Hydrocarbon Traps and Seals

Revision

What are geological elements of petroleum system?

What are geological processes of petroleum system?

How organic matters is converted into petroleum?

• Source Rock - A rock with abundant hydrocarbon-prone organic matter

• Reservoir Rock - A rock in which oil and gas accumulates:

- Porosity - space between rock grains in which oil accumulates

- Permeability - passage-ways between pores through which oil and gas moves

• Seal Rock - A rock through which oil and gas cannot move effectively (such as mudstone and claystone)

• Migration Route - Avenues in rock through which oil and gas moves from source rock to trap

• Trap - The structural and stratigraphic configuration that focuses oil and gas into an accumulation

Top of oil window

Top of gas window

Generation Expulsion Accumulation

GOC

OWC

Migration Preservation

The processes

(modified from Tissot and Welte, 1984)

Organic Debris

Kerogen

Carbon

Initial Bitumen

Oil and Gas

Methane

Oil Reservoir

MigrationThermal Degradation

Cracking

Diagenesis

Catagenesis

Metagenesis

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gSchematic Representation of the Mechanism

of Petroleum Generation and Destruction

i. Diagenesis is chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperature and pressure than the former. ii. Catagenesis Catagenesis is the cracking process which results in the conversion of organic kerogens into hydrocarbons 

iii. Metagenesis is the last stage of maturation and conversion of organic matter to hydrocarbons. Metagenesis occurs at temperatures of 150° to 200°C. At the end of metagenesis, methane, or dry gas, is evolved along with nonhydrocarbon gases such as CO2, N2, and H2S, as oil molecules are cracked into smaller gas molecules.

What is TrapA trap is the place where oil and gas are barred

from further movement….(Levorsen, 1967)

.

Seals or Cap RocksFor a trap to have integrity, it must be overlain

by an effective seal.Any rock that is impermeable can act as seal or

cap rock but commonly mudstone

Introduction: Oil Traps• Some rocks are permeable and allow oil and gas to freely pass through them

• Other rocks are impermeable and block the upward passage of oil and gas

• Where oil and gas rises up and capped by impermeable rocks it can’t escape. This is one type of an Oil Trap.

Impermeable

Permeable

• The permeable rocks than contain oil and gas within the oil trap are known as the Reservoir Rock.

• Reservoir rocks have lots of interconnected holes called pores. These allow them to absorb the oil and gas like a sponge.

Earth Science World Image Bank Image #h5innl

This is a highly magnified picture of a sandy reservoir rock (water-filled pores are shown in blue)

As oil migrates it fills up the pores(oil-filled pores shown in black)

Carbonate as Reservoir

How to find oil: Source rock, reservoir rock, traps

Hydrocarbon Trapsi. Structural traps

Structural traps are caused by structural features. They are usually formed as a result of tectonics.

ii. Stratigraphic traps

Stratigraphic traps are usually caused by changes in rock quality.

iii. Combination traps

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.

Structural Hydrocarbon Trapsi. Fold ii. Fault

iii. Salt Dome

Salt flows up as a weak mass

Petroleum trapped in top of dome

Oil and gas trapped on sides of impermeable dome

Rocks above salt dome are bowed up

iii. Salt Dome

Fault Trap

Oil / GasSand

Shale

In this normal fault trap, oil-bearing sandstone is juxtaposed against impervious shale.

Stratigraphic Hydrocarbon Traps

(modified from Bjorlykke, 1989)

Stratigraphic hydrocarbon traps occur where reservoir facies pinch into impervious rock such as shale, or where they have been truncated by erosion and capped by impervious layers above an unconformity.

Asphalt Trap

Hydrodynamic Trap

Water

MeteoricWater

BiodegradedOil/Asphalt

PartlyBiodegraded Oil

Shale

OilWater

HydrostaticHead

(modified from Bjorlykke, 1989)

Other Traps 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.

Summary

Questions?