Petroleum System P5

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Petroleum System

( Part Five )

Petroleum Traps

Oil

Oil & & Gas Gas Engineering Engineering Dept. Dept. Petroleum Geology Petroleum Geology //

77

MSc. Hussein A.MSc. Hussein A.

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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

the presence of porous rock covered impermeable rocks.

.

W

ater is

a

key factor

key factor in

in

guiding

the oil

and

gas

to trap

in

the majority of cases, as it helps in removing oil and gas

to

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Petroleum traps are divided into three types:

1.

1. Structural traps.

Structural traps.

2.

2. Stratigraphic traps.

Stratigraphic traps.

3.

3. Combination traps.

Combination traps.

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1-Structural traps:

Structural traps: are created by the deformation of rock strataare created by the deformation of rock strata within

within the the earthearth’s ’s crust. crust. This This deformation deformation can can be be caused caused byby horizontal compression or tension, vertical movement and horizontal compression or tension, vertical movement and differential compaction, which results in the folding, tilting and differential compaction, which results in the folding, tilting and faulting within sedimentary rock formations.

faulting within sedimentary rock formations.

Structural Traps

There are three basic There are three basic forms of a structural traps

forms of a structural traps in petroleum geology:in petroleum geology:

1.

1. Anticlinal and Dome Trap.Anticlinal and Dome Trap. 2.

2. Fault Trap.Fault Trap. 3.

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The rock layers in an anticlinal trap were originally laid down The rock layers in an anticlinal trap were originally laid down horizontally then folded upward into an arch or dome. Later, horizontally then folded upward into an arch or dome. Later, hydrocarbons migrate into the porous and permeable reservoir rock; the hydrocarbons migrate into the porous and permeable reservoir rock; the low density of petroleum causes it to buoyantly migrate upward to 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 highest parts of the fold. A cap or seal (impermeable layer of rock) is required to permit the accumulation of the hydrocarbons.

required to permit the accumulation of the hydrocarbons.

1.

1. Anticlinal an Dome

Anticlinal an Dome

T

rap:

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The faulting of

The faulting of stratified rock occurs as a result of vertical andstratified rock occurs as a result of vertical and horizontal stress. At some point the rock layers break, resulting in the 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 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 offset position. A fault trap is formed when the faulted formations are tilted toward the vertical. When a nonporous rock face is moved into tilted toward the vertical. When a nonporous rock face is moved into a position above and opposite a porous rock face, it seals off the natural a position above and opposite a porous rock face, it seals off the natural flow of the

flow of the hydrocarhydrocarbons allowing bons allowing them to accumulate.them to accumulate.

2.

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A trap created by piercement or intrusion of stratified rock layers from A trap created by piercement or intrusion of stratified rock layers from below by ductile nonporous salt. The intrusion causes the lower below by ductile nonporous salt. The intrusion causes the lower formations nearest the intrusion to be uplifted and truncated along the formations nearest the intrusion to be uplifted and truncated along the sides of the intrusion, while layers above are uplifted creating a dome sides of the intrusion, while layers above are uplifted creating a dome or anticlinal folding. Hydrocarbons migrate into the porous and or anticlinal folding. Hydrocarbons migrate into the porous and permeable beds on the sides of the column of salt. Hydrocarbons 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 accumulate in the traps around the outside of the salt plug if a seal or cap rock is present.

cap rock is present.

3.

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The term "

The term "stratigraphystratigraphy" basically means the study of the rocks" basically means the study of the rocks and their variations. One thing stratigraphy has shown us is that and their variations. One thing stratigraphy has shown us is that many layers of rock change, sometimes over short distances, even many layers of rock change, sometimes over short distances, even within the same rock layer. As an example, it is possible that a layer 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 of rock which is a sandstone at one location with siltstone or a shale. We learned that sandstones make a good reservoir because of shale. We learned that sandstones make a good reservoir because of the many pore spaces contained within. On the other hand, shale, the many pore spaces contained within. On the other hand, shale, made up of clay particles, does

made up of clay particles, does NOTNOT make a good reservoir,make a good reservoir, because it does not contain large pore spaces. Therefore, if oil because it does not contain large pore spaces. Therefore, if oil migrates into the sandstone, it will flow along this rock layer until migrates into the sandstone, it will flow along this rock layer until it hits the low-porosity shale.

it hits the low-porosity shale.

2.

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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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1-1- _{Pinch-out or lateral graded Traps.}_{Pinch-out or lateral graded Traps.}

2-2- _{Carbonate Reef Traps.}_{Carbonate Reef Traps.}

3-3- _{Sandstone Lens or Lenticular Traps.}_{Sandstone Lens or Lenticular Traps.}

4-4- _{Facies Change Traps.}_{Facies Change Traps.}

5-5- _{Angular nconfor!it" Traps.}_{Angular nconfor!it" Traps.}

Exampl

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1-1- Pinch-out or lateral graded Traps#Pinch-out or lateral graded Traps#

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2-2- Carbonate Reef Traps#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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3-3- Sandstone Lens or Lenticular Traps#Sandstone Lens or Lenticular Traps#

Lenses - Layers of sand often form lens like bodies that pinchout. If the Lenses - Layers of sand often form lens like bodies that pinchout. If the rocks surrounding these lenses of sand are impermeable and rocks surrounding these lenses of sand are impermeable and deformation has produced inclined strata, oil and natural gas can deformation has produced inclined strata, oil and natural gas can migrate into the sand bodies and will be trapped by the impermeable migrate into the sand bodies and will be trapped by the impermeable rocks. This kind of trap is also difficult to locate from

rocks. This kind of trap is also difficult to locate from the the surface, surface, andand

requires subsurface exploration techniques.

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%-%- Angular nconfor!it" Traps#Angular nconfor!it" Traps#

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

with younger, less permeable strata.

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3.

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

common in petroleum reservoirs.



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

common. In

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

usually comes from a source such as rain falls or rivers.or rivers.

Combination Traps

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A reservoir needs a cap rock. A reservoir needs a cap rock.

•

• Impermeable cap rock keeps the fluids trapped in the reseroir.Impermeable cap rock keeps the fluids trapped in the reseroir.

•

• It must hae !ero permeabilit".It must hae !ero permeabilit".

•

• Some examples are##Some examples are



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



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



 &ero'porosit" carbonates.&ero'porosit" carbonates.

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AP

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.

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Exampl

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19 19 R&C'S R&C'S S(AL S(AL

1-1- Anh"drite seal.Anh"drite seal.

2-2- )olo!ite !udstone seal.)olo!ite !udstone seal.

R(S(R*&+R R&C'S

1-1- Fractured Lst., channel porosit".Fractured Lst., channel porosit".

2-2- Fossiliferous Lst., Fossiliferous Lst., intergranular porosit".intergranular porosit".

S&RC( R&C'S

1-1- Fora!iniferal acstone.Fora!iniferal acstone.

2-2- Fine grained Fine grained Lst., disse!inated bitu!en.Lst., disse!inated bitu!en.

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