In the early days, the search for oil was very haphazard. Apart from drilling in places where oil seeped to the surface, many 'wildcat' wells were sunk on the basis of a 'hunch', often with disappointing results. Nowadays, exploration for oil has become much more scientific, but even with modern technology and the know-how of highly skilled geologists and geophysicists, the search for oil remains fraught with uncertainties. The United States is often regarded as the birthplace of the modern oil industry. By the 1920s, the USA had become a leading oil producer.
Oil exploration has to contend with an earth surface which has had a complicated history.
Geoscientists know that parts of the earth's crust, involving whole continents and oceans, move relative to one another. When collisions occurred, immense forces built mountain chains, crumpling the rocks into folds and thrusting them over each other to form complex structures. Some of these are favorable to the collection of oil. One of the commonest structures is the anticline, where rocks have the form of a dome or arch. Oil may be present in a reservoir below the anticline, sealed in by an impermeable layer.