Petroleum reservoir

An oil field with dozens of wells. This is the Summerland Oil Field, near Santa Barbara, California, before 1906

Oil Field Mittelplate in the North Sea

Eagle Ford Shale flares visible from space (green and infrared wavelengths), in the arc between "1" and "2", amid cities in southeast Texas in 2012.

An oil field is a pool of oil under the surface of the earth, trapped in a sealed hollow of impermeable rock. As actually used in practice, the term implies the possibility of sufficient economic benefit worthy of commercial attention. Secondarily, the area on the surface above where oil lies trapped underground, is also called an oil field.[1][2]

Because oil reservoirs typically extend over a large area, possibly several hundred km across, full exploitation entails multiple wells scattered across the area. In addition, there may be exploratory wells probing the edges, pipelines to transport the oil elsewhere, and support facilities.

Because an oil field may be remote from civilization, establishing a field is often an extremely complicated exercise in logistics. This goes beyond requirements for drilling, to include associated infrastructure. For instance, workers require housing to allow them to work onsite for months or years. In turn, housing and equipment require electricity and water. In cold regions, pipelines may need to be heated. Also, excess natural gas may be burned off if there is no way to make use of it—which requires a furnace, chimney and pipes to carry it from the well to the furnace.

Thus, the typical oil field resembles a small, self-contained town in the midst of a landscape dotted with drilling rigs or the pump jacks, which are known as "nodding donkeys" because of their bobbing arm. Several companies, such as Hill International, Bechtel, Esso, Weatherford International, Schlumberger Limited, Baker Hughes and Halliburton, have organizations that specialize in the large-scale construction of the infrastructure and providing specialized services required to operate a field profitably.

More than 40,000 oil fields are scattered around the globe, on land and offshore. The largest are the Ghawar Field in Saudi Arabia and the Burgan Field in Kuwait, with more than 60 billion barrels (9.5×10⁹ m³) estimated in each. Most oil fields are much smaller. According to the US Department of Energy (Energy Information Administration), as of 2003 the US alone had over 30,000 oil fields.

In the modern age, the location of oil fields with proven oil reserves is a key underlying factor in many geopolitical conflicts.[3]

The term "oilfield" is also used as a shorthand to refer to the entire petroleum industry. However, it is more accurate to divide the oil industry into three sectors: upstream (crude production from wells and separation of water from oil), midstream (pipeline and tanker transport of crude) and downstream (refining, marketing of refined products and transportation to Oil stations).

location of Gas Fields of Iran

Vučkovec Gas Field facility, Croatia

The drillship Discoverer Enterprise is shown in the background, at work during exploratory phase of a new offshore field. The Offshore Support Vessel Toisa Perseus is shown in the foreground, illustrating part of the complex logistics of offshore oil and gas exploration and production.

Natural gas originates by the same geological thermal cracking process that converts kerogen to petroleum. As a consequence, oil and natural gas are often found together. In common usage, deposits rich in oil are known as oil fields, and deposits rich in natural gas are called natural gas fields.

In general, organic sediments buried in depths of 1,000 m to 6,000 m (at temperatures of 60 °C to 150 °C) generate oil, while sediments buried deeper and at higher temperatures generate natural gas instead. The deeper the source, the "drier" the gas (that is, the smaller the proportion of condensates in the gas). Because both oil and natural gas are lighter than water, they tend to rise from their sources until they either seep to the surface or are trapped by a non-permeable stratigraphic trap. They can be extracted from the trap by drilling.

The largest natural gas field is South Pars/Asalouyeh gas field, which is shared between Iran and Qatar. The second largest natural gas field is the Urengoy gas field, and the third largest is the Yamburg gas field, both in Russia.

Like oil, natural gas is often found underwater in offshore gas fields such as the North Sea, Corrib Gas Field off Ireland, and near Sable Island. The technology to extract and transport offshore natural gas is different from land-based fields. It uses a few, very large offshore drilling rigs, due to the cost and logistical difficulties in working over water.

Rising gas prices in the early 21st century encouraged drillers to revisit fields that previously were not considered economically viable. For example, in 2008, McMoran Exploration passed a drilling depth of over 32,000 feet (9754 m) (the deepest test well in the history of gas production) at the Blackbeard site in the Gulf of Mexico.[4] Exxon Mobil's drill rig there had reached 30,000 feet by 2006, without finding gas, before it abandoned the site.

Crude oil is found in all oil reservoirs formed in the Earth's crust from the remains of once-living things. Evidence indicates that millions of years of heat and pressure changed the remains of microscopic plant and animal into oil and natural gas.

Roy Nurmi, an interpretation adviser for Schlumberger oil field services company, described the process as follows:

Plankton and algae, proteins and the life that's floating in the sea, as it dies, falls to the bottom, and these organisms are going to be the source of our oil and gas. When they're buried with the accumulating sediment and reach an adequate temperature, something above 50 to 70 °C they start to cook. This transformation, this change, changes them into the liquid hydrocarbons that move and migrate, will become our oil and gas reservoir.[5]

In addition to the aquatic environment, which is usually a sea but might also be a river, lake, coral reef, or algal mat, the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps:[6]

• Deep burial under sand and mud.
• Pressure cooking.
• Hydrocarbon migration from the source to the reservoir rock
• Trapping by impermeable rock.

Timing is also an important consideration; it is suggested that the Ohio River Valley could have had as much oil as the Middle East at one time, but that it escaped due to a lack of traps.[6] The North Sea, on the other hand, endured millions of years of sea level changes that successfully resulted in the formation of more than 150 oilfields.[7]

Although the process is generally the same, various environmental factors lead to the creation of a wide variety of reservoirs. Reservoirs exist anywhere from the land surface to 30,000 ft (9,000 m) below the surface and are a variety of shapes, sizes, and ages.[8] In recent years, igneous reservoirs have become an important new field of oil exploration, especially in trachyte and basalt formations. These two types of reservoirs differ in oil content and physical properties like fracture connectivity, pore connectivity, and rock porosity.[9]

After the discovery of a reservoir, a petroleum engineer will seek to build a better picture of the accumulation. In a simple textbook example of a uniform reservoir, the first stage is to conduct a seismic survey to determine the possible size of the trap. Appraisal wells can be used to determine the location of oil–water contact and with it the height of the oil bearing sands. Often coupled with seismic data, it is possible to estimate the volume of an oil-bearing reservoir.

The next step is to use information from appraisal wells to estimate the porosity of the rock. The porosity, or the percentage of the total volume that contains fluids rather than solid rock, is 20–35% or less. It can give information on the actual capacity. Laboratory testing can determine the characteristics of the reservoir fluids, particularly the expansion factor of the oil, or how much the oil expands when brought from the high pressure and high temperature of the reservoir to a "stock tank" at the surface.

With such information, it is possible to estimate how many "stock tank" barrels of oil are located in the reservoir. Such oil is called the stock tank oil initially in place (STOIIP). As a result of studying factors such as the permeability of the rock (how easily fluids can flow through the rock) and possible drive mechanisms, it is possible to estimate the recovery factor, or what proportion of oil in place can be reasonably expected to be produced. The recovery factor is commonly 30–35%, giving a value for the recoverable resources.

The difficulty is that reservoirs are not uniform. They have variable porosities and permeabilities and may be compartmentalised, with fractures and faults breaking them up and complicating fluid flow. For this reason, computer modeling of economically viable reservoirs is often carried out. Geologists, geophysicists, and reservoir engineers work together to build a model that allows simulation of the flow of fluids in the reservoir, leading to an improved estimate of the recoverable resources.

Reserves are only the part of those recoverable resources that will be developed through identified and approved development projects. Because the evaluation of "Reserves" has a direct impact on the company or the asset value, it usually follows a strict set of rules or guidelines (even though loopholes are commonly used by companies to inflate their own share price). The most common guidelines are the SPE PRMS guidelines, the SEC Rules, or the COGE Handbook. Government may also have their own systems, making it more complicated for investors to compare one company with another.

To obtain the contents of the oil reservoir, it is usually necessary to drill into the Earth's crust, although surface oil seeps exist in some parts of the world, such as the La Brea tar pits in California and numerous seeps in Trinidad.

A virgin reservoir may be under sufficient pressure to push hydrocarbons to the surface. As the fluids are produced, the pressure will often decline, and production will falter. The reservoir may respond to the withdrawal of fluid in a way that tends to maintain the pressure. Artificial drive methods may be necessary.