AN OVERVIEW OF THE PETROLEUM SYSTEMS IN THE IONIAN ZONE, ONSHORE NW GREECE AND ALBANIA

A. Zelilidis¹,  A.G. Maravelis², P.  Tserolas¹ and P.  A. Konstantopoulos³

¹ Laboratory of Sedimentology, Department of Geology, University of Patras, Greece.

² School of Environmental and Life Sciences, University of Newcastle, Callaghan 2308 NSW,  Australia.

³ Energean Oil & Gas, Kifissias Avenue 32, Athens, Greece.

 *Corresponding author: a.zelilidis@upatras.gr

The Ionian and Gavrovo Zones in the external Hellenide fold-and-thrust belt of western Greece are a southern extension of the proven Albanian oil and gas province. Two petroleum systems have been identified here: a Mesozoic mainly oil-prone system, and a Cenozoic system with gas potential. Potential Mesozoic source rocks include organic-rich shales within Triassic evaporites and dissolution-collapse breccias; marls at the base of the Early Jurassic (lower Toarcian) Ammonitico Rosso; the Lower and Upper Posidonia beds (Toarcian–Aalenian and Callovian–Tithonian respectively); and the Late Cretaceous (Cenomanian–Turonian) Vigla Shales, part of the Vigla Limestone Formation. These potential source rocks contain Types I-II kerogen and are mature for oil generation if sufficiently deeply buried. The Vigla Shales have TOC up to 2.5% and good to excellent hydrocarbon generation potential with kerogen Type II. Potential Cenozoic gas-prone source rocks with Type III kerogen comprise organic-rich intervals in Eocene–Oligocene and Aquitanian–Burdigalian submarine fan deposits, which may generate biogenic gas. The complex regional deformation history of the external Hellenide foldbelt, with periods of both crustal extension and shortening, has resulted in the development of structural traps. Mesozoic extensional structures have been overprinted by later Hellenide thrusts, and favourable trap locations may occur along thrust back-limbs and in the crests of anticlines. Trapping geometries may also be provided by lateral discontinuities in the basal detachment in the thin-skinned fold-and-thrust belt, or associated with strike-slip fault zones. Regional-scale seals are provided by Triassic evaporites, and Eocene-Oligocene and Neogene shales.

Onshore oil- and gasfields in Albania are located in the Peri-Adriatic Depression and Ionian Zone. Numerous oil seeps have been recorded in the Kruja Zone but no commercial hydrocarbon accumulations. Source rocks in the Ionian Zone comprise Upper Triassic – Lower Jurassic carbonates and shales of Middle Jurassic, Late Jurassic and Early Cretaceous ages. Reservoir rocks in both oil- and gas-fields in general consist of silicilastics in the Peri-Adriatic Depression succession and the underlying Cretaceous–Eocene carbonates with minimal primary porosity improved by fracturing in the Albanian Ionian Zone. Oil accumulations in thrust-related structures are sealed by the overlying Oligocene flysch whereas seals for gas accumulations are provided by Upper Miocene–Pliocene shales. Thin-kinned thrusting along flysch décollements, resulting in stacked carbonate sequences, has clearly been demonstrated on seismic profiles and in well data, possibly enhanced by evaporitic horizons.

Offshore Albania in the South Adriatic basin, exploration targets in the SW include possible compressional structures and topographic  highs proximal to the relatively unstructured  boundary of the Apulian platform. Further to the north, there is potential for oil accumulations both in the overpressured siliciclastic section and in the underlying deeply buried platform carbonates. Biogenic gas potential is related to structures in the overpressured Neogene (Miocene–Pliocene) succession.

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