Traps.doc

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, 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 contact, 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 watersaturated 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 welllogging 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 (e.g. Sarir field in Libya), a mat of heavy tar is present at the oilwater 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 pools within 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 Traps

Tectonic 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 postdepositional, 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 previouslydefined 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 earth's 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-Z, Wilmington 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 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)

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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 the underlying 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 deepseated 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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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 unmetamorphosed sediment 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 impermeable. Second, 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 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

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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 on one of the producing sands, illustrating the downthrown anticlinal development.

Figure 8

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 , Northsouth cross-section of the Vermillion Block 76 field, offshore Louisiana).

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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 several kilometers 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 is,will 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 nonsealing.The 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 are transitional 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 case:Unconformity 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 manner,strongly 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 facieschange 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, ranging from meandering fluvial deposits through deltaic distributary channels to deepsea 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 fluvial-channel 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

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Muddy stream channels).

Figure 2

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. 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. 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).

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Figure 3

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).

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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 AZ 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, 1968; McGregor 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 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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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.

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 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 well as within individual reefs. Such changes are due to variations in lithofacies and diagenetic effects, and are typical features of reefal traps ( Figure 2 ,

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Cross-section of pinnacle reef showing complex lithofacies,Rainbow area, Alberta, Canada).

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).

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

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Table 1

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).

Figure 1

Traps which occur above an unconformity will be discussed first. 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

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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 Bank field of Montana, with recoverable reserves of over 200 million barrels of oil (MacKenzie, 1972). Here the Cretaceous Cut Bank sand unconformably onlaps Jurassic shales and is itself onlapped by younger shales (Blixt, 1941; Shelton, 1967). Figure 3, (Southwest-northeast E-log correlation section A-Z, Cut Bank sandstone, Montana) is a cross section through this field.

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).

Figure 4

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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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).

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Figure 6

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 traps, closure is needed in both directions along the strike (Figure 7, Schematic of trap below unconformity, featuring closure provided by the intersection 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 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).

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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 flow.Such 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 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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Figure 1

Movement of salt develops several structural shapes, from deepseated 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 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 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). 57

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 U.S. 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 even reach the surface.

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