ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 3 docx

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.. Similarly,

Chapter 3: TRAP

3.2 Classification: four major

3.1.Definitions and Concepts

• 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

• 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)

Figure 1: Nomenclature of a trap using a simple anticline as an example

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

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 FAULT

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 (Compressional )

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

geosynclinal troughs (mangs)

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-03-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 )

• 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

• 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

There are many differences between the fold traps caused by

compression, and those caused by compaction

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

(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

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.

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 back trap (Oxley and Herling, 1972)

turtle-Schematic cross-section showing a turtleback structure (anticline)

developed betw two adjacent piercement salt domes

Figure 23

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 24

cross-section of Nigerian field, showing traps and possible

shows a complex

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

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