Geology Libya
Transcript of Geology Libya
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The Main Oil Provinces of Libya
From 1958 to 1968, 16 major oil fields were discovered in theSirte Basin, each with recoverable reserves greater than 500million barrels of oil. Four of those fields had recoverable
reserves of more than two billion barrels.The Amal Field discovered in 1959 is the largest field, withrecoverable reserves of over 4.2 billion barrels. The Gialo
Field, discovered in 1961, has approximately 4 billion barrelsof recoverable reserves.
Based on July 2001 figures, Libya has 12 oil fields each withreserves of more and two others with reserves of 500 million-l
billion barrels.
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Gas Production
Production of natural gas in Libya received a major boost in
1971, when a law was passed requiring oil companies to store
and liquefy the natural gas condensate from their wells, ratherthan simply burning it off. However, natural gas production
has lagged far behind oil because the high costs of transport
and liquefaction have made it a less attractive alternative. A
large liquefaction plant was built at Marsa al Burayqah in
1968, but export performance has been variable. About 70percent of Libya's natural gas production is consumed
domestically.
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Major gas-producing fields include Attahadi, Hatiba, Zelten,
Sahl, and Assumud. In recent years large new discoveries havebeen made in the Ghadames and el-Bouri fields, as well as in
the Sirte basin. Libyan natural gas development projects
currently underway include as- Sarah and Naha, Fargo, Wafer,
offshore block NC-4l, abu-Attifel, Intisar, and block NC- 98.
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The Sirte Basin
The Sirte Basin while the most mature of Libya's basins,
remains an important exploration region.The Sirte is a Cretaceous to Tertiary basin with a series of
horst and graben structures that developed during the
Cretaceous and continued through the Paleocene, and became
active again in Eocene and Oligocene time. A good Sirte playis the reefs that grew on the horst structures.
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The structural development of the basin, together with spacial
distribution of the most important sediments, provides optimal
preconditions for an oil province. There are a wide variety of
structures present in the basin, ranging from pure reefs, to
anticlines, to pinch outs. Oil fields in the basin produce mainlyfrom Cretaceous and Paleocene strata but oil has been found
extensively from Cambro-Ordovician up to Oligocene, and
even from Pre-Cambrian rocks. Source rocks in the basin are
late Cretaceous shales and possibly some Paleocene shales
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Murzuk Basin
The Murzuk Basin is highly prospective, with a horst and
graben structural set-up similar to that of the Sirte, but of a
different age. In addition, there are excellent late Ordovicianreservoirs of glacial origin. Three giant fields already have
been found in the basin. The Murzuk's disadvantage is its
remote location hundreds of miles from the oil infrastructure
and contractor support found in eastern Libya.
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Ghadames Basin
The Ghadames Basin in northwest Libya is a large Paleozoic
basin that extends into Algeria and the source rocks there are
the same shales that source the largest North African oil field,located in Algeria. There have been a number of discoveries in
the basin but no giant fields. The structure of the basin is
relatively under-developed when compared with Sirte and
Murzuk Further exploration will be required to learn more
about this location.
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Offshore Exploration
Libya has a relatively narrow continental shelf and slope in the
Mediterranean and Gulf of Sirte, which widens in the west in
the Gulf of Gabes. The northern part of the Gulf of Gabes, alsoknown as the November Seventh concession, lies on the
Libyan- Tunisian border and is considered rich in oil and gas.
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Offshore Exploration
Following the settlement of maritime boundary disputes with
Tunisia in 1982 and Malta in 1983 Libyan access to offshore
deposits in these formerly disputed areas may be significant,since these may contain as much as 7 billion barrels of oil. The
offshore region bordering Tunisia has already yielded the large
Bouri field.
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Offshore Exploration
As part of a 1988 settlement to a long- standing territorial
dispute, the area (which contains an estimated 3.7 billion
barrels of oil and nearly 12 trillion cubic feet of natural gas) isset to be exploited by the Libyan-Tunisian Joint Oil Company
(JOC), a 50-50 venture of Libya's NOC and Tunisia's ET AP.
The Libyan side of the zone contains the Omar structure,
which is estimated to contain more than 65% of the zone's
total oil and gas reserves.
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Offshore Exploration
However, recent exploration has been light in the other
offshore areas of Libya. It is too soon to say whether any
significant discoveries are likely to be made here. However,the Benghazi-Derna basin in the eastern offshore has tested oil
at commercial rates in at least one well.
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The Kufra Basin
An example of an incomplete play
The Kufra Basin has been studied recently and the results
published in a paper by Luning et a/ (1999). Structurally, theKufra basin is not dissimilar to the Murzuk Basin discussed
briefly above with potential structural traps in seismically
defined fault blocks. Thick Paleozoic sandstones of Cambrian
to Ordovician age display good porosity and would provide
potentially good reservoir conditions. Seals above these
reservoirs are provided by Lower Silurian shales (non-source
rock).
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The Kufra Basin
This is also of Lower Silurian age, but its distribution is
limited to depressions in the topography of the seabed
immediately below the Lower Silurian. In most cases, the
shales were deposited directly above upper Ordovician glacial
sandstones during the initial early Silurian transgression that
was a result of the melting of the late Ordovician icecap. Thus
it may accumulate locally to thicknesses of up to 130 m, but in
a real terms it is often absent from the stratigraphic succession.
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The Kufra Basin
Changes in the depositional conditions across the basin have
also caused the sediment character to alter, with siltstones and
sandstones of similar age found in exploration wells drilled
twenty years ago by AGIP in the northern sector of the basin.
Thus, there is the potential for smaller, isolated fields to
develop in association with "fossil" valleys, assuming that a
migration pathway has been created to link the Tanezzuft
Formation with the older sandstone reservoirs.
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The Kufra Basin
Identification of prospects will depend upon the ability of
seismics to delineate the valley structures. However, the
economic risk in drilling these exploration wells does not atpresent appear to be justified while other basins are yielding
reasonably large discoveries from more predictable geological
settings.
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The Sirte Basin
A report published by Ahlbrandt (2003) ranks Libya's Sirte
Basin as the fifteenth largest hydrocarbon province in the
world, with reserves of 43.1 billion barrels of oil equivalent, of
which 36.7 billions barrels are of oil itself. To date 16 giant
fields (>500 million bbls) and 23 large fields (>100 million
bbls) have been discovered.
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The Sirte-Zelten System
The dominant system in the Sirte Basin is called the Sirte-
Zelten System. This is comprised of a number of different
plays. The Sirte-Zelten is not necessarily the only system that
operates in this area, but the alternatives are not yet fully
understood and so cannot be confirmed as contributing to the
Libyan national oil reserve.
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Structural Setting
The Sirte Basin is shaped by a failed attempt by the North
African plate to split into a new oceanic system during the
period between late Mesozoic and early Tertiary. These splits
occur as triple-junctions, i.e. three separate rift systems
radiating from a common origin. In this case, the full
development of an oceanic crustal plate was prevented by
stronger tectonic forces acting elsewhere on the planet surface.
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One arm of this system
extends roughly northwards
towards the ocean, and is called
the Sirte arm. Running south-
west, the second arm is calledthe Tibesti, while the remaining
arm trends eastwards and is
called the Sarir arm. It is
believed that the Sirte and
Tibesti arms formed first withthe Sarir developing later in the
Cretaceous.
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The structures within the areas defining these arms are
alternating horsts and grabens. As horst blocks rise relative to
the descending graben blocks, the horsts are eroded with the
resulting sediments being transported down the slopes of the
horst. They settle in the extended basins in the grabens, oftenforming fan-shaped clastic structures. Formations of this type
commonly result in stratigraphic traps (not dependent on
deformation). The Sarir field is an example of this.
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While the horsts themselves remained below sea-level, they
provided a relatively shallow environment that promoted
abundant marine life. In the time period during which the Sirte
basin structures were formed, the sea was significantly rich in
carbonates leading to the deposition of limestones anddolomites. The climatic conditions were also important since
they encouraged the development of reefs and other carbonate
based biological colonies on the structural highs. Deposition of
sediments continued into the Oligocene to Miocene periods,after structural activity had ceased.
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Source Rock
Without an adequate source rock, no play can be successful. In
the Sirte Basin, geochemical analysis of the crude oil produced
from the various different reservoir units has revealed a
common origin corresponding to the Upper Cretaceous Sirte
Shale Formation. This is located at relatively deep levels
within the grabens, with easy access to the fault planes
defining the horst-graben boundaries.
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Source Rock
Without an adequate sourcerock, no play can be successful (seesection 7.3). In the Sirte Basin,geochemical analysis of the crude oil
produced from the various differentreservoir units has revealed acommon origin corresponding to theUpper Cretaceous Sirte ShaleFormation. This is located atrelatively deep levels within the
grabens, with easy access to the faultplanes defining the horst-grabenboundaries.
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The total organic content (TOC) of the Sirte Shale throughout
the Sirte Basin area varies from 0.5% to 7.8%, with average
values in the order of 1.5% to 1.75%. The organic material
itself is oil-prone, i.e. more likely to yield oil than hydrocarbon
gases. Additionally, the oils discovered to date have tended tobe low-sulphur and of high gravity (average value 36).
Geochemical analysis of 81 separate oil samples from across
the basin revealed that all but three samples belonged to the
same oil "family". The remaining samples were inconclusivein that they could be representative of either younger Tertiary
oil, or may simply be biodegraded Sirte Shale oil.
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For any organic-rich shale to become a hydrocarbon source
rock, it must be buried such that the temperature and pressure
regime place it in the "oil window" (figure 7-4). The Sirte
Shale is buried to depths between 2700 m and 3400m in the
central and eastern areas of the basin. This is ideal from the
point of view of this location.
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Pre-Cambrian
The Pre-Cambrian rocks depend on the occurrence of fractures
within the formation for both porosity and permeability .Thenature of the rocks themselves is therefore largely irrelevant.
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Cambro-Ordovician
The Cambrian and Ordovician rocks also pre-date the riftingphase. These are primarily sandstones such as the Gargaf
(western Sirte) and Hofra Formation (central Sirte). This group
of formations is very significant in terms of reservoir volume,containing around 29% of the total known petroleum volume.The Cambrian and Ordovician sands are located in association
with rifted fault blocks, allowing them to escape the late-Paleozoic erosion phase that removed much of the Paleozoic
rocks from the central Sirte area. Most of these reservoirs aretightly cemented ortho-quartzites, and are therefore relatively
poor in both porosity and permeability .These requirefracturing to enhance their productive capabilities.
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Nubian (Sarir)
These deposits are largely associated with the development of
the rifts themselves with the emergent horst blocks being
eroded to provide the sediments that gathered in the adjacent
grabens. Some geologists have suggested that there is
alternation with marine sediments within the Sarir that
correspond with the initiation of the rifting system.
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Late Cretaceous and Early Tertiary Marine
SedimentsThis group represents the greatest single reservoir type. The
sediments developed in response to cycles of marine
transgression (flooding in response to sea-level rise) andregression (sea-level fall). Although the older reservoir unitsare more restricted in their distribution, the sediments of this
group are distributed across the entire Sirte Basin. Thesedeposits were laid down while the rifting was still active, but
at a reduced rate. The periods of regression correspond to
uplift of the rocks above sea-level, leading to removal ofsignificant quantities of older rocks and the creation of
unconformities.
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Late Cretaceous and Early Tertiary Marine
Sediments
The first cycle began as the sea flooded the land in the Upper
Cretaceous, depositing the marine Bahi Formation. This wasfollowed by the deposition of carbonates such as Lidam
Formation dolomites. After this the sea began to recede,
depositing carbonates followed by evaporates and shales (of
the Etel formation).
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Late Cretaceous and Early Tertiary Marine
Sediments
After this cycle, a further transgression began leading to the
deposition of the shallow marine Rachmat Formation. Aswater deepened, the enormously important, deeper marine
Sirte Shale was deposited. Elsewhere in the basin, shallower
water sediments such as the Kalash limestone are buried by
shales including the Hagfa Formation, and by carbonates such
as the extremely important reservoir of the Beda Formation.
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Late Cretaceous and Early Tertiary Marine
Sediments
This last is of Paleocene age. The Beda Formation is composed
of a variety of carbonate forms including fine-grained calcilutites,sand-sized calcarenites, oolites, and biological debris.
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Late Cretaceous and Early Tertiary Marine
Sediments
The Beda Formation is found along the southern margin of the
Sirte Basin. These formations are associated with the shallower
platforms formed by the submerged horst blocks. As the
Paleocene period continued, further sedimentation occurred, most
significantly including the development of reef structures on the
margins of the horsts (the Zelten Formation). This formation is
the most important of all the reservoir units in the Sirte Basin,
with many of the giant reservoirs located here (e.g. Intistar, Beda,
Zelten (or Nasser), and Hofra fields.
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Late Cretaceous and Early Tertiary Marine
Sediments
Once again the sea-level fell in a regressive phase leading to the
deposition of the Harash and Kheir Formations. This in turnwas followed by yet another marine transgression leading to
the deposition of the Facha dolomite. These are not
particularly significant in reservoir terms, although some
stratigraphic traps do occur within the Facha Dolomites.
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Seal Formations
The most significant seal in the basin is the Hon EvaporatesMember of the Gir Formation laid down during the final
regression discussed in the reservoir section above. The other
Tertiary rocks are mainly shales and carbonates that providesome limited seal capability. The Hon member is up to 1305mthick in places and widespread across the basin. The formationitself includes dolomites and other evaporitic minerals (such as
anhydrite) apart from halite (rock salt).
Where the salt thins near to the onshore margins of the basinno significant oil fields have been discovered. Various shales
and clays within the older formations provide reasonablyeffective seals at the local level.
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Migration
The existence of an active source rock, suitable reservoir rock,and efficient seal are of little value without a migrationmechanism to allow the oil and gas to travel between source
and reservoir.In the Sirte Basin, the hydrocarbons are able to migrate from asingle Upper Cretaceous source shale to a variety of different-aged reservoir units thanks to the presence of the faultsforming the horst and graben structures. These are open
enough to permit fluids to pass easily. The near vertical faultplanes enable the hydrocarbons to come into contact withreservoirs at several different levels.
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