Triassic evaporites and the structural architecture of the External Hellenides and Albanides (SE Europe): controls on the petroleum and geoenergy systems of Greece and Albania

A combination of well data, seismic information on thrusting and tectonic shortening, plus analyses of the nature and depth of the Mesozoic units related to main detachment horizons (Triassic evaporites, flysch), are used to review, and further update the complex structural styles of the External Hellenides and Albanides Orogenic Belts, SE Europe. In the study area, the late Alpine orogenic evolution resulted in a structural architecture characterised by the successive westward thrusting of tectonic nappes (Gavrovo/Kruja, Ionian), with an imbricate tectonic style prevailing in all external zones. In the internal and central Ionian zone, imbricate thrusts and duplex structures are recorded, especially where about 24 km of stacked Mesozoic-Tertiary successions have been formed in Albania. Triassic evaporites, up to 3.5 km thick, acted as a detachment horizon for the internal deformation of the Ionian and Pre-Apulia zones (1.7-1.8 km thick in Greece), whereas the flysch unit of Upper Eocene-Oligocene (External Hellenides) and Oligocene-Aquitanian (Albanides) contributed to the deformation of the Ionian zone. In the Ionian zone, Triassic evaporites are laterally continuous and act as an effective seal unit when thrusted above Mesozoic carbonate/Tertiary units. Most of the oil and gas fields, oil shows and surface seeps have been developed in association with the relative more complex structure styles in the internal and the central Ionian zone of Albania and Greece. The Triassic evaporite detachment is detected at depths of 8-22 km in Albania, and about 5 km to 12-13 km in Greece. Thin and probably combination of thin- and thick-skinned deformation (SW Greece) of the Meso-Cenozoic succession above the Triassic evaporite and Permian sequences better depicts the complex structural architecture of the External Hellenides and Albanides, later affected by strike-slip tectonics and extensional deformation active since the Early Pliocene times. The main source rock levels, Lower Cretaceous shale/carbonate, Toarcian Posidonia and Triassic shales, are common in the Albanides and the Hellenides forelands. The thickness and Total Organic Carbon of these levels significantly increase in Albania, whereas trapping mechanism is almost the same and the Triassic evaporites play a significant role in both forelands. The hydrocarbon migration primarily followed developed thrust faults and important halokinesis, to charge fractured Cretaceous-Eocene carbonate reservoirs in the overthrust unit (Ionian zone), likely beneath the thrusted anticlinal belts. These sub-thrust structural models are related to the evaporites. Sub-thrust plays, referred to the autochthonous units (Apulia, Pre-Apulia/Paxos, Sazani zones), may also present reservoir potential in Albania and Western Greece. Upper Miocene and Pliocene deposits (sands) record reservoir potential in stratigraphic traps and good caprock characteristics in the Peri-Adriatic depression (Albania), whereas Messinian evaporites and clays may document seal potential for Upper Miocene sands in the southern areas of the Ionian Sea (Kyparissiakos Gulf, SW Kythira).

Automated summary

This study assesses and updates the complex structural characteristics of the External Hellenides and Albanides Orogenic Belts in Southeast Europe, using well data, seismic information, and analyses of the nature of the Mesozoic units. The late Alpine orogenic evolution resulted in a structural architecture characterized by the westward thrusting of tectonic nappes, with imbricate tectonic style prevailing in all external zones. Triassic evaporites and flysch units played significant roles in the deformation of the Ionian and Pre-Apulia zones. The study highlights the importance of these zones in the development of oil and gas fields.