Oil Exploration is the search by petroleum geologists for hydrocarbon deposits beneath the Earth's surface.
When organic-rich rock such as oil shale is subjected to high pressure and temporature over an extended period of time, hydrocarbons form. Being less dense that the surrounding water table, these are expelled and forced upwards. Normally such hydrocarbons will seep to the surface, but particular formations of non-porous rock known as traps may allow them to accumulate. Such accumulations are known as reservoirs.
The objective of oil exploration is to find such reservoirs.
While visible surface features such as pock-marks or craters caused by escaping gas may provide evidence of very shallow reservoirs, most exploration depends on highly sophisticated technology to detect larger, deeper deposits. Areas thought to contain hydrocarbons are initially subjected to a gravity survey or magnetic survey to detect large scale features of the sub-surface geology. Features of interest (known as prospects) are subjected to more detailed seismic surveys and those unlikely to be mature traps discarded. Finally, an exploration well is drilled to conclusively determine the presence or absence of oil or gas.
Oil exploration is an expensive, high-risk operation. It is generally only undertaken by very large corporations or national governments.