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Oil and Gas n Describe and explain the origin of oil and natural gas and migration from source-rock to reservoir-rock under a cap rock. Define and recognise.

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Presentation on theme: "Oil and Gas n Describe and explain the origin of oil and natural gas and migration from source-rock to reservoir-rock under a cap rock. Define and recognise."— Presentation transcript:

1 Oil and Gas n Describe and explain the origin of oil and natural gas and migration from source-rock to reservoir-rock under a cap rock. Define and recognise the trap structures; anticline, fault, salt dome, unconformity and lithological.

2 Formation of Oil and Gas n They are formed by the partial decomposition of: n Marine planktonic organisms (plants/algae and bacteria). n This decomposition usually happens at low temperature < 200  C, above this the plant matter would completely decompose. n Where does the energy come from initially? n The Sun.

3 Formation of Oil and Gas 2 n This partial decomposition begins when plankton accumulate on the sea floor. n This accumulation needs to be in a low energy environment often in deep basins. n In this environment anaerobic (like coal) conditions occur and the bacteria that do exist can only partially break down the organic material. n Sometimes deeply buried coal can also break down to form gas.

4 STAGES OF OIL FORMATION Stage 1: n Organic mud. Present day forms in the Black Sea and contains 35% organics.  Draw a simplified version of diagram 2 page 2.

5 Stage 2: n The organic mud is buried and compressed. n Forms a black “oil” shale. For example the Jurassic (140mya) Kimmeridge shale, Dorset.

6 Stage 3/Maturation: n If heated and compressed enough the organic material changes to kerogen (diagrams 4 & 5 page 3). n With further burial chains of C and H break off the kerogen to form heavy oil. n As heating continues chains break off the heavy oil to produce light oil and finally break further to produce gas (Often Methane).

7 Maturation 2 n The process of oil and gas formation from a rock is called maturation. n Petroleum can exist as: – natural gas – crude oil – asphalt (a solid). n Petroleum consists of H and C with varying amounts of O, S and N, usually only consisting of a total of 10%.

8 Maturation 3 n If in stage 3 the depth of burial is shallow then heavy crude oil will form. n With deep burial light crude oil will form as the greater temperatures and pressures will break down the long hydrocarbon chains into smaller lighter molecules such as gas.

9 Maturation in the North Sea n Therefore the type of oil and gas varies depending on depth of burial. n In the N. Sea: 1. Oil forms at km depth 2. Gas forms at km depth 3. >6 km the kerogen is carbonised with no hydrocarbons.  Look at diagram 6 page 3.

10 REQUIREMENTS FOR OIL AND GAS TO SURVIVE UNDERGROUND

11 SOURCE ROCK: n Rock in which the oil formed. n Oil Shale. n Not necessarily where it is found. n As this rock is compacted more the liquid and gas can be squeezed out and migrates up slope, as it is less dense than the surrounding solid rock.

12 Oil Migration n The usual route is via permeable rocks (see diagrams 7, 8 & 9 page 4), lines of weakness such as faults, joints and unconformities. n This is a slow process, probably only a few Kms. in a million years.

13 RESERVOIR ROCK:

14 CAP ROCK:

15 OIL TRAPS


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