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Petroleum Systems for Geologists

1) Petroleum systems involve elements like source rocks, reservoir rocks, seals, and traps that work together through processes of generation, migration, and accumulation of hydrocarbons. 2) Source rocks are organic-rich sediments that generate hydrocarbons during thermal maturation; hydrocarbons then migrate through porous reservoir rocks and accumulate below impermeable seal rocks in structural or stratigraphic traps. 3) The key processes are generation of hydrocarbons in the source rock kitchen during heating, primary and secondary migration out of the source rock into reservoir rocks driven by buoyancy, and preservation of the hydrocarbons in traps below seals.

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314 views19 pages

Petroleum Systems for Geologists

1) Petroleum systems involve elements like source rocks, reservoir rocks, seals, and traps that work together through processes of generation, migration, and accumulation of hydrocarbons. 2) Source rocks are organic-rich sediments that generate hydrocarbons during thermal maturation; hydrocarbons then migrate through porous reservoir rocks and accumulate below impermeable seal rocks in structural or stratigraphic traps. 3) The key processes are generation of hydrocarbons in the source rock kitchen during heating, primary and secondary migration out of the source rock into reservoir rocks driven by buoyancy, and preservation of the hydrocarbons in traps below seals.

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Bright Moon
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© © All Rights Reserved
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Petroleum

Systems
Source, Generation, and
Migration

Petroleum Systems

Elements
Source Rock
Migration Route
Reservoir Rock
Seal Rock
Trap

Processes
Generation
Migration
Accumulation
Preservation

Petroleum System Summary


Source (Material and Rocks)
Generation
Migration
Trap
Reservoir

(Maturation)

Source
Hydrocarbon
originates from minute organisms in seas and
Rocks

lakes. When they die, they sink to the bottom where they form
organic-rich "muds" in fine sediments.

These "muds" are in a reducing environment or "kitchen",


which strips oxygen from the sediments leaving hydrogen and
carbon.
The sediments are compacted to form organic-rich rocks with
very low permeability.
The hydrocarbon can migrate very slowly to nearby porous
rocks, displacing the original formation water.

Temperature
Window
If the temperature is too low, the
organic material cannot transform
into hydrocarbon.
If the temperature is too high, the
organic material and hydrocarbons
are destroyed.

Hydrocarbon
Hydrocarbon migration takes place in two stages:
Migration
Primary migration -

from the source rock to a porous rock.

This is a complex process and not fully understood. It is probably limited to a few
hundred metres.

Secondary migration -

along the porous rock to the trap.

This occurs by buoyancy, capillary pressure and hydrodynamics through a


continuous water-filled pore system.
It can take place over large distances.

Types of Petroleum
Oil and gas are formed by the thermal cracking of
organic compounds buried in fine-grained rocks.

Algae = Hydrogen rich = Oil-prone


Wood = Hydrogen poor = Gas-prone

Kerogen Types
Type 1

Type 2

Type 3

Interpretation of Total Organic


Carbon (TOC)
(based on early oil window
maturity)
Hydrocarbon
TOC in Shale
. %(wt)

TOC in Carbonates
. %(wt)

Poor

0.0-0.5

0.0-0.2

Fair

0.5-1.0

0.2-0.5

Good

1.0-2.0

0.5-1.0

Very Good

2.0-5.0

1.0-2.0

5.0<

2.0<

Generation
Potential

Excellent

Petroleum System
1) Early Generation

Spill Point

Migration from
Kitchen
2) Late Generation

Spill Point

Reservoir Rock
(Sandstone)

Seal Rock
(Mudstone)

Gas beginning to
displace oil

Displaced oil
accumulates
Gas displaces all
oil

Generation, Migration, and


Trapping of Hydrocarbons

Fault
(impermeable)

Oil/water
contact (OWC)
Migration route
Seal

Hydrocarbon
accumulation
in the
reservoir rock
Top of maturity
Source rock

Reservoir
rock

Petroleum
System
Processes
Petroleum System Elements
Petroleum System Elements

Gas
Cap

Entrapment
Accumulation

Oil
Water

Seal Rock
Reservoir
Rock

Migration
120 F
350 F

Generation
Source
Rock

24803

Hydrocarbon Traps
Structural traps
Stratigraphic traps
Combination traps

Structural Hydrocarbon Traps


Shale

Oil

Trap

Se
al

Oil/Gas
Contact
Oil/Wat
er
Contact

Fracture Basement

Salt
Dome

Gas

Closure

Oil
Fold Trap

Salt
Diapir

Oil

Hydrocarbon Traps - Dome


Gas

W
at

er

Oil

Sandstone

Shale

Fault Trap
Oil / Gas

Sand

Shale

Stratigraphic Hydrocarbon Traps


Unconformity

Pinch out

Oil/Gas

Uncomformity

Oil/Gas

Channel Pinch Out

Oil/Gas

(modified from Bjorlykke, 1989)

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