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Our transcription: Lighter than both rock and water, petroleum drifts upward through the porous spaces in fractures in rock. Some finds its way all the way to the surface of the Earth where it dissipates into the oceans or collects in pools. The remainder, however, becomes trapped by geologic structures within the Earth. One of the most effective structures for creating a petroleum reservoir is known as an "anticlinal trap". Here an impermeable layer of rock forms a cap over a layer of porous permeable sedimentary rock. If petroleum is present, it drifts upward and is caught within the fold of the anticline. Natural gas, which is the lightest form of petroleum, collects at the top. Next comes oil. Water, which is heavier than petroleum, forms a layer underneath. Other geologic structures can also create oil traps -- a fault, for example, or an unconformity.
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