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Marine and Petroleum Geology 26 (2009) 1513–1527

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

Evaporites, petroleum exploration, and the Cenozoic evolution of the Libyanshelf margin, central North Africa

Joseph Carl Fiduk*

CGGVeritas, 10300 Town Park Drive, Houston, TX 77072, United States

a r t i c l e i n f o

Article history:Received 29 April 2008Received in revised form4 December 2008Accepted 20 April 2009Available online 9 June 2009

Keywords:North AfricaLibyaPelagian ShelfSirt EmbaymentOffshore CyrenaicaEvaporitesSalt tectonicsMessinian salinity crisisMargin evolutionOffshore petroleum potential

* Tel.: þ1 (832) 351 8576.E-mail address: carl.fiduk@cggveritas.com

0264-8172/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.marpetgeo.2009.04.006

a b s t r a c t

The Mediterranean margin of Libya can be divided into three physiographic provinces, the Pelagian Shelf,the Sirt Embayment, and Offshore Cyrenaica. The petroleum potential of the Pelagian Shelf has beeninvestigated but the Sirt Embayment and Offshore Cyrenaica are almost unexplored. During 2004–2005,new 2D pre-stack time-migrated seismic data were acquired and used to examine the large-scalestructural, depositional, and salt tectonic features of the Libyan shelf and slope. The data coverapproximately 38,000 line kilometers in water depths ranging from under 50 to over 3000 m.

Cenozoic strata along much of the Libyan margin have a demonstrable progradational characterpunctuated by surfaces of erosion and margin failure. Within the Sirt Embayment the most visible ret-rogradational surface becomes seismically coincident with the top of Messinian unconformity. Thisretrogradational surface extends for over 700 km along strike and cuts both sides of the Sirt Embayment.Over 5000 cubic kilometers of material are missing from above this surface. There are two ideas for howretrogradation occurred. One idea relates the retrogradation to margin erosion during the Messiniansalinity crisis. The other idea suggests that a large part of the Libyan margin experienced a cataclysmicfailure during the late Miocene.

Some existing models for offshore Libya have interpreted a widespread layer of halite lying within theMessinian which thickens basinward. This interpretation was probably based on the fact seismicreflection continuity was lost over much of the Sirt Embayment on older data beneath the top Messinianunconformity. The loss of good reflection character adversely affected exploration efforts by obscuringdeep structures. Recent seismic data and a current understanding of salt behavior do not support theinterpretation of thick halite within the Messinian section. Regional observations do not indicate anymobile halite present in the Sirt Embayment. There is a relatively thin, high amplitude and high velocitylayer of non-halite evaporites (mainly anhydrite) which caps the Messinian section. Where this highamplitude and high velocity layer is absent or eroded, seismic continuity within the Messinian interval isrestored. Limited available well data support this interpretation.

True mobile halite is interpreted to exist in offshore Libya only in the far west of the Pelagian Shelf nearthe Tunisian border. Beneath the Pelagian Shelf are a series of tight contractional folds that are inter-preted to be salt cored. Basins adjacent to the folds display geometries characteristic of salt withdrawal.The fold crests formed bathymetric highs which served as nucleation sites for nummulitic shoaldevelopment. These shoals are the principal reservoirs of the Pelagian Shelf.

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1. Introduction

The Mediterranean waters of Libya cover in excess of 500,000square kilometers and reach depths greater than 3000 m. The vastmajority of this area is underexplored and undrilled. It is only in thepast few years that regional seismic data covering offshore Libyahave become available. More sparse vintages of seismic coverage

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date back to the 1980s and late 1970s. Exploration activities in Libyawere severely curtailed after the United States imposed economictrade sanctions in 1986. The lifting of the embargo in 2004 hasallowed a resurgence of activity both onshore and offshore.

Offshore Libya can be divided into three large physiographicregions, the Pelagian Shelf, the Sirt Embayment, and offshoreCyrenaica (Fig. 1). All three regions show the influence of Tethyanand Alpine tectonic deformation (Hallett, 2002). At a regionalperspective the Pelagian Shelf lies mostly west of 16� East longi-tude, the Sirt Embayment lies mostly between 16� and 20� Eastlongitude, and offshore Cyrenaica lies mostly east of 20� East

Fig. 1. Base map showing northern Libya and the Mediterranean Sea. The map shows CGGVeritas’ regional seismic data coverage over the Pelagian Shelf, Sirt Embayment, andOffshore Cyrenaica. Black line across the Sirt Embayment refers to the cross-section A–A0 in Fig. 2. Bathymetry contours increase in non-linear steps from 200m, 500m, and 1000m,then linear steps from 2000 m, 3000 m, and 4000 m. Author’s note: I have found a variety of spellings in the literature for Sirt (e.g. Sirte and Syrte). As I am unsure which is correctI have merely tried to be consistent throughout the text of this paper in using the single form ‘‘Sirt’’. I ask for the reader’s indulgence on this matter.

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longitude. But it should be understood that these boundaries aregradational. Of the three regions the Pelagian Shelf, within Libyanwaters, has the smallest area (Fig. 1). However, at present it is themost significant in terms of petroleum discoveries. There are overtwenty oil and gas discoveries within the Libyan Pelagian Shelf andmore exist in the Tunisian portion of the Pelagian Shelf (IHS, 2001).The most significant of the Libyan oil fields, discovered in 1976, isthe giant Al Bouri field with an estimated five billion barrels of oil inplace and 670 million barrels recoverable (Thomas, 1995). Thismakes it one of the largest offshore fields in the Mediterranean Sea.

In contrast, the Sirt Embayment and offshore Cyrenaica, eachwith greater area, have no commercial production. Several smallgas pools were drilled in southernmost block NC 201 within the SirtEmbayment that did not spark continued exploration efforts. A1984 well just offshore from Benghazi tested oil in the LowerCretaceous (Hallett, 2002), but several offset wells came in drywhich curtailed further interest in the area. In fact, few wells havebeen drilled in either region and most of those have been in shallowwater close to shore. Recently, in 2007, Woodside drilled four wellsin the Sirt Embayment, the A-1-35/3 spudded in April, the A-1-36/2spudded in early June, the A-1-53/2 spudded in early July, and theA-1-52/4 spudded in late July (Fig. 1). Each of these wells targetedthe Lower Cretaceous Sarir (Nubian) formation. Although drilled indeeper water and testing large structures, all four wells wereplugged and abandon. Only the A-1-36/2 well found a show of oil inthe Sarir (Nubian) formation.

Petroleum systems considerations aside, the early explorationactivity on the Pelagian Shelf and lack thereof in the Sirt Embay-ment and offshore Cyrenaica can be attributed to two factors: waterdepth and seismic data quality. Early exploration efforts focused onshallow water because it was within companies’ technical ability todrill there and produce hydrocarbons (Fig. 1). Almost all of thePelagian Shelf is less than 500 m deep and where most ofthe discoveries are located it is less than 250 m (Fig. 1). The shelf inthe Sirt Embayment is relatively narrow in comparison. Fewer thantwenty wells have been drilled along the Sirt shelf with nocommercial success (IHS, 2001). Beyond the shelf, water depthsquickly deepen to greater than 1000 m and in places exceed3000 m. The northern margin of the Cyrenaica Platform is verysteep and drops off to abyssal depths in just a few 10s of kilometers.

Just a handful of wells have been drilled along this margin and onlya few of those were drilled offshore. No wells have been drilled indeep water offshore Cyrenaica.

Seismic data quality was the other limiting factor to earlyexploration activity along the Libyan margin. Data quality hastypically been good on the Pelagian Shelf, even back in the 1970s.This allowed early explorationists to see deep structures, mapthem, and drill high quality prospects. Fortunately, hydrocarbonswere also present in these structures. In the Sirt Embayment,however, this was not the case. Below a relatively thin zone ofcoherent shallow reflections data quality was poor. On seismic itappeared as good shallow data with a wedge of distortion thick-ening basinward into deeper water below it. Correlations to wellsshowed that the coherent data ended at about the top Messinian. Ithas thus been interpreted that a wedge of Messinian age salt mayexist within the Sirt Embayment (Gibbs, 2004; Fig. 2).

Data from multiple wells across offshore Libya document thepresence of evaporites (gypsum, anhydrite) within the Messiniansection (Hallett, 2002), but none documents the presence ofsubstantial halite. Current knowledge about salt behavior informsus that a dipping wedge of halite within the Messinian sectioncould not exist for five million years without becoming highlydeformed. Modern seismic data should show this deformation butsuch deformation is not seen. It is interpreted by this author thattrue mobile halite only exists within the cores of folds under thePelagian Shelf.

2. Data

The seismic data used in this study were collected by CGG (nowCGGVeritas) in 2004–2005. A total of nearly 38,000 line kilometersof high quality 2D seismic were acquired using two seismic vessels.Ten seconds of data were recorded by an 8000 m Sercel streamerwith a group interval of 12.5 m and a shot interval of 25 m. Thisyielded data with 162 fold. In the deepest waters of the SirtEmbayment record lengths were increased to twelve seconds andshot intervals to 37.5 m to produce 108 fold data. Dip lines orientedN–S or NE–SW have a 12–13 km spacing. Strike lines oriented E-Wor NW-SE have a 25 km spacing that is increased to 50 km in the

Fig. 2. Cross-section A–A0 is a south to north oriented schematic interpretation drawn across the Sirt Embayment. It shows the progradational character of Cenozoic strata along theSirt margin and a hypothetical wedge of halite associated with the Messinian section. The existence of the salt wedge was postulated to explain the loss of seismic reflections belowthe top Messinian unconformity. A wedge of salt, as shown, would be unstable and should show significant deformation. Cross-section modified from Gibbs (2004).

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deepest water areas (see Fig. 1). In blocks of high interest infill lineswere shot giving a tighter grid spacing.

3. Pelagian shelf

3.1. Tectonics

The Pelagian Shelf is a broad shallow region that represents thenorthern extent of the African continent in western Libya (Fig. 3).The lithology is dominantly carbonate but there is clastic inputalong the margins and shale deposition in the basin center (Fig. 4A).It is considered to be an extensional regime with a dextral strike-slip component. During the Miocene, eastward movement of theCalabrian Arc to the north caused the Pelagian Shelf to subside andaccumulate several thousand meters of Miocene to Recent strata(Hallett, 2002). The Miocene to Recent section is highly faulted andsome faults show obvious inversion owing to an alternatingtranstensional – transpressional regime. The Miocene interval isless evaporitic than to the east and consequently does not distortseismic continuity of the deeper section. Pelagian Shelf stratamildly show the progradational character dominant in the SirtEmbayment.

3.2. Observations and interpretation

Underlying the western Pelagian Shelf lie an important group ofstructures. On seismic they appear as regularly spaced folds withoccasional interference structures (Fig. 5). They are most clearlyvisible in the high amplitude reflections of the Cretaceous intervalaround 2 s two-way travel time (TWT). Below the top Cretaceous(Blue horizon) the folds are tight. Adjacent to some of the folds are

asymmetrically expanded basins. Above the Cretaceous most of thefolds broaden through the Cenozoic interval. Some of the foldsmerge to form single broad structures. However, some folds persistas individual structures all the way to the sea floor. On these foldsthe entire section from below the Cretaceous to Present is elevatedabove regional. Most of the folds are faulted. Inverted normal faultssuggest that some folds have been subjected to recent transpres-sional stress (see fold at sea floor between 35 and 40 km, Fig. 5).Fold trains from line to line can be linked to produce structures 20–30 km long in roughly an east to west (strike) orientation. Asa group the folds extend approximately 80–100 km in a north-south direction (Fig. 3).

Collectively, these structures are interpreted to be a contrac-tional fold belt (Fig. 5). The folds are detached on and cored withmobile salt thought to be Late Triassic to Early Jurassic in age. Thissalt is related to the breakup of Pangaea and opening of the TethysSea (Dercourt et al., 1986; Ricou, 1994). Within the limitations of thedata coverage, no diapirs are observed. Salt cored folds extendacross the border into Tunisian waters (Klett, 2001) but it isundocumented whether there are any true diapiric structuresassociated with this fold belt. Even though halite has not beentested by the drill bit, withdrawal geometries found in basinsadjacent to folds attest to deeper salt evacuation. Reflectionselevated above regional suggest fold development was underwayduring the Late Cretaceous and possibly earlier. Deformation ofthese folds is ongoing as many structures have an expression on thepresent-day sea floor (Fig. 5).

The presence of a contractional fold belt is extremely importantto the commercial exploration success found on the Pelagian Shelf.Many of the large discoveries are associated with fold structures.The lower Eocene (Ypresian) Jdeir Formation within the Farwah

Fig. 3. Tectonics and structural elements map for Libya and surrounding offshore areas. Structural highs and lows within the Pelagian Shelf, Sirt Basin, and Sirt Embayment areshown by lighter or darker shades respectively of the same color. Structures in subbasins of the Cyrenaica region have not been so designated and are shaded brown. Map modifiedfrom Gibbs (2004).

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Group is the primary hydrocarbon reservoir in the region (Ham-muda et al., 1985; El Ghoul, 1991). It is a highly fossiliferous lime-stone composed of calcareous algae, gastropods, pelecypods,ostracods, and the abundant coiled benthic foraminifera Nummu-lites. Subtle bathymetric highs over fold crests were key nucleationsites for Nummulites dominated shoals (Bernasconi et al., 1991).Shoals may repeatedly occupy the same fold crest creating stackedreservoirs. Stacked shoals of the Jdeir Formation in well B2-NC 41(Al Bouri field) contribute to a reservoir zone 214 m thick (Hallett,2002). On some data the lower Eocene interval displays a rapidseismic character change with adjacent carbonates giving a bank orreef-like appearance (Fig. 6). This type of feature is imaged onseveral lines but precisely coincides with just one of the knowndiscoveries, NC 41-A, on our data grid. It may be a rare morphologyor possibly one difficult to image unless the stacking velocities andline orientation are precisely correct. However, Loucks et al. (1998)has argued based on facies relationships that nummulite accumu-lations are not mounds at all during deposition and that there is‘‘negligible relief’’ to these deposits. Their study, based on outcrop,core, and thin section analysis, concluded the regional depositionalsetting to be a gently basinward dipping carbonate ramp. The mainreservoir interval corresponds to thick but low-relief nummulites

banks in the middle ramp environment. Their study did not includeseismic data and their discussion did not mention the underlyingfold belt. Additional research is required to resolve this apparentconflict.

4. Sirt Embayment

4.1. Tectonics

The Sirt Embayment is the offshore extension of the Sirt Basinonshore (Fig. 3). Rifting of the Late Paleozoic age Sirt Arch begin-ning in the Triassic led to its collapse in the Middle to Late Creta-ceous, forming the Sirt Basin (Dercourt et al., 1986; Anketell, 1996).Coincident with the collapse, northward movement of the Apuliaplate commenced the opening of the Mediterranean Sea and SirtEmbayment. This origin gives the Sirt Embayment a more typicalpassive margin profile of shelf to slope to deep basin (Fig. 2). TheSirt Basin petroleum province is ranked 13th in the world withproven reserves of 43.1 billion barrels of oil equivalent (Alhbrandt,2001). Onshore structural trends of horsts and grabens are inter-preted to continue offshore into the Sirt Embayment (Finetti, 1982;

Fig. 4. Generalized stratigraphic columns for (A) the Pelagian Shelf, (B) the Sirt Embayment, and (C) the Cyrenaica Offshore areas. Filled or open diamonds indicate proven orpotential source zones respectively. Filled or open circles indicate proven or potential reservoir zones respectively. Columns modified from Gibbs (2004).

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Jongsma et al., 1985; Anketell, 1996). It is hoped that the prolific oilproduction found onshore in the Sirt Basin also continues offshore.

Structurally, the Sirt Embayment descends from shorelineacross a series of platforms, troughs, ridges, and slopes into deepwater (Figs. 2 and 3). From an exploration perspective this setting isadvantageous in that it places trapping structures adjacent todeeper source kitchens. The most important of these features is theSirt Trough (Fig. 3). It is here that key possible source beds,equivalent to the Upper Cretaceous Sirt shale in the Sirt Basin, mayhave reached the oil window. The Sirt Embayment terminates tothe north where the lower Sirt slope, underlain by extended Africancontinental crust, meets the Ionian Abyssal Plain, underlain byJurassic oceanic crust (Figs. 1 and 3). Water depths in this area aregreater than 3000 m.

4.2. Observations

The shelf margin around the Sirt Embayment has a demon-strably progradational character to it and this has been the casethroughout most of the Cenozoic. Early systems during the Paleo-cene and Eocene advanced the margin 10s of kilometers into theembayment (Fig. 7). These carbonate sequences (Fig. 4B) sit justabove the top Cretaceous unconformity and deeper rift geometries.They are time equivalent to the major reservoirs found to the southin the Sirt Basin. Oligocene to Miocene progradation advanced themargin considerably farther into the Sirt Embayment. Multipleprogradational systems built the margin out into deep waterincreasing the steepness of the margin (Fig. 8). This was accom-panied by a change from carbonate to clastic dominated sequences(Fig. 4B). The shelf margin section culminates in a thoroughly clasticPlio-Pleistocene deltaic system.

The Plio-Pleistocene deltaic system is quite extensive occupyingthe entire shelf margin of the Sirt Embayment along more than

700 km of coastline. However, it was not the first deltaic system tooccupy this position on the Libyan margin. A late Miocene deltaicsystem previously occupied this margin but is now almostcompletely missing (Fig. 9). Deltaic progradation is a self-limitingautocyclic process and some signs of abandonment, lobe switching,or retrogradation are expected. However, the scale of retrograda-tion documented here is immense and suggests possible allocycliccontrols. This will be discussed in more detail once some obser-vations are presented.

Removal of the older delta has left an erosionally basedcontainer wherein sits the younger delta system (Fig. 10). At thesouthwest (updip) end of the profile the erosional surface (purple),hereafter called the retrogradational surface, curves up to the seafloor where it is bound by two main faults and truncates hori-zontal reflections. The two main faults offset the sea floor andseveral smaller faults penetrate the retrogradational surfaceattesting to continued fault activity. Moving northeast along theretrogradational surface, underlying reflections and some smallfaults are truncated as the surface turns sub-horizontal. The ret-rogradational surface continues to the northeast until about the43 km mark. Between 35 and 43 km the retrogradational surfacetruncates some reflections dipping steeply basinward. These arethe distal clinoforms of the now missing late Miocene deltaicsystem. They constitute direct seismic evidence of the older sys-tem’s existence. For comparison, distal clinoforms of the youngerPlio-Pleistocene deltaic system lie almost directly above. Beyondthe 43 km mark the interpreted retrogradational surface ends asthere is no further evidence for missing section above. Continuingnortheast from the end of the retrogradational surface is a highamplitude reflection that is the top Messinian unconformity.Below the top Messinian unconformity is a faint reflection whichalso appears to cut section. This reflection could be the basalMessinian unconformity (Fig. 10).

Fig. 5. Seismic profile showing a salt-cored contractional fold belt underlying the Pelagian Shelf. Faulting is pervasive through the section and only a few have been interpreted forclarity. The top Cretaceous (TK) is interpreted in dark blue. Salt withdrawal basins are highlighted in light blue. Deformation of the folds has continued up to the present asevidenced by the sea floor expression of some folds. The horizontal and vertical scales for all seismic data will be in kilometers (Km) and two-way travel time (TWT) respectively.

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The physical relationships and observations made on Fig. 10 areconsistent across the seismic data in the Sirt Embayment. The onenotable difference is that faults at the updip end of the missingdelta are limited to a 65 km stretch of margin centered in Block 53.In this area the retrogradational surface sits in the hanging wall ofthe major bounding faults. The close proximity of bounding faultsand missing delta complex could lead one to speculate that failure/collapse of the margin had a tectonic trigger. If one uses minimumvalues for thickness (500 m), width (500 km), and dip length(20 km) to calculate the missing sediment volume, it yields a valueof 5000 cubic kilometers. The volume could easily be twice thisvalue. If margin failure were to happen as one single event, theeffects would be felt catastrophically around the entire Mediter-ranean basin.

Alternatively, margin retrogradation could be largely related todrawdown of the Mediterranean Sea during the Messinian salinitycrisis (MSC). A discussion of the various MSC theories is beyond thescope of this paper. Fortunately, a recent paper by Rouchy andCaruso (2006) summarizes the main theories and integrates thekey elements into a single consistent scenario. This will be pre-sented after the remaining observations.

The top Messinian unconformity over much of the Sirt Embay-ment is a single, high amplitude, positive (peak) reflection. Abovethe unconformity seismic reflections are clearly visible and belowthe unconformity seismic reflections are highly degraded orcompletely wiped out (Fig. 11). This is probably the main reasonsome previous workers associated a wedge of salt with the Mes-sinian interval in the Sirt Embayment. However, it is known fromthe B1 NC 35A well that 10 m of anhydrite lie at the top Messinian

unconformity (Hallett, 2002). Though it lies outside the SirtEmbayment, seismic data at this location has the same single, highamplitude, positive (peak) reflection seen on Fig. 11. Also, at variouslocations throughout the embayment the Messinian surface can befaulted or incised by channels/canyons providing narrow windowsof illumination below. This would not occur if a thick wedge of saltwere causing the disruption. In the shallowest areas along themargin the high amplitude reflection at the top Messinian uncon-formity is completely eroded/missing. This allows a clear windowinto the deeper section reveling the older prograding margins (Figs.7, 8, and 10).

Fig. 12 emphasizes the empirical relationship between thepresence of the high amplitude peak reflection (interpreted asanhydrite) at the top Messinian unconformity and the loss of dataunderneath. On the right half of the image the high amplitude peakreflection is clearly visible as is the loss of seismic resolution belowit. On the left half of the image data quality is very good. The changeoccurs at the precise location where the high amplitude peakreflection terminates. The top Messinian unconformity continuespast this point climbing section on the left half of the image. Whatis missing is the high amplitude peak reflection at the unconfor-mity. Either the conditions necessary for anhydrite deposition weremissing at this shallower level or it was subsequently removed byerosion. Below the top Messinian unconformity is a faint reflectionwhich appears to cut the deeper section. This reflection could bethe basal Messinian unconformity. It also disappears at the samelocation where the high amplitude peak reflection terminates.

Some readers may notice a subtle vertical and lateral change inseismic character of reflections overlying the high amplitude peak

Fig. 6. Uninterpreted (A) and interpreted (B) seismic profile showing the seismic expression of a well imaged nummulitic carbonate bank (shown in green). The bank is early Eocene(Ypresian) in age and sits above the crest of a salt-cored fold. Fold crests were bathymetric highs during the Eocene and possibly preferred sites for Nummulities shoal development.The top Cretaceous (TK) is interpreted in dark blue.

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Fig. 7. Seismic profile showing Paleocene (Pal) and Eocene (Eo) age shelf margins prograding into the Sirt Embayment from the northeast (near the city of Benghazi). These marginsdirectly overlie faulted, tilted, and eroded basement blocks created by extension during Cretaceous rifting. The purple horizon identifies the top Messinian unconformity surfacenow overlain by a Plio-Pleistocene clastic deltaic complex.

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reflection of the top Messinian unconformity. This is real andcorresponds to a change in seismic facies, seen in strike orientation,from prograding clinoforms immediately above the peak reflectionto discontinuous erosional deposits at the prodelta toe higher up(see dashed box on Fig. 9 for a dip section through these seismicfacies).

A regional line across the Sirt embayment shows the effect ofseismic disruption at the top Messinian unconformity (Fig. 13).Most of the reflections and seismic character between the topMessinian unconformity and the top Cretaceous has been wipedout. The rifted basement and Mesozoic section are visible onlybecause of modern processing techniques and an 8000 m cable.Older data shot with short cables saw little of the deeper sectionand nothing just below the top Messinian unconformity. Thisproblem would have seriously limited exploration efforts andcontributed to the idea of a thick salt wedge associated with theMessinian interval.

Fig. 8. Seismic profile showing one Oligocene (Olig) and three Miocene (Mio) shelf margradational system #3 is truncated by the Messinian erosion. The gold horizon is the intesurface now overlain by a Plio-Pleistocene clastic deltaic complex.

Modern ideas concerning salt behavior and observations visibleon Fig. 13 (and all the seismic data in the basin) argue against thepresence of thick mobile salt at/below the top Messinian uncon-formity. The first observation is the pronounced regional dip intothe basin. A salt layer deposited with such a dip would haveinherent instabilities. Those instabilities would cause salt to beginmoving relatively soon after deposition. The second observationfrom Fig. 13 is the presence of a sediment interval onlapping the topMessinian unconformity starting at 81 km and thickening basin-ward. This differential load would also cause underlying salt tomove relatively soon after its deposition. If mobile salt werepresent, we should expect to find deformation or structures relatedto salt movement and withdrawal basins created by salt evacuation(similar to those seen on the Pelagian Shelf). Where thick Messinianhalite has been found in other Mediterranean basins seismic datashows deformational structures and withdrawal geometries (Lofi,2002; Rouchy and Caruso, 2006; Roberts and Peace, 2007). Another

gin systems prograding into the Sirt Embayment from the southwest. Miocene pro-rpreted top Oligocene. The purple horizon identifies the top Messinian unconformity

Fig. 9. Map showing the extent of late Miocene shelf margin retrogradation. Brown shading highlights the area where stratigraphic terminations indicate missing section. The widthof the brown shading serves as a good measure of how far the shelf edge retreated.

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point worth noting is that where thick Messinian halite has beenfound in other Mediterranean basins there is no trouble imaging it.Regional observations find tectonic deformation, faulting, andmagmatism but do not find any obvious evidence for salt swells,salt walls, diapers or related withdrawal basins on this data set. Inshort, there does not appear to be any thick, mobile salt in the SirtEmbayment. These observations do not, however, rule out possiblethin halite interbedded within the Messinian section.

Fig. 10. Seismic profile showing the modern Plio-Pleistocene delta and the remnants of thsurface which extends for approximately 40 Km down dip. This surface is coincident with thto 55 km) below the top Messinian unconformity where it has bright amplitude and the cleshading highlights interpreted Messinian evaporite interval. The dashed box highlights a fa

4.3. Messinian salinity crisis

The summary study by Rouchy and Caruso (2006) incorporatesdata and observations from numerous previous studies (Butleret al., 1995, 1999; Clauzon et al., 1996; Riding et al., 1998; Krijgsmanet al., 1999; Rouchy, 1982; Rouchy and Saint Martin, 1992) on theMSC and integrates these data into a consistent scenario. Thisscenario follows the major ideas of the classical deep basin-shallow

e mostly missing late Miocene delta. The purple line highlights the retrogradationale top Messinian unconformity. Note the degrading image at the end of the line (from 45ar imaging (between 34 and 45 km) where the bright amplitudes are missing. Yellowcies change discussed in text.

Fig. 11. Seismic profile showing typical high amplitude seismic character of the top Messinian surface in the Sirt Embayment. Wipe out of data immediately below the surface leadsome to associate a layer of salt with the Messinian section.

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water model first presented by Hsu et al. (1973, 1978). It signifi-cantly advances these earlier models by integrating glacio-eustaticchanges and tectonic controls with an increased understanding ofwater exchange threshold effects between sub-basins, polyphasederosion around the basin margin, diachronous evaporite deposi-tion, changes in circum-Mediterranean climate, and incorporatesa new more detailed chronology. Key points from the Rouchy andCaruso (2006) study are outlined in the following paragraphs. Thereader is referred to their paper for additional references.

The model of Rouchy and Caruso (2006) has three main phases.Phase 1 includes isolation of the Mediterranean Sea from theworld’s ocean, evaporative draw down of the sea, and deposition ofa lower evaporite sequence. From the onset of Phase 1 (~5.96 Ma)the margins of the Mediterranean were eroded and the climatebecame increasingly cold and arid. As water levels eventually fellmore than 1000 m, erosion followed deep into the basin untilreaching the edge of brine pools in the deepest sub-basins. Evap-orites were deposited in these deep basins with gypsum occurring

Fig. 12. Seismic profile showing the relationship between the top Messinian surface, the hiquality. Erosion or nondeposition of the high amplitude reflection at the top Messinianinterpreted Messinian evaporite interval.

along the periphery and halite in the centers (Fig. 14A). Wherecomplete desiccation occurred K–Mg salts were deposited withhalite. Through time salt accretion filled the basin and forced waterlevels higher (Fig. 14B). The top of these deposits is marked bya terminal desiccation, dissolution, and erosional event. Depositionof the lower evaporites in Phase 1 ended after the glacial maximumTG 12 (~5.57 Ma) (Fig. 15).

Deposition of Phase 2 evaporites, dominantly gypsum, beganwith an interval of global warming and sea level rise. Conditionswere quite different from Phase 1 as non-evaporitic siltstones,marlstones, and claystones became more common than gypsum.Incursions of marine water still occurred but there was nowa detectable influence by freshwater that increased upsection.Sedimentation occurred under a variety of salinity conditions fromhypersaline to brackish to freshwater. The Mediterranean basinfilled as erosion of the margins fed sediments into low relief,shallow water sub-basins where evaporite deposition wasbecoming less and less common.

gh amplitude reflection typically found at the top Messinian surface, and seismic dataresults in much improved data quality below the surface. Yellow shading highlights

Fig. 13. Seismic profile across Sirt Embayment showing regional scale relationships. Note the relatively undeformed nature of the sediment wedge onlapping the top Messinian.Transparent zones in the wedge are canyon incisions. Yellow shading highlights interpreted Messinian evaporite interval.

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Phase 3 represents the final stage of Messinian deposition.Referred to as the Lago-Mare event, it is characterized by fresh-water dilution of the uppermost Messinian deposits. They containmostly freshwater to brackish faunal assemblages but occasionalhave calcareous nanoplankton and foraminifers suggestingepisodes of marine incursion. Yet at times the basin dried outcompletely allowing weathering processes to develop paleosoils. Itappears that depositional conditions within the Mediterraneanbasin changed quite rapidly and often as the Phase 2 and Lago-Mare event together lasted only 240,000 years. Deposits of theLago-Mare event end abruptly at the Messinian/Pliocene boundarywith the restoration of fully marine deep water conditions(~5.33 Ma).

Erosion was a widespread and continually occurring processduring the MSC. Surfaces of erosion stretched from the marginsdown into the deep basins (Fig. 14C). The sedimentologicalevidence indicates that the MSC was the result of not one but manysea level fluctuations. It also indicates that much of the interbeddedsediment in Phase 2 was brought in by freshwater drainage systemsaround the Mediterranean periphery. This suggests that many ofthe erosional surfaces, especially those along the margins, wereactivated and reactivated many times with at least three peaksperiods of enhanced activity. Thus the single seismic horizonobserved around the basin margin on modern data must be a pol-yphased surface resulting from many erosional events (Fig. 14C).

4.4. Interpretation

Based on the Rouchy and Caruso (2006) model for the MSC,many of the observations from the Sirt Embayment can now beinterpreted in a regional context. If we limit our observations to theMessinian and younger section, Fig. 14C is a reasonable genericschematic cross-section for the seismic data in Fig. 13. The datacorrespond closely to the margin and peripheral basins portion ofthe schematic. In the updip part of Fig. 13 (0–80 km, basin margin)the retrogradational/top Messinian surface is equivalent to seismichorizon M. In this position it would be interpreted as the poly-phased surface with the accumulated effects from several mainepisodes of erosion, desiccation, and possible karstification. In thedowndip part of Fig. 13 (80–150 km, peripheral basin) the contin-uation of the top Messinian unconformity is onlapped by a sedi-ment wedge. The alternating high and low amplitude reflections ofthe sediment wedge equate to the combined Phase 2 evaporitesand Phase 3 Lago-Mare event deposits. Since the top of the Mes-sinian section now correlates to the top of the sediment wedge, atthe point of onlap the top Messinian unconformity becomes an

unconformity separating Phase 2 evaporites from pre-evaporitestrata.. The unconformity continues basinward until about the115 km mark after which it becomes difficult to interpret. The top ofthe sediment wedge (Phase 2–3 deposits) is itself an unconformityand has been incised by several canyons of Miocene and possiblyrecent origin. The Miocene canyons may reflect the terminaldesiccation and erosion event that occurred at the end of Lago-Mare deposition. The transparent reflections above the sedimentwedge are basinal sediments deposited after restoration of normalmarine conditions and are equivalent to the Plio-Pleistocene deltasystem. There are no deep basin deposits on Fig. 13.

One observation does not exactly fit the model in Fig. 14C. OnFigs. 10, 12, and 13 there is an apparent unconformity visible belowthe top Messinian unconformity. If this is a basal Messinianunconformity then there is some section equivalent to Phase 1evaporites. Following the Rouchy and Caruso (2006) model thereshould be no Phase 1 evaporites outside the deep basins. Thisinterval has not been mapped because the lower unconformity ispoorly/not imaged on much of the data. The interval could becanyon fill related to Phase 1 erosion or it may predate the Messi-nian completely. It will likely take 3-D seismic or well data toanswer this question.

Two points remain to be addressed: 1) Did the Libyan marginsuffer a catastrophic failure? and 2) Where are the sediments fromthe missing Miocene delta? At present the evidence for large-scalefailure of the margin is circumstantial and speculative. Theerosional scar exists and the late Miocene delta is mostly missing.However, no large deep water fan deposit has been traced back tothis margin and no coeval tsunami deposits have been identified ofwhich the author is aware. Failure of the Libyan margin in the lateMiocene remains a hypothesis.

As for the missing sediments, some, if not the majority, arecontained in the sediment wedge. The pathway from the retro-gradational margin via the top Messinian unconformity to thesediment wedge is straight downdip. The wetter climate afterPhase 1 would contribute to eroding the margin and transportingsediment into the basin. Plus the majority of Phase 2 and Lago-Mareevent deposits are non-evaporite sediments. The simplest answeris that sediment in the wedge came from the missing delta.

5. Offshore Cyrenaica

5.1. Tectonics

Offshore Cyrenaica includes all Libyan waters north of theCyrenaica Platform between the city of Benghazi and the Egyptian

Fig. 14. Schematic diagram of sedimentary, hydrological, and erosional processes in an evolving evaporitic basin. In the initial drawdown stage (A) erosion of the margin follows theincreasingly saline brine down into the basin. Once the water volume has been reduced by 80% or consentrations have increased five fold gypsum deposition commences along themargin of deep basins. At 90% water reduction halite deposition commences in the centers of deep basins. During the second stage (B) cycles of marine influx and evaporativedrawdown lead to salt accretion. As salt deposition proceeds, halite onlaps the earlier gypsum deposits and eventually fills the deep basins. A simple schematic cross-section (C)shows regional relationships between the three depositional phases of the MSC and important erosional surfaces. Seismic horizon M is an erosional surface seen truncating themargins surrounding the Mediterranean. Modified from Rouchy and Caruso (2006).

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boarder (Figs. 1 and 3). The shallowest strata are prograding deltacomplexes like those seen in the Sirt Embayment. Deeper stratamore closely resemble the bedded carbonates of the Pelagian Shelf.Although Offshore Cyrenaica comprises a very large area (>12,000square kilometers), only a fairly narrow fringe along the coast isconsidered immediately prospective. The continental shelf andslope are much less extensive here than in the Sirt Embayment

(Fig. 3). The margin edge is steep and water depths rapidly changefrom 100s of meters to 1000s of meters in a few 10s of kilometers.Within 50–80 km of shore the continent ends where thrust sheetsof the Hellenic fore-arc impinge on Africa from the north. Nocommercial production has been found in this region to date nor isthere any seismic evidence, structural or stratigraphic, suggestingthe presence of mobile salt.

Fig. 15. Seismic examples from Offshore Cyrenaica showing (A) the Marmarica Shelf, (B) the Darnah Basin, and (C) the Benghazi Basin and Mediterranean Ridge.

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5.2. Observations

There are three areas within Offshore Cyrenaica that haveimmediate exploration potential: the Marmarica Shelf, the DarnahBasin, and the Benghazi Basin (Figs. 1, 3 and 14A, B, C). Essentially,all three areas must be considered rank wildcat risks because theyare virtually unexplored. Fewer than ten wells have been drilled inthese three areas combined and that includes onshore wells (IHS,2001). The most important of these tests was AGIP’s 1984 A1-NC120 well in the Benghazi Basin (location on Fig. 3). This wellencountered a nearly complete marine succession from middleJurassic to middle Miocene (Hallett, 2002). A similar stratigraphicsection is thought to exist in the other two areas. Section youngerthan this is quite thin and often missing beyond the shelf edge dueto erosion or slope failure (Fig. 14A, B, C). Visible faulting andfolding along the margin have created numerous structural trap-ping opportunities. The unstable slope and water depths will provea challenge to drilling these traps.

The A1-NC 120 well mentioned above encountered an impor-tant but enigmatic show of petroleum. Out of multiple zones inlower Cretaceous through Turonian marine carbonates the wellflowed over 5000 barrels per day of 36� API gravity oil (Duronioet al., 1991). Disappointingly, the follow-up well B1-NC 120 drilledalong the shelf to the northeast was declared a dry hole. Anotheroffset well, the A1-NC 128, drilled to the southwest by Lasmo in1996 was also declared a dry hole. The one positive point is thatthere is a working petroleum system, presumably with a Jurassic tolower Cretaceous source, in the Benghazi Basin (Fig. 4C). Continuedexploration is needed to find commercial success.

One last area of exploration potential in Offshore Cyrenaicabears some mention. North of the deep trench separating NorthAfrica from the thrusted terrain of the Hellenic fore-arc lies an areacalled the Mediterranean Ridge (Figs. 3 and 14C). It is an accre-tionary wedge composed of sediment scraped from a tectonic platebeing subducted beneath Crete (Dewey et al., 1973). The ridge isbathymetrically high compared to the sea floor to the north. Theshallowness and thickness of the wedge is attributed to theaccretion of thick North African continental margin sediments tothe wedge (Emeis et al., 1996). The entire Mediterranean Ridge hasa shallow Messinian section and its attendant data wipeoutunderneath (Fig. 14C). Yet, strata below the wipeout zone are wellimaged and shows structure. Despite the deep water and poorlyknown section, the Mediterranean Ridge should not be completelyoverlooked. It stretches over a very large area and contains as yetundiscovered potential.

6. Conclusions

The Libyan shelf margin can be divided into three physiographicregions: the Pelagian Shelf, the Sirt Embayment, and OffshoreCyrenaica. Each of these regions has a modern progradational shelfmargin but in the Sirt Embayment demonstrably extending backover the entire Cenozoic Era. Of these regions only the PelagianShelf is an established petroleum province. Production comes fromlower Eocene nummulitic carbonate reservoirs. The reservoirsoverlie a contractional fold belt detached on and cored by LateTriassic–Early Jurassic mobile salt.

Some pre-existing models for offshore Libya have interpreteda widespread, basinward thickening, layer of mobile salt lyingwithin the Messinian interval. Recent seismic data and a currentunderstanding of salt behavior do not support the interpretation ofthick mobile halite within the Messinian section. There is a rela-tively thin, high amplitude and high velocity, layer of non-haliteevaporites (mainly anhydrite) associated with the top Messinianunconformity. This surface is responsible for degrading seismic

reflection data character across much of the Sirt Embayment.Regional observations do not indicate any mobile halite present inthe Sirt Embayment or offshore Cyrenaica.

Within the Sirt Embayment there is a major retrogradationalsurface that is coeval with the Messinian section. This retrograda-tional surface extends 700 þ kilometers along strike and cuts bothsides of the Sirt Embayment. The volume of missing shelf marginmaterial is a minimum of 5000 cubic kilometers. The majority ofmissing sediments are thought to have been eroded during the lateMessinian and redeposited within a sediment wedge onlapping thedeep Sirt Embayment. But it is possible that this part of the Libyanmargin experienced a cataclysmic failure during the late Miocenejust prior to or during the Messinian salinity crisis.

Acknowledgements

I need to pay tribute to friends and colleagues who madecontributions to my understanding of the Libyan margin. Inalphabetical order my thanks go out to Lynn Anderson, Gene Brush,Peter Gibbs, and Tom Schultz. I am indebted to Philip Rowell, MarkRowan, Steve Schulz, Gene Brush, and Gabor Tari for their criticalreviews and suggestions which greatly improved the paper. I wouldalso like to thank Gabor Tari for his invitation to submit thisresearch to Marine and Petroleum Geology. Lastly, I would like tothank the management of CGGVeritas for their permission topublish this manuscript. This paper is dedicated to the memory ofLynn Anderson, my longstanding friend and mentor.

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