The Zelten oil field (now known as the Nasser field) is located at the foot of the Zelten Mountains, about south of
Brega
Brega , also known as ''Mersa Brega'' or ''Marsa al-Brega'' ( ar, مرسى البريقة , i.e. "Brega Seaport"), is a complex of several smaller towns, industry installations and education establishments situated in Libya on the Gulf of Sidra, ...
in Concession 6. Zelten holds the title as the largest
oil field
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 presence ...
in the
Gulf of Sidra
The Gulf of Sidra ( ar, خليج السدرة, Khalij as-Sidra, also known as the Gulf of Sirte ( ar, خليج سرت, Khalij Surt, is a body of water in the Mediterranean Sea on the northern coast of Libya, named after the oil port of Sidra or ...
. The 229 wells in Zelten use a
gas lifting
Gas is one of the four fundamental state of matter, states of matter (the others being solid, liquid, and plasma (physics), plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), chemical element, elemental molec ...
system. The Zelten oil field is not associated with the town
Zelten, which is located in the North-West of Libya.
Background
Discovered in 1956, Zelten was Libya's first major
oil well
An oil well is a drillhole boring in Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas may ...
, producing about . In 1959, its reserves were estimated at of 38°
API
An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software Interface (computing), interface, offering a service to other pieces of software. A document or standa ...
oil, lying at 5,500-7,600 feet.
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 p ...
Standard began production of Zelten in 1961. The Zelten gas-lift system was installed during the period from 1969 to 1971 and represented one of the first uses of
gas lift
Gas lift or bubble pumps use the artificial lift technique of raising a fluid such as water or oil by introducing bubbles of compressed air, water vapor or other vaporous bubbles into the outlet tube. This has the effect of reducing the hydrosta ...
on a fieldwide basis to produce the high volume wells typical of
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 ...
Oil fields. In 1979, approximately of total liquid was being produced by gas lift. By 2006, the accumulated production of the Zelten field reached 2,426 mmb of oil and of associated gas.
Sirte Oil Company
The
Sirte Oil Company (SOC), a subsidiary of the
National Oil Corporation
The National Oil Corporation (NOC; ar, المؤسسة الوطنية للنفط) is the national oil company of Libya. It dominates Libya's oil industry, along with a number of smaller subsidiaries, which combined account for around 70% the cou ...
, was originally created in 1981 to take over
Exxon
ExxonMobil Corporation (commonly shortened to Exxon) is an American multinational oil and gas corporation headquartered in Irving, Texas. It is the largest direct descendant of John D. Rockefeller's Standard Oil, and was formed on November 30, ...
's holdings in Libya. In 1986, SOC took over the assets of
Grace Petroleum, one of the five US companies forced by the US government to leave Libya in 1996. SOC operates the Raguba field in the central part of the Sirte Basin. The field is connected by pipeline to the main line between the Zelten field and Brega. Besides Zelten, SOC is in charge of two other gas fields (Attahadi and Assumud) plus the Brega
liquefied natural gas
Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled down to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the volu ...
(LNG) plant.
Sirte-Zelten Petroleum System
There is one dominant
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 ...
system in the
Sirte Basin
The Sirte Basin is a late Mesozoic and Cenozoic triple junction continental rift (extensional basin) along northern Africa that was initiated during the late Jurassic Period. It borders a relatively stable Paleozoic craton and cratonic sag basins a ...
sourced by the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
Sirte Shale. In the Central Sirte Basin,
carbonates
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
of
Upper Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
,
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
and
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
age produce in 150 fields on significant
horst blocks or platforms (e.g., the Beda and Zelten platforms). Reservoirs are largely carbonate build-ups and reefs with some offshore production in the Gulf of Sidra. Carbonate reservoirs are largely related to two Upper Cretaceous and two Paleocene cycles of sedimentation in a
syn-rift
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.
Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...
fill sequence; however, there is minor Mid-Cretaceous
calcarenite
Calcarenite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) sand-size (0.0625 to 2 mm in diameter), carbonate grains. The grains consist of sand-size grains of either corals, shells, ooi ...
production. Giant oil fields in this unit include Intistar, Beda, Defa, Waha, Haram, Zelten (Nasser), Hofra and Nafoora.
Upper Cretaceous Sirte Shale of the Rakb Group is the dominant source rock. Geochemical data of 81 oils demonstrate that the Sirte Shale generates a low
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
, high gravity oil with low gas oil ratios. Generation began in Eocene era and continues to present. Onshore the petroleum is dominantly oil; however, offshore increased thermal gradients and deeper burial suggest higher gas oil ratios 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 carbo ...
potential. The Upper Cretaceous carbonate reservoirs are commonly
dolomites
The Dolomites ( it, Dolomiti ; Ladin: ''Dolomites''; german: Dolomiten ; vec, Dołomiti : fur, Dolomitis), also known as the Dolomite Mountains, Dolomite Alps or Dolomitic Alps, are a mountain range located in northeastern Italy. They form par ...
whereas the lower Paleocene cycles include
calcilutite
Calcilutite (also known as cementstone) is a type of limestone that is composed of predominantly, more than 50 percent, of either clay-size or both silt-size and clay-size detrital (transported) carbonate grains. These grains consist either of foss ...
s, calcarenites,
oolites
Oolite or oölite (''egg stone'') is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Ancient Greek word for egg (ᾠόν). Strictly, oolites consist of ooids of diameter 0.25–2 ...
and skeletal debris. The petroleum system is named after the
Zelten Formation, which was deposited in this latter Paleocene transgression.
[Central Sirte Carbonates Assessment Unit 20430102, USGS]
See also
*
List of oil fields
This list of oil fields includes some major oil fields of the past and present.
The list is incomplete; there are more than 25,000 oil and gas fields of all sizes in the world. However, 94% of known oil is concentrated in fewer than 1500 giant ...
Notes
References
*Baird, D.W., ''Geohistory and petroleum in the central Sirt Basin'', in ''The geology of Sirt Basin'': p. 3-56.
*Baric, G., ''Geochemical characterization of source rocks in NC-157 Block (Zaltan Platform), Sirt Basin'', (Ibid), p. 541-553.
Oil fields of Libya
Tripolitania
Horsts (geology)
{{Oilfield-stub