A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s contained in
porous or fractured
rock formations.
Such reservoirs form when
kerogen (ancient plant matter) is created in surrounding rock by the presence of high heat and pressure in the Earth's crust. Petroleum reservoirs are broadly classified as ''conventional'' and ''
unconventional'' reservoirs. In conventional reservoirs, the naturally occurring hydrocarbons, such as
crude oil
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
or
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
, are trapped by overlying rock formations with lower
permeability, while in
unconventional reservoirs, the rocks have high
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
and low permeability, which keeps the hydrocarbons trapped in place, therefore not requiring a
cap rock. Reservoirs are found using
hydrocarbon exploration
Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology.
Exploration methods
Visi ...
methods.
Oil field
An oil field is an area of accumulation of liquid oil underground in multiple (potentially linked) reservoirs, trapped as it rises by impermeable rock formations. In industrial terms, an oil field implies that there is economic benefit worthy of commercial attention. Oil fields themselves may extend up to several hundred kilometers across the surface, meaning that extraction efforts can be large and spread out across the area. In addition to extraction equipment, there may be exploratory wells probing the edges to find more reservoir area,
pipelines to transport the oil elsewhere, and support facilities.
Oil fields can occur anywhere that the geology of the underlying rock allows, meaning that certain fields can be far away from civilization, including at
sea
The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...
. Creating an operation at an oil field can be a logistically complex undertaking, as it involves not only the equipment associated with
extraction Extraction may refer to:
Science and technology
Biology and medicine
* Comedo extraction, a method of acne treatment
* Dental extraction, the surgical removal of a tooth from the mouth
Computing and information science
* Data extraction, the pr ...
and
transportation
Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land ( rail and road), water, cable, pipelin ...
, but infrastructure such as roads and housing for workers. This infrastructure has to be designed with the lifespan of the oil field in mind, as production can last many years. Several companies, such as
Hill International,
Bechtel
Bechtel Corporation () is an American engineering, procurement, construction, and project management company founded in San Francisco, California, and headquartered in Reston, Virginia. , the '' Engineering News-Record'' ranked Bechtel as the se ...
,
Esso
Esso () is a trading name for ExxonMobil. Originally, the name was primarily used by its predecessor Standard Oil of New Jersey after the breakup of the original Standard Oil company in 1911. The company adopted the name "Esso" (the phonetic ...
,
Weatherford International
Weatherford International plc, an American Irish public limited company, together with its subsidiaries, is a multinational oilfield service company and one of the largest companies in the world in oil services. Weatherford is a wellbore and p ...
,
Schlumberger Limited,
Baker Hughes and
Halliburton, have organizations that specialize in the large-scale construction of the
infrastructure
Infrastructure is the set of facilities and systems that serve a country, city, or other area, and encompasses the services and facilities necessary for its economy, households and firms to function. Infrastructure is composed of public and priv ...
to support oil field exploitation.
The term "oilfield" can be used as a shorthand to refer to the entire
petroleum industry
The petroleum industry, also known as the oil industry or the oil patch, includes the global processes of exploration, extraction, refining, transportation (often by oil tankers and pipelines), and marketing of petroleum products. The larg ...
. However, it is more accurate to divide the oil industry into three sectors: upstream (
crude oil production
Petroleum is a fossil fuel that can be drawn from beneath the earth's surface. Reservoirs of petroleum was formed through the mixture of plants, algae, and sediments in shallow seas under high pressure. Petroleum is mostly recovered from oil d ...
from wells and
separation of water from oil),
midstream (
pipeline and
tanker transport of crude oil) and downstream (
refining of crude oil to products, marketing of refined products, and transportation to oil stations).
More than 65,000 oil fields are scattered around the globe, on land and offshore. The largest are the
Ghawar Field in
Saudi Arabia
Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in Western Asia. It covers the bulk of the Arabian Peninsula, and has a land area of about , making it the fifth-largest country in Asia, the second-largest in the Ara ...
and the
Burgan Field
The Burgan field is an oil field situated in the desert of southeastern Kuwait. Burgan field can also refer to the Greater Burgan—a group of three closely spaced fields, which includes Burgan field itself as well as the much smaller Magwa and ...
in
Kuwait
Kuwait (; ar, الكويت ', or ), officially the State of Kuwait ( ar, دولة الكويت '), is a country in Western Asia. It is situated in the northern edge of Eastern Arabia at the tip of the Persian Gulf, bordering Iraq to Iraq–Ku ...
, with more than 66 to 104
billion barrels
A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids, u ...
estimated in each. In the modern age, the location of oil fields with proven
oil reserves is a key underlying factor in many
geopolitical conflicts.
Gas field
Natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
originates by the same geological
thermal cracking process that converts
kerogen to
petroleum
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
. 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
A seep or flush is a moist or wet place where water, usually groundwater, reaches the earth's surface from an underground aquifer.
Description
Seeps are usually not of sufficient volume to be flowing beyond their immediate above-ground location ...
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
Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...
and
Qatar
Qatar (, ; ar, قطر, Qaṭar ; local vernacular pronunciation: ), officially the State of Qatar,) is a country in Western Asia. It occupies the Qatar Peninsula on the northeastern coast of the Arabian Peninsula in the Middle East; it sh ...
. The second largest natural gas field is the
Urengoy gas field, and the third largest is the
Yamburg gas field, both in
Russia
Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...
.
Like oil, natural gas is often found underwater in offshore gas fields such as the
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...
,
Corrib Gas Field off
Ireland
Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...
, 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
Offshore drilling is a mechanical process where a wellbore is drilled below the seabed. It is typically carried out in order to explore for and subsequently extract petroleum that lies in rock formations beneath the seabed. Most commonly, the t ...
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.
Exxon Mobil's drill rig there had reached 30,000 feet by 2006, without finding gas, before it abandoned the site.
Formation
Crude oil
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
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
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
changed the remains of
microscopic
The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded as the scale be ...
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...
and
animal
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
into oil and
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
.
Roy Nurmi, an interpretation adviser for
Schlumberger
Schlumberger Limited (), doing business as SLB, is an oilfield services company. Schlumberger has four principal executive offices located in Paris, Houston, London, and The Hague.
Schlumberger is the world's largest offshore drilling comp ...
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.
In addition to the
aquatic environment, which is usually a sea but might also be a
river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...
,
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
,
coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
, or
algal mat, the formation of an oil or gas reservoir also requires a
sedimentary basin
Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsiden ...
that passes through four steps:
* 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
The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabian Peninsula, Arabia (including the Arabian Peninsula and Bahrain), Anatolia, Asia Minor (Asian part of Turkey except Hatay Pro ...
at one time, but that it escaped due to a lack of traps.
The
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...
, on the other hand, endured millions of years of sea level changes that successfully resulted in the formation of more than 150
oilfield
A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations.
Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presenc ...
s.
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 below the surface and are a variety of shapes, sizes, and ages.
In recent years, igneous reservoirs have become an important new field of oil exploration, especially in
trachyte
Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...
and
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
formations. These two types of reservoirs differ in oil content and physical properties like
fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...
connectivity, pore connectivity, and rock
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
.
Geology
Traps
A
trap forms when the
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
forces driving the upward migration of
hydrocarbons
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
through a
permeable rock cannot overcome the
capillary forces of a sealing medium. The timing of trap formation relative to that of petroleum generation and migration is crucial to ensuring a reservoir can form.
Petroleum geologists broadly classify traps into three categories that are based on their geological characteristics: the
structural trap, the stratigraphic trap and the far less common
hydrodynamic trap. The trapping mechanisms for many petroleum reservoirs have characteristics from several categories and can be known as a combination trap.
Traps are described as
structural traps (in deformed strata such as folds and faults) or
stratigraphic traps (in areas where rock types change, such as unconformities, pinch-outs and reefs). A trap is an essential component of a petroleum system.
Structural traps
Structural traps are formed as a result of changes in the structure of the subsurface due to processes such as folding and faulting, leading to the formation of
domes
A dome () is an architectural element similar to the hollow upper half of a sphere. There is significant overlap with the term cupola, which may also refer to a dome or a structure on top of a dome. The precise definition of a dome has been a m ...
,
anticline
In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is t ...
s, and
folds. Examples of this kind of trap are an
anticline trap, a
fault trap, and a
salt dome trap (see
salt dome
A salt dome is a type of structural dome formed when salt (or other evaporite minerals) intrudes into overlying rocks in a process known as diapirism. Salt domes can have unique surface and subsurface structures, and they can be discovered usi ...
).
They are more easily delineated and more prospective than their stratigraphic counterparts, with the majority of the world's petroleum reserves being found in structural traps.
source rock
In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depo ...
File:Structural Trap Fault.svg, Structural trap along a fault plane
File:StratigraphicTrap5.png, Structural-stratigraphic trap in a
traps are formed as a result of lateral and vertical variations in the thickness, texture,
of the reservoir rock. Examples of this type of trap are an
.