For many years, petroleum companies stopped at the water's edge or sought and developed oil and gas accumulations only in inland waters or shallow seas bordering onshore producing areas. Exploration deeper under the sea, and production from the continental shelves beyond territorial limits, did not begin in earnest until the world's increasing demand for petroleum energy sources, coupled with a lessening return from land drilling, provided the incentives for the huge investments needed for drilling in the open sea. See also Continental margin; Oil and gas well drilling; Petroleum reserves.
Today offshore oil exploration and production is a worldwide industry. By the early 1990s, offshore sources accounted for 30% of worldwide crude oil production and 14% of worldwide natural gas. Until now, most offshore production came from reservoirs located under the continental shelf, in water depths up to 600 ft (180 m). Spurred by technology and a need to find additional secure sources of energy, exploration and production are now moving even farther from shore and into the deeper waters of the continental slopes. Exploration wells have been drilled in water deeper than 9000 ft (2750 m), and hydrocarbons are being produced from offshore fields in waters deeper than 5000 ft (1500 m).
There is a sound geologic basis for the petroleum industry turning to the continental shelves and slopes. Favorable sediments and structures exist beneath the present seas of the world in geologic settings that have proven highly productive onshore. In fact, the subsea geologic similarity, or in some cases superiority, to geologic conditions on land has been a vital factor in the expansion of the world's investment in offshore exploration and production. See also Marine geology.
Significant exploration is taking place in water depths up to 10,000 ft (3050 m) in the Gulf of Mexico, offshore Brazil, offshore West Africa, and in the northern Atlantic off the coasts of Norway, Ireland, and the United Kingdom. Oil and natural gas fields in waters as deep as 5500 ft (1675 m) are already on-stream in the Gulf of Mexico and offshore Brazil. In the North Atlantic west of the Shetland Islands, where weather conditions are particularly severe, production has started from reservoirs in water depths up to 1500 ft (450 m). See also Petroleum geology.
The underwater search has been made possible only by vast improvements in offshore technology. Drillers first took to sea with land rigs mounted on barges towed to location and anchored, or with fixed platforms accompanied by a tender ship. As the search for oil and natural gas advanced worldwide and farther away from shore, types of exploration rigs evolved which could move easily between locations and operate in a wide range of water depths.
The move into the open and often hostile sea has required not only the development of drilling vessels but also a host of auxiliary equipment and techniques. An entire industrial complex has developed to serve the offshore industry, including construction of fixed platform structures from which the majority of the world's offshore oil and gas production is presently drilled and produced.
The United States Gulf coast, where a large percentage of the world's offshore drilling has taken place, is regularly hit by hurricanes that damage structures in their path, and a number of platforms were lost early on due to high wind and waves. However, improved understanding of the environment, as well as more advanced materials and structural analysis techniques, has significantly reduced failures due to environmental forces. The major causes of accident are now due to human error, process hazards, and transportation to and from offshore facilities by helicopter. See also Hurricane.
Improved reservoir management and further discoveries have increased production from onshore fields, where costs are less than for offshore. However, the world's increasing demand for petroleum energy continues to force the search for new reserves into even deeper waters and more remote corners of the world.
