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Cretaceous Shallow Water Limestone Reservoirs and Possible Analogues in Eastern and Central Mediterranean


A study has been carried out on isolated carbonate platforms in Central and Eastern Mediterranean area, in order to compare them with the recent Eastern Mediterranean HC discoveries located in Zohr and Calypso fields. The study focuses on the high-relief isolated carbonate platforms of the Mediterranean region, trying to understand the key factors of these potential reservoirs. The E&P activity carried out in Cretaceous carbonate reservoirs in Eastern Mediterranean is less developed if compared with the coeval and similar reservoir in Western and Central Mediterranean, which host the well-studied reservoirs of Apulian platform (Italy), Sirte basin (Libya), offshore Spain, onshore and offshore Tunisia and Western Desert (Egypt). The Cretaceous paleogeography of the Peri-Mediterranean region was characterized by the alternation of wide platforms and relatively deep and narrow troughs, and was mainly influenced by tectonic activity and eustatic fluctuations. The Cretaceous environmental settings were heavily affected by accelerated break up of Pangaea occurred during Early Cretaceous, associated with the formation of a multitude of rift basins, intensified spreading, and important volcanic activity inland and offshore. The deposits were after involved in the Alpine deformation and nowadays can be found in several geodynamic domains, from thrust belts to rift basins. The Cretaceous carbonate platforms show different characteristics in function of their evolution through time. If the isolated carbonate platform showed more or less continuous and similar carbonate deposition throughout the Cretaceous, the attached carbonate platforms, during Lower Cretaceous, were more sensitive to the variable siliciclastic input from emerged land, while they experienced, from the Cenomanian transgression onwards, a more continuous and predominant carbonate deposition. In addition, studies carried out on the geometries of the margins in several Peri-Adriatic areas (Maiella Mountain in Central Apennines, Monte Cavallo in Southern Alps, Adriatic Sea subsurface, for instance at Medusa wells) highlight the occurrence of two different type of margins: Lower Cretaceous platforms were muddy and flat topped, with no bioconstructed margin, bypass slope controlled by tectonic and generally located in front of a starved basin; on the contrary, Upper Cretaceous factory locally switched to production of large amount of biocalcarenites that shed into the basin and, combined with a strong decrease in subsidence, resulted into slope to ramp geometries and progressive filling of the basins. In order to found the best analogues for the Zohr and Calypso structures, the available dataset, involving all the shallow water carbonates of Central and Eastern Mediterranean area, underwent a selection, applying three fundamental criteria of comparison with Zohr structure, in terms of: siliciclastic input (isolated platform or attached platform), paleogeography and size. All the isolated platforms, characterized by almost pure carbonate sedimentation, located within 25° of paleolatitude from Zohr and with a size comparable with Zohr itself, were described as possible analogues, also taking into account their geological evolution and the nature of the substratum. The more promising analogues of the Eastern Mediterranean carbonate discoveries resulted to be the isolated platforms and buildups present in the Adriatic and Ionian Seas subsurface: Barbara Platform (offshore Italy/Croatia), A4-1X and nearby buildups (offshore Albania), Cephalonia Island (Greece), Isis, Miskar and others buildups in offshore Tunisia, and the Bagnolo platform in Italian onshore, with proved oil and strong similarities with Zohr in terms of size, age and main characteristics. The selected potential analogues, however, show several reciprocal differences and are not the ‘perfect’ analogue of Zohr, because their facies and geometries are the result of a complex interplay between global and local effects, like different carbonate production, sediment reworking, variable accommodation rate, eustasy, paleoceanographic and paleoclimatic variations, trophic resources availability and distribution, biological evolution and interaction with tectonic deformation and thermal subsidence.