Special conditions and great lengths of time were needed for these remains to undergo complex chemical changes to form oil and gas. These are sometimes concentrated in accumulations which man can detect and exploit. Oil exploration began more than a hundred years ago, when drilling was carried out near oil seeps which indicated that oil lay below the surface. Today, much more sophisticated techniques are employed, such as seismic surveying and satellite imaging.
Powerful computers assist geologists in interpreting their findings.
At the end of the day, however, only the drill can determine whether or not oil lies below the ground. Millions of years ago, countless microscopic plants and animals known as phytoplankton and zooplankton floated in the surface waters of ancient seas. After death, the remains of these minute organisms settled to the seabed where, together with mud and silt, they formed, over millions of years, organic-rich sedimentary layers. The continued accumulation of younger sediments buried the organic layers to depths of thousands of feet, compressing them into a rock that was to become the source for oil. As the depth of burial increased, the temperature rose.