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When organic-rich rock such as oil shale is subjected to high pressure and temperature over an extended period of time, hydrocarbons form. Hydrocarbons are expelled from source rock by three density-related mechanisms: the newly-matured hydrocarbons are less dense than their precursors, which causes overpressure; the hydrocarbons are less dense than the ubiquitous water medium, and so migrate upwards due to buoyancy, and the fluids expand as further burial causes increased heating. Almost all (99%) of such hydrocarbons 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.
Visible surface features such as oil seeps, natural gas seeps, pockmarks (underwater craters caused by escaping gas) provides basic evidence of reservoirs (be it shallow or deep in the Earth); however, most exploration depends on highly sophisticated technology to detect and determine the extent of these 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 which works on the principle of the time it takes for reflected sound waves to travel through matter (rock) of varying densities and using the process of Depth conversion to create a profile of the substructure. 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. Offshore and remote area exploration is generally only undertaken by very large corporations or national governments. Hundreds of smaller companies search for onshore hydrocarbon deposits worldwide, with some wells costing as little as $500,000 USD.
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