LIBYA BASINS-1 GEOLOGY, MURZUK OIL DEVELOPMENT COULD BOOST S.W. LIBYA PROSPECTS

David Thomas
Thomas & Associates
Hastings, England

With the recent involvement of Repsol, Total, and OMV in developing the 2 billion bbl oil-in-place Murzuk field complex, an infrastructure will be finally constructed in western Libya which will act as a precursor to more exploration activity and development projects in the Murzuk and Ghadames basins.

Murzuk, an intra-cratonic sag basin, is a huge ladle-shaped structural basin covering more than 400,000 sq km and extending beyond the borders of southern Libya (Fig. 1) (124582 bytes). The sedimentary fill is predominantly marine and continental Paleozoic, with some Mesozoic and Cenozoic sediments overlying Precambrian crystalline basement. In the central part of the basin the total sedimentary thickness exceeds 3,500 m.

The whole sedimentary sequence is well exposed along much of the edge of the basin, as well as on the southern flank of the Al Qarqaf arch (Fig. 2) (92576 bytes). The full sedimentary sequence is present in only a few outcrop areas due to regional erosion connected with the Caledonian and Hercynian orogenies, and other lesser unconformities affecting all formations. In the core of the Al Qarqaf arch the crystalline basement outcrops in relatively small areas.

The structure of the area is quite simple. The sub-horizontal or gently dipping strata are faulted and the faults are most frequently parallel to the anticlinal axis. Tectonic movements affected the basin to a greater or lesser degree from early Paleozoic (Caledonian) to post-Eocene (Alpine) times.

EXPLORATION HISTORY

Although geographically remote, the basin has had an exploration history since the late 1950s. Gulf Oil, Amoseas, British Petroleum, and Wintershall drilled exploration wells during the 1960s with very limited success. This was in the main a response to the Esso Standard discovery made in 1957 in the Atshan area, which was the first Libyan oil discovery. The Atshan 2 well tested oil from Devonian and Carboniferous sandstones, and the play was a continuation of the Paleozoic trend found productive in the neighboring Edjeleh region of the Illizi basin in adjacent eastern Algeria. As exploration moved to the Sirte basin of north central Libya, the Murzuk basin was virtually forgotten. In the mid-1970s Occidental acquired concession NC34, and drilled A1-NC34 in 1977, which encountered oil indications in Cambro-Ordovician sandstones. The interval was not tested. Meanwhile Braspetro was awarded concession NC58. This was a relatively large permit which encompasses most of the west-central Murzuk basin. Braspetro acquired some 3,000 line km of seismic and drilled seven wells with one, A1-NC58, a minor, sub-commercial oil discovery in Cambro-Ordovician sandstone reservoirs. Permit NC58 was relinquished in the early 1980s.

Rompetrol, the state oil company of Romania (NC115), and BOCO, the state oil company of Bulgaria (NC101), were subsequently awarded exploration permits on the northwestern and north central flanks of the basin, respectively. Oil discoveries were made on both permits in the Ordovician Memouniat formation. In late 1994, a group led by Repsol as operator was formally awarded the ex-Rompetrol Murzuk field complex permit. Plans are for field development and oil pipeline construction to the El Hamra terminus in the southeast Ghadames basin, where a pipeline system to the coast is already in place.

BOCO, holder of permit NC101, acquired approximately 4,300 line km of seismic and drilled 18 exploration and appraisal wells since 1981. Sixteen prospects have been drilled, resulting in the discovery of five oil accumulations in Cambro-Ordovician and two minor oil accumulations in Devonian.

Lasmo was awarded the open acreage (NC174) between the Rompetrol and BOCO permits in 1990. Approximately 2,700 line km of seismic were acquired, and in 1992-93 four wildcats were drilled. Results are confidential, but two were reported as Ordovician oil discoveries, with one well flowing 450 b/d of oil. This program fulfills the work commitment on the license.

No other acreage is currently held under license in the Murzuk basin.

STRATIGRAPHY

As with most of the African intracratonic basins, the general depositional history is relatively uncomplicated, with stratigraphic changes attributed to tectonic phases and major sea-level cyclical movement.

The basement complex underlying the sedimentary basin is highly heterogeneous in composition. The Cambro-Ordovician section is laterally very extensive in North Africa. It extends into Algeria in the west and the Sirte basin to the east, and in both cases it is an important petroleum reservoir. The Early to Middle Cambrian Hassaouna formation has a continental origin, with the remaining Paleozoic section being predominantly marine with peri-glacial cycles in the Ordovician to Early Silurian.

The Upper Ordovician (Ashgillian) Memouniat formation is a regressive sandstone sequence composed of massive, cross-bedded clastic units, with occasional high percentages of kaolinitic matrix which degrade reservoir porosity from good to very poor. Local silicification also affects reservoir poro-permeability characteristics.

The Silurian represents a transgressive and regressive depositional cycle, with rhythmic alterations of euxinic, organic-rich shales and siltstones (Tanezzuft formation) which grade upwards into very fine sandstones of the Acacus formation (Fig. 3) (105015 bytes). This represents the regressive phase of the Early Silurian transgression. The Silurian sediments are usually conformable with the underlying Memouniat formation, but the partial to total truncation of the Silurian by an Early to Middle Devonian unconformity is recorded in outcrop.

During the basinwide Early-Middle Devonian tectonic phase, which is time-related to the onset of the Hercynian orogeny, the Cambro-Ordovician and Silurian sequences were uplifted and the basin's depocenter began to shift eastwards (Fig. 2) (92576 bytes), creating a sub-basin which filled mostly in Carboniferous and Permian time.

The Early Devonian Tadrart formation illustrates a typical seaward progradation and basinward-thinning of a clastic wedge to the northwest and shows changes from fluvio-continental to estuarine and marine-neritic environment. Significant thickness variations reflect strong tectonic control on sedimentation. The Tadrart sandstones are usually poorly cemented and exhibit good reservoir characteristics. It is a major oil and gas producing reservoir in the Ghadames and Illizi basins of Algeria.

The Middle to Upper Devonian sequence in the Murzuk basin is the result of a major transgression which progressed into the Carboniferous albeit with brief regressive phases. The Aouinet Ouenine and Tahara formations represent fluvial-deltaic and marine conditions. They also exhibit good reservoir facies and are important oil and gas producing sequences in neighboring Algeria.

The Lower to Middle Carboniferous system represents the last full marine cycle in the Paleozoic of the Murzuk basin and is followed by the stand-still phase of the Late Mississippian and the prograding continental Tiguentourine formation of Late Carboniferous-Permian. From this time onwards fluvio-continental conditions prevailed.

A great part of the Murzuk basin is covered by Recent aeolian sands forming dune systems separated by deflation valleys. The highest dunes reach over 300 m and prove to be a major seismic acquisition and processing problem.

PETROLEUM RESERVOIRS

ORDOVICIAN

Ordovician sandstones of the Memouniat formation are the only known commercial oil-bearing reservoirs in the basin. They were found in wells drilled by BOCO (NC101) and Rompetrol (NC115). Formation damage is common, as well as relatively low productivity rates of between 100 b/d and 2,200 b/d of oil. Braspetro flowed minor amounts of oil and water from the Memouniat formation during open-hole drillstem tests in exploration well A1-NC58.

In general, the Cambro-Ordovician reservoirs are described as predominantly massive sandstones with irregular intercalations of siltstone. The sandstones are fine to coarse grained, with low to medium to good porosity values, and weak to strong fracturing. The fractures are mainly vertical or slightly inclined, occasionally horizontal.

Very low porosities, due to intense silicification, have been measured in outcrops, and porosities of the Ordovician reservoir in some wells are in the order of 2-5%, with permeabilities of 0.014 md. However, in other wells in the Murzuk basin porosities are locally better, and reach as high as 25% in concession NC115. The porosity-permeability character of the Memouniat formation is therefore patchy, and more study is required to confidently delineate reservoir porosity-permeability fairways for oil exploration.

SILURIAN AND DEVONIAN

Hydrocarbon shows in Silurian and Devonian sandstones have been recorded in wells drilled in the Atshan area, but their limited net-porous thickness, combined with lateral facies variations, have reduced exploration interest. On the other hand, the Devonian is a principal reservoir in the eastern Illizi basin of Algeria and the southern Ghadames basin.

Two oil-bearing units in the Upper Devonian Tahara formation were tested in the Murzuk basin proper. Open-hole tests indicated good permeability but unfortunately with indication of formation damage. Subsequent production tests were disappointing.

CARBONIFEROUS

Sandstone intervals in the Carboniferous Mrar formation are primary objectives in the Atshan area, which is part of the structural saddle separating the Illizi/Ghadames and Murzuk basins. Basement occurs at shallow depths (around 1,300 m), and some hydrocarbon bearing reservoirs are above sea level. In the Atshan wells (NC151), net permeable reservoir thickness ranges from 1.5 m to 15 m, and porosity from 15-25%.

SOURCE ROCKS

The Silurian Tanezzuft shale is regarded as the most consistently mature and organic-rich source rock in the Murzuk, southern Illizi, and Ghadames basins. The basal Tanezzuft shows a gradational section of rhythmical alternation of euxinic, good oil-source shales and sandstones. The recognizable "Hot shale" gamma ray readings record the basinwide occurrence of the shale unit, with local concentrations of high amounts of hydrocarbon-prone organic matter.

TOC values in excess of 2% have been recorded in wells geographically spread across the basin, and in well E1-NC58 a thin highly radioactive shale interval (1,676-82 m) was identified as a very good potential source rock with TOC values of 4.9-8.4%.

However, due to the basin's tectonic history the Silurian shale is not always mature, especially in certain flank areas. Care is therefore required in the evaluation of regional hydrocarbon prospectivity.

High TOC values are also recorded in Carboniferous and Devonian shale intervals, with a maximum of 3.8% in the latter.

OIL GRAVITY, GAS CONTENT

Gas-oil ratios are low. For example, in the BOCO fields, GOR varies from 10-15 scf/bbl, whereas in the adjacent southern Illizi and Ghadames basin hydrocarbon distribution reveals the near total predominance of gas and gas/condensate.

Oil gravity in the Ordovician reservoirs are 36-45API. Gravity is 47 in Atshan field and even higher in the Carboniferous reservoirs.

THE HYDROGEOLOGY PROBLEM

The Cambro-Ordovician lithostratigraphic unit is present in the whole Murzuk basin with a fairly constant thickness and very uniform facies. Resting on Precambrian crystalline rocks, these sandstones form extensive outcrops along the border of the basin (Fig. 2) (92576 bytes).

The occurrence of ground water in the Cambro-Ordovician is well documented in the region of Ghat as well as from exploration wells in the basin proper. Groundwater movements across and beyond parts of the Al Qarqaf arch is uncertain but probable. Further north, and to the northwest of the Ghadames basin, water wells located on structural highs have tested confined, fresh ground water in the Cambro-Ordovician or undifferentiated Paleozoics. Based on water-level elevations, the situation would appear to indicate a northward groundwater outflow from the Murzuk basin, that is, over the Al Qarqaf arch and Jabal Fezzan and into the Ghadames basin.

In the southern Illizi and Ghadames basins, relatively saline ground water is present in the Cambro-Ordovician. The boundary between fresh and saline water apparently coincides with the northeast-southwest ridge connecting the Al Qarqaf arch and Tihemboka uplift. Saline water indicates either no flow or restricted flow of ground water in these areas.

In large parts of the basin the thick Tannezuft shales of Gotlandian age cover the Ordovician and provide confinement. On early Paleozoic structures, Devonian sandstones directly overlie the Ordovician or Cambrian. Under such circumstances hydraulic interconnection between the two permeable formations is likely. Definitive confinement of the lower Paleozoic sandstones is ensured by the thick Carboniferous, mainly shaly formations. Consequently, there is no likelihood at all of the Paleozoic hydraulic system being related to that of the continental Mesozoic sandstones which are important water-bearing formations in the central part of the Murzuk basin.

The Cambro-Ordovician fresh water aquifer is prominent in the north. Its thickness is about 500 m. in the Sabha region. The sequence slopes to the south, and is mainly a sandstone bed with occasional shaly layers in its upper parts. Average water salinity in this aquifer is 1,200 ppm sodium chloride equivalent, with a porosity of 13-16%. In the southern part of the area the sequence becomes thinner but more consolidated, with porosity of less than 10%.

The Carboniferous aquifers are saline, 7,000-10,000 ppm NaCl equivalent. The bottom part of this aquifer becomes increasingly consolidated to the south.

A thick shale-siltstone bed separates the Lower Cretaceous-Jurassic aquifer unit from the saline Carboniferous aquifer below. Another semi-impervious bed of shale-siltstone of about 50 m thickness separates the Carboniferous and the Cambro- Ordovician aquifers.

AQUIFER INFLUENCE ON ACCUMULATIONS

The Ordovician sandstones appear to display primary porosity/fracture interconnections throughout the Murzuk basin.

Low formation water salinities are prevalent in Ordovician reservoirs. An apparent exception to the general pattern is the salty water (24,600-26,900 ppm NaCl equivalent) recovered in DST No. 5 (1,517-24 m) of exploration well A1-NC115, just below the oil-bearing zone.

Potentiometric evidence suggests a hydraulic continuity within the Ordovician reservoir throughout the basin, with importantly, water flow from south to north. This could imply long-distance lateral migration possibility of hydrocarbon fluids from the deepest parts of the basin to the north, with entrapment of hydrocarbons in Caledonian/Hercynian-created traps, which also preserved a relatively saline connate water environment.

Silurian and Devonian reservoirs generally display higher salinities than those observed from deeper and older reservoirs, illustrating a salinity inversion in the Murzuk basin.

STRUCTURES

Early structural growth during the Caledonian orogeny can be demonstrated on seismic profiles, with additional growth during the Hercynian. Later Alpine structural movements, which are of prime consideration in the Ghadames basin, are minor in the Murzuk.

Leakage and breaching of structural and stratigraphic traps did not occur after the Hercynian orogeny (Fig. 3) (105015 bytes).

The structural grain is approximately north-south, with related northwest-southeast and northeast-southwest oriented horst-type structures. Some of the Caledonian/Hercynian horst features are quite large, with areal closure of 80 sq km and up to 80 msec vertical closure. There are a number of these large undrilled structures in open acreage.

MURZUK FIELD DEVELOPMENT

In December 1994 the Libyan People's Committee ratified the Repsol, Total, and OMV consortium proposal for the development of the Murzuk field complex. It encompasses the development of the A, B, and H oil accumulations within NC115 and estimated to hold up to 2 billion bbl of oil in-place. Phased development is expected to achieve 50,000-70,000 b/d within a few years. Water injection will be needed under the second phase and could produce 180,000 b/d within 10 years.

A 30 in., 250,000 b/d capacity pipeline is to be constructed to the El Hamra terminus and linked to the El Hamra to Az Zawiyah pipeline (Fig. 4) (61813 bytes) for first phase production transportation. The coastal pipeline was built in the mid 1980s and has a capacity of 120,000 b/d. The proposed phase two development of the Murzuk field complex, and any additional Ghadames and Murzuk developments, would probably require a parallel pipeline construction.

NC101 DEVELOPMENT CANDIDATE

Five oil accumulations in the Ordovician and two minor oil accumulations in the Devonian have been discovered by the Bulgarian state oil company in concession NC101. Total oil in place is estimated to be in excess of 500 million bbl. The main factors influencing development viability are fiscal regime, pipeline costs, and well productivity. With the building of the Murzuk pipeline, one of the considerations can be addressed.

ACREAGE AVAILABILITY

There is plenty of open space available. The trick is to identify the regions with commercial oil bearing structures. The Murzuk basin, apart from preserving world class prehistoric cave paintings, may also shelter world class oil reserves.

BIBLIOGRAPHY

Bellini, E., Massa, D., A stratigraphic contribution to the Paleozoic of the southern basins of Libya, in The geology of Libya, Vol. I, Salem, M.J., et al., eds., Proc. of 2nd Symposium on the Geology of Libya, Tripoli, Sept. 16-21, 1978, pp. 3-56.

Hammuda, O.S., Geologic factors controlling fluid trapping and anomalous freshwater occurrence in the Tadrart sandstone, Al Hamadah al Hamra area, Ghadames basin, in The geology of Libya, Vol. II, Salem, M.J., et al., eds., Proc. of 2nd Symposium on the Geology of Libya, Tripoli, Sept. 16-21, 1978, pp. 501-508.

Meister, E.M., et al., The origin and migration fairways of petroleum in the Murzuq basin, Libya: an alternative exploration model, in The geology of Libya, Vol. VII, Salem, M.J., et al., eds., Proc. of 3rd Symposium on the Geology of Libya, Tripoli, Sept. 27-30, 1987, pp. 2,725-42.

Pierobon, E.S.T., Contribution to the stratigraphy of the Murzuq basin, S.W. Libya, in The geology of Libya, Vol. V, Salem, M.J., et al., eds., Proc. of 3rd Symposium on the Geology of Libya, Tripoli, Sept. 27-30, 1987, pp. 1,767-83.

Sinha, S.C., Pandey, S.M., Hydrogeological studies in part of Murzuq basin using geophysical logs, in The geology of Libya, Vol. II, Salem, M.J., et al., eds., Proc. of 2nd Symposium on the Geology of Libya, Tripoli, Sept. 16-21, 1978, pp. 629-634.

THE AUTHOR