Hydrocarbons form when organic-rich rocks, like oil shale or coal, are subjected to high underground pressures and temperatures for an extended period of geological time. Eventually, due to the pressures and temperatures, the hydrocarbons escape from source rock and seep toward the surface. When the liquid or gas hydrocarbons become trapped, a petroleum reservoir is formed between other layers of rock. For a trap to occur, the layer of rock on top has to be impermeable in order to prevent these hydrocarbons from migrating further. Inside an oil or gas reservoir, the hydrocarbons are mixed with porous sandstone or limestone. The petroleum collects in the pores of these rocks.
When a reservoir is discovered, wells are drilled in order to release the petroleum or for the purpose of pumping it to the surface. Oil and gas of varying grades may be found near the surface or tens of thousands of feet below the ocean. Oil, gas and water are sometimes mixed in the same reservoir and must be separated and refined.
Oil exploration involves extensive economic risks. For that reason, as much geological information as possible is gathered before a well is drilled. Sophisticated technology is usually required to detect underground deposits of oil and gas. Seismic, magnetic and gravitational surveys are often used to find deposits. By studying the time it takes for sound waves to travel and reflect through rock of varying densities, for instance, geologists and geophysicists can determine if a reserve of oil or gas is located beneath the surface. If such a reserve does exist, its size and depth can also be estimated.
Much of the world’s reserves are located in the Middle East, but oil and gas fields in the Gulf of Mexico produce 30 percent of the crude oil consumed in the United States and 21 percent of the natural gas. Approximately 3,900 conventional drilling platforms are attached to the ocean floor in the shallow regions of the gulf. But more than half of the oil and gas harvested from the gulf comes from the 50 or so larger facilities floating in deeper regions.
Offshore exploration is generally done by large companies or governments. Drilling a well for oil or gas in relatively shallow water, depending on conditions, costs from $10 to $30 million. Deep water wells can cost more than $100 million. A big oil field is capable of yielding billions of dollars worth of petroleum over its lifespan. Some of the huge drilling platforms in the Gulf of Mexico produce up to 150,000 barrels of crude oil per day.
Before drilling begins offshore or onshore, reservoir engineers try to determine how much oil or gas can actually be recovered during the lifespan of the proposed well. In an oil field, 10-50 percent of the petroleum is typically recoverable. For gas, 50-80 percent is usually recoverable.
More than 80 percent of the oil and gas extracted from the Earth’s crust is used for fuel, but petroleum is also used as the raw material for other products, including fertilizers, pesticides and plastics.
The Earth’s reserves of hydrocarbons are finite. As the global demand for oil increases, it has been estimated that the world’s known reserves will be depleted in another 40 years or so. With this in mind, companies are investing more than ever before on the discovery of new deposits.