Geology Libya

8/12/2019 Geology Libya

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Petroleum Geology in 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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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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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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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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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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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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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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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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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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Geology Libya

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