Trap

PETROLEUM GEOLOGY Series 1 Introduction and Overview F. TRAPS F.1. Hydrocabon Traps: Introduction Definitions and Concepts Many terms are used to describe the various parts of a trap. The anticlinal trap, the simplest 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, stringers

shale within

a

reservoir unit contribute to gross pay but not to net pay ( Figure 2 , Facies change in an Figure 2.

Dicky Haris Hidayat’s Library: Traps

anticlinal

trap,

1

PETROLEUM GEOLOGY Series 1 Introduction and Overview illustrating the difference between net pay and gross pay). Net pay refers only to the possibly productive reservoir. A trap may contain oil, gas or a combination of the two. The oil-water contact, OWC, is the deepest level of producible oil within an individual reservoir (

Figure 3.

Figure 3a , Fluid contacts within a reservoir in an oil-water system). It marks the interface between predominately oil-saturated rocks and water-saturated rocks. Similarly, either the gaswater 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.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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 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 (e.g. 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 , 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.

Figure 4.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

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. Fluid contacts in a trap are

almost

always

planar but are by no means horizontal.

always Should

a

tilted fluid contact be present,

its

early

recognition is essential for correct evaluation of Figure 5.

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).

Figure 6.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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 pools within an oil and gas field). Each individual pool may contain one or more pay zones.

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

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PETROLEUM GEOLOGY Series 1 Introduction and Overview Fold Traps: Compressional Folds Compactional Folds Diapir Folds

Tectonic processes Depositional / Tectonic processes Tectonic Processes

F.2. Structural Traps Structural Traps The geometry of a structural trap is due essentially to some post-depositional modification of the reservoir. In the words of Levorsen (1967) "A structural trap is one whose upper boundary has been made concave, as viewed from below, by some local deformation, such as folding, or faulting, or both, of the reservoir rock." Selley (1982) has further defined the boundaries of a structural trap, thus "The edges of a pool occurring in a structural trap are determined wholly or in part by the intersection of the underlying water table with the roof rock overlying the deformed reservoir rock." Structural traps are divided into those due to folding, and those due to faulting.

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 earth's crust ( Figure 1 , Active continental margin with net shortening of crust- subduction zone). 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 organicrich basinal muds are overlain by deepsea sands and capped by younger continental beds as shown by Figure 2

Dicky Haris Hidayat’s Library: Traps Figure 1.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview (Generalized west-southwest-east-northeast structural cross-section), a cross- section of the Los Angeles basin. 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.

Figure 2.

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. 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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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 3.

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-Z, Wilmington field). The main

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PETROLEUM GEOLOGY Series 1 Introduction and Overview reservoir occurs beneath the Pliocene unconformity in Miocene- and Pliocene-age deep-sea sands. 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).

Individual anticlines Figure 5.

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 3.5 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) 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).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 6.

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 Figure 7.

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) beneath a thrust plane, which itself is involved in thrusting along its southeastern border.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

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.

Figure 9.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

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 ).

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 the underlying structural high ( Figure 2a , Developmental stages of compactional anticlines--initial stage of deposition). 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).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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 deepwater muds present further down the flanks. For this reason, reservoir quality often diminishes down the flank of such structures. 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 Figure 2.

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.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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 crosssection A-Z through Forties field, North Sea). 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 occurring

folds over

deep-

seated horst blocks have been

discussed.

Compaction

folds,

however, may also occur over

reefs

deep-seated

and

other rigid

features. Figure 4.

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. 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.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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

Figure 1.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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). The fault is downthrown to the southeast

with

metamorphosed

impermeable Tertiary

flysch

comprising the upthrown block and younger Tertiary unmetamorphosed sediment

comprising

downthrown

block.

It

the 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).

Figure 2.

There are two requirements for this trap to be valid. First, the rocks in the upthrown fault block adjacent reservoir

to

the

rocks

impermeable.

down-thrown must

Second,

be 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.

Figure 3.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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). 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

characteristic

fault)

downthrown

illustrates reversal

the of

regional dip as the beds slump into the fault plane. 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. Figure 4.

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

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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 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

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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). 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, pinchouts, and the fault itself provide traps for an estimated 3 billion barrels of recoverable oil and 20 trillion cubic feet of gas. In southern Louisiana's 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 anticlinal feature down-thrown to a major

growth

fault.

Figure 8

(Structural contours on top of Pliocene 10 sand, Vermilion Block 76 field, offshore Louisiana) is a structure map Figure 8.

on one of the producing sands, illustrating the downthrown anticlinal

development. A north-south cross section of the field shows the downthrown anticlinal structure as well as the downthrown expansion of the sedimentary column ( Figure 9 , North-south crosssection of the Vermillion Block 76 field, offshore Louisiana).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

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 several kilometers in length, and their curved traces develop scalloped fault patterns ( Figure 10 , Structural styles and hydrocarbon distribution, Niger Delta).

Figure 10.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Exercise 1. Label the parts of a trap indicated in Figure 1 .

Figure 1.

See Figure 2 . (same as figure above) Exercise 2. What is the difference between gross pay and net pay, and why is it important? Cross pay refers to the total thickness of a given-reservoir unit, whether it is a productive reservoir or not. Net pay refers only to the possibly productive reservoir. This distinction is very important since two neighboring wells may penetrate the same gross pay interval (e.g. 50 ft.), but the net pays may differ. For example, well 1 penetrates 40 ft of producing sand and 10 ft of non-producing shale in the "A" reservoir, while neighboring well 2 penetrates 10 ft of sand and 40 ft of shale in the same "A" reservoir. Exercise 3. Construct a cross section through a compactional anticline.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview Hint: Use a horst-and-graben basal layer. The sediments should be thinner on the crest, and thicker in the troughs, with the anticlines developed over the crests. ( Figure 1 )

Figure 1.

Exercise 4. There are some important differences between compactional and compressional fold traps. Below is a listing of 12 features, each pertaining to only one of the two fold types. Please fill in the appropriate title. _____ folds form contemporaneously with sedimentation and initial burial. _____ folds commonly form in one major tectonic event. _____ folds sometimes have secondary porosity (diagenetic) developed on their crests, due to leaching and sub-aerial exposure. _____ folds sometimes have secondary porosity which is fracture-related. _____ folds are often irregularly shaped due to shape of underlying features. _____ folds may have a lengthy history due to rejuvenation of underlying basement faults _____ folds are usually lithified before deformation, so the rocks are brittle.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview _____ folds have their primary porosity often preserved throughout. _____ folds contain sediments which deform soon after deposition, while still soft. _____ folds are generally oriented with their long axis perpendicular to direction of crustal shortening. _____ folds sometimes have primary porosity which may vary between crest and flank. _____ folds usually form long after sedimentation and initial burial.

Compaction and Compression Folds Compaction folds: •

form contemporaneously with sedimentation and initial burial.

•

sometimes have secondary porosity (diagenetic) developed on their crests, due to leaching and sub-aerial exposure.

•

are often irregularly shaped due to shape of underlying features.

•

may have a lengthy history due to rejuvenation of underlying-basement faults.

•

contain sediments which deform soon after deposition, while still soft.

•

sometimes have primary porosity which may vary between crest and flank.

Compression folds: •

commonly form in one major tectonic event.

•

sometimes have secondary porosity which is fracture-related.

•

are usually lithified before deformation, so the rocks are brittle.

•

have their primary porosity often preserved throughout.

•

are generally oriented with long axis perpendicular to direction of crustal shortening.

•

usually form long after sedimentation and initial burial.

Exercise 5. Construct a cross section through a mature growth fault showing the various types of traps which may be present. Your answer should show a mature growth fault and a secondary, antithetic fault ( Figure 1 ).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 1.

The sedimentary units should increase in thickness across the growth fault, since the faulting occurred during deposition. The antithetic fault should separate equivalent unit thicknesses, since this faulting occurred after deposition. The hydrocarbon traps will be found in two locations: 1. major accumulations are in the rollover anticlines, down-thrown to the growth fault; and 2. there are accumulations trapped in pure fault traps up against the main fault and the antithetic fault.

F.3. Stratigraphic Traps Depositional Traps 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 stratigraphic traps can be found in King (1972), Busch (1974), and Conybeare (1976).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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 are transitional between clearly-defined types.

Table 1.

Table 1 , (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 , Schematic of trap within normal conformable sequence) and those that are associated with unconformities ( Figure 2 , Schematic of traps associated with unconformaties). 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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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 1.

Figure 2.

Figure 3.

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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PETROLEUM GEOLOGY Series 1 Introduction and Overview Depositional Traps: Channels Many oil and gas fields occur trapped within channels of various types, ranging from meandering fluvial deposits through deltaic distributary channels to deep-sea channels. Many good examples of stratigraphic traps in channels can be found in the Cretaceous basins along the eastern flanks of the Rocky Mountains, from Alberta, through Montana, Wyoming, Colorado and New Mexico. These channels occur both cut into a major pre-Cretaceous unconformity and within the Cretaceous strata. The South Glenrock oil field in Wyoming contains oil trapped in both marine-bar and fluvialchannel reservoirs. The channel reservoir has a width of some 1500 meters and 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 a distance 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 Muddy stream channels). The SP curves on some of the well logs (e.g. wells #5 and #6 on Figure 2 ) display upward-fining point-bar sequences, a characteristic of meandering channel deposits.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

The South Glenrock field illustrates an important

points

about

channel

stratigraphic traps. Because of their limited areal extent and thickness, such reservoirs seldom contain giant accumulations.

Figure 2.

The deltaic distributary channel of Oklahoma, is a good example of channel traps in sands other than the meandering fluvial variety ( Figure 3 , Isopach map of Booch sandstone, greater

Seminole

district,

eastern

Oklahoma).

Figure 3.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview Depositional Traps: Bars Because of their clean well-sorted texture, marine barrier bars often make excellent reservoirs (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 shales, forming stratigraphic traps in shoestring sands elongated parallel to the paleo shoreline ( Figure 1 , Schematic of barrier bars, showing interconnedted bars forming blanket reservoir and one isolated bar set).

Figure 1.

Figure 2.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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). 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). The SP log in some wells shows the

typical

upward-coarsening

grain-size

motif

which

characterizes barrier bars. (See inset,

Figure 3

examples

of

.)

Two

other

barrier

bar

stratigraphic traps are the Bell Creek field, Montana (Berg and Davies, Biggs,

1968; 1970,

McGregor 1972);

and

and the

Recluse field, Wyoming (Woncik, 1972). Figure 3.

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 be more properly classified as a combination trap (Selley, 1982).

Depositional Traps: Reefs The reef or carbonate build-up trap has a rigid stoney framework containing high primary porosity (Maxwell, 1968; Jones and Endean, 1973). Reefs grow as discrete domal or elongated barrier features, and have long been recognized as one of the most important types of stratigraphic traps. Reefs are often later transgressed by organic-rich marine shales (which may act as source 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 excellent seals.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview In Alberta, Canada, the Devonian-age Rainbow reefs in the Black Creek Basin provide an excellent example of reef traps (Barss et al., 1970). More than seventy individual reefs, containing various amounts of oil and gas, were discovered within an area about 50 kilometers long and 35 kilometers wide. Total reserves of these reefs are estimated in excess of 1.5 billion barrels of oil in place and one trillion cubic feet of gas. As shown in Figure 1 (Schematic cross-section through Middle Devonian reefs, Rainbow area, Alberta, Canada), two basic geometric forms of reefing are present: the pinnacle reef and the broader elliptical form of the atoll reef. 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 for hydrocarbon entrapment. There is a wide range of net pays found in the Rainbow reefs ( Figure 1 ). Some reefs 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

individual

well

as

reefs.

within Such

changes are due to variations Figure 1.

in lithofacies and diagenetic

effects, and are typical features of reefal traps ( Figure 2 , Cross-section of pinnacle reef showing complex lithofacies,Rainbow area, Alberta, Canada).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 2.

There are many other reef hydrocarbon provinces around the world, notably in the Arabian Gulf and Libya. In Libya, the Intisar reefs in the Sirte basin have been well documented (Terry and William, 1969; Brady et al., 1980).

Diagnenetic Traps Diagenetic traps are formed by the creation of secondary porosity in a non-reservoir rock by replacement, solution or fracturing with the tight unaltered rock forming the seal for the trap (Rittenhouse, 1972).

Figure 1.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview An example of a diagenetic trap formed by replacement is the Deep River field in Michigan, in which dolomitization of a preexisting limestone deposit has resulted in the formation of secondary intercrystalline porosity ( Figure 1 ). The development of solution porosity is commonly associated with carbonate rocks ( Figure 2 ), but may involve sandstones as well. Fracturing

can

cause

secondary

porosity in any brittle rock — whether carbonate, sandstone, shale, igneous or metamorphic

rock

Ravenscroft,

1977).

(Kostura The

and

Spraberry

trend in west Texas forms a series of diagenetic traps (with oil reserves of about one billion barrels) within a producing fairway about 240 kilometers Figure 2.

long and 80 kilometers wide (Wilkinson 1953). A structure map contoured on

the productive Spraberry formation, a 300-meter-thick section of tight Middle Permian shales, siltstones, limestones, and fine-grained sandstones shows that in the southern Midland basin, the areas of oil production have little relationship to structure ( Figure 3 ). Production comes from areas of fracturing throughout the otherwise tight Spraberry formation. The depositional and diagenetic stratigraphic traps just considered occur in normal comformable sequences, although they may also occur at unconformities.

Figure 3.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview Unconformity-Related Traps Another major group of stratigraphic traps is represented by traps for which an unconformity is essential ( Table 1 ) (Levorsen, 1934).

Table 1. (no table exist) Significantly large percentages of the known global petroleum reserves are trapped adjacent to major unconformities. In addition to being held in pure stratigraphic traps, many of these reserves are held in structural and combination traps as well. Unconformity-related traps can be subdivided into those which occur above the unconformity and those beneath ( Figure 1 , Schematic of traps located above and below an unconformity). Traps which occur above an unconformity will be discussed first.

Figure 1.

Shallow-marine or fluvial sands may onlap a planar unconformity. A stratigraphic trap can occur where such sands are overlain by shales and are underlain by impermeable rock which provides a seat seal. Onlapping updip pinch-out sands such as these could occur as sheets ( Figure 2a , Schematic of onlapping pinch-out sands-- occurring as a sheet deposit) , or as discrete paleogeomorphic traps ( Figure 2b , Schematic of onlapping pinch-out sands--occurring as a discrete paleogeomorphic sand).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview A good example of an onlap stratigraphic trap is provided by the Cut Bank field of Montana, with recoverable reserves of over 200

million

barrels

(MacKenzie,

1972).

Cretaceous

Cut

unconformably

of

oil

Here

the

Bank

onlaps

sand

Jurassic

shales and is itself onlapped by younger Shelton,

shales

(Blixt,

1967).

1941;

Figure 3

(Southwest-northeast

,

E-log

correlation section A-Z, Cut Bank sandstone, Montana) is a cross section through this field.

Figure 2.

Figure 3.

One type of paleogeomorphic trap is represented by channels which cut into the unconformity; another occurs where sands are restricted within strike valleys 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). It is important to note that closure is necessary along the strike of such traps, not just updip as shown in Figure 2a . In Figure 5 (Schematic of sandstone pinch-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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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 4.

Figure 5.

The second group of traps associated with unconformities is truncation traps which occur beneath the unconformities ( Figure 6 , Schematic of traps below unconformity).

Figure 6.

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36

PETROLEUM GEOLOGY Series 1 Introduction and Overview Again, it is generally overlying shales which provide a seal (and often the source as well) for such traps. As with onlap, pinch-out, and

paleogeomorphic

closure directions

is

needed along

the

traps, in

both

strike

(

Figure 7 , Schematic of trap below unconformity, Figure 7.

featuring

closure

provided by the intersection of a dipping structural nose and a flat

unconformity). This may be structural or stratigraphic but for many truncation traps, it may be provided by the irregular topography of the unconformity itself, such as a buried hill providing closure for a subcropping sandstone formation ( Figure 8 , Schematic of trap below unconformity, featuring closure provided by buried hill).

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).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview 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).

Figure 9.

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). 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. Figure 10.

Dicky Haris Hidayat’s Library: Traps

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PETROLEUM GEOLOGY Series 1 Introduction and Overview Exercise 1. On the left is a listing of stratigraphic trap types (a-i). Match each with the appropriate stratigraphic trap categories (1-4) from the right column. (There may be some types which occur in more than one category.) TRAP TYPE

TRAP CATEGORIES

a) Barrier bar trap

_____

b) Truncation trap

_____

c) Reef trap

_____

1) Depositional

d) Fracture-related trap

_____

2) Diagenetic

e) Seconary-solution trap

_____

Related to Uncomformities

f) Onlap trap

_____

3) Above unconformity

g) Replacement trap

_____

4) Below unconformity

h) Channel trap

_____

i) Strike-valley trap

_____

Within Normal Conformable Sequences

Solution 1: a) Barrier bar trap

1

b) Truncation trap

4

c) Reef trap

1

d) Fracture-related trap

2

e) Secondary-solution trap

2&4

f) Onlap trap

1&3

g) Replacement trap

2

h) Channel trap

1&3

I) Strike-valley trap

3

Exercise 2. Construct a cross section through an unconformity showing some of the different types of stratigraphic traps which can occur.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Exercise 3. Your company is developing the concession on the maps shown in Figure 1 , Figure 2 and Figure 3. A seismic reflector is contoured on the map; it conforms to the paleoslope of the sediments. Well A found oil in a barrier bar sand above the reflector at 2100m. Well B found oil in a channel sand below the reflector at 2300m, but did not encounter the barrier bar sand above the reflector. Indicate on the map the most likely extent of the two fields. Then construct a dip-cross section along line S-T, and a strike-cross section along line U-V. Pick the best well location for reservoir development.

Figure 1.

Dicky Haris Hidayat’s Library: Traps

40

PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 2.

Figure 3.

Figure 3. The approximate extent of the two traps is shown in Figure 4 , Figure 5 and Figure 6 , along with the next location. This combines the opportunities of testing the southeastward extension of the barrier bar sand, and the southwestward extention of the channel sand.

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41

PETROLEUM GEOLOGY Series 1 Introduction and Overview F.4. Other Trap Types Hydrodynamic Traps In a hydrodynamic trap, a downward movement of water prevents the upward movement of oil or gas. Pure hydrodynamic traps are extremely rare, but a number of traps result from the combination of hydrodynamic forces and structure or stratigraphy. An ideal hydrodynamic trap is shown in Figure 1 (Schematic cross-section of an ideal hydrodynamic trap). A monoclinal flexure is developed which has no genuine vertical closure; oil could not be trapped within it in a normal situation. Groundwater, however, is moving down through a permeable bed and is preventing the upward escape of oil. Oil is trapped in the monoclinal flexure above a tilted oil-water contact. Pure hydrodynamic traps like this, however, are very rare.

Figure 1.

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42

PETROLEUM GEOLOGY Series 1 Introduction and Overview There are a number of fields with tilted oil-water contacts where

entrapment

is

a

combination of both structure and

hydrodynamic

forces

(

Figure 2 , Schematic crosssection from

showing both

hydrodynamic

entrapment

structural

and

forces).

For

further discussion of the effect of hydrodynamic conditions on hydrocarbon traps, the reader is referred

to

Goebel

(1950),

Hubbert (1953), Yuster (1953), Figure 2.

and Eremako and Michailov (1974).

Combination Traps Combination traps result from two or more of the basic trapping mechanisms ( structural, stratigraphic, and hydrodynamic ). Since there are many ways in which combination traps can occur, a few examples must suffice for explanation. In the Main Pass Block 35 field of offshore Louisiana, a rollover anticline has developed to the south of a major growth fault (Hartman, 1972) ( Figure 1 , Structural contours on top of 'G2' sandstone, Main Pass Block 35, offshore Louisiana). The rollover anticline, however, is crosscut by a channel. Oil with a gas cap occurs only within the channel; thus, the trap is due to a combination of structure and stratigraphy.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 1.

Figure 2.

An excellent example of a combination trap is provided by the Prudhoe Bay field on the North Slope of Alaska (Morgridge and Smith, 1972; Jones and Speers, 1976; Jamison et al., 1980; Bushnell, 1981). A series of Carboniferous-through-basal-Cretaceous strata were folded into a westerly-plunging anticlinal nose ( Figure 2 , Structural contours on top of Sadlerochit reservoir, Prudhoe Bay, Alaska). This nose was truncated progressively from the northeast, and overlain by Cretaceous shales which acted as source and seal to the trap. Oil and gas were trapped in

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44

PETROLEUM GEOLOGY Series 1 Introduction and Overview reservoir beds subcropping the unconformity, primarily in the Triassic Sadlerochit sandstone. Major faulting on the northern and southwestern side of the structure provided additional closure. Fault-unconformity combination

traps

characterize the northern North

Sea.

Jurassic

sandstone reservoirs exist in

numerous

blocks

tilted

which

fault were

truncated and overlain by Cretaceous

shales.

The

resulting traps include such fields

as

Brent

(Bowen,

1972), Ninian (Albright et al.,

1980),

and

(Maher,

1980).

section

through

Piper

A

cross

one

of

these, the Piper field, is Figure 3.

shown

in

Figure 3

(Southwest-northeast

structural cross-section, Piper field, North Sea).

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. Recentlydeposited clay and sand have densities less than salt which has a density of about 2.16 g/cm3. 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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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Figure 1.

Figure 2.

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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46

PETROLEUM GEOLOGY Series 1 Introduction and Overview 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). There may be simple structural anticlinal or domal traps over the crest of 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 complexlyfaulted domal traps, stratigraphic pinch-out and truncation traps , or unconformity truncation traps.

Figure 3.

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). The Bryan field of Mississippi is an example of a turtle-back trap (Oxley and Herling, 1972).

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PETROLEUM GEOLOGY Series 1 Introduction and Overview

Major oil and gas production from salt-dome-related

traps

comes

from the U.S. Gulf Coast, Iran, the Arabian Gulf and the North Sea. Diapiric mud structures, not just salt domes, may also generate hydrocarbon diapirs

of

traps.

Sometimes

overpressured

clay

intrude the younger, denser cover and, just like salt domes, mud lumps Figure 4.

may

even

reach

the

surface.

Exercise 1. Construct a cross section through: a) an ideal-hydrodynamic trap, and b) a combination trap, combining structure and hydrodynamic flow.

Figure 1.

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48

PETROLEUM GEOLOGY Series 1 Introduction and Overview Exercise 2. Why do diapirs form? What two sediments can flow diapirically? Diapirs form when relatively low-density sediments are overlain by denser ones. The two diapir-forming sediments are evaporites (salt), and overpressured clays. Exercise 3. Below is a profile of a piercement-salt diapir ( Figure 1 ) . Complete the cross section by drawing the various trap types which these piercement structures can cause.

Figure 1.

Ideally your cross section should contain the following trap types, as illustrated in Figure 2 A = anticlinal or domal trap; B = faulted domal trap;

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49

PETROLEUM GEOLOGY Series 1 Introduction and Overview C = other faulted trap; D = stratigraphic-pinch-out trap; E = stratigraphic-truncation trap; F = unconformity-truncation trap. Exercise 4. Why are many more giant oil fields known in anticlines than in all other trap types? Anticlines may contain multiple pay horizons while stratigraphic traps usually only contain one pay horizon. Also, it is easier to locate anticlinal traps than other types. They can be mapped at the surface, or can be detected seismically in the subsurface. Exercise 5. Why are stratigraphic traps generally smaller than structural traps? Stratigraphic traps usually occur in lenticular reservoirs, such as channels and barrier bars or in wedge shaped reservoirs, such as pinch-outs and truncations, or in pinnacle or atoll reefs. Thus they tend to be smaller than structural traps which usually occur in laterally extensive reservoirs, later deformed structurally. Also, stratigraphic traps generally contain a single pay horizon.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview References ALBRIGHT, W.A., TURNER, W.L., and WILLIAMSON, K.R., 1980, Ninian Field, U.K. Sector, North Sea, In: Giant Oil and Gas Fields of the Decade, 1968-1978, M.T. Halbouty, ed., A.A.P.G., Mem. 30, p. 173-193. BAILEY, R.J., and STONELEY, R., 1981, Petroleum: Entrapment and Conclusion, In: Economic Geology and Geotectonics, D.H. Tarling, ed., Blackwell, Oxford, p. 73-97. BARBAT, W.F., 1958, The Los Angeles Basin Area, California, In: The Habitat of Oil, L.G. Weeks, ed., A.A.P.G., Tulsa, OK, p. 62-77. BARSS, D.L., COPLAND, A.B., and RITCHIE, W.D., 1970. Geology of Middle Devonian Reefs, Rainbow Area, Alberta, Canada, In: Geology of Giant Petroleum Fields, M. Halbouty, ed., A.A.P.G., Mem. 14, p.19-49. BERG, R . R., and DAVIES, D.K., 1968, Origin of Lower Cretaceous Muddy Sandstone at Bell Creek Field, Montana, A.A.P.G. Bull., v. 52, p.1888-1898. BISHOP, R.S., 1978, Mechanism for Emplacement of Piercement Diapirs, A.A.P.G. Bull., v. 62, p. 1561-1584. BLIXT, J .E., 1941, Cut Bank Oil and Gas Field, Glacier County, Montana, In: Stratigraphic Type Oil Fields, A.A.P.G., Tulsa, OK, p.327-381. BOWEN, J.M., 1972, The Brent Oil Field, In: Stratigraphic Oil and Gas Fields, R.E. King, ed., A.A.P.G., Mem. 16, p. 353-360. BRENNAND, T.P., and VAN VEEN, F .R., 1975, The Auk Oil Field, In: Petroleum and the Continental Shelf of Northwest Europe, A.W. Woodland, ed., Applied Science Publishers, London, p. 275-284. BRUCE, C. H., 1973, Pressure Shale and Related Sediment Deformation: Mechanism for Development of Regional Contemporaneous Faults, A.A.P.G. Bull., v. 57, p. 878-886. BUSCH, D.A. , 1974, Stratigraphic Traps in Sandstones: Exploration Techniques, A. A. P.G., Mem. 21, 174 p.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview BUSHNELL, H., 1981, Unconformities - Key to N. Slope Oil, Oil & Gas Jl, Jan 12, p. 112-118. CHRISTIAN, H.E. Jr., 1969, Some Observations on the Initiation of Salt Structures of the Southern British North Sea, In: The Exploration for Petroleum in Europe and North Africa, P. Hepple, ed., Inst. Pet. London, p. 231-250. CLAPP, F.G., 1910, A Proposed Classification of Petroleum and Natural Gas Fields Based on Structure, Econ. Geol, v. 5, p. 503-521. CLAPP, F.G., 1917, Revision of the Structural Classification of the Petroleum and Natural Gas Fields, Geol. Soc. Am. Bull., v. 28, p.553-602. COHEE, G.V., and LANDES, K.K., 1958, Oil in the Michigan Basin, In: Habitat of Oil, L.G. Weeks, ed., A.A.P.G, Tulsa, OK, p. 473-494. COLMAN-SADD, S.P., 1978, Fold Development in Zagros Simply-Folded Belt, Southwest Iran, A.A.P.G. Bull., v. 62, p. 984-1003. CONYBEARE, C.E.B., 1976, Geomorphology of Oil and Gas Fields in Sandstone Bodies, Elsevier, Amsterdam, 341 p. CURRY, W. H., and CURRY, W.H. III, 1972, South Glenrock Oil Field, Wyoming: Prediscovery Thinking and Postdiscovery Description, In: Stratigraphic Oil and Gas Fields, R.E. King, ed., A.A.P.G., Mem. 16, p. 415-427. DEWEY, J. F., and BIRD, J.M., 1970, Mountain Belts and the New Global Tectonics, in Plate Tectonics and Geomagnetic Reversals, W.H. Freeman and Company, p. 610-631. DOTT, R.H., and REYNOLDS, M. J. , 1969, Sourcebook for Petroleum Geology, A.A.P.G. , Mem. 5, 471 p. EMERY, K.O., 1980, Continental Margins - Classification and Petroleum Prospects, A.A.P.G. Bull., v. 64, p. 297-315. EREMAKO, N.A., and MICHAILOV, 1974, Hydrodynamic Pools at Faults, Bull. Can. Petrol. Geol, v. 22, p. 106-118.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview EVAMY, D.D., HAREMBOURE, J., KAMERLING, P., KNAPP, W.A., MOLLOY, F.A., and ROWLANDS, P.H., 1978, Hydrocarbon Habitat of Tertiary Niger Delta, A.A.P.G. Bull., v. 62, p. 139. FALCON, N.L., 1958, Position of Oil Fields of Southwest Iran with Respect to Relevant Sedimentary Basins, In: The Habitat of Oil, L.G. Weeks, ed., A.A.P.G., Tulsa, OK, p. 1279-1293. FALCON, N. L, 1969, Problems of the Relationship Between Surface Structure and Deep Displacements Illustrated by the Zagros Range, In: Time and Place in Orogeny, Geol. Soc. Lond., Sp. Pub. No. 3, p. 9-22. FOWLER, C., 1975, The Geology of the Montrose Field, In: Petroleum and the Continental Shelf of North-West Europe, Vol. 1 - Geology, A.W. Woodland, ed., Applied Science Publishers, LTD, 501 p. GALLOWAY, W.E., HOBDAY, O.K., and MAGARA, K., 1982, Frio Formation of Texas Gulf Coastal Plain: Depositional Systems, Structural Framework, and Hydrocarbon Distribution, A.A.P.G. Bull., v. 66, p. 649-688. GOEBEL, L.A., 1950, Cairo field, Union County, Arkansas, A.A.P.G. Bull., v. 34, p. 1954-1980. HALBOUTY, M.T., 1967, Salt domes, Gulf Region, United States and Mexico, Gulf Pub. Corp., Houston, 425 p. HALBOUTY, M .T., 1972, Rationale for Deliberate Pursuit of Stratigraphic, Unconformity and Paleogeomorphic Traps, A.A.P.G. Bull., v. 56, p. 537-541. HALBOUTY, M.T., and HALBOUTY, J.J., 1982, Relationships Between East Texas Field Region and Sabine Uplift in Texas, A.A.P.G. Bull., v. 66, p. 1042-1054. HARMS, J.C., 1966, Valley Fill, Western Nebraska, A.A.P.G. Bull., v. 50, p. 2119-2149. HARTMAN, J.A., 1972, "G2", Channel Sandstone, Main Pass Block 35 Field, Offshore Louisiana, A.A.P.G. Bull., v. 56, p. 554-558. HERITIER, F.E., LOSSEL, P., and WATHNE, E., 1980, Frigg Field -Large Submarine-Fan Trap in Lower Eocene Rocks of the Viking Graben, North Sea, In: Giant Oil and Gas Fields of the Decade: 1968-1978, M.T. Halbouty, ed., A.A.P.G., Mem. 30, p. 59-79.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview HILL, P.J. and WOOD, G.V., 1980, Geology of the Forties Field, U.K. Continental Shelf, North Sea, In: Giant Oil and Gas Fields of the Decade: 1968-1978, M.T. Halbouty, ed., A.A.P.G., Mem. 30, p. 81-93. HOLLENSHEAD, C.T. & PRITCHARD, R.L., 1961, Geometry of Producing Mesaverde Sandstones, San Juan Basin, In: Geometry of Sandstone Bodies, J.A. Peterson and J.C. Osmond, eds. , A.A.P.G. , p. 98-118. HOBSON, G.D., and TIRATSOO, E.N., 1975, Introduction to Petroleum Geology, Scientific Press Ltd., Beaconsfield, England, 300 p. HUBBERT, M.K., 1953, Entrapment of Petroleum Under Hydrodynamic Conditions, A.A.P.G. Bull., v. 37, p. 1954-2026. HULL, C.E., and WARMAN, H.R., 1970, Asmari Oil Fields of Iran, In: Geology of Giant Petroleum Fields, M.T. Halbouty, ed., A.A.P.G., Mem. 14, p. 428-437. JAMISON, H.C., BROCKETT, L.D., and McINTOSH, R.A., 1980, Prudhoe Bay - A 10-Year Perspective, In: Giant Oil and Gas Fields of the Decade: 1968-1978, M.T. Halbouty, ed., A.A. P. G. , Mem. 30, p. 289-314. JANOSCHEK R., 1958, The Inner Alpine Vienna Basin, an Example of Small Sedimentary Area with Rich Oil Accumulation. In: The Habitat of Oil, L.G. Weeks, ed. , A.A.P.G. , Tulsa, OK, p. 1134-1152. JONES, H.P., and SPEERS, R.G., 1976, Permo-Triassic Reservoirs of Prudhoe Bay Field, North Slope, Alaska, In: North American Oil and Gas Fields, J. Braunstein, ed., A.A.P.G., Tulsa, OK, p. 23-50. JONES, O.A., and ENDEAN, R., 1973, Biology and Geology of Coral Reefs, (2 vols.), Academic Press, London, 410 p. & 435 p. KENT, P., 1979, The Emergent Hormuz Salt Plugs of Southern Iran, Jl. Petrol. Geol., v. 2, p. 117144. KING, R.E., ed., 1972, Stratigraphic Oil and Gas Fields -Classification, Exploration Methods, and Case Histories. A.A P. G., Mem. 16, 687 p.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview KOSTURA, J.R., and RAVENSCROFT, J.H., ed., 1977, Fracture-Controlled Production, A.A.P.G., Reprint Series No. 21, 221 p. LAMB, C.F., 1980, Painter Reservoir Field - Giant in Wyoming Thrust Belt, In: Giant Oil and Gas Fields of the Decade: 1968-1978, A.A.P.G. Mem. 30, Halbouty, .T., ed., p. 281-288. LEES, G.M., 1952, Foreland Folding, Quart. Jl. Geol. Soc., Lond., 108, p. 1-34. LEVORSEN, A .I., 1934, Relation of Oil and Gas Pools to Unconformities in the Mid-Continent Region, In: Problems of Petroleum Geology, Wrather, R.E. and Lahee, F.H., eds., A.A.P.G., Tulsa, OK, p. 761-784. LEVORSEN, A.I., 1966, The Obscure and Subtle Trap, A.A.P.G. Bull., v. 50, p. 2058-2067. LEVORSEN, A.I., 1967, Geology of Petroleum (2-nd Edition), S.H. Freeman & Co., San. Fran., 724 p. LOVELY, H.R., 1943, Classification of Oil Reservoirs, A.A.P.G. Bull., v. 27, p. 224. MACKENZIE, D.B., 1972, Primary Stratigraphic Traps in Sandstone, In: Stratigraphic Oil and Gas fields, R.E. King, ed., A.A.P.G., Mem. 16, p. 47-63. MAHER, C.E., 1980, Piper Oil Field, In: Giant Oil and Gas Fields of the Decade, 1968-1978, M.T. Halbouty, ed., A.A.P.G., Mem. 30, p. 131-172. MARTIN, R., 1966, Paleogeomorphology and its Application to Exploration for Oil and Gas (with examples from Western Canada), A.A.P.G. Bull., v. 50, p. 2277-2311. MAYFIELD, B.M. , 1973, Vermilion Block 76 Field, Vermilion Area, Offshore Louisiana, In: Offshore Louisiana Oil and Gas Fields, Lafayette Geological Society, p. 97-104. MAYUGA, M.N., 1970, California's Giant - Wilmington Oil Field, In: Geology of Giant Petroleum Fields, M.T. Halbouty, ed., A.A.P.G. , Mem. 14, p. 158-184. MAXWELL, W.G.H., 1968, Atlas of the Great Barrier Reef, Elsevier, Amsterdam, 242 p. McCUBBIN, D.G., 1969, Cretaceous Strike-Valley Sandstone Reservoirs, Northwestern New Mexico, A.A.P.G. Bull., v. 53, p. 2114-2140.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview McGREGOR, A.A., and BIGGS, C.A., 1970, Bell Creek Field, Montana: A Rich Stratigraphic Trap, In: Geology of Giant Petroleum Fields, M.T. Halbouty, ed., A.A.P.G., Mem. 14, p. 128-146. MOODY, J.D., 1975, Distribution and Geological Characteristics of Giant Oil Fields, In: Petroleum and Global Tectonics, A.G. Fischer and S. Judson, eds., Princeton Univ. Press., p. 307-320. MORGRIDGE, D. L., and SMITH, W.B., 1972, Geology and Discovery of Prudoe Bay Field, Eastern Arctic Slope, Alaska, In: Stratigraphic Oil and Gas Fields, R.E. King, ed., A.A.P.G., Mem. 16, p. 489-501. National Research Council, Ad Hoc Panel to Investigate the Geologic and Geolphysical Research Needs and Problems of Continental Margins, 1979, Continental Margins Geological and Geophysical Research Needs and Problems, Nat. Acad. Sci., Washington, 302 p. OXLEY, M.L., and HERLING, 0. E., 1972, The Bryan Field - a Sedimentary Anticline, Geophysics, 37, p. 59-67. RITTENHOUSE, G., 1972, Stratigraphic Trap Classification. Stratigraphic Oil and Gas Fields, R.E. King, ed., A.A.P.G., Mem. 16, p. 14-28. SABINS, F.F., Jr., 1963, Anatomy of a Stratigraphic Trap, A.A.P.G. Bull., 47, p. 193-228. SABINS, F.F., Jr., 1972, Comparison of Bisti and Horseshoe Canyon Stratigraphic Traps, San Juan Basin, New Mexico, In: Stratigraphic Oil and Gas Fields, R.E. King, ed., A.A.P.G., Mem. 10, p. 610-622. SCHWADE, I.T., CARLSON, S.A., and 0'FLYNN, J.B., 1958, Geologic Environment of Cuyuma Valley Oil Fields, California, In: The Habitat of Oil, L.G. Weeks, ed., A.A.P.G., Tulsa OK, p. 78-98. SELLEY, R.C., 1982, Principles of Petroleum Geology, W.H. Freeman and Co., In Press. SHELTON, J.W., 1967, Stratigraphic Models and General Criteria for Recognition of Alluvial, Barrier Bar and Turbidity-Current Sand Deposits, A.A.P.G. Bull., v. 51, p. 2441-61. SIMONSON, R.R., 1958, Oil in the San Joaquin Valley, In: The Habitat of Oil, L.G. Weeks, ed., A.A.P.G., Tulsa, OK, p. 99-112. SMITH, D.A., 1980, Sealing and Non-Sealing Faults in Louisiana Gulf Coast Salt Basin, A.A.P.G. Bull., v. 64, p. 145-172.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview STANLEY, T.B., 1970, Vicksburg Fault Zone, Texas. In: Geology of Giant Petroleum Fields, M.T. Halbouty, ed., A.A.P.G., Mem. 14, p. 301-308. TERRY, C.E., and WILLIAMS, J.J., 1969, The Idris 'A' Bioherm and Oilfield, Sirte Basin, Libya, In: The Exploration for Petroleum in Europe and North Africa, P. Hepple, ed., Inst. Pet., London, p. 31-48. VAN der BARK, E., and THOMAS, 0.0., 1980, Ekofisk: First of the Giant Oil Fields in Western Europe, In: Giant Oil and Gas Fields of the Decade, 1968-1978, M.T. Halbouty, ed., A.A.P.G., Mem. 30, p. 195-224. WEBER, K.J., and DAUKORU, F., 1975, Petroleum Geology of the Niger Delta, Trans. 9th Wld. Petrol. Congr., Tokyo, 2, p. 209-221. WILKINSON, W.M., 1953, Fracturing in Spraberry Reservoir, West Texas, A.A.P.G. Bull., v. 37, p. 250-265. WILLIAMS, J.J., 1968, The Stratigraphy and Igneous Reservoirs of the Augila Field, Libya, In: Geology & Archeology of Northern Cyrenaica, Libya, T.F. Barr, ed., p. 197-206. WILLIAMS, J., 1972, Augila Field, Libya: Depositional Environment and Diagenesis of Sedimentary Reservoir and Description of Igneous Reservoir, In: Stratigraphic Oil & Gas Fields, R.E. King, ed., A.A.P.G., Mem. 16, p. 623-632. WONCIK, J., 1972, Recluse Field, Campbell County, Wyoming. In: Stratigraphic Oil & Gas Fields, R.E. King, ed., A.A.P.G., Mem. 16, p. 367-382. YUSTER, S.T., 1953, Some Theoretical Considerations of Tilted Water Tables, Tech. Paper 3564, Trans. Amer. Inst. Min. Engrs, 198, p. 149-153.

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PETROLEUM GEOLOGY Series 1 Introduction and Overview Recommended Reading For major compilations of oil field case histories see: Braunstein, J., ed., 1976, North American Oil and Gas Fields, A.A.P.G., Mem. 24, 360 p. Halbouty M.T., ed., 1970, Geology of Giant Petroleum Fields, A.A.P.G., Mem. 14, 575 p. Halbouty, M.T., ed., 1980, Giant Oil and Gas' Fields of the Decade: 1968-1978, A.A.P.G., Mem. 30, 596 p. King, R.E., ed., 1972, Stratigraphic Oil and Gas Fields -Classification, Exploration Methods, and Case Histories, A.A.P.G., Mem. 16, 685 p. Woodland, A.W., ed., 1975, Petroleum and the Continental Shelf of North-West Europe, Volume I, Geology, Applied Science Publishers Ltd., 501 p.

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