Remember: Oil was formed millions of years ago from the remains of tiny plants and animals that died in the ancient seas (see "What is Crude Oil?"). Movements in the Earth trapped the oil and natural gas in the reservoir rocks between layers of impermeable rock, or cap rock, such as granite or marble.

The task of finding oil is assigned to geologists who search for the right conditions for an oil trap -- the right source rock, reservoir rock and entrapment.

Charges are drilled into the ground and detonated to produce shock waves. The reflections travel at different speeds depending upon the type or density of rock layers through which they must pass. These reflections are detected by sensitive microphones or vibration detectors -- hydrophones over water, geophones over land.

Test bore tractor drill	Workers drill and insert charges in the ground
Geophones detect vibrations created by charges	Information then received by Data Collector

The readings are interpreted by seismologists for signs of possible oil and gas traps such as anticlines.

An anticline is formed when rock layers fold in the shape of an arch.

Deep underground, anticlines are highly desired by petroleum prospectors. The rock beds trap rising hydrocarbon fluids, allowing oil and gas to build up in the pore space of the anticline rocks. The largest oilfields occur in large, gentle anticlines in thick sedimentary rock sequences.

http://geology.about.com/library/bl/images/blanticline.htm

http://science.howstuffworks.com/oil-drilling1.htm

PETEX 2005, A Dictionary of Oil & Gas Industry