africa is a growth fault province containing major

accumulations of oil and gas Individual faults are seldom more than severalkilometers in length and their curved traces develop scalloped fault patterns (

Figure 10 Structural styles and hydrocarbon distribution Niger Delta)

Fault traps

(from other book) We indicated above that a trap may be formed where a

dipping reservoir is cut off up-dip by a fault setting it against something

impermeable The proviso is that we also have lateral closure this may be provided

by further faulting or by opposing dips The large Wytch Farm oilfield of southern

England offers a splendid example

Cretaceous T Tertiary (2-28)

We do not propose to discuss fault traps in detail although there are many problems

in trying to locatethem in the subsurface and in understanding them Whether or not

there is a trap and how big it iswill depend on the dip of the reservoir as compared

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Figure 9

Similarly the Niger Delta of West Africa is a growth fault province containing major

accumulations of oil and gas Individual faults are seldom more than severalkilometers in length and their curved traces develop scalloped fault patterns (

Figure 10 Structural styles and hydrocarbon distribution Niger Delta)

Fault traps

(from other book) We indicated above that a trap may be formed where a

dipping reservoir is cut off up-dip by a fault setting it against something

impermeable The proviso is that we also have lateral closure this may be provided

by further faulting or by opposing dips The large Wytch Farm oilfield of southern

England offers a splendid example

Cretaceous T Tertiary (2-28)

We do not propose to discuss fault traps in detail although there are many problems

in trying to locatethem in the subsurface and in understanding them Whether or not

there is a trap and how big it iswill depend on the dip of the reservoir as compared

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Figure 9

Similarly the Niger Delta of West Africa is a growth fault province containing major

accumulations of oil and gas Individual faults are seldom more than severalkilometers in length and their curved traces develop scalloped fault patterns (

Figure 10 Structural styles and hydrocarbon distribution Niger Delta)

Fault traps

(from other book) We indicated above that a trap may be formed where a

dipping reservoir is cut off up-dip by a fault setting it against something

impermeable The proviso is that we also have lateral closure this may be provided

by further faulting or by opposing dips The large Wytch Farm oilfield of southern

England offers a splendid example

Cretaceous T Tertiary (2-28)

We do not propose to discuss fault traps in detail although there are many problems

in trying to locatethem in the subsurface and in understanding them Whether or not

there is a trap and how big it iswill depend on the dip of the reservoir as compared

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with thatofthe fault whether the fault is normal orreverse and it will depend on the

amount of displacement on the fault whether or not the reservoir is completely or

only partially offset It also depends on whether the fault itself is sealing or non-

sealingThe reader may care to think through the various situations sketched as bits

of cross-sections in the

following figure in which the faults themselves are non-sealing thus causing sand

against sand topermit migration and sand against shale to be sealing The sealing

capacity of faults is a major difficulty confronting us We know that sometimes as at

Wytch Farm a fault can provide a seal but we also know that sometimes faults are

pathways formigrating petroleum and non-sealing at all Occasionally indeed it

seems that one and the same faultmay act or have acted in the past in both ways

All very puzzling Although attempts have been madeto investigate the problem in

Nigeria and elsewhere and naturally we have some ideas on the subject

we still do not fully understand what the difference is due to It adds further

uncertainties to ourpredictions of the subsurface occurrence of oil and gas

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Figure 10

Stratigraphic Traps

Depositional Traps

Petroleum may be trapped where the reservoir itself is cut off up-dip thus

preventing further migration no structural control is needed The variety in size and

shape of such traps is enormous to a large extent reflecting the restricted

environments in which the reservoir rocks were deposited

Stratigraphic trap geometry is due to variations in lithology These variations may be

controlled by the original deposition of the strata as in the case of a bar a channel

or a reef Alternatively the change may be post-depositional as in the case of a

truncation trap or it may be due to diagenetic changes

For reviews on the concept of stratigraphic traps the reader is referred to Dott and

Reynolds (1969) and Rittenhouse (1972) Major sources of specific data on

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stratigraphic traps can be found in King (1972) Busch (1974) and Conybeare

(1976)

Levorsen (1967) defines a stratigraphic trap as one in which the chief trap-making

element is some variation in the stratigraphy or lithology or both of the reservoir

rock such as a facies change variable local porosity and permeability or an

upstructure termination of the reservoir rock irrespective of the cause

Stratigraphic traps are harder to locate than structural ones because they are not as

easily revealed by reflection seismic surveys Also the processes which give rise to

them are usually more complex than those which cause structural traps

A broad classification of the various types of stratigraphic traps can be made

However classifying traps has its limitations because many oil and gas fields aretransitional between clearly-defined types

Table 1

Table 1

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Figure 10

Stratigraphic Traps

Depositional Traps

Petroleum may be trapped where the reservoir itself is cut off up-dip thus

preventing further migration no structural control is needed The variety in size and

shape of such traps is enormous to a large extent reflecting the restricted

environments in which the reservoir rocks were deposited

Stratigraphic trap geometry is due to variations in lithology These variations may be

controlled by the original deposition of the strata as in the case of a bar a channel

or a reef Alternatively the change may be post-depositional as in the case of a

truncation trap or it may be due to diagenetic changes

For reviews on the concept of stratigraphic traps the reader is referred to Dott and

Reynolds (1969) and Rittenhouse (1972) Major sources of specific data on

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stratigraphic traps can be found in King (1972) Busch (1974) and Conybeare

(1976)

Levorsen (1967) defines a stratigraphic trap as one in which the chief trap-making

element is some variation in the stratigraphy or lithology or both of the reservoir

rock such as a facies change variable local porosity and permeability or an

upstructure termination of the reservoir rock irrespective of the cause

Stratigraphic traps are harder to locate than structural ones because they are not as

easily revealed by reflection seismic surveys Also the processes which give rise to

them are usually more complex than those which cause structural traps

A broad classification of the various types of stratigraphic traps can be made

However classifying traps has its limitations because many oil and gas fields aretransitional between clearly-defined types

Table 1

Table 1

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stratigraphic traps can be found in King (1972) Busch (1974) and Conybeare

(1976)

Levorsen (1967) defines a stratigraphic trap as one in which the chief trap-making

element is some variation in the stratigraphy or lithology or both of the reservoir

rock such as a facies change variable local porosity and permeability or an

upstructure termination of the reservoir rock irrespective of the cause

Stratigraphic traps are harder to locate than structural ones because they are not as

easily revealed by reflection seismic surveys Also the processes which give rise to

them are usually more complex than those which cause structural traps

A broad classification of the various types of stratigraphic traps can be made

However classifying traps has its limitations because many oil and gas fields aretransitional between clearly-defined types

Table 1

Table 1

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(Classification of stratigraphic type hydrocarbon traps) based on the scheme

proposed by Rittenhouse (1972) shows that a major distinction can be made

between stratigraphic traps which occur within normal conformable sequences (

Figure 1

Figure 1

Schematic of trap within normal conformable sequence) and those that are

associated with unconformities ( Figure 2 Schematic of traps associated with

unconformaties)

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Figure 2

This distinction is rather arbitrary since there are some types such as channels that

can occur both at unconformities and away from them ( Figure 3 Schematic of two

channel traps)

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Figure 3

A dipping reservoir cut across by erosion and later covered above the unconformity

by impermeable sediments provides the classic caseUnconformity traps can also be

found above the break Consider the sea gradually encroaching over theland as sea level

rises the beach sands will spread progressively over the land surface becomingyounger

as time goes on until perhaps the supply of sand runs out We would be left with a

sandstonereservoir dying out above the unconformity to provide a trap when later

covered with say claystone

More esoterically but nevertheless known a hill on the old land surface may be formed

of permeablerock if drowned by shales the porosity could be preserved beneath the

unconformity In this mannerstrongly weathered basement rock (granites gneisses)under an unconformity

Of the traps occurring within normal conformable sequences a major distinction is made

between traps due to deposition and those due to diagenesis The depositional or facies-

change traps include channels bars and reefs

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Depositional Traps Channels

Many oil and gas fields occur trapped within channels of various types rangingfrom meandering fluvial deposits through deltaic distributary channels to deep-sea channels

Many good examples of stratigraphic traps in channels can be found in theCretaceous basins along the eastern flanks of the Rocky Mountains from Alberta through Montana Wyoming Colorado and New Mexico These channelsoccur both cut into a major pre-Cretaceous unconformity and within theCretaceous strata

The South Glenrock oil field in Wyoming contains oil trapped in both marine-bar and

fluvial-channel reservoirs The channel reservoir has a width of some 1500 metersand a maximum thickness of approximately 15 meters ( Figure 1 Isopach map of

Lower Muddy interva South Glenrock oil field Wyoming) It has been mapped for adistance of over 15 kilometers and can be clearly seen to meander

Figure 1

A cross section of the field shows that the channel is only partially filled by sand and

is partly plugged by clay ( Figure 2 West-east cross-section A-Z of two Lower

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Depositional Traps Channels

Many oil and gas fields occur trapped within channels of various types rangingfrom meandering fluvial deposits through deltaic distributary channels to deep-sea channels

Many good examples of stratigraphic traps in channels can be found in theCretaceous basins along the eastern flanks of the Rocky Mountains from Alberta through Montana Wyoming Colorado and New Mexico These channelsoccur both cut into a major pre-Cretaceous unconformity and within theCretaceous strata

The South Glenrock oil field in Wyoming contains oil trapped in both marine-bar and

fluvial-channel reservoirs The channel reservoir has a width of some 1500 metersand a maximum thickness of approximately 15 meters ( Figure 1 Isopach map of

Lower Muddy interva South Glenrock oil field Wyoming) It has been mapped for adistance of over 15 kilometers and can be clearly seen to meander

Figure 1

A cross section of the field shows that the channel is only partially filled by sand and

is partly plugged by clay ( Figure 2 West-east cross-section A-Z of two Lower

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Muddy stream channels)

Figure 2

The SP curves on some of the well logs (eg wells 5 and 6 on Figure 2 ) display

upward-fining point-bar sequences a characteristic of meandering channel deposits

The South Glenrock field illustrates an important points about channel stratigraphic

traps Because of their limited areal extent and thickness such reservoirs seldomcontain giant accumulations

The deltaic distributary channel of Oklahoma is a good example of channel traps insands other than the meandering fluvial variety ( Figure 3 Isopach map of Booch

sandstone greater Seminole district eastern Oklahoma)

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Figure 3

Depositional Traps Bars

Because of their clean well-sorted texture marine barrier bars often make excellentreservoirs (Hollenshead and Pritchard 1961)

The barrier sands may coalesce to form blanket reservoirs Oil may then be

structurally or stratigraphically trapped within these blanket sands Sometimes

however isolated barrier bars may be totally enclosed in marine or lagoonal shalesforming stratigraphic traps in shoestring sands elongated parallel to the paleo

shoreline ( Figure 1 Schematic of barrier bars showing interconnedted barsforming blanket reservoir and one isolated bar set )

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Figure 1

The Rocky Mountain Cretaceous basins contain many barrier bar stratigraphic traps

The Bisti field in the San Juan basin New Mexico is a classic barrier bar sand

(Sabins 1963 1972) The field is about 65 kilometers long and 7 kilometers wide (Figure 2 Bar sandstone isopach map of Bisti field Colorado)

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Figure 2

It consists of three stacked sandbars with an aggregate thickness of 15 meters

totally enclosed in the marine Mancos shale ( Figure 3 North-south cross-section A-

Z of Bisti field using electric logs)

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Figure 3

The SP log in some wells shows the typical upward-coarsening grain-size motif which

characterizes barrier bars (See inset Figure 3 ) Two other examples of barrier bar

stratigraphic traps are the Bell Creek field Montana (Berg and Davies 1968McGregor and Biggs 1970 1972) and the Recluse field Wyoming (Woncik 1972)

During a regressive stage barrier bars often develop as sheet sands which may

pass updip into lagoonal or intertidal shales causing pinch-out or feather-edge traps(Selley 1982) As with many sheet reservoirs lateral closure must occur for the trap

to be valid This may be stratigraphic as for example where an embayment occurs

in a shoreline Alternatively it may be structural in which case the trap might bemore properly classified as a combination trap (Selley 1982)

Depositional Traps Reefs

The reef or carbonate build-up trap has a rigid stoney framework containing highprimary porosity (Maxwell 1968 Jones and Endean 1973) Reefs grow as discrete

domal or elongated barrier features and have long been recognized as one of themost important types of stratigraphic traps

Reefs are often later transgressed by organic-rich marine shales (which may act assource rocks) or the reefs may be covered by evaporites Oil or gas may be trapped

stratigraphically within the reef with the shales or evaporites providing excellentseals

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In Alberta Canada the Devonian-age Rainbow reefs in the Black Creek Basinprovide an excellent example of reef traps (Barss et al 1970) More than seventy

individual reefs containing various amounts of oil and gas were discovered withinan area about 50 kilometers long and 35 kilometers wide Total reserves of these

reefs are estimated in excess of 15 billion barrels of oil in place and one trillion cubicfeet of gas

As shown in Figure 1 (Schematic cross-section through Middle Devonian reefsRainbow area Alberta Canada) two basic geometric forms of reefing are present

the pinnacle reef and the broader elliptical form of the atoll reef

Figure 1

The individual reefs are up to 15 square kilometers in area and up to 250 meters

high in relief At the end of reefal growth evaporite sediments infilled the basin The

evaporites completely covered the reefs thereby providing an excellent seal forhydrocarbon entrapment

There is a wide range of net pays found in the Rainbow reefs ( Figure 1 ) Somereefs are nearly full of oil and gas while others contain a very small column of oil or

gas at the very crest of the reef Porosities and permeabilities also differ greatly from

reef to reef as well as within individual reefs Such changes are due to variations inlithofacies and diagenetic effects and are typical features of reefal traps ( Figure 2

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Cross-section of pinnacle reef showing complex lithofaciesRainbow area AlbertaCanada)

Figure 2

There are many other reef hydrocarbon provinces around the world notably in theArabian Gulf and Libya In Libya the Intisar reefs in the Sirte basin have been well

documented (Terry and William 1969 Brady et al 1980)

UNCONFORMITY-RELATED TRAPS Another major group of stratigraphic traps is represented by traps for whichan unconformity is essential (Table 1) (Levorsen 1934)

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Table 1

Significantly large percentages of the known global petroleum reserves aretrapped adjacent to major unconformities In addition to being held in pure

stratigraphic traps many of these reserves are held in structural andcombination traps as well Unconformity-related traps can be subdivided intothose which occur above the unconformity and those beneath (Figure 1Schematic of traps located above and below an unconformity)

Figure 1

Traps which occur above an unconformity will be discussed first

Shallow-marine or fluvial sands may onlap a planar unconformity Astratigraphic trap can occur where such sands are overlain by shales and areunderlain by impermeable rock which provides a seat seal Onlapping updippinch-out sands such as these could occur as sheets (Figure 2a Schematicof onlapping pinch-out sands-- occurring as a sheet deposit) or as discrete

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paleogeomorphic traps (Figure 2b Schematic of onlapping pinch-out sands--occurring as a discrete paleogeomorphic sand)

Figure 2a 2b

A good example of an onlap stratigraphic trap is provided by the Cut Bankfield of Montana with recoverable reserves of over 200 million barrels of oil(MacKenzie 1972) Here the Cretaceous Cut Bank sand unconformablyonlaps Jurassic shales and is itself onlapped by younger shales (Blixt 1941Shelton 1967) Figure 3 (Southwest-northeast E-log correlation section A-ZCut Bank sandstone Montana) is a cross section through this field

Figure 3

One type of paleogeomorphic trap is represented by channels which cut intothe unconformity another occurs where sands are restricted within strikevalleys cut into alternating hard and soft strata (Figure 4 Schematic of channel and strike valley sands above an unconformity) (Harms 1966

Martin 1966 and McCubbin 1969)

Figure 4

It is important to note that closure is necessary along the strike of such trapsnot just updip as shown in Figure 2a In Figure 5 (Schematic of sandstonepinch-out intersecting with a structural nose) closure is provided by the

intersection of a sandstone pinch-out with a structural nose

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Figure 5

The second group of traps associated with unconformities is truncation trapswhich occur beneath the unconformities (Figure 6 Schematic of traps below

unconformity)

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Figure 6

Again it is generally overlying shales which provide a seal (and often thesource as well) for such traps As with onlap pinch-out and paleogeomorphic

traps closure is needed in both directions along the strike (Figure 7Schematic of trap below unconformity featuring closure provided by theintersection of a dipping structural nose and a flat unconformity)

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Figure 7

This may be structural or stratigraphic but for many truncation traps it may beprovided by the irregular topography of the unconformity itself such as a

buried hill providing closure for a subcropping sandstone formation (Figure 8Schematic of trap below unconformity featuring closure provided by buriedhill)

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Figure 8

Many truncation traps have had their reservoir quality enhanced by secondary

solution porosity due to weathering Secondary solution porosity induced by

weathering is most common in limestones but also occurs in sandstones and

even basement rock Examples in limestones are found in Kansas and in the

Auk field of the North Sea (Brennand and van Veen 1975) Development of

subunconformity solution porosity in sandstones has occurred in the Brent

Sand of the North Sea (Bowen 1972) and in the Sarir group of Libya (Selley

1982) Basement rock weathering is found in the Augila field of Libya

(Williams 1968 1972) One of the best known truncation traps in the world is

the East Texas field (Halbouty 1972 Halbouty and Halbouty 1982) which

contained over 5 billion barrels of recoverable oil The trap is caused by the

truncation of the Cretaceous Woodbine sand by the overlying impermeable

Austin chalk (Figure 9 Generalized west-east cross-section East Texas

basin)

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Figure 9

It has a length of some 60-70 kilometers and a width of nearly ten kilometers

Figure 10 (Structural contours on top of Woodbine sand East Texas field)

illustrates the structural closure at the northern and southern ends of the field

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Figure 10

HYDRODYNAMIC TRAPS

Imagine surface water perhaps from rain entering a reservoir formation or

aquifer up in the hills andpercolating downwards towards a spring Oil has found its way

into the reservoir and is battling tomigrate upwards to the surface against the flow of

water Depending on the balance of forces acting onthe oil it may find itself caught

against an unevenness of the reservoir surface where there is noconventional trap at all

This is what has been described as a hydrodynamic trap It is totally dependenton the

flow of water and is effective of course only for as long as the water keeps coming dry

up thesupply of water and the oil will be free to move again This may be one of the

reasons why oilaccumulations trapped hydrodynamically are rare a regime of water flow

cannot normally be expectedto remain constant for long geologically speaking

The oil-water contact in such a hydrodynamic trap is normally tilted in the direction of

water flowSuch tilted contacts in say ordinary anticlinal traps are not all that rare they

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are known in a numberof parts of the world In this sort of situation we would have to be

careful where we locate and drill ouroil production wells as we do not want to waste the

money drilling wells that would miss the oilaltogether Furthermore cases are known

where flowing water has apparently been able totally to flushoil out of an anticlinal trap

We would recognize this from residual traces of oil in a water-bearingreservoir

indicating the former presence of an oil accumulation now lost It is therefore

alwaysimportant to get a handle on the hydrodynamic regime in a reservoir for both

exploration andoilfielddevelopment purposes

Diapir Associated Traps

Diapirs are a major mechanism for generating many types of traps

Diapirs are produced by the upward movement of less dense

sediments usually salt or overpressured clay Recently-deposited clay

and sand have densities less than salt which has a density of about

216 gcm3

As most sediments are buried they compact gaining density

ultimately a depth is reached where sediments are denser than salt

This generally occurs between 800 and 1200 meters When this

situation is reached the salt tends to flow upwards to displace the

denser overburden If this movement is triggered tectonically the

resulting structure may show some alignment such as that displayed

by the salt domes in the North Sea ( Figure 1 Salt structures of the

southern North Sea) However in many cases the salt movement is

apparently initiated at random

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are known in a numberof parts of the world In this sort of situation we would have to be

careful where we locate and drill ouroil production wells as we do not want to waste the

money drilling wells that would miss the oilaltogether Furthermore cases are known

where flowing water has apparently been able totally to flushoil out of an anticlinal trap

We would recognize this from residual traces of oil in a water-bearingreservoir

indicating the former presence of an oil accumulation now lost It is therefore

alwaysimportant to get a handle on the hydrodynamic regime in a reservoir for both

exploration andoilfielddevelopment purposes

Diapir Associated Traps

Diapirs are a major mechanism for generating many types of traps

Diapirs are produced by the upward movement of less dense

sediments usually salt or overpressured clay Recently-deposited clay

and sand have densities less than salt which has a density of about

216 gcm3

As most sediments are buried they compact gaining density

ultimately a depth is reached where sediments are denser than salt

This generally occurs between 800 and 1200 meters When this

situation is reached the salt tends to flow upwards to displace the

denser overburden If this movement is triggered tectonically the

resulting structure may show some alignment such as that displayed

by the salt domes in the North Sea ( Figure 1 Salt structures of the

southern North Sea) However in many cases the salt movement is

apparently initiated at random

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Figure 1

Movement of salt develops several structural shapes from deep-

seated salt pillows which generate anticlines in the overlying sediment

to piercement salt domes which actually pierce the overlying strata (

Figure 2 Schematic cross-section showing two salt structures a salt

pillow on the right and a piercement salt dome on the left ) (Bishop

1978) In extreme cases salt diapirs can actually penetrate to the

surface as in Iran (Kent 1979)

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Figure 2

There are many ways in which oil can be trapped on or adjacent to salt

domes (Halbouty 1972) ( Figure 3 Schematic cross-section showing

the varieties of hydrocarbon traps associated with piercement salt

domes)

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Figure 3

There may be simple structural anticlinal or domal traps over the crestof the salt dome Notable examples of this type include the Ekofisk

field (Van der Bark and Thomas 1980) and associated fields of

offshore Norway and Denmark There may also be complexly-faulted

domal traps stratigraphic pinch-out and truncation traps or

unconformity truncation traps

Occasionally anticlinal structures known as turtle-back structures are

developed between adjacent salt domes When the salt moves into a

dome the source salt is removed from its flanks thereby developing

rim synclines Thus anticlines develop above the remaining salt (

Figure 4 Schematic cross-section showing a turtleback structure

(anticline) developed between two adjacent piercement salt domes)

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The Bryan field of Mississippi is an example of a turtle-back trap

(Oxley and Herling 1972)

Figure 4

Major oil and gas production from salt-dome-related traps comes from

the US Gulf Coast Iran the Arabian Gulf and the North Sea

Diapiric mud structures not just salt domes may also generate

hydrocarbon traps Sometimes diapirs of overpressured clay intrude

the younger denser cover and just like salt domes mud lumps may