Heat combined with increased pressure aids the transformation of organic matter into petroleum products. But too much heat or too much pressure can prevent any oil from forming. Many geologists use petroleum formation to guide them on how much heat was in an area. For example, if oil is found, the temperature never exceeded 200E C or (392E F).

As oil is being produced in source rocks like diatomite or limestone, the oil and gas will move into reservoir rocks that can trap the oil as shown in the diagram below.