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Sedimentological Review of Depositional Environments in the External Hellenides Isopic Zones, Ionian Sea and Western Greece margin

Abstract

The studied area of the Ionian Sea and the Western Greece margin, running from the Diapondia Islands-north of Corfu Island, to Filiatra in Southern Peloponnese, is characterized by the most external Hellenides zones and the Apulian Platform. It is well known that during the Tethyan rifting, taphrogenetic processes divided the uniform carbonate platform into shallower parts (the Apulian Platform, its margin the Pre-Apulian zone, and the Gavrovo-Tripolitza Platform) and deeper parts (the Ionian basin). From that time, different sedimentary evolution processes took place in each part, thus the zones exhibit distinct lithostratigraphic columns with different petroleum systems. The aim of this study is to distinguish through well data the depositional environments of the Mesozoic and Cenozoic Era in the study area, considering its paleogeographic evolution, and to specify prospective facies associations for the hydrocarbon exploration. The external Hellenides units are separated by NW-SE trending thrusts. In the offshore areas of the Ionian Sea, a system of NE-SW strike-slip faults cross-cuts the Apulian, Pre-Apulian and Ionian units. The most important of these strike-slip faults from North to South are the Borsh-Kardhiqit Fault, the Kefalonia Transform Fault Zone (KTFZ) and the projection of the Achaia strike slip fault zone to the South Ionian Sea, dividing the offshore area in three (3) segments; the north, the central and the south segments. For the purpose of this study, ten (10) wells have been selected in order to cover both geographically the three segments and stratigraphically the external Hellenides zones which are present in the area. In the north segment: East Ericoussa-1, Yanadhes-1, Paxos Gaios-1X, Archangelos-3 and Parga-1; In the central segment: South Kefallinia-1, Sosti-1 and Zante-2 and in the south segment: Peristeri-1 and Filiatra-1. A hierarchical approach to sedimentological description and interpretation has been applied to generate the present lithotypes/lithofacies (LTs/LFs) through cuttings and core data. The LTs/LFs have been grouped in bed-set scale units-depositional packages (DPs) with similar characteristics and internal organization. The vertical and lateral organization of the depositional packages can define the bed-set stack scale facies associations (FAs) with specific geometries and dimensions. These facies can be assigned to specific depositional environments. The wells which have been drilled on the Ionian zone, exhibit a wide range of both clastic and carbonate depositional environments, ranging from shallow marine, continental and coastal, to deep-water. On the other hand, the wells which have been drilled on the Pre-Apulian and the Gavrovo-Tripolitza units, exhibit shallow marine carbonate deposits overlaid by deep-water to shallow-marine clastic sediments. Consequently, the aforementioned depositional environments have their distinct FAs and DPs in different proportions through geological time. The available log data can give a first impression of the FAs with the best reservoir properties, such as the upper shoreface, proximal lower shoreface, channel, lobe and shallow shelf deposits. The tectonism of the External Hellenides, which is moving to the SW, along with the presence of the major strike slip faults, leads to a juxtaposition of depositions from different geological environments, making them present quite far from their initial deposited place and even further from the source of sedimentation regarding the clastic sediments. Due to the structural setting of the area and the distance between the wells, any attempt to correlate specific FAs among wells is very speculative, leading us to correlate only coeval deposits in terms of age. The implications in terms of reservoir quality and perspectivity of these FAs in each depositional environment are discussed.