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

Abstract

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 thrusting westwards of tectonic nappes (Gavrovo/Kruja, Ionian), with an imbricate tectonic style prevailing in all external zones. In the internal and central Ionian zone, imbricate 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 Miocene flysch unit 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 km to 22 km in Albania, and about 5 km to 12-13 km in Greece). The above mentioned structure styles were later affected by strike-slip tectonics and extensional deformation active since the Early Pliocene times. We propose that 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 and in the southern areas of the Ionian Sea (Kyparissiakos gulf, SW Kythira).

Space and Time Variability of Detachment‐ Versus Ramp‐Dominated Thrusting: Insights From the Outer Albanides

“…New balanced and restored geological sections across the southern outer Albanides are presented together with a strike section in order to provide a comprehensive picture of detachment‐dominated thrusting of the sedimentary cover that largely characterizes this sector of the fold and thrust belt. As recent and active deformation of the entire frontal zone of the belt appear to be governed by ramp‐dominated, basement‐involved thrusting (Kamberis et al., 2022; Teloni et al., 2021; Vittori et al., 2021), our results also provide new insights into the development of mixed thrust tectonics styles (Figure 1b) that may result from space‐time variations of both stratigraphic and rheological factors.…”

“…This process has also major implications for the active tectonic setting of Albania: the demonstrated activity of frontal basement structures in the northern region indicates that aseismic shear strain accumulation along the middle crustal décollement triggers episodic seismic rupture of otherwise locked upper crustal faults splaying out from it, as it occurred during the 2019–2020 Durrës earthquake sequence (Teloni et al., 2021; Vittori et al., 2021). Recent crustal seismicity in the Greek offshore south of our study area suggests that active basement thrusting characterizes also the frontal part of the Hellenides fold and thrust belt (Kamberis et al., 2022, and references therein). Since active basement thrusts of the southern outer Albanides are not equally well constrained by recent seismicity, our crustal section D‐D′ may be used (together with the other balanced sections of Figures 10 and 11) to model less known/elusive potential seismic sources.…”

“…The largest of them is the Dumre diapir, which is clearly involved in thrust‐related deformation (Figure 3b). Besides the diapirs, the structural architecture is dominated by faulted and WSW‐transported, often doubly verging anticlines (Kamberis et al., 2022, and references therein). The analysis of syn‐tectonic growth strata indicates that deformation of the Ionian basin progressed forelandward, starting in late Oligocene‐Aquitanian times to the east and continuing during Langhian–Serravallian times more to the west (Lacombe et al., 2009).…”

“…Single folds showing abrupt plunge terminations (Figure 7) form part of fold arrays typically associated with buckling (Price & Cosgrove, 1990, and references therein). It is worth noting that almost all published detailed geological sections across the outer southern Albanides show variably offset and/or truncated folds, with the common occurrence of faulted fold trains, well‐developed footwall synclines, and reverse faults cutting across the intermediate limbs of antiform‐synform pairs (e.g., Durmishi et al., 2014; Kamberis et al., 2022; Frasheri et al., 2009; Velaj, 2001, 2011, 2015b; Velaj et al., 1999; Figure 3b). These features indicate that in most cases folds were not generated passively (i.e., as forced folds; Butler et al., 2019) by the motion of rock panels above stepped thrust as postulated in fault‐related folding models (e.g., Jamison, 1987).…”

“…The latter, interposed between the Ionian basin to the west and the ocean‐continent transition to the east (Figure 4), can be regarded as an extensional ribbon like the Apennine platform of the southern Apennines (Mazzoli et al., 2014) or the Bisotun‐Avalon platform of the Lurestan area of the Zagros Mountains (Vergés et al., 2011). The base of the Ionian basin stratigraphic succession, overlying an unknown basement generally assumed of crystalline—high grade metamorphic and/or igneous—nature (e.g., Kamberis et al., 2022), is formed by Permo‐Triassic (Velaj, 2015b) to Upper Triassic (Burano Fm.) evaporites consisting of intercalated gypsum, anhydrides, dolomitic limestone, salt and varicolored shales and breccias (Velaj, 2001).…”