Petroleum Geoscience - Volume 18, Issue 4, 2012

Oceanic and continental lithosphere distribution within the eastern Mediterranean is not well understood. A gravity inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to map Moho depth, crustal thickness and lithosphere thinning for the eastern Mediterranean, from which the distribution of oceanic and continental lithosphere, the structure of the ocean–continent transition (OCT) and the location of the continent–ocean boundary (COB) can be determined. The gravity inversion results show thin crust and high continental lithosphere thinning under the Ionian Sea and the Herodotus Basin, consistent with these basins being underlain by oceanic crust. Moho depths from gravity inversion are in agreement with seismic refraction estimates in these basins. Highly thinned continental crust is predicted under the offshore Sirte and Levant basins. The sharp decrease in crustal thickness predicted by gravity inversion off the Libyan and Egyptian coast gives an indication of COB location. Crustal thickness and continental lithosphere thinning determined from gravity inversion have also been used to explore the relationship between the Cretaceous West and Central African Rift System (WCARS: Benue Trough, Chad, Central African Shear Zone (CASZ) and Sudan basins) and the eastern Mediterranean basins; continuity between the Cretaceous WCARS and the eastern Mediterranean basins is not apparent in the gravity inversion results.

Valid palaeotectonic and palaeogeographical reconstructions of the easternmost Mediterranean and adjacent region involve a long-lived Tethys (Rheic, Palaeotethyan and Neotethyan oceans), northward subduction beneath Eurasia and rifting of continental fragments from Gondwana. Rifted microcontinents bordering Gondwana were separated (from south to north) by the Southern Neotethyan ocean, the Berit ocean (new name), the Inner Tauride ocean and the İzmir–Arkara–Erzincan ocean. Mid-Permian to Mid-Triassic pulsed rifting culminated in Late Triassic–Early Jurassic spreading of the Southern Neotethyan oceans (the main focus here). After Early–Mid-Jurassic passive subsidence, the Late Jurassic–Early Cretaceous was characterized by localized alkaline, within-plate magmatism related to plume activity or renewed rifting. Late Cretaceous ophiolites formed above subduction zones in several oceanic basins. Ophiolites were emplaced southwards onto the Tauride and Arabian platforms during the latest Cretaceous. The Southern Neotethys sutured with the Arabian margin during the Early–Middle Miocene, while oceanic crust remained in the Eastern Mediterranean further west. The leading edge of the North African continental margin, the Eratosthenes Seamount, collided with a subduction trench south of Cyprus during the Late Pliocene–Pleistocene, triggering rapid uplift. Coeval Plio-Quaternary uplift of the Taurides may relate to break-off or delamination of a remnant oceanic slab.

There is significant opportunity in re-working older data with modern geophysical technology to develop new plays and concepts in the northern and eastern Mediterranean. New exploration success and higher commodity prices have encouraged both majors and independents to reconsider the Mediterranean as a viable entry point to North African and Southern European energy markets. Two case studies in the Adriatic Sea and Levantine Basin are examined for play concepts and leads using vintage seismic data re-imaged with modern imaging techniques integrated together with a geologically driven workflow. The Adriatic was selected based upon maturity of hydrocarbon discovery; in comparison the Levantine Basin ranks as a region of new territory which we are only just beginning to understand.

This paper reconstructs drainage systems with outlets close to the present-day Nile system, honouring both onshore and offshore evidence and attempts a source to sink quantification. A large river is evidenced to have extended the length of the Red Sea Hills from Eritrea to the current outlet since the Oligocene. The early route of the river is uncertain through Sudan but a more westerly course is proposed through Egypt. The largest contributor of clastic sediment was the Red Sea Hills, where average erosion of the order of 1200–1500 m is constrained by a combination of Apatite Fission Track Analysis, planation surface analysis, and Red Sea sink volumes. Nubia was a significant supplier of sand-rich sediment during wet periods. This sediment supply pattern contrasts with the present-day situation where the Ethiopian Highlands contribute the vast majority of sediments, this contrast being validated by available mineralogical data. This is a consequence of wetter climates in the past and of the younger Ethiopian topography. The interpretations presented here illustrate the importance of hinterland climate change on clastic supply and allow the reservoir fairways in the Nile Cone to be more precisely mapped out in time and space.

Giant gas discoveries in the vicinity have sparked increased interest in the petroleum potential of the Eastern Mediterranean. However, a lack of well data from the area means that a number of crucial unknowns about the region remain. Many of these unknowns relate to uncertainties regarding the sedimentary infill of the main basins, including the sediment provenance areas and elastic properties.

The petroleum potential of this region has been investigated through the acquisition and analysis of multiple 2D and 3D seismic surveys offshore Cyprus and Lebanon and a thorough review of available literature regarding the regional geology. An amplitude extraction study indicates sourcing of probable clastic sediments from the NE, from present-day Syria. In addition evidence of conduits coming off onshore Lebanon can be seen in the seismic data. Use of released data from one of the closest wells penetrating the same stratigraphy as the new discoveries enabled us to build a synthetic model over one of these discoveries (Tamar discovery) and to perform a fluid-substitution over this model. Comparison of these results with characteristics of identified leads and prospects in the northern, undrilled part of the Levantine Basin may improve our understanding of the rock properties in this area as similarities in between these may also indicate similarities in the elastic properties.

The offshore Matruh and Herodotus basins of NW Egypt represent an underexplored region of the Eastern Mediterranean with only two wells drilled along the margin to date. The Matruh Canyon segment in the broader Matruh Basin appears to be unique in Egypt having a large gravity-driven linked system detached on shale. The updip extension with blocks bounded by listric normal faults (rafts) in the onshore part of the system transitions into downdip contraction with toe-thrust imbrications in the ultra-deep water part of the Herodotus Basin. The structures above a prominent Cretaceous shale detachment level within the basin fill of the Matruh Canyon developed during two major periods. The Syrian Arc regional-scale inversional episodes appear to trigger and reactivate the gravity-driven linked system during the Santonian and the mid-Cenozoic. Whereas the Messinian unconformity post-dates the formation of the rafts, an offhore segment of the linked system shows neotectonic activity.

Interpretation of 2D long-offset multi-client seismic data acquired by CGGVeritas in 2004–5 has allowed the distribution and composition of the Messinian salinity crisis (MSC) facies to be mapped across the offshore Sirt Basin, Libya. The results reveal that only the Lower and Upper Evaporites are present within the marginal offshore Sirt Basin, with the middle halite unit confined to the deeper basin. The Upper Evaporites, ‘Lago Mare’, are characterized by a period of fluctuating base level and strong water salinity changes controlled by astronomical precession. They consist of interbedded evaporites and clastics with a total of seven precessional cycles recognized, each associated with erosional sub-aerial channels interpreted to have been created by the Eosahabi rivers sourced from the flooding of Neogene Lake Chad. The Lower Evaporites display a high relief, irregular topography which strongly controls the distribution of the overlying Lago Mare facies. They have an overall chaotic high amplitude response with very little internal structure and are interpreted to represent mass transport complex deposits of the Re-sedimented Lower Gypsum unit. There is a strong correlation between the distribution and composition of the MSC facies and the quality of seismic imaging.

In sedimentary basins, compaction disequilibrium generates overpressure during rapid burial of fine-grained sediments in the mechanical compaction regime, at temperatures below ~70°C. Mudstones behave differently at greater depths in the chemical compaction regime, at temperatures above ~100°C, where evidence suggests that porosity reduction with increasing depth and temperature continues independently of effective stress up to high values of overpressure. We offer an explanation for this behaviour. The horizontal alignment of clay mineral grains is enhanced during clay diagenesis, creating sub-horizontal, flat pores. Because of their flexibility, the flat pores tend to close even under low values of normal effective stress acting across them. Thus, chemical compaction can proceed unless the net expulsion of pore water from the mudstones is inhibited sufficiently for the flat pores to be held open, which necessarily requires the pore pressure to approach the lithostatic stress.

In the Lower Kutai Basin, density log reversals are encountered in mudstones in the chemical compaction regime at depths of 3–4 km, where the pore pressure is close to the lithostatic stress. We attribute these reversals to the inhibition of dewatering during clay diagenesis at shallower depths, when the pore pressure was already close to lithostatic stress. Porosity was preserved by the very high pore pressure holding the flat pores open while the mudstone matrix was being cemented by the products of clay diagenesis. We coin the term ‘chemical undercompaction’ for this process.