What is an oil or natural gas reservoir?

Oil and natural gas were formed from the remains of prehistoric plant and animals. Hundreds of millions of years ago, prehistoric plant and animal remains settled into the seas along with sand, silt, and rocks. As the rocks and silt settled, layer upon layer piled up in rivers, along coastlines, and on the sea bottom. Geologic shifts resulted in some of these layers being buried deep in the earth. After time, the layers of organic material were compressed beneath the weight of the sediments above them, and the increasing pressure and temperature changed the mud, sand, and silt into rock and the organic matter into petroleum. This rock containing the organic matter that turned into petroleum is referred to as source rock. The oil and natural gas are contained in the tiny pore spaces in these source rocks, similar to water in a sponge.

Not lakes of oil

During millions of years, the oil and gas that formed in the source rock deep within the earth moved upward through tiny, connected pore spaces in the rocks. Some seeped out at the surface of the earth. But most of the petroleum hydrocarbons were trapped by nonporous rocks or other barriers that would not allow them to migrate any further. These underground traps of oil and gas are called reservoirs. Reservoirs are not underground “lakes” of oil; reservoirs are made up of porous and permeable rocks that can hold significant amounts of oil and gas within their pore spaces. The properties of these rocks allow the oil and natural gas within them to flow through the pore spaces to a producing well.

Where they are located

Some reservoirs are only hundreds of feet below the surface, while others are thousands of feet underground. In the United States, a few reservoirs have been discovered at depths greater than 30,000 ft (9.15 km). Many offshore wells are drilled in thousands of feet of water and penetrate tens of thousands of feet into the sediments below the sea floor.

Most reservoirs contain oil, gas, and water. Gravity acts on the fluids and separates them in the reservoir according to their density, with gas being on top, then oil, then water. However, other parameters, such as fluid or rock properties and solubility will restrict complete gravitational separation. When a well produces fluids from a subsurface reservoir, typically oil and water, and often some gas, will be recovered.

The larger subsurface traps are the easiest deposits of oil and gas to locate. In mature production areas of the world, most of these large deposits of oil and gas have already been found, and many have been producing since the 1960s and 1970s. The oil and gas industry has developed new technology to identify and access smaller, thinner bands of reservoir rock that may contain oil and gas. Improved seismic techniques (such as 3D seismic) have improved the odds of correctly identifying the location of these smaller and more difficult to find reservoirs. There are still a lot of oil and gas reserves left to be discovered and produced. Future discoveries will be in deeper basins and in more remote areas of the earth. Advanced technologies also can be used to locate small reservoirs found in existing oil and gas areas.

Technological innovation also makes it to get more oil or gas from each reservoir that is discovered. For example, new drilling techniques have made it feasible to intersect a long, thin reservoir horizontally instead of vertically, enabling the oil or gas from the reservoir to be recovered with fewer wells. Technology advances have greatly enhanced the oil and gas industry’s ability to find and recover more of the finite amount of oil and gas created millions of years ago.