|BRITAIN'S OFFSHORE OIL & GAS
Much oil and gas moves away or migrates from the source rock. Migration is triggered both by natural compaction of the source rock and by the processes of oil and gas formation. Most sediments accumulate as a mixture of mineral particles and water. As they become buried, some water is squeezed out and once oil and gas are formed, these are also expelled. If the water cannot escape fast enough, as is often the case from muddy source rocks, pressure builds up. Also, as the oil and gas separate from the kerogen during generation, they take up more space and create higher pressure in the source rock. The oil and gas move through minute pores and cracks which may have formed in the source rock towards more permeable rocks above or below in which the pressure is lower (F8).
Oil, gas and water migrate through permeable rocks in which the cracks and pore spaces between the rock particles are interconnected and are large enough to permit fluid movement (F5). Fluids cannot flow through rocks where these spaces are very small or are blocked by mineral growth; such rocks are impermeable (F6). Oil and gas also migrate along some large fractures and faults which may extend for great distances if, or when as a result of movement, these are permeable.
Oil and gas are less dense than the water which fills the pore spaces in rocks so they tend to migrate upwards once out of the source rock. Under the high pressures at depth gas may be dissolved in oil and vice versa so they may migrate as single fluids. These fluids may become dispersed as isolated blobs through large volumes of rock, but larger amounts can become trapped in porous rocks. Having migrated to shallower depths than the source rocks and so to lesser pressures the single fluids may separate into oil and gas with the less dense gas rising above the oil. If this separation does not occur below the surface it takes place when the fluid is brought to the surface (F7). Water is always present below and within the oil and gas layers, but has been omitted from most of the diagrams for clarity.
Migration is a slow process, with oil and gas travelling between a few kilometres and tens of kilometres over millions of years. But in the course of many millions of years huge amounts have risen naturally to sea floors and land surfaces around the world (F8). Visible liquid oil seepages are comparatively rare, most oil becomes viscous and tarry near the surface as a result of oxidation and bacterial action, but traces of natural oil seepage can often be detected if sought.