Petroleum System P5

Oil & Gas Engineering Dept.

Petroleum Geology / 7

MSc. Hussein A.

Petroleum System    8    1    0    2     /    1    1     /    5    2

( Part Five )

Petroleum Traps 25 / 11 / 2018

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Trap: is geometric pattern layers of sedimentary allows oil or gas or both to assemble in economic quantities, and prevents escape them, and take this geometric pattern several forms, but remain the main feature of the trap is the presence of porous rock covered impermeable rocks. rocks.  Water  W ater is a key factor in guiding the oil and gas to trap in the majority of cases, as it helps in removing oil and gas to wells holes in i n the production stage. 25 / 11 / 2018

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Petroleum traps are divided into three types: 

1.  Structural traps. 2.  Stratigraphic traps. 3.  Combination traps.

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1Structural traps: are created by the deformation of rock strata

 within the earth’s earth’s crust. This deformation can be caused by horizontal

compression

or

tension,

vertical

movement

and

differential compaction, which results in the folding, tilting and faulting within sedimentary rock formations. There are three basic forms of a structural traps in petroleum geology: 1. Anticlinal and Dome Trap. 2. Fault Trap. 3. Salt Dome or Salt Plug Trap. 25 / 11 / 2018

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1.  Anticlinal an Dome Trap: The rock layers in an anticlinal trap were originally laid down horizontally

then

folded

upward

into

an

arch

or

dome.

Later,

hydrocarbons migrate into the porous and permeable reservoir rock; the low density of petroleum causes it to buoyantly migrate upward to the highest parts of the fold. A cap or seal (impermeable layer of rock) is required to permit the accumulation of the hydrocarbons.

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2. !ault Trap: The faulting of  stratified rock occurs as a result of vertical and horizontal stress. At some point the rock layers break, resulting in the rock faces along the fracture moving or slipping past each other into an offset position. A fault trap is formed when the faulted formations are tilted toward the vertical. When a non- porous rock face is moved into a position above and opposite a porous rock face, it seals off the natural flow of the hydrocarbons hydrocarbons allowing them to accumulate.

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3. Sa Salt lt Do Dome me or Sa Salt lt Pl Plug ug Tra rap: p: A trap created by piercement or intrusion of stratified rock layers from below

by

ductile

nonporous

salt.

The

intrusion

causes

the

lower

formations nearest the intrusion to be uplifted and truncated along the sides of the intrusion, while layers above are uplifted creating a dome or

anticlinal

folding.

Hydrocarbons

migrate

into

the

porous

and

permeable beds on the sides of the column of salt. Hydrocarbons accumulate in the traps around the outside of the salt plug if a seal or cap rock is present.

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2. The term "stratigraphy" basically means the study of the rocks and their variations. One thing stratigraphy has shown us is that many layers of rock change, sometimes over short distances, even within the same rock layer. As an example, it is possible that a layer of rock which is a sandstone at one location with siltstone or a shale. We learned that sandstones make a good reservoir because of the many pore spaces contained within. On the other hand, shale, made up of clay particles, does NOT make a good reservoir, because it does not contain large pore spaces. Therefore, if oil migrates into the sandstone, it will flow along this rock layer until it hits the low-porosity shale. 25 / 11 / 2018

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Stratigraphic traps: are formed as a result of differences

or variations between or within stratified rock layers, creating a change or loss of permeability from one area to another. These traps do not occur as a result of movement of the strata.

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1-

Pinch-out or lateral graded Traps.

2-

Carbonate Reef Traps.

3-

Sandstone Lens or Lenticular Traps.

4-

Facies Change Traps.

5-

Angular nconfor!it" Traps.

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1- Pinch-out or lateral graded Traps#

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2- Carbonate Reef Traps#

Are great places to trap oil? The open cavities between the corals create excellent reservoirs, and when the reef is buried by mud, the oil becomes trapped. Many of the large oil and gas fields in west Texas are found in buried reef.

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3- Sandstone Lens or Lenticular Traps# Lenses - Layers of sand often form lens like bodies that pinchout. If the rocks

surrounding

these

lenses

of

sand

are

impermeable

and

deformation has produced inclined strata, oil and natural gas can migrate into the sand bodies and will be trapped by the impermeable rocks. This kind of trap is also difficult to locate from the surface, and requires subsurface exploration techniques.

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$- Facies Change Traps#

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%- Angular nconfor!it" Traps# An angular unconformity is one in which older strata dips at an angle different from that of younger strata. An angular unconformity trap occurs when inclined; older petroleum bearing rocks are subjected to the forces of younger non-porous formations. This condition may occur whenever an anticline, dome or monocline are eroded and then overlain with younger, less permeable strata.

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3. Combination traps that combine more than one type of trap are common in petroleum reservoirs.  

Other types of traps (such as hydrodynamic traps) are usually less common.

In hydrodynamic traps, the hydrocarbon is trapped by the action of  water movements. Tilted contacts are common in this case. The water usually comes from a source such as rain falls or rivers.

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Seals A reservoir needs a cap rock. •

Impermeable cap rock keeps the fluids trapped in the reseroir. It must hae !ero permeabilit".

•

•

Some examples are# are#

 

Sedimentar" elastic and incompetent rocks such as marl$ mudstone and shale.

 

%aporites rocks such as salt or anh"drite.

 

&ero'porosit" carbonates.

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"nhydrite #ormation . !otnia "nhydrite

$. %ata %atawi #ormati ormation on 'ubair #ormation ormation &. 'ubair ahr *mr #orma ormati tion on (. )ahr

+. hasib hasib #ormati ormation on anuma #ormation ormation -. Tanuma /hirani anish sh #ormati ormation on . /hir

0. 1ower #ars 2#atha3 #ormation 25 / 11 / 2018

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S(AL R&C'S 1- Anh"drite seal. 2- )olo!ite !udstone seal.

R(S(R*&+R R&C'S 1- Fractured Lst., channel porosit". 2- Fossiliferous Lst., intergranular porosit".

S&RC( R&C'S 1- Fora!iniferal acstone. 2- Fine grained Lst., disse!inated bitu!en. 25 / 11 / 2018

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