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*Trapping Oil and Gas

F9: Traps

Oilfields and gasfields are areas where hydrocarbons have become trapped in permeable reservoir rocks, such as porous sandstone or fractured limestone. Migration towards the surface is stopped or slowed down by impermeable rocks such as clays, cemented sandstones or salt which act as seals. Oil and gas accumulate only where seals occur above and around reservoir rocks so as to stop the upward migration of oil and gas and form traps (F10), in which the seal is known as the cap rock. The migrating hydrocarbons fill the highest part of the reservoir, any excess oil and gas escaping at the spill point (F9) where the seal does not stop upward migration. Gas may bubble out of the oil and form a gas cap above it; at greater depths and pressures gas remains dissolved in the oil (F9). Since few seals are perfect, oil and gas escape slowly from most traps. In many fields incoming oil and gas balance this loss, as in the Brent and Ekofisk fields in the North Sea. Gas migrates and escapes from traps more readily than oil, but the salt layers beneath the southern North Sea where much gas is trapped have proved a very efficient seal because salt contains no pore spaces, and fractures reseal themselves.

Figure 10 shows the main types of traps. Structural traps are formed where rocks are folded into suitable shapes (A) or reservoir and sealing rocks are juxtaposed across faults (B). Traps may also form when rocks are domed over rising salt masses (C). Stratigraphic traps originate where a suitable combination of rock types is deposited in a particular environment (D), for example, where a reservoir rock of permeable river sand is sealed by clays accumulated in the swamps which formed to cover the river channel. In reality most traps are formed by more complex sequence of events and cannot be classified so rigidly. For example (E), the reservoir rock was first folded and eroded, then sealed by an impermeable rock which was deposited later over the eroded structure. Where a particular set of circumstances has combined to produce a group of oil or gasfields with similar trap structures or reservoir rock, this is termed a play. There are several important plays in the North Sea, which are described on pages 18 to 23.

F10: Trap structures

In order to trap migrating oil and gas, structures must exist before hydrocarbon generation occurs. In some parts of the North Sea trap structures existed 125 million years ago, but were not filled with oil until 100 million years later. The rocks beneath the North Sea are sinking only a few millimetres in ten years, so generation only occurred after very long periods of burial and 'cooking'.

All oil and gas fields form by a chance combination of events that produces the right sorts of rocks and structures, together with the right timing. The forces that shaped Britain's offshore oil and gas fields also created oceans and transported Britain from south of the equator to its present latitude. The origin of these forces and their effect on the crust, both globally and around Britain, are described in the next section.

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