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# Long description

These are two similar sketches of vertical sections through the Earth's surface. They show a gentle upwards fold of three rock strata below the surface. There are narrow bands of impermeable rock top and bottom and a central larger section of permeable rock. The permeable rock has oil filling the upper part, floating on water below, with a horizontal line separating them. In (a), there is an drilling rig with a pipe down into the oil layer and arrows showing that the oil flows under its own pressure into the pipe and upwards to the surface. In (b), in addition to the oil production well seen in (a), there is also a water tank at the surface with a pipe down into the water layer, labelled water injection well. There are arrows to show that the water is pumped down the pipe and out into the water layer below the oil. This results in the water level below the oil being higher in (b) than in (a). Oil is then forced up into the pipe of the oil production well and up to the surface in the same way as in (a).