GIAO TRINH CHAPTER 3

TRAP TYPES CAUSESTectonic Processes Fault Traps Tectonic Processes Stratigraphic Traps Depositional morphology or diagenesis Hydrodynamic Traps Water flow Combination Traps Combin

Chapte r 3: TRAP

HCMUT-AU-2011

3.2 Clas s ific ation: four major

type s : Struc tural, Stratigraphic , Hydrodynamic and Combination

3.1 Definitions and Concepts

3.1.De finitio ns and Co nc e pts

• A trap is subsurface configuration of

reservoir rock and cap rock or seal that has potential to concentrate petroleum in the

pores of a reservoir rock

• A trap is a geological feature of a reservoir

rock that restricts the flow of fluids

• A trap can content one or more reservoirs

Fig ure 1: No me nc lature o f a trap us ing a s imple antic line as an e xample

Figure 2

Figure 3

• Boundaries between oil, gas and water may

be sharp ( Figure 4a , Transitional nature of

fluid contacts within a re s e rvoir- s harp

contact

• Gradational ( Figure 4b , Transitional nature

of fluid contacts within a re se rvoir-

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

Gradational contacts usually indicate low

permeability reservoirs with high capillary

pressure.

Figure 4

• 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 fie ld ) Each individual pool may contain one or more pay zones.

Figure 7

3.2.Clas s ific atio n

Basically, traps can be classified into four major types:

 Structural,

 Stratigraphic,

 Hydrodynamic and

 Combination

TRAP TYPES CAUSES

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

BASIC HYDROCARBON TRAPS

UNCONFORMITY

ANTICLINAL

SUB-SALT SEDIMENT TRUNCATION

3.2.1 Struc tural 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."

Fo ld Traps

Fo ld Traps (Compre s s io nal )

• Anticlinal traps which are due to

compression are most likely to be found in

or near geosynclinal troughs

Example s of Compre s s io nal Fold Traps

• 01-The Wilmington oil field in the Los

Angeles basin ( Figure 9, Oil fields of the

Los Ange le s bas in ) 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 10, S tructural contours

on top of R ange r z one , Wilm ington fie ld,

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 ,

S outhwe s t-northe as t cros s-s e ction A-Z,

Wilmington fie ld ) The main reservoir

occurs beneath the Pliocene unconformity

in Miocene- and Pliocene-age deep-sea sands.

Figure 11

Fold Traps ( Co mpac tio nal )

• Compactional fold frequently occurs where crus tal

tens ion as s ociated with rifting causes a

s edimentary bas in 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

drape d ove r unde rlying s tructurally high horst

blocks )

Figure 16

Examples of c ompac tional 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, S tructural contours on top of

Pale oce ne re s e rvoir, Fortie s fie ld are a, North

S e a)

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, S chematic north-south

cross-s e ction A-Z through Fortie cross-s fie ld, North

S e a )

Figure 19

Fold Traps : Comparis on of Major Type s

There are many differences between the fold traps

caused by compres s ion, and those caused by

compaction

• Compres s ional 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 : Comparis on of Major Type s

(cont.)

• Compres s ional 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

• Compres s ional folds commonly form from one major tectonic event, while compactional folds

may have a complex history due to rejuvenation

of underlying basement faults

Diapir As s oc iate d Traps

• Diapirs are a major mechanism for

generating many types of traps Diapirs

are produced by the upward movement

of le s s de ns e s e dime nts , 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.

Figure 21

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

(Halbouty, 1972)

• ( Figure 22, S chematic cross-section

s howing the varie tie s of hydrocarbon traps ass ociate d with pie rce m e nt s alt dom e s )

Figure 22

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 mig ratio n, thus pro viding a

while others do not

• In general, faults have more tendency to s eal in

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 24

s-se ction of Nige rian fie ld, showing traps and poss ible

shows a complex

faulted situation in the Niger Delta in which some faults seal while others are conduits.

Figure 24

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 29 (S tructural contours on top of Pliocene 10

s and, Ve rmilion Block 76 fie ld, offs hore 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-s outh cros s-s e ction of the Ve rm illion Block 76 fie ld, offs hore Louis iana )

Figure 30: North-south cross-section of the Verm illion

3.2.2 Stratigraphic trap

Classification of stratigraphic type hydrocarbon trap

De po s itional 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 re vie ws on the conce pt of stratigraphic traps ,

the re ade r is re fe rred to Dott and Re ynolds

(1969) and Ritte nhouse (1972) Major s ource s of spe cific data on stratigraphic traps can be found

in King (1972), Busch (1974), and Conybe are

(1976).

Depositional Traps

• Levorsen (1967) defines a stratigraphic trap as "one in

which the chief trap-making element is s ome variation

in the s tratig raphy, or litholo g y, or both, of the

res ervo ir ro ck, s uch as a facies chang e, variable local

po ros ity and permeability, or an ups tructure

terminatio n o f the res ervo ir rock, irres pective of the

caus e."

• 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.

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 31

S che matic of traps that are associated with unconformities

Figure 32

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

( Figure 33: S chematic of two channel traps ).

Figure 33

Depos itio nal Traps : Channe ls

• 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

inte rva, S outh Gle nrock oil fie ld, Wyoming) It

has been mapped for a distance of over 15

kilometers and can be clearly seen to meander

(bending).

Figure 34, Isopach map of Lower Muddy interva

Figure 34

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

The SP curves on some of the well logs (e.g wells #5

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

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

Muddy stream channels

Depos itional 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 (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, S che matic of barrier bars, showing interconnected bars forming blanket rese rvoir

and one isolate d bar set

Figure 36

• The Rocky Mountain Cretaceous basins

contain many barrier bar stratigraphic

traps The Bisti field in the San J uan 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, Bar sands tone is opach map

of Bisti fie ld, Colorado

Figure 37

North-s outh cross-se ction A-Z of Bis ti fie ld us ing

e le ctric logs

Figure 38

Depo s itio nal Traps : Re efs

• The reef or carbonate build-up trap has a rigid

stoney framework containing high primary porosity (Maxwell, 1968; J ones 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 g as

• As shown in Figure 39 (S chematic cross-section

through Middle De vonian re e fs , Rainbow are a, Albe rta, Canada), two basic geometric forms of reefing are present: the pinnacle reef and the

broader elliptical form of the atoll reef

Figure 39, S chem atic cross-section through Middle Devonian reefs , Rainbow area, Alberta, Canada

Figure 39

Diag ene tic 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)

• 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

Unc onfo rmity-Relate d Traps

• 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).