The Levantine Basin is located in the easternmost region of the Mediterranean Sea between Cyprus and the Nile Delta marine cone in Egypt. Based on an analysis of more than 20,000 line-km of 2-D seismic data, the basin appears to contain up to 10,000 metres of Mesozoic and Cenozoic rocks above a rifted Triassic-Lower Jurassic terrain. Although many hydrocarbon discoveries have been made in the Nile Delta and the near-offshore areas in the southeastern Mediterranean Sea, no exploration wells have been drilled in its deep offshore or anywhere offshore Lebanon, Syria and Cyprus. Widespread occurrence of oil seeps (over 200) that closely correlate to hydrocarbon indication on seismic (e.g. bright spots and gas chimneys associated with possible migration pathways) suggest that the undrilled parts of the Levantine Basin can be prospective. Thirteen potential exploration plays are identified in this study and illustrated with seismic examples. The plays range in age from the Triassic to the Neogene-Pliocene.
The Levantine Basin is situated in the eastern part of the Mediterranean Sea (Figure 1; Breman, 2006). Its northern boundary is defined by Cyprus and the Larnaca Thrust Zone, and its northwestern margin by the Eratosthenes Seamount. The Nile Delta Cone and the East Mediterranean coast define its southwestern and eastern margins. Evidence from seismic and other geophysical studies show that the Levantine Basin contains up to 10,000 metres of Mesozoic and Cenozoic sequences above a rifted Triassic-Lower Jurassic terrain. The basin is structurally complex with evidence of both compression and extension due to plate motions, salt tectonics and other gravitational processes. Along the eastern Lebanon offshore margin, there is good evidence of a NS-trending fault that runs semi-parallel to the Dead Sea Transform. The latter fault accommodates the sinistral movement between the Levant and Arabian plates further inland (Aksu et al., 2005).
Apart from hydrocarbon discoveries made in the Nile Delta in Egypt, and the near-offshore areas in the southeastern Mediterranean Sea, the Levantine Basin is essentially an exploration frontier region. To date no exploration wells have been drilled in its deep offshore or anywhere offshore Lebanon, Syria and Cyprus. This paper is based on an analysis of over 20,000 line-km of 2-D seismic data from the Levantine Basin and adjacent areas acquired and processed by GGS-Spectrum and its co-venturers (Figure 2). It concentrates on describing the offshore exploration plays in Lebanon and Syria.
REGIONAL TECTONIC SETTING
It is generally considered that the East Mediterranean Basin formed during the tectonic break-up of the Pangea Supercontinent during the mid-Permian to Middle Jurassic times. However, the history and nature of the crustal evolution of the Levantine Basin is not generally agreed upon and falls into two models. The first model interprets the underlying crust as oceanic and the product of rifting and drifting (e.g. Garfunkel, 1998, 2005; Robertson et al., 1998), while the second considers it as a stretched continental-transitional crust (e.g. Vidal, 2000; Gardosh and Druckman, 2005). Reported geophysical estimates of the depth to the Moho (approximately 20 km versus 35–40 km for the true continental crust to the east) can be used to support either model.
Our modern seismic data suggests that the crust beneath the Levantine Basin is probably transitional in nature since no evidence is seen of oceanic crust. Instead, deep seismic lines show a faulted terrain of Jurassic or possibly Triassic age. This is illustrated in Figures 3a and 3b, which are sections from the southern and central parts of the Levantine Basin, and which are approximately orthogonal to the basin margins (i.e. parallel to the assumed Mesozoic extensional direction).
In addition, Figure 3b shows the ‘onlapping’ relationship of the Levantine Basin with the Eratosthenes Seamount. Further study of the seamount, and the incorporation of other geological and geophysical evidence, would be necessary to throw further light on the nature and history of this significant feature.
Following the mid-Permian to Early Jurassic break-up of the Pangea Supercontinent, the lithosphere stretched and thinned (Flexer et al., 2000), and clastics, carbonates and evaporites were deposited in grabens throughout the Levantine Basin. During the Middle Jurassic to Late Cretaceous, the basin was in a passive continental-margin setting. This phase was characterised by normal faulting, sub-parallel to the present-day East Mediterranean coastline, and basin subsidence. The area was dominated by shallow to deep-marine carbonate deposition, alternating with clastics on the basin margin (May, 1991).
In Late Cretaceous to Paleogene times, the region experienced compression due to the convergence of the African and Eurasian plates. This led to the inversion of the previously formed NE-trending grabens and to strike-slip faulting due to differential plate motion.
At the end of the Miocene, the Mediterranean Sea became isolated from the Atlantic Ocean, which led to the deposition of up to 1,500 m of evaporites in the Levantine Basin (Gradmann et al., 2005). This event lasted about 1.5 million years, and is known as the “Messinian Salinity Crisis” (Butler et al., 1999). The crisis was followed by inundation of the basin with oceanic waters and Pliocene to Recent sedimentation.
The present-day Levantine Basin can be described as a foreland basin on the African Plate. To the north the thrust belt caused by the Africa-Eurasia plate collision is seen in the Cyprus Arc (or Larnaca/Latakia Ridge system). In total about 14,000 metres of Mesozoic to Recent sediments were deposited in the Levantine Basin. North of this thrust belt, several ‘piggy-back’ basins can be recognised, such as the Latakia Trough, Iskenderun, Adana and Cilicia basins (Montadert et al., 1988).
STRATIGRAPHY AND POTENTIAL PETROLEUM SYSTEMS
Overview of the Stratigraphy
The stratigraphy of the northern part of the Levantine Basin is summarised in Figure 4 (Breman, 2006). Well evidence from the southeastern part of the Levantine Basin, shows that the basin was established since the Middle Jurassic. Along the present-day coastal area, a shallow-water platform developed, whereas to the west a deeper-water basin formed (Garfunkel, 1998; Gardosh and Druckman, 2006), which was possibly up to 2,000 metres below the platform by the end of Jurassic times (Garfunkel, 2005).
Six second-order composite depositional sequences of Jurassic and Cretaceous age have been described by Gardosh et al. (2002) for the Levantine Basin. The highstand system tracts consist of various types of aggrading and back-stepping carbonate platforms. The lowstand system tracts consist of siliclastic and carbonate deep-water turbidite complexes. This sequence stratigraphic architecture provides a model for sandstone and carbonate reservoirs and for sealing lithofacies, either in a deepwater setting or associated with major flooding events (May, 1991).
Potential Reservoirs and Seals
Suitable Cenozoic sandstone reservoirs are expected in the basal Pliocene-Pleistocene, intra-Messinian, and Middle to Lower Cenozoic successions. Cretaceous reservoirs may include both sandstones and limestones (including carbonate reefs). Jurassic reservoirs may consist of sandstones and limestones, including fractured dolomites and oolitic limestones. The oldest potential reservoirs are in the Triassic sandstones.
Impermeable sedimentary rocks that could provide top and/or lateral seals are found in the Messinian Salt, shales and marls of the Paleogene, Neogene, Cretaceous and Jurassic as well as Triassic evaporites.
Potential Source Rocks
Several source rocks have been described in the literature for the Levantine Basin and adjacent areas. Pliocene shales are the source of dry biogenic gas in the Pliocene deep-water sands of the Nile Delta, and southeastern Mediterranean Sea, where the sediments are believed to have been supplied by local canyons draining the coastal areas (Maddox, 2000). To date more than 1.0–2.0 trillion cubic feet (TCF) of gas has been proven in the southern Levantine Basin in these biogenic gas systems.
The most common source rocks in the region occur in rocks of Mesozoic age. Oil-prone source rocks are found in the Upper Cretaceous (Cenomanian, Turonian or Senonian – see Lipson-Benitah, 1988; Tannenbaum and Lewan, 2003). Triassic-Jurassic source rocks are often gas-prone (Nader and Swennen, 2004).
Indications of a Mesozoic hydrocarbon system have been found in onshore Lebanon where, for example, in-situ Senonian hydrocarbon shows (asphalt) have been recorded from marly-chalky carbonates, which are rich in organic material and are believed to have been deposited in an anoxic basin. These are not mature onshore, probably due to the shallow depth of burial (Nader and Swennen, 2004). Hydrocarbon shows have also been reported in the Cenomanian (El Qaa borehole) and Kimmeridgian (Terbol-1 well, where the Total Organic Carbon (TOC) was 10%). Offshore both of these sources could generate hydrocarbons due to increased maturity.
This possibility is supported in the southern part of the Levantine Basin by Mango-1. This well tested 10,000 barrels oil/day (BOPD) from Lower Cretaceous sandstones. Two other wells in the area tested light oil at 500 BOPD levels from small tight complex structures, showing further evidence of excellent oil potential further offshore. Other wells in this area tested thermogenic gas in the Pliocene-Pleistocene rocks. This gas is believed to be associated with deeper oil accumulations (Feinstein et al., 1993; Horscroft and Peck, 2005), possibly from the Middle Jurassic.
HYDROCARBON HABITAT AND PLAY CONCEPTS
A satellite seep study undertaken by Infoterra over the East Mediterranean Sea (Figure 5a) has shown that seep features are widespread (over 200) and diverse (Peace and Johnson, 2001). Combining this information with GGS-Spectrum’s seismic data, provided additional support for the presence of a working petroleum system in the ‘deep’ Levantine Basin. Often the seeps have a close correlation to Direct Hydrocarbon Indicators (DHI), bright spots, flat spots and gas chimneys seen on the seismic data (Figures 5b and 6). Some of the seeps are associated with clear migration pathways through deep-seated major faults.
A large number of potential hydrocarbon plays have been recognised in the seismic data. From younger to older these are:
(1) post-salt (Pliocene to Recent) channel sands;
(2) intra-salt (Messinian) sand plays such as bright spots and channels;
(3) sub-Messinian salt plays;
(4) anticlines and faulted anticlines in the middle Cretaceous to Paleogene;
(5) onlaps in the middle Cretaceous to Paleogene;
(6) fault blocks and combined fault/stratigraphic traps in the middle Cretaceous to Paleogene;
(7) large inversion structures in the middle Cretaceous to Paleogene;
(8) carbonate build-ups in the Cretaceous (e.g. rudist reefs) to Miocene;
(9) onlap and drape onto Jurassic highs;
(10) Jurassic sediments in anticlines/horsts or inverted grabens;
(11) Jurassic carbonate build-ups on highs;
(12) Jurassic karst plays; and
(13) Triassic plays.
The EW-oriented, depth-migrated seismic line in Figure 7 is located in offshore Lebanon. It is only 40 km in length and shows examples of nine of these 13 plays (plays 1–5 and 8–11). Other examples of the enumerated plays occur commonly and are discussed below.
(1) Post-salt (Pliocene to Recent) channel sands (Figure 8)
These are generally found in the near-shore areas but, not unexpectedly, are nowhere as deep-lying or extensive as those described for the Nile Delta (Aal et al., 2001), which are due to the huge influx of deltaic deposits in that area. Potential reservoirs could be sourced from either the surrounding Pliocene shales, or if Messinian salt is absent or has been breached, from deeper levels as seen in the southern part of the Levantine Basin (Feinstein et al., 1993; Horscroft and Peck, 2005).
The Messinian Salt sedimentation appears to have been controlled by basin topography and to possibly include sand-like bodies within the salt; one interpretation is a shallow water or sabkha paleo-environment with the incursion of clastics into the system. An alternative interpretation of the higher amplitude events within the salt is also possible; for example, they could be due to alternating successions of different types of evaporites such as halite and anhydrite/gypsum as a result of several different transgressive events (Gradmann et al., 2005). These plays would also need to rely on a breach in the underlying salt to allow oil migration.
(3) Sub-Messinian salt plays (Figure 9)
Messinian salt provides a first class seal for potential Miocene reservoirs immediately below the base of the salt. In many cases these rocks are either gently folded, creating four-way dip closures, or pinch-outs below the base salt contact.
(4) Anticlines and faulted anticlines in the middle Cretaceous to Paleogene (Figure 10)
In the basinal areas, the Cretaceous to Cenozoic rocks are seen to be gently folded and faulted. The tectonic trend is SW-NE and is believed to be due to ‘Syrian Arc’ deformation and regional basin inversion, which occurred in several phases from the Late Cretaceous (Turonian) to the Eocene (Moustafa, 2002). The deformation has been reported to be extensive in the area from Syria through to northern Egypt.
(5) Onlaps in the middle Cretaceous to Paleogene (Figure 11)
Onlapping sequences are extensive along the eastern and western margin of the Levantine Basin (as seen in Figure 7 and 11) as well as being found deeper offshore over the Jurassic highs (Figure 15). Potential reservoirs could thus be found if suitable top/bottom seals are present.
This example shows a syn-rift play on the eastern margin of the Levantine Basin. The potential reservoir sands show amplitude brightening and are a prime candidate for further geophysical work such as pre-stack amplitude analysis (e.g. AVO).
(7) Large inversion structures in the Cretaceous to Paleogene (Figure 13)
These are a larger version of the play described in (4) above and are typically around 10 km in width and 20 to 30 km in length.
(8) Carbonate build-ups in the Cretaceous (e.g. rudist reefs) to Miocene (Figure 9 and 14) Carbonate build-ups are seen on the platform margin in a number of areas, including the fringe of a large Jurassic high in the southern part of offshore Lebanon (Figure 14). It is suggested here that some of these build-ups may have originated as rudist reefs. Rudist reefs of Albian to Turonian age have been reported to outcrop south of the study area in the Carmel region (Bein, 1976) and elsewhere in the Mediterranean (Philip, 1988). Rudists are bivalve reef builders, which are believed to have formed topographic wave-resistant banks or reefs. Dissolution of the aragonite skeletons and dolomitisation can produce extensive secondary porosity. With the rudists dying-out in the Turonian, we postulate that growth of some of the carbonate mounds continued, by another as yet unidentified reef-builder, into Paleogene-Neogene time – until the start of the Messinian Salinity Crisis. This is illustrated in Figure 14 where the reef complex sits on a large, presumably very stable, Jurassic high in the southern part of offshore Lebanon, and is overlain by Messinian salt. Reefs of Cenozoic age have been recognised elsewhere in the Mediterranean, e.g. on the Balearic Islands (Pomar, 2001a, b).
(9) Onlap and drape onto Jurassic highs (Figure 15)
The Jurassic-Cretaceous boundary is marked by an angular unconformity with Cretaceous sequences onlapping and draping over a faulted Jurassic terrain. These structural and stratigraphic plays rely on the presence of Cretaceous reservoir rock and seal, and could be sourced from either the Jurassic or Cretaceous successions.
Subaerial exposure and erosion of the Jurassic sediments may have enhanced the reservoir properties of the rocks and created talus and alluvial plays. The overlying Cretaceous marls would act as the seal and the play could be sourced either from the underlying Jurassic, or from structurally deeper Cretaceous sequences.
(11) Jurassic carbonate build-ups on highs (Figure 9)
This play relies on the presence of pre-Jurassic or Lower Jurassic highs on which carbonate reefs were built up.
(12) Jurassic Karst plays (Figure 14)
Changes in the seismic character of data on the top of Jurassic highs strongly indicate the possibility of karstification and a possible play sealed by overlying Lower Cretaceous mudstones (Breman, 2006).
(13) Triassic plays (Figure 17)
Triassic plays are common onshore Syria and could be expected along the continental margin. The deeper water area also shows evidence of faulted pre-Jurassic terrain, which could be related to the early opening of the east Mediterranean basin during the Triassic. One of the critical components in any of these deep plays would be depth of burial and the possibility of over-maturity in the deeper parts of the basin.
The location of some of these extensive plays and leads are shown in Figure 18.
The Levantine Basin is a large, thick sedimentary basin with rocks from Triassic to Recent age, which has exhibited passive-margin processes and sedimentation for more than a 100 million years. Over this period, subsidence, uplift and tectonic processes have created a favourable regime for hydrocarbon generation and trapping. Offshore Lebanon and Syria is very much an under-explored province with numerous plays from the Triassic to Tertiary in shallow to deep waters. These plays have been highlighted by modern seismic data whose availability will spur-on exploration efforts in the area and aid the authorities and oil companies in future petroleum licensing rounds.
The authors thank GGS-Spectrum, Fugro Geoteam, staff at Spectrum Energy and Information Technology (now GGS-Spectrum), Alan Taylor (A.T. Energy Ltd.), Dave Meaux (AOA Geophysics Inc.), Paul Chandler (Infoterra), the Lebanese and Syrian authorities, and two anonymous referees. GeoArabia’s Editor-in-Chief, Moujahed Al-Husseini and Designer Arnold Egdane are thanked for preparing the final editing and designs.
ABOUT THE AUTHORS
Glyn Roberts is the New Ventures Manager of GGS-Spectrum’s Non-Exclusive Survey Department. He is a Geologist with 30 years experience for Geophysical Contractors from GSI/HGS to Nopec International to TGS-Nopec and GGS-Spectrum.
David Peace is an Independent Consultant with SD Exploration Services. His career started nearly 40 years ago with Esso Exploration in EAME region and covered evaluation of many play types in the greater Mediterranean and North Africa region. He later worked extensively as a consultant with AGIP-ENI based in Milan and the UK where he evaluated many regions around Italy and the central Mediterranean region. In 1987 he was a new venture opportunity specialist with Texaco for the EAME region and in the 1990 was appointed as Exploration Director of Texaco Italiana based in Rome where he first started looking at the Eastern Mediterranean potential. In 1998 he left Texaco and started his own consulting business SDES. He has subsequently carried out regional scale interpretation of the Spectrum - GGS seismic data covering the entire Eastern Mediterranean region. He has arranged regional SAR natural oil seep studies and potential fields evaluations of the Eastern Mediterranean region. More recently he has been involved in more detailed prospect evaluation and licence work in the Levantine Basin region.