Earth Systems 3209
Unit: 5
Earth’s Resources
Reference:
Chapters 21; Appendix A & B
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Unit 5:
Topic 3.3
Components of Petroleum Traps
Focus on . . .
 identifying source rock, cap rock, and reservoir rock as
the components required to form petroleum traps.
 relating porosity and permeability to reservoir rock.
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Components Involved in the Formation of Petroleum Traps
1) Source Rock
2) Reservoir Rock
3) Cap Rock
Note:
 Liquids within the reservoir rock separate by density
(Water - Oil - Gas)
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Components Involved in the Formation of Petroleum Traps
1) Source Rock
 Source rocks must contain an abundance of organic matter
 Petroleum is often created and released from the source
rock while lithification is occurring
 Examples of source rocks are shale and limestone
Where sediment contains
more than 5% organic matter,
it eventually forms a rock
known as a Black Shale
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Components Involved in the Formation of Petroleum Traps
2) Reservoir Rock

requires high porosity and high permeability since it is the
rock which petroleum moves through and is stored in.
Note:


Porosity is the volume of pore spaces or holes between
sediment grains.
Permeability is the interconnection of the pore space,
thereby allowing the movement of the petroleum. This
directly determines the rate of flow.
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Components Involved in the Formation of Petroleum Traps
2) Reservoir Rock
 Porosity of a material is influenced by:
1) particle shape
2) particle size
3) the degree of sediment sorting.
 Large, rounded, well-sorted particles offer higher porosity,
particularly if the amount of cement between them is limited.
 Usually, the higher the porosity and the larger the pore
spaces, the higher the permeability.
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Components Involved in the Formation of Petroleum Traps
2) Reservoir Rock
 Examples of reservoir rocks are sandstone, dolomite, and
conglomerate, all have both high porosity and permeability.
 Note: petroleum exists within reservoir rock between sediment
as opposed to being confined as a whole volume of liquid
petroleum.
 “There are no ponds of petroleum in the ground”.
As oil migrates it fills up the pores
(oil-filled pores shown in black)
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Components Involved in the Formation of Petroleum Traps
3) Cap Rock:
 is an impermeable rock that serves to trap
petroleum from either escaping to the
surface or spreading throughout the rock.
 Example: Shale
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Example 1:
Which rock type would have high porosity and high permeability?
A) granite
B) quartzite
C) sandstone
D) shale
Which has the highest porosity and highest permeability?
A) cap rock
B) reservoir rock
C) source rock
D) trap rock
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Example 2:
Which rock is most likely to be the target for an oil company?
A) fractured basalt
B) continuous shale layer
C) porous shale
D) porous sandstone
Which would be the best cap rock in an oil trap?
A) porous and permeable sandstone
B) permeable shale
C) impermeable shale
D) porous and permeable dolomite
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Your Turn . . .
Take the time and complete the following questions . . .
(Solutions to follow)
Questions:
Which characterizes the best reservoir rocks?
A)
B)
C)
D)
high permeability and high porosity
impermeable
low permeability and high porosity
nonporous
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Solutions . . .
Questions:
Which characterizes the best reservoir rocks?
A)
B)
C)
D)
high permeability and high porosity
impermeable
low permeability and high porosity
nonporous
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Summary . . .
Overview of Points covered:
Three (3) components of a petroleum trap include:
1) Source Rock
 Contain abundant organic matter. Ex. Shale and Limestone
2) Reservoir Rock
 Have high porosity high permeability. Ex. Sandstone
3) Cap Rock
 Impermeable rock that traps the oil. Ex. Shale
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