Facts, Figures and Explanations Oil and Gas on the Move Once it has formed, oil and gas moves away (migrates) from the source rock. This movement happens for two reasons. First the oil and gas expand to take up more room than the original organic matter (algae, bacterial and leaf skins) therefore their pressure on the rock increases and they try to escape. Second, being less dense than the surrounding rock and water, they tend to rise upwards. This migration is a slow process. It takes millions of years for the oil and gas to rise a few kilometres. What happens to the migrating oil and gas? If there is no seal above the rock source, oil and gas slowly rise and escape at the surface. This does happen on occasions but, when it does, no oil and gas deposits can build up. However because oil and gas formation takes such a long time it often spans millions of years and during this extended period of time a range of different environmental conditions can apply. For example, a desert could develop whose sands could compact to form a layer of sandstone over the source rock before any or all of the oil and gas have escaped. In this case, the migrating oil and gas would rise up into the sandstone, which would act as reservoir rock. Sandstone can store oil and gas because it is both porous and permeable. Porous means that liquids and gas can be held and stored. The pores in rock are the spaces that occur between the individual rock particles. These spaces are created because the rock particles are irregularly shaped and so don't fit together exactly or closely together. This porous rock is called the 'reservoir'. Permeable means that liquid can flow through. A permeable rock has pores that are connected and so allows oil and gas to flow through. In swampy areas and in water, the muddy sediment that was sinking at the same time as the organic material that went on to become oil, formed rocks known as sedimentary rocks. Some sedimentary rocks are porous and others are impermeable. Lots of people think that crude oil is like a lake beneath the earth's surface but actually it's all held in the pores of porous rock rather like water is held in a sponge. Once formed, crude oil and gas slowly rise upwards, seeping into the tiny pores in porous rock. Eventually the oil and gas reaches a layer of impermeable rock and is trapped. Unless an impermeable rock stops its progress, oil and gas will eventually migrate from porous rock through permeable rock to the surface. So oil and gas deposits will only remain in the reservoir rock if another layer forms on top which is impermeable i.e. does not allow the liquids or gases to pass through or escape. This impermeable layer is called a seal. So although there are many places in the world where crude oil seeps to the surface it is more common to find oil-containing rocks deep below the land or seabed in rock structures called traps. Traps Click on the thumbnail to view a larger image How oil and gas were trapped by sedimentary rocks Just as oil has formed throughout the Earth's history, so have rocks. The movements of rocks on a massive scale over millions of years has resulted in the formation of different types of rock structures. A trap can occur where rocks have been pushed or folded by the powerful forces within the Earth's crust. When this happens- this is known as an anticline trap. The impermeable rock traps the crude oil preventing it from flowing away - like an upturned bathtub. A fault trap is created when rocks slide past each other - an impermeable rock then forms a sort of dam trapping the flow of the moving oil. Salt domes are formed by a combination of rock movements forcing a layer of rock salt up through the layers of rock above. If one of these layers of rock is porous, oil and gas may become trapped by the salt dome which forms an impermeable plug. Facts, Figures and Explanations Index Next Page

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