There are three geological features that need to be present before oil may be present underground: source rock, reservoir rock, and geological traps. The source rock is where the oil was formed(if you accept the organic theory), but because it is relatively non-porous, it can not hold oil in appreciable amounts. Instead, the oil migrates to more porous rock like sandstone or limestone. These are examples of reservoir rock. It is possible for the oil to move through the reservoir rock all the way to the surface of the earth. However, this rarely happens because its progress is blocked by some impermeable rock barrier. This causes the oil to accumulate to form a reservoir. The barrier and the resulting reservoir form what is known as a trap.

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