Abstract

This thematic set of papers is derived from a workshop, dedicated to the Messinian sequence, which was convened during the 77th EAGE Conference and Exhibition in Madrid in 2015. The main goal of the workshop was to introduce a geological exploration topic to the EAGE meeting. With this objective, and in collaboration with Héctor González of REPSOL and Jean-Loup Rubino of TOTAL, we co-ordinated a dedicated meeting to bring together people studying the Mediterranean region, particularly those dealing with the Messinian sequence, from both academic and exploration viewpoints.

This thematic set of papers is derived from a workshop, dedicated to the Messinian sequence, which was convened during the 77th EAGE Conference and Exhibition in Madrid in 2015. The main goal of the workshop was to introduce a geological exploration topic to the EAGE meeting. With this objective, and in collaboration with Héctor González of REPSOL and Jean-Loup Rubino of TOTAL, we co-ordinated a dedicated meeting to bring together people studying the Mediterranean region, particularly those dealing with the Messinian sequence, from both academic and exploration viewpoints.

The discourse generated by the EAGE workshop continues in this thematic set of papers, capturing a dialogue between colleagues working in various sectors of the Mediterranean Sea (western, central and eastern Mediterranean) and the Black Sea, using complementary and often integrated approaches such as stratigraphy, sedimentology, structural geology, basin modelling and petroleum systems analysis. Within the 11 papers of this set, discussion continues on some of the unresolved scientific questions related to the distribution, origin and tectonic influence of the Messinian Salinity Crisis (MSC) throughout the Mediterranean area, and its impact on exploration.

The antique and vivid mare nostrum was also the cradle of western civilization. This basin has attracted a renewed interest for subsurface exploration, with many offshore hydrocarbon prospection activities in regions like the Nile Delta of Egypt, offshore Libya and Croatia, the Levantine Basin and also in the near Black Sea. In most of these areas, the hydrocarbon targets are related to the Messinian, as in the deep-water Levantine Basin where important gas accumulations (e.g. Leviathan, Tamar and Aphrodite) have been found beneath the Messinian evaporite sequence.

Since the early 1970s, the scientific community has explored most of the onshore basins surrounding the Mediterranean Sea, analysing the stratigraphic records of the Messinian sediments and seeking better understanding of the age and nature of the sedimentation during the latest Miocene. A major breakthrough for the study of the Messinian in the Mediterranean was made by the scientific expedition of the Deep Sea Drilling Project, on-board the D/V Glomar Challenger, during Leg 13 from August to October 1970. The expedition, led by William B. F. Ryan, Kenneth J. Hsü and Maria B. Cita, drilled 28 holes at 15 sites (Ryan et al. 1973). Most of their scientific objectives are relevant today, because they looked to unravel the ‘genesis of the Mediterranean basins, the sedimentary and diagenetic history of the Mediterranean’, as well as the ‘biostratigraphy of the Mediterranean’. One of their major findings is summarized by the sentence, ‘5.5 million years ago, the Mediterranean was a desert’, which is also the title of the famous book written by Kenneth Hsü in 1982 (The Mediterranean Was A Desert: A Voyage Of The Glomar Challenger). The basin, as well as most of the objectives of the DSDP-Leg 13, was revisited in 1995 by two further drilling expeditions of the Ocean Drilling Program during Legs 160 and 161 (Comas et al. 1996; Emeis et al. 1996).

The papers in this thematic set evidently constitute a small but, we hope, relevant drop in the mare nostrum, especially in the variety of topics related to the Messinian, because they illustrate different aspects of a vivid discussion on how the geological records, in onshore and offshore regions, can be used to unravel the Mediterranean desiccation, the so-called MSC.

The papers in this thematic set are organized geographically, moving from west to east (Fig. 1). The first five papers deal with the western Mediterranean, from southern Spain to Sicily. Roveri et al. suggest an alternative deep-water, non-desiccated scenario for the MSC, in contrast to the common interpretation of a high-amplitude sea-level oscillation of the Mediterranean during the Messinian. Martínez del Olmo & Martín interpret observations from outcrops and subsurface, concluding that the Messinian sediments in southern Spain resulted from three major depositional sequences, caused by climatic change and a subsequent sea-level drop. Dal Cin et al. compare the seismic stratigraphy of the eastern Sardo-Provençal Basin and the northern Algero-Balearic Basin, two basins that were completely opened during the MSC. Granado et al. show the crustal architecture of the highly extended western margin of the Provençal Basin, documenting how the style of the crustal deformations as well as the Messinian salt deformations change from the shelf to the deeper portion of this margin. Catalano et al. present new observations on the Sicilian Messinian Tripoli formation, suggesting that the lower part of this sedimentary sequence could be older than the MSC, and infer that sedimentation evolved from an initially open-marine environment to a confined basin with short episodes of marine water incursion.

Three contributions are dedicated to the eastern Mediterranean, particularly in the Levantine Basin and its continuation to the Nile Delta. Feng & Reshef analyse the seismic properties of the thick Messinian salt layer in the eastern Mediterranean. Using newly acquired 3D seismic datasets, they discuss how some of the migration procedures better imaged the internal structure of the salt layer. They also document that the reflective layers within the MSC salt correspond to highly deformed, clay-rich intervals. Allen et al. also use seismic data to illustrate the deformations related to the MSC salt from the Levantine Basin to the Nile Delta, and assert that gliding in conjunction with spreading are not mutually exclusive mechanisms to explain the observed deformation in this margin. In fact, these two mechanisms could overlap through time and space to explain the gravity-driven deformations related to the thick, intra-sedimentary MSC salt. They also argue that intra-salt deformations can be explained by a single, post-Messinian period of salt-related deformation. Al-Balushi et al. study the petroleum system in the Levant Basin associated with a rapid unloading and loading cycle caused by the MSC. They modelled the impact of a rapid pressure drop on the sub-Messinian (Miocene) sequences, inferring that this was the primary control for the occurrence of a ‘preservation window’ for biogenic gas accumulations.

Papers on the Black Sea are also part of this set, represented by three complementary contributions that discuss the magnitude of the sea-level drop during the MSC in that basin. Tari et al. illustrate the occurrence of a deeply incised, erosive unconformity formed during the MSC. In contrast with the situation in the Mediterranean, they argue for the absence of major river incisions during this epoch and demonstrate the absence of Messinian evaporites in the Black Sea. In addition, they discuss the overall impact of a moderate sea-level drop in the Black Sea on the hydrocarbon traps. Based on seismic interpretation and well data, Krezsek et al. reconstruct the magnitude of the sea-level drop in the Black Sea basin during the MSC, estimating a value of c. 500 m. The Black Sea, in consequence, did not get close to the conditions for evaporite deposition during this epoch. Finally, the paper by Schleder et al. analyses the deformations in the western margin of the Black Sea during the intra-Pontian (Messinian), documenting the occurrence of a large, downslope gravitational failure above a thick, Oligocene–Lower Miocene shale-rich sequence. During the MSC, it was formed by a linked system of deformations, with counter-regional extension in the marginal areas and downslope shortening with thrusting and erosion in the deeper portions of the margin.

We thank all the contributors to this set for their efforts in preparing their research for publication, and all the reviewers who have given their time to provide comprehensive evaluations to improve the papers. On behalf of all the workshop convenors, we hope that you will enjoy reading the papers in this set as much as we have enjoyed preparing them, and that there may be some insights that will prove stimulating for further research and/or exploration.