The structural style and hydrocarbon exploration of the subthrust in the Berati Anticlinal Belt, Albania

Abstract

The Albania thrust belt consists of both internal and external tectonic zones, which are westward overthrusts with large amplitude (50–100 km), above the Apulian Patform and the South Adriatic Basin. The External Albanides are subdivided into distinct tectonic-stratigraphic units: the Kruja zone (shallow carbonate water) in the east and the Ionian zone in the west, which is represented by pelagic limestone. The Ionian zone, located between the Sazan (Pre-Apuliane) and Kruja zones, consists of three anticlinal belts: the Berati, Kurveleshi and Cika anticlinal belts. Their western margin is bordered on overthrust faults, which have produced large westward overthrusts on each of them, with amplitudes of 10–30 kms, and are utilized through a large diapir masses. Therefore, they mask perspective plays in subthrusts. The Berati anticlinal belt is the most eastern in the Ionian zone and lies mostly in the Albanian territory and partly in the Greek territory. It has a length of about 225 kms and a width 12–22 kms, and is differentiated into two sectors: the southern sector (south of the Qafa Kicoku) and the northern sector (north of the Qafa Kicoku). Understanding the evolution of evaporite tectonics is important in determining the principal features of the structural model on the Berati anticlinal belt. The changes in the lithostatic pressure on the space have a greater role in the development of evaporite tectonics, which are conditioned by the deposition thickness of the cover sediments above the evaporites and the presence of weak tectonic loops. These loops belong to the intersecting places of transverse and longitudinal fault generated since the rifting phase (T3-Cr1). Such intersecting places are the Dumrea (Albania) and Zavrohon (Greece) on the surface. We stress that evaporites are also extended over regional thrust faults of the Berati belt, helping its overthrusting westward about 20–30 km (depicted by seismic and deep wells data). The tectonic evaporite effect differs from the expansion of the Berat thrust belt. Thus, in its southern part, as a result of a diapirism that has not emerged on the surface, it is helping orogenies in developing large fan-type anticline structures (Golika, Nemercka, Mitsikeli anticlines, etc.), that have backthrusts in their eastern flank. Moreover, we encountered the presence of a carbonate massive in the middle of the Epiro- Akarnani syncline (e.g. Timfeu massive in Greece), caused by the “blind” evaporite body effect. The northern sector (north of Qafa e Kicoku), is characterized by a smaller evaporite effect, which is reflected in increasing density of the anticlinal structures on the surface units and their normal passing to each other. The Dumrea diapir is a considerable ellipsoid-shape evaporite mass outcropping with a surface area of about 210 km2 and axes 20 × 10 km. It is an ordinary diapir with a westward overthrust (20–30 km). Most oil fields in Albania are in carbonate sections (Cr2-Pg2) in the anticline structures delivered to the thrust of adjacent units. There is another new prospect of the high perspective: The carbonate structures under overthrust of different units (subthrust complex) of the Ionian zone. The subthrust in the Ionian zone is divided into three types: 1. The subthrust beneath overthrust of thrust belts. These overthrusts are related to the evaporite detachment horizon, and they mask folded structures of subthrust. 2. The subthrusts beneath the overthrusts of the anticline units. These overthrusts have a local character and they are more developed in the Kurveleshi thrust belt. 3. The subthrusts beneath the overthrusts of the tectonic zones of the orogenetic front (Ionian and Kruja Zones) above the western autochthonous (Apulian Platform and South Adriatic Basin). These structures display duplex and triplex style. Currently, the more interesting opportunity lies in the subthrust beneath the Berati thrust belt. This has a higher perspective than the other subthrusts beneath the Kurvelesh and Cika anticlinal belts. Actually, the Berati subthrust is in the processes of being the subject of a seismic survey and exploration drilling. The discovery of a new oil field in the subthrust of the Berati anticlinal belt, with drilling of the Shpiragu-2 well, which in the first test gave 800–1300 barrels (light oil with density 35–37°; API) and 2–5 mmcfd gas per day, is of great importance for the entire tectonic ensemble of the Dinaric-Albanid-Hellenic. The presence of prospective structures such as the Dumrea anticline in the north and its continuation southward (the Shpiragu oil field, Molishti anticline, etc.), beneath the thrust belt, as shown by both drilling and seismic data, together with the presence of the surface structures (e.g. the Kasidhiaris, Kurenton and Thesprotikon anticlines) in Greece, shows the regional scale potential of petroleum plays. These prospects for new oil and gas discoveries constitute the object of this important paper, offering completely new ideas.