TRAP Chapter 3: TRAP Chapter 3:

Definitions and 3.1 3.1 Definitions and Concepts Concepts

3.2 3.2 Classification:

Classification: four four major types: major types: Structural, Structural, Stratigraphic, , Stratigraphic Hydrodynamic and Hydrodynamic and Combination Combination

33..11..Definitions

Concepts Definitions andand Concepts

A trap is subsurface configuration of ••A trap is subsurface configuration of reservoir rock and cap rock or seal that has reservoir rock and cap rock or seal that has potential to concentrate petroleum in the potential to concentrate petroleum in the pores of a reservoir rock pores of a reservoir rock

A trap is a geological feature of a reservoir ••A trap is a geological feature of a reservoir rock that restricts the flow of fluids rock that restricts the flow of fluids

A trap can content one or more reservoirs ••A trap can content one or more reservoirs

• 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

• Net pay refers only to the possibly productive

reservoir.

(Figure 2, Facies change in an anticlinal trap,

illustrating the difference between net pay and gross pay).

Nomenclature of a trap using a simple anticline as an example Figure 11: : Nomenclature of a trap using a simple anticline as an example Figure

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). It marks the interface between predominately oil-saturated rocks and water-saturated rocks. Similarly, either the gas- water contact, GWC ( Figure 3b , Fluid contacts within a reservoir in a gas-water system),

• or the gas-oil contact, GOC ( Figure 3c , Fluid contacts within a reservoir in a gas-oil-water system) is the lower level of the producible gas. The GWC or GOC marks the interface between predominately gas-saturated rocks and either water-saturated rocks, or oil-saturated rocks, as the case may be.

Figure 3

• 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

• 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

• 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)

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.

Figure 7

Classification 33..22..Classification

Basically, traps can be classified into four Basically, traps can be classified into four major types: major types:

Structural, Structural,

Stratigraphic

Stratigraphic, ,

Hydrodynamic and Hydrodynamic and

Combination Combination

TRAP TYPES CAUSES

Structural Traps

Fold Traps:

Compressional Folds Compactional Folds

Diapir Folds

Tectonic processes Depositional / Tectonic processes Tectonic Processes

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

SALT DOME

B-S

ALT S

EDIMENT TRUNC

TION

S U A

PINCH-OUT

SAND LENS

FAULT

UNCONFORMITY

GRADING

ANTICLINAL

BURIED REEF

Figure 8

BASIC HYDROCARBON TRAPS BASIC HYDROCARBON TRAPS

3.2.1 Structural Traps 3.2.1 Structural Traps

• "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."

Fold Traps Fold Traps Fold Traps (Compressional ) Fold Traps (Compressional )

• Anticlinal traps which are due to

compression are most likely to be found in or near geosynclinal troughs.

Examples of Compressional Fold Traps Examples of Compressional Fold Traps

• 01-The Wilmington oil field in the Los

Angeles basin ( Figure 9 , Oil fields of the Los Angeles basin) is a giant anticlinal trap with ultimate recoverable reserves of about 3 billion barrels of oil.

Figure 9

• 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 10, Structural contours on top of Ranger zone, Wilmington field, CA). Notice also the cross-cutting faults.

Figure 10

• From a southwest-northeast cross section of the Wilmington field, we can see the broad arch of the anticline ( Figure 11, Southwest-northeast cross-section A-Z, Wilmington field). The main reservoir occurs beneath the Pliocene unconformity in Miocene- and Pliocene-age deep-sea sands.

Figure 11

02-Reservoir in the Zagros mountains • The foothills of the Zagros mountains in

Iran contain one of the best-known hydrocarbon provinces with production from compressional anticlines ( Figure 12, Location map, southwest Iran and Persian Gulf).

Figure 12

• 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 13 (Southwest-northeast generalized

sections through Asmari oil fields)

Figure 13

Anticline related to thrust faults-- Anticline related to thrust faults Painter Reservoir field 0303--Painter Reservoir field

• In areas of still more intense structural deformation,

anticlinal development may be associated with thrust faulting. 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 14, 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.

Figure 14

• In cross section, the anticline is overturned and thrust faulted on its southeastern flank ( Figure 15, 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 15

Fold Traps ( Compactional ) 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 16, Compactional anticlines in sediments draped over underlying structurally high horst blocks ).

Figure 16

• At the time of deposition, thickness of a given

sedimentary unit is thinner over the crest of the underlying structural high

• (Figure 17a , Developmental stages of compactional anticlines--initial stage of deposition).

• ( Figure 17b , Developmental stages of

compactional anticlines--compactional stage). ( Figure 17c , Developmental stages of compactional anticlines--structural closure enhanced by recurrent fault movement).

Figure 17

Examples of compactional anticline traps Examples of compactional anticline traps

• 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 18, Structural contours on top of

Paleocene reservoir, Forties field area, North Sea).

Figure 18

• A north-south cross section depicts the

anticline developed at the Paleocene level where the reservoir sands are sealed by overlying Tertiary clays

( Figure 19, Schematic north-south cross- section A-Z through Forties field, North Sea).

Figure 19

Fold Traps: Comparison of Major Types Fold Traps: Comparison of Major Types

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

• 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.

Fold Traps: Comparison of Major Types Fold Traps: Comparison of Major Types (cont.) (cont.)

• 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.

Associated Traps Diapir Associated Traps Diapir

• 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.

Diapir Associated Traps (cont.) Diapir Associated Traps (cont.)

• 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 20 , Salt structures of the southern North Sea). However, in many cases, the salt movement is apparently initiated at random.

Figure 20

• 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 21, 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)

Figure 21

• There are many ways in which oil can be trapped on or adjacent to salt domes (Halbouty, 1972)

• ( Figure 22, Schematic cross-section

showing the varieties of hydrocarbon traps associated with piercement salt domes).

Figure 22

• 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 23),

• The Bryan field of Mississippi is an example of a

turtle-back trap (Oxley and Herling, 1972).

section showing a turtleback structure Schematic cross--section showing a turtleback structure Schematic cross (anticline) developed betw. two adjacent piercement salt domes (anticline) developed betw. two adjacent piercement salt domes

Figure 23

Fault Traps 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 Figure 2424 Schematic cross-- ((Schematic cross section of Nigerian section of Nigerian field, showing traps field, showing traps and possible and possible accumulation accumulation modelmodel) shows a ) shows a complex faulted complex faulted situation in the situation in the Niger Delta in Niger Delta in which some faults which some faults seal while others seal while others are conduits. are conduits.

Figure 24

• In the Gaiselberg field of Austria the Steinberg fault, trends northeast-southwest, and provides the trap for this field ( Figure 25, Structural contours on top of Sarmatian horizon 18 of the Gaiselberg field).

• 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 26 , West-northwest-east- southeast cross-section A-Z through the Gaiselberg field).

Figure 25

Figure 26

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 27, (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 27a , Initial rapid progradational depositionclay).

• This results in downwarping of the under-compacted clay under the heavier sand body ( Figure 27b , 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 27c , 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 27d , Mature growth fault)

• Figure 28 (Schematic cross-section of a mature growth

fault) illustrates the characteristic downthrown reversal of regional dip as the beds slump into the fault plane.

Figure 27

Figure 28

In southern Louisiana's deltaic depositional province, In southern Louisiana's deltaic depositional province, growth faults provide traps for considerable oil and growth faults provide traps for considerable oil and gas reserves. 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 29 (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 29

• A north-south cross section of the field

shows the downthrown anticlinal structure as well as the downthrown expansion of the sedimentary column (Figure 30), North-south cross-section of the Vermillion Block 76 field, offshore Louisiana).

section of the Vermillion south cross--section of the Vermillion

Figure 30:30: NorthNorth--south cross Figure

Block 76 field, offshore Louisiana). ). Block 76 field, offshore Louisiana

3.2.2 Stratigraphic trap 3.2.2 Stratigraphic trap

Classification of stratigraphic type hydrocarbon trap Classification of stratigraphic type hydrocarbon trap

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).

Depositional Traps Depositional Traps

• 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.

Figure 31

Classification of stratigraphic type hydrocarbon traps based on Classification of stratigraphic type hydrocarbon traps based on the scheme proposed by Rittenhouse (1972), shows that a the scheme proposed by Rittenhouse (1972), shows that a major distinction can be made between stratigraphic traps major distinction can be made between stratigraphic traps which occur within normal conformable sequences which occur within normal conformable sequences

Schematic of trap within normal conformable sequence Schematic of trap within normal conformable sequence) ) and those that are associated with unconformities and those that are associated with unconformities Figure 32 32 , , Schematic of traps associated with Schematic of traps associated with ((Figure unconformities). ). unconformities

Figure 32

This distinction is rather arbitrary since there are some types, This distinction is rather arbitrary since there are some types, such as channels, that can occur both at unconformities and such as channels, that can occur both at unconformities and away from them away from them

Figure 33

( Figure 33: Schematic of two channel traps).

Channels Depositional Traps: Channels Depositional Traps:

• 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 fluvial-channel reservoirs. The channel reservoir has a width of some 1500 meters and a maximum thickness of approximately 15 meters ( Figure 34, 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 (bending).

Figure 34

Figure 34, Isopach map of Lower Muddy interva

Figure 3535) display SP curves on some of the well logs (e.g. wells #5 on some of the well logs (e.g. wells #5 ) display upward upward--fining point fining point--bar bar

meandering channel a characteristic of meandering channel

traps. Because of stratigraphic traps. Because of

A cross section of the field shows that the channel is A cross section of the field shows that the channel is only partially filled by sand and is partly plugged by only partially filled by sand and is partly plugged by clayclay The The SP curves and #6 on Figure and #6 on sequences, a characteristic of sequences, deposits. deposits. The South Glenrock field illustrates an important The South Glenrock field illustrates an important points about channel stratigraphic points about channel their limited areal extent and thickness, such their limited areal extent and thickness, such reservoirs seldom contain giant accumulations. reservoirs seldom contain giant accumulations.

Figure 35: West-east cross-section A-Z of two Lower Muddy stream channels

Depositional Traps: BarsBars Depositional Traps: • Because of their clean well-sorted texture, marine barrier bars often make excellent reservoirs (Hollenshead and Pritchard, 1961). • The barrier sands may coalesce (co-oporatae)

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 36)

Figure 36

Schematic of barrier bars, showing Figure 36,36, Schematic of barrier bars, showing Figure interconnedted bars forming blanket reservoir bars forming blanket reservoir interconnedted and one isolated bar set and one isolated bar set

• 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 37)

• It consists of three stacked sandbars, with

an aggregate thickness of 15 meters, totally enclosed in the marine Mancos shale (Figure 38), The SP log in some wells shows the typical upward-coarsening grain- size motif which characterizes barrier bars.

Figure 37

Figure 3737, , Bar sandstone isopach map of Bisti Bar sandstone isopach map of Bisti Figure field, Colorado field, Colorado

Figure 38

NorthNorth--south cross south cross--section A

section A--Z of Bisti field using Z of Bisti field using electric logs electric logs

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: ReefsReefs Depositional Traps:

• 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.

• 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 39 (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 ( Fig. 39). 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 (Fig. 40, Cross-section of pinnacle reef showing complex lithofacies,Rainbow area, Alberta, Canada).

• 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

Figure 39

section through Middle Schematic cross--section through Middle

Figure 39, Schematic cross Figure 39,

Devonian reefs, Rainbow area, Alberta, Canada Devonian reefs, Rainbow area, Alberta, Canada

Figure 40

Fig.Fig. 4040, , CrossCross--section of pinnacle reef showing complex section of pinnacle reef showing complex lithofacies,Rainbow area, Alberta, Canada lithofacies,Rainbow area, Alberta, Canada

Traps Diagnenetic Traps Diagnenetic

• 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).

• 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 (Fig. 41).

Figure 41

), but may Figure 4242), but may

The development of solution porosity is commonly The development of solution porosity is commonly associated with carbonate rocks ( associated with carbonate rocks (Figure involve sandstones as well involve sandstones as well

Figure 42

• Fracturing can cause secondary porosity in any brittle rock — whether carbonate, sandstone, shale, igneous or metamorphic rock (Kostura and Ravenscroft, 1977). The 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 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 43). Production comes from areas of fracturing throughout the otherwise tight Spraberry formation.

Figure 43

Related Traps Unconformity--Related Traps Unconformity

• The depositional and diagenetic

stratigraphic traps just considered occur in normal comformable sequences, although they may also occur at unconformities. • Another major group of stratigraphic traps

is represented by traps for which an unconformity is essential (Fig. 44) (Levorsen, 1934).

Figure 44

• 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 45, Schematic of traps located above and below an unconformity).

• Above an unconformity: 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 (Fig.46, Schematic of onlapping pinch-out sands- occurring as a sheet deposit)

Schematic of traps located above and below an Figure 4545, , Schematic of traps located above and below an Figure unconformity unconformity

out sands-- Schematic of onlapping pinch--out sands Figure 4646, , Schematic of onlapping pinch Figure occurring as a sheet deposit occurring as a sheet deposit

• Schematic of onlapping pinch-out sands- occurring as a sheet deposit) , or as discrete paleogeomorphic traps (Figure 47, Schematic of onlapping pinch-out sands-occurring as a discrete paleogeomorphic sand).

• 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 47, Schematic of channel and strike valley sands above an unconformity)

• It is important to note that closure is necessary along the strike of such traps, not just updip as shown in Figure 46 .

• In Figure 48 (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.

Figure 47, Schematic of channel and strike valley sands above an unconformity

Figure 48

• The second group of traps associated with

unconformities is truncation traps which occur beneath the unconformities (Figure 49, Schematic of traps below unconformity). • 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 50, Schematic of trap below unconformity, featuring closure provided by the intersection of a dipping structural nose and a flat unconformity).

Schematic of traps below unconformity Figure 49 49 Schematic of traps below unconformity Figure

Figure 50

• 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 51, Schematic of trap below unconformity, featuring closure provided by buried hill).

• 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

• One of the best known truncation traps in the world is the East Texas field 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 52, 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 51

Figure 52

3.2.3 Hydrodynamic Traps 3.2.3 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 53 (Schematic cross-section of an ideal hydrodynamic trap).

Figure 53

• 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.

• There are a number of fields with tilted oil-water contacts where entrapment is a combination of both structure and hydrodynamic forces (Figure 54, Schematic cross-section showing entrapment from both structural and hydrodynamic forces).

Figure 54

3.2.4 Combination Traps 3.2.4 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 55, 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.

Figure 55

Structural contours on top of 'G2' sandstone, Figure 55,55, Structural contours on top of 'G2' sandstone, Figure Main Pass Block 35, offshore Louisian Main Pass Block 35, offshore Louisian

• 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 56, 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 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.

Figure 5656, , Structural contours on top of Sadlerochit Structural contours on top of Sadlerochit Figure reservoir, Prudhoe Bay, Alaska reservoir, Prudhoe Bay, Alaska

• Fault-unconformity combination traps characterize

the northern North Sea.

• Jurassic sandstone reservoirs exist in numerous

tilted fault blocks which were truncated and overlain by Cretaceous shales. The resulting traps include such fields as Brent, Ninian, and Piper. A cross section through one of these, the Piper field, is shown in Figure 57 Southwest-northeast structural cross-section, Piper field, North Sea).

section, northeast structural cross--section,

Figure 57 57 Southwest Figure

Southwest--northeast structural cross Piper field, North Sea Piper field, North Sea

Exercise Exercise