LECTURE OUTLINE

• Definition and Terminology

• Classification of Traps

• Structural Traps

• Stratigraphic Traps

• Hydrodynamic (fluid-barrier) Traps

• Combination Traps

ACCUMULATION OF PETROLEUM:

THE PETROLEUM TRAP

Elements of stuctural geology (review of basic concepts)

DEFINITION AND NOMENCLATURE OF TRAPS

A trap is a place where oil and gas (HCs) are prevented from

further movement.

For example an impermeable roof rock that is concave

upward (anticline) prevents petroleum from escaping either

vertically and laterally, thus localizing the HCs.

Traps may be related to:

Deformation (stuctural; folding and faulting),

Lateral changes in permeability (stratigraphic),

Difference in fluid potentials (hydrodynamic), or

A combination of two or more stated above

In general, therefore, the accumulation of HCs in a trap is

accomplished when the physical properties and geometry of

the rocks prevent further migration.

Anticlinal Traps (a structural type of trap)

Terminology:

The most simplest trap is an anticlinal trap.

The highest point of a trap is the crest.

The lowest point at which HCs may be contained in a trap is the

spill point, which lies on a horizontal contour, the spill plane.

The vertical distance from the crest to the spill plane is the closure of the trap.

Crest

Structural relief is the highest level to which the folded bed

rises above the regional slope.

Pay: Within the trap, the productive reservoir is termed the pay.

Gross pay: The vertical distance from the the top of the crest to

oil-water contact (OWC) is termed the gross-pay.

The net-pay: the cumulative vertical thickness of the reservoir

from wich the petroleum may be produced. Determination of

the gross-net pay ratio is important in decision making for the

development of a reservoir

The time of formation of a trap relative to the petroleum

migration is very important:

If traps predate migration they will be productive.

If they postdate migration they will be barren.

Gross pay>

Net pay

Gross pay=

Net pay

Stratigraphic traps with tilted oil/water contact

CLASSIFICATION OF TRAPS

Structural traps: These are traps formed by folding and

faulting. They can be found by surface and subsurface

mapping using geological and geophysical mapping and well-

log data. Structural study of the type of folds, faults, salt and

mud intrusions are important in petroleum exploration.

Stratigraphic traps: A lateral variation in permeability due to

facies changes (i.e., lithology), truncations, onlaps and

diagenetic changes, together with the roof rock, will form an

internal barrier, or a stratigraphic trap.

Hydrodynamic traps: Difference in fluid potentials causes a

down-dip flow of water to oppose up-dip flow of HCs.

Increased fluid gradients usually exist where the flow rate is

constricted due to thinning of reservoir or reduction in its

permeability

STRUCTURAL TRAPS

These traps are formed mainly by folding and faulting, and are

recognized by structural mapping, using geological, geophysical

methods and well-log data.

Many structures change with depth in shape, size and

amplitude. Thus, the surface data may not always be reliable.

Structural traps can further be classified according to the type of

folds and faults.

Fold traps (anticlines): Anticlines are the most common type

of traps containing majority of the world’s giant oil fields.

Structural closure and structural relief are important

parameters to consider for anticlinal traps.

FOLDING:

Compressional or Compactional folds

Compressional:

These folds occur by crustal shortening in or close to the

mountain chains (e.g., Taurus or Zagros mountains)

Compressional folds are elongated perpendicular to the axis of

crustal shortening (along the direction of mountain chains).

These folds are usually associated with faults. En-echelon folds

develop along transpressional segments of transform faults.

Examples: Anticlines in the Lower Miocene Asmari Lmst.

capped by evaporites of the Lower Fars Group in the Zagros

Mountain belt.

Tectonic compression

Compression

Compactional folds:

These folds are formed by crustal tension.

In extentional basins horsts and grabens develop. Anticlines

form over deep-seated horsts.

Closure may be enhanced by differential compaction and

sedimentation, the amount of compaction being greater for the

thicker flank (graben) sediments.

Examples include compactional anticlines in the Paleocene

deep sea sands draped over Mesozoic horsts in the North Sea.

Compactional folds are irregularly shaped, reflecting the

intersection of fault trends in the basement

Compactional folds

A) Deposition over topographic high and low: variable sediment thickness

B) After uniform compaction of 10% of total sediment thickness

Extensional “roll-over“ anticlines

HorstGraben

Horst Horst

Structural contour

Thrust fault

Map

Cross

section

N

Map

Cross

section

Diapiric: These folds are formed by upward movement of

sediments (salt or overpressured plastic clay) that are less

dense than those rocks overlying them.

Movement of salt or mud could be triggered tectonically or by

density contrast. The intrusion can produce a variety of traps

that are sometimes not strictly structural.

The overlying strata are up-domed, or pierced through, and

usually faulted. In many salt domes the surrounding

sediments thin towards the salt plug, so pinch-outs are

common.

Nearly all the salt domes have an overlying 100-200 m-thick,

draping cap rock. It is made up of an anhydrite layer at the

base, a gypsum + anhydrite layer in the middle and a calcite

layer at the top.

The surface expression of salt domes are:

Topographic mound, or a depression (caused by salt

solution),

Sulfurous springs, and

Oil and gas seeps.

Examples of salt dome-associated traps occur in the Gulf

Coast of USA and the Zagros Mountains of Iran.

Deposition of younger rocks on top of evaporite

Differential compaction

Traps associated with salt domes

Classification of fault traps

Fault traps can be classified according to the type of faults:

normal,

reverse,

thrust,

transform, and

growth faults

Faults can juxtapose (bring together) different rocks of

differing permeabilities, and can seal or allow migration

between different reservoir levels.

Fault traps

Fault traps:

Faulting plays an indirect role in formation of HC traps. Faults

can seal and act as a barrier to migration of fluids. However,

some faults or fault zones are permeable.

Criteria for a fault to seal or be permeable:

If the throw of the fault is less than the thickness of the

reservoir, it is unlikely to seal.

Faults in brittle rocks (e.g. Quartz sanstone) are less likely

to seal than in plastic rocks (e.g., mudstones).

In unlithified (soft) sediments the faults tend to seal.

Permeable sand

Reservoir

STRATIGRAPHIC TRAPS

Stratigraphic traps form as a result of lateral changes in the

lithology of a reservoir rock, or a break in its continuity.

Such changes may be:

1. Depositional (channels, offshore bars, reefs, and pinchouts)

2. Post-depositional (caused by diagenesis)

3. Unconformity related (stratigraphic traps overlying and

underlying unconformities)

Stratigaphic traps are less well known and harder to locate than

structural traps.

Classification of stratigraphic traps

Channel traps:

River channel deposits enclosed in impervious muds provide

a trap.

Channel fills are long and narrow sand and gravel deposits.

Stream meandering back and forth within the flood plain often

changes its channel when it becomes clogged.

Channel traps have concave-down profile with a flat top and

sinuous, meandering pattern.

The sediments are poorly sorted.

They can be up to 50-60 m thick and several tens of km long.

With high resolution 3-D seismic methods they are easy to

locate and map.

Channel traps

Channel traps

Offshore barrier sand bars:

These are lenticular sand lenses, often arranged en echelon.

The sand is well sorted.

The bars have flat base and bulged upper surface.

The sand bars show a sharp contact on the seaward side

and a gradational contact on the lagoonal side, with the

muds.

Sand bar Channel sand

Regressive

upward coarsenning

barier bar

Isopach map

Cross section

Pinchout traps:

Isolated, transgressive and regressive barrier sands pass up-

dip into lagoonal and intertidal shales, and form the pinchout

traps.

Transgressive sands form the best sealed traps.

The regressive sands develop better, but they usually lack

the up-dip seals.

The closure of sand bars may be stratigraphic if a shoreline

forms an embayment, or it can be stuctural.

Regressive and transgessive barrier sands

Pinchout trap

Map

Cross section

İsopach map

Reef traps:

Reefs are carbonate built-ups forming the most important

type of stratigraphic traps.

They are porous and permeable, sealed laterally and on top

by shales, carbonate muds and sometimes evaporites.

There are different types of reefs: fringing, barrier, and atoll.

Barrier

reef

Lagoon

Open seaFringing

reef

Atoll

Sea mountOpen sea Open

sea

Reef traps

Reef traps

Traps related to unconformities:

Unconformities may bring together permeable and

impermeable rocks, thereby causing the formation of traps.

The channel, sand bar and diagenetic traps can occur in both

conformable and unconformable sequences.

Traps may form above or below unconformities.

Marine and fluvial channel sands, as well as reefs, can onlap

a planar unconformity.

Where the unconformity surface is irregular, sand often infills

strike valleys that cut into soft units over the old land surface.

Supra-unconformity traps: Strike valleys

Where the unconformity surface is irregular, sand often infills strike

valleys that cut into soft units over the old land surface.

Sub-unconformity traps

Stratigraphic traps also occur below the unconformities

where porous and permeable beds have been truncated and

overlain by impervious clays.

In many cases the seal below is provided by the impervious

strata beneath the reservoir.

Weathering of basement granites and limestones can provide

high permeability below the unconformity surface that later

become sealed by onlapping shales.

Traps related to unconformity: truncation trap

Traps related to unconformity

Diagenetic traps:

Solution and precipitation of mineral cements by diagenesis

can form traps.

Solution is especially common in carbonates.

Migrating oil towards surface can be oxidized by bacterial

degradation and form an impervious tar residue that act as a

seal.

Traps solely due to diagenesis are rare. Other factors are

also involved in their formation.

HYDRODYNAMIC (FLUID-BARRIER) TRAPS

In these traps hydrodynamic downward movement of water is

essential to prevent the upward movement of oil and gas.

The local fluid potential gradients become large when there is

a local reversal of dip, or a facies change.

Reduction in permeability or thinning of reservoir rock causes

constriction of flow rate and result in high fluid gradients.

Hydrodynamic traps are very rare and difficult to locate.

A tilted OWC is a sign of a hydrodynamic trap.

HYDRODYNAMIC TRAPS

Diagenetic

trap

Hydrodynamic

trap

HYDRODYNAMIC TRAPS

COMBINATION TRAPS

Many traps are not solely of structural, stratigraphic, or

hydrodynamic origin, but due to a combination of the two or

more of these factors.

Most of the traps are caused by a combination of structural

and stratigraphic processes.

Structural-hydrodynamic and stratigraphic-hydrodynamic

traps are rare.

Pinchout, onlap, truncation traps all require closure, which

are often structural, along the strike.

Likewise folded and faulted beds are often sealed by

unconformities to form another group of combination traps.

Stratigraphic (channel)-

structural (anticline) type

combination trap

Unconformity-

Fault type

Combination

trap