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.

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.

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.

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.

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

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).

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%.

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.

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.

REQUIREMENTS FOR OIL AND GAS TO SURVIVE UNDERGROUND

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.

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.

RESERVOIR ROCK:

CAP ROCK:

OIL TRAPS