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Formation of Oil and Gas, Sources,

Reservoirs and Traps

• Origin of Oil and Gas

• Sources – what are good source conditions?

• Migration – how does hydrocarbon migrate and whatinfluences migration?

• Reservoirs – what makes for good storage potential?

• Traps – how is hydrocarbon trapped?

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Formation of Oil and Gas

• Decay of algae andbacteria

• Burial

• Cooking (temp+press)

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Formation of Hydrocarbon

Favourable Preservation Conditions

•High organic production (over 10%)

•Anaerobic depositional systems

•Moderate to low rate of sedimentation

Lake (thermal stratification)ocean strat.

Barred basinSalinity contrast

Continental shelfupwelling

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Hydrocarbon type

Gas

(e.g. S. North Sea,Carboniferous)

CoalPlants

Gas

(e.g. N. North Sea,Jurassic)

OilAlgae and Bacteria

Deep burialShallow burialMaterial

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Changes on Burial

Diagenesis (up to 50ºC)

• Biological

• Chemical

• PhysicalCatagenesis (50-200ºC)

Metagenesis (200-250ºC)

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Formation of Oil and Gas

Fats, waxes, oils Kerogen

Cellulose, woodWoodyKerogen

HEAT +PRESSURE

HeavyOil

Coal

Light

oil

Gas

Gas

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The Oil Window

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Hydrocarbon Needs

Source Rock

Reservoir Rock

Trapping Mechanism

Timing

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North Sea

Oil – 3.5-5kmGas –4-6km

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Migration

• Physical Compaction - burial

• Oil and gas formation (chemical)

Kerogen expansion

Increase in pressure

Expulsion

Factors controlling migration?

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Migration

• Porosity

• Permeability – primary and secondary

High Low

Connected Unconnected

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Migration

Differentiation of

Water, oil and gas

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Hydrocarbon Traps

Types of Trap

• Structural• Stratigraphic

• Diagenetic

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Trap type

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North Sea – Key Hydrocarbon Events

• Mesozoic extension and rifting, Jurassic-Cretaceous highorganic input to rift valley

• Permian (Pangaea) basin formation (crustal subsidence) onVariscan folding

• Variscan Orogeny – Southern North Sea• Crustal Extension – Carboniferous, equatorial swamps, highorganic content

• Caledonian Orogeny – closing of Iapetus, volcanic activity inback-arc basin, land of lakes (Devonian ORS)

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North Sea Examples –Salt Gas Traps, Southern North Sea

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North Sea Examples –Salt Gas Traps, Southern NorthSea

High organic inputCarboniferous Coal

Permian desert lakebordered by vast sand dunes

Rotliegend Sandstone

Deposition of salt layer

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North Sea Examples –Salt Gas Traps, Southern North Sea

Indefatigable and Leman Fields in Rotliegend Sand

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North Sea Examples – Salt Movement and Basin Inversion

Salt flow through Permian e.g. Fulmar and Gannet Fields

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North Sea Examples – Deep (Permian) Basins

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North Sea Examples – Deep (Permian) Basins

Half-graben“trap-door mechanism

e.g. Faeroe-Shetland Basin,giant Morecambe field

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North Sea Examples – Mesozoic Rift Valleys & Extension

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North Sea Examples – Rift

 Valleys & Extension

Rifting for 100MYMost active in JurassicJurassic - highest source potentialKimmeridge Clay (140Ma) sourceKimmeridge (Brent) delta sands reservoirfrom upland volcanic centre

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North Sea Examples – Rift Valleys & Extension

Source – slow subsidence and large accumulationMaturation still in progress today

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North Sea Examples – Rift Valleys & Extension

Reservoir in delta sands off active rivers e.g. Brent Field

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North Sea Examples – Rift Valleys & Extension

150ma (Jurassic-Cretaceous)•Rapid subsidence and slumping –•Large low relief deposits ofsheet-like sandse.g. Brae, Galley,Claymore and Magnus fields