*5.1. Microfacies Types and Depositional Environments*

The main textural and compositional characteristics, as well as the sedimentary features of the distinguished microfacies, are summarized in Table 3, corresponding to different depositional environments or facies zones (FZ) defined by [84,85] (Figure 7). More specifically, the Pantokrator Limestones were classified as boundstone of algae (SMF 7) and bioclastic grainstone (SMF 11) in the Agios Georgios and Perivleptos sections respectively, giving evidence of a depositional environment characterized by a platform, with both intertidal and subtidal environments (FZ5-6). Recrystallization-dolomitation and fracturing of some redeposited carbonate clasts points a subaerial exposure of parts of the platform. Vigla Limestones consist of mudstone-wackestone with radiolarians and planktonic foraminifera (SMF 2-3), characterizing a low energy, relatively deep environment, such as the toe of slope (FZ3) and/or deep shelf (FZ2). Senonian Limestones present a variety of lithofacies, which correspond to environments ranging from the slope to deep shelf. They mostly include in-situ wackestone-packstone with planktonic foraminifera along with microbreccia bioclastic packstone with fragments of shallow water fauna (rudists and benthic foraminifera) (SMF 4), and micrite with transported ooids (SMF 13), which in total represent a medium-to-high-energy environment (e.g., slope), possibly due to the transportation of the sediments within the basin from the platform.

The above facies distribution reflects the separation of the deep Ionian Basin into a central topographically-higher area characterized by reduced sedimentation, and two surrounding talus slopes with increased sedimentation [66]. Locally, micritic peloids and dark-gray intraclasts, floating in an overall micritic matrix were also observed within these carbonates. The range of depositional interpretations of these formations includes supratidal settings, vadose-marine inorganic precipitation in inter- and subtidal environments of formation in marine seepage or groundwater springs [85]. Overall, Late Cretaceous calciturbidites suggest relatively deep-slope depositional conditions (FZ3-4). Relatively similar and possibly deeper depositional conditions apply for the Paleogene biomicritic packstones with radiolaria (mostly at the Early Paleocene) and planktonic foraminifera (through the Eocene) (SMF3), suggesting a basinal environment (FZ2).

**Table 3.** Detailed description of representative thin sections, where sedimentary facies, lithology, formation, age, and the depositional environments of the studied deposits are presented.


**Figure 7.** Depositional distribution model in the Ionian zone. The sedimentary changes display good correlation with the energy level of environments and facies changes.

#### *5.2. Reservoir Potential of the Early Jurassic to Eocene Carbonate Rocks of the Ionian Zone*

Reservoir characterization deals with physical characteristics of the reservoir, including petrophysics (porosity–bulk density and grain density measurements, capillary pressure measurements), fluid properties (e.g., reservoir fluid saturations) and reservoir drive mechanisms [51,91]. Moreover, reservoir quality is defined as the amount of porosity and bulk density in a reservoir and can be a function of many control factors for both carbonates and sandstones [92–94]. In the present study, the reservoir properties, porosity and bulk density, are comparatively examined in the studied sections on a large intra-basin scale in order to assess the quality of carbonate reservoirs of the Ionian zone. Measuring porosity and bulk density of a given reservoir is a direct measure for the storage and flow capacity. Though porosity seems to be a main contributor to the flow capacity, bulk density is mostly controlled by the pore throat distribution [95]. However, they are difficult to predict, since they depend on both initial depositional processes and diagenetic overprinting. Particularly, their complexity in carbonate reservoirs should be attributed to the different interplays, among other factors, of hydrodynamic conditions, carbonate cementation or dissolution, and tectonic setting that form the architecture of the marine setting [96].

The surveyed carbonates showed remarkable average porosities (3.02–6.50%; Table 2) accompanied by even lower bulk densities (2.62–2.70%), and therefore, the quality of the reservoir has been described as poor to fair. Low and homogeneous bulk density values are in good agreement with the literature data for pure calcite rocks [97–99]. Average porosity values are quite variable in the study sections, with a tendency to slightly higher values in section E and upper part of section D both of Senonian age, as well as in the upper part of section B corresponding to the Vigla Shales. On the contrary, the lowest porosity (and bulk density) values were reported for the biomicritic mudstone-wackestone depositional facies with pelagic fauna of Vigla formation. The Pantokrator Limestones present significant porosity and bulk density values only on dolomitized horizons, explaining the significant variability observed into this formation. Generally, this intra-zone original porosity evolution characterized by an increased tendency from the Early Cretaceous pelagic limestones to the Late Cretaceous calciturbidites, which are also slightly more permeable, is consistent with previous findings in the Ionian zone of NW Greece [33,100,101]. Particularly, the overall evolution in the study area display a quite variable average porosity pattern, characterized mostly by low to moderate

values (~3–7%), while some high individual values around 10% also recorded. The levels with such a sudden porosity increase may be related to burial diagenesis and dolomitization, which increase the reservoir quality to good.

Factors related to sequence architecture, including the occurrence of intervals with clay laminations (e.g., Vigla Shales; promoting chemical compaction and associated cementation) and the distribution of early dolomitization (promoting porosity preservation during burial), generally increases secondary fracture porosity. A higher increase of porous within the marly limestones of Vigla Shales formation than in Senonian calciturbidites, could be caused due to fracturing due to the nodule's development, in the manner described by [43]. Intracrystalline porosity could be also developed within dolomitized intervals and therefore some breccia limestones may form potential reservoirs. Especially for the Ionian zone, it is documented that the dolomitization front has changed through time [33]. In the internal and external parts of the Ionian zone, dolomitization continued well into the Cretaceous, whilst in the central part, it did not continue after the Middle Jurassic. However, it is worth noting that the low porosity and bulk density identified within the study formations could either imply high fluid pressures or fluid migration through permeable "fracture conduits" in the vicinity of fault zones. The observed porosity values correspond to microporosity that does not take into consideration the fracture porosity related to thrusts which considerably increases the carbonate reservoir quality. Some of the porosity variations observed in this study may due to the vicinity/distance of the samples with the tectonic zones. In any case, distribution of Ionian tectonic zones is related to the prevailing tectonic style of the Ionia zone, which is a combination of thick- and thin-skinned deformation [33]. The elucidation of the predominance of tectonic style has not been achieved to date, due to the fact that deep seismic surveillance is hindered by the subsurface Ionian evaporites [34,58,70,71].

The incorporation of the depositional with the carbonate reservoir quality data indicate that there is not a clear view for the reservoir quality that can be associated with a specific depositional environment. Generally, in western Greece, most of the fair to good potential reservoirs are deposited in shallow to restricted platforms (e.g., Gavrovo platform carbonates close to its transition to the Ionian zone, in the thrust sheets of Ionian and pre-Apulian zones) [33,34,40]. In the Ionian zone, similar medium to high energy environments, such as tidal domains, reef barriers, and slopes were recorded during Jurassic (e.g., Pantokrator Limestones, Posidonia beds) and Late Cretaceous to Paleocene/Eocene (e.g., Limestones with microbreccia), respectively [35,40]. However, Jurassic studied sediments do not contain any proven reservoirs with the most significant porosities to be associated with the development of fracture and/or diagenetic zones. This study shows that deeper depositional (deep marine basinal and/or slope) environments can also be associated with the deposition of good potential reservoirs. On this regard, potential reservoir rocks within the Ionian zone further include the upper part of pelagic Vigla Limestones, Senonian Limestones, and the microbrecciated intervals of the Paleocene/Eocene limestones, all presented very good porosity values up to 10%.

## *5.3. Paleogeographic Analysis of the Ionian Zone*

In Epirus area only the Jurassic to Eocene carbonate succession occurs. The carbonate platform sediments begin at the base with thick-bedded neritic Jurassic Pantokrator Limestones, which feature remarkable facies homogeneity, indicating that an extensive shallow sea was spread all over the study area during that time. In Perivleptos and Agios Georgios sections, this facies association is mostly represented by biolithitic boundstones and biosparite grainstones with calcareous algae and benthonic foraminifera, implying a carbonate margin platform with both intertidal and subtidal environments (Figure 8A). These extensive platforms are developed until the Hettangian–Pliensbachian age, when the overlying synrift sequence begins. Pliensbachian Siniais Limestones correspond to the general deepening of the Ionian Basin. The structural differentiation that followed caused the fragmentation of the initial basin into smaller paleogeographic units with half–graben geometry. This is recorded in the abruptly changing thickness of the synrift formations that take the form of syn-sedimentary wedges [34]. In the deeper parts of the half grabens, these wedges include complete

Toarcian-Tithonian successions, whereas in the shallower parts of the half grabens, the successions are interrupted by unconformities. However, this topmost part of the Jurassic is not recorded in the studied sections. In Koloniati section, there is an unconformity between the base of the Vigla Shales and the topmost horizons of the underlying Pantokrator formation, which marks a period of uplift and erosion at the beginning of the Toarcian. This led to occasional karstification of Pantokrator Limestones.

**Figure 8.** Paleo-environmental map of the study area during the (**A**) Early Jurassic, (**B**) Early Cretaceous, (**C**) Late Cretaceous, and (**D**) Paleocene-Eocene based on outcropped- (red circles from this study and white ones from the literature) and well-data (white-red symbols) [33,35,42,59,86].

The post-rift sequence begins with the pelagic Vigla Limestones, whose deposition was synchronous throughout the Ionian Basin, beginning in the Early Berriasian [73,102]. The basal sequence of the Vigla limestones, consisted of thin layered, sub-lithographic, pelagic limestones, with abundant radiolarian and frequent cherty beds enriched with radiolarian, is related to the Early Cretaceous subsidence caused deepening in the entire basin (Figure 8B). Towards the upper part of this formation, chert layers become more abundant, containing intercalations of green, red, and locally black shales, named as the "Vigla Shales" member, indicative of basinal sedimentation. The microfacies analysis of the Koloniati (lower part) and Vigla sections suggest that these carbonate sediments represent a low energy, relatively deep environment, like the toe of slope and the deep shelf of the basin. Apart from the halokinetic movements, which probably caused the variation in thickness of Vigla Limestones [34] from the western (external) to the eastern (internal) parts of the basin, the pelagic depositional conditions persisted until the Late Eocene, when flysch sedimentation began.

During the Late Cretaceous, the Senonian Limestones formation consisted of hemipelagic calciturbidites and resedimented microbreccia, reflect similar deep marine slope environments. In particular, sedimentary facies analysis of the Koloniati (upper part) and Asprageli-2 samples suggests that they were deposited in a deep-water toe of slope and platform margin environment, respectively, where miccrobrecciated carbonates were transported and accumulated (Figure 8C). The allochthonous bioclastic material identified in the analysed samples consists mainly of rudist (typical reef builder) fragments and benthic foraminifera (e.g., forereef dweller *Orbitoides*; [103] and/or inner platform taxa *Cuneolina*, Textulariids and Miliolids; [85,103,104]), originated and transported from a nearby shallow shelf environment (e.g., platform or reef). Such shelf margins of the nearby pre-Apulian and Gavrovo platforms [75] and/or internally to the Ionian basin [101] were characterized by high productivity of such skeletal material, transported and redeposited in the deeper parts of the Ionian basin [34]. These bioclasts are also accompanied by pelagic Globotruncanids and radiolarian specimens observed in the in situ micritic matrix. Moreover, ooid lithofacies and some reworked lithoclasts observed in the upper part of the Koloniati Senonian Limestones indicate shallow-water conditions that were exposed during the uppermost interval of the Cretaceous (Maastrichtian). This is further reinforced by extensional tectonics, while possible sea level effects cannot be ruled-out and could also be related to the eustatic sea level low-stand (~150 m sea-level drop) that took place between the Late Cretaceous and the Paleocene [105,106]. Our paleoenvironmental observations from the Late Cretaceous interval in Epirus fully agree with recent sedimentary findings of [42] for the Araxos area (internal Ionian zone), as well as with the previous literature [34,35,75] for the entire Ionian zone. Overall, the facies distribution of the Senonian reflects the separation of the Ionian Basin into a central area (middle and outer part of the Ionian Zone) characterized by deeper water sedimentation and two surrounding talus slopes, issued from western Gavrovo platform and western Apulian platforms. Both platforms provided the clastic carbonate material that was transported by turbidity currents into the Ionian Basin.

The study of the Asprageli-1 samples provides evidence that the supply of clastic material due to tectonics diminished significantly during the Paleocene/Eocene. However, despite the reduced tectonic activity during that time, the slumping of platform edge sediments produced turbidity currents resulting to the deposition of Limestones with microbreccia and calciturbidites. The main depositional facies of platy mudstone-wackestone with Globigerinidae, Globorotaliidae and rare siliceous nodules, analogous to those of the Vigla Limestones, imply that the depositional environments during that period did not change significantly from the Late Cretaceous (Figure 8D). The greatest thicknesses of the Eocene units can be found in the marginal parts of the Ionian Zone, where the microbreccias are more frequent.

#### **6. Conclusions**

The Ionian zone consists of a heterogeneous multi-layered calciclastic reservoir in Epirus region (western Greece). The identified carbonate formations display various facies ranging in a full spectrum of depositional conditions, from shallow platforms (reefs) to slope (platform margin) environments, even to the open marine settings, with different lithologies, sedimentary features, energy conditions, and diagenetic overprints. This heterogeneity also explains the rock petrophysical/geomechanical variation of these carbonate rocks. The Early Jurassic limestones (biolithites boundstone) do not contain any proven reservoirs due to relatively low porosities, with the exception of the microcrystallized or dolomited horizons, which increase the reservoir quality in a local scale. The Early Cretaceous limestones and cherts (biomicrites mudstone-wackestone) of Vigla formation has been described as the poorest of the studied carbonates, in terms of their reservoir potential. On the contrary, the Late Cretaceous (Senonian Limestones) and the Paleocene/Eocene carbonate units can be considered the primary target for oil/gas exploration in the study area, since they contain calciturbidites deposited mainly in the slope (bioclastic packstone-rudstone with rudist fragments and benthic foraminifera) and the deep shelf (planktonic foraminiferal biomicrites mudstone-wackstone). The highest porosity values recorded in those carbonates may be further associated with the development of fracture networks and/or diagenetic zones. Overall, the results of this study may have implication for reservoir- and/or source-rock-geologists and diagenetic modelling approaches in the presented area and elsewhere within the eastern Mediterranean Sea [42,86,100,101,107,108], implying that sample specific analyses or a very well understood regional diagenetic framework are required for accurate prediction of reservoir quality.

**Author Contributions:** Conceptualization, G.K., L.M.; methodology, G.K., L.M.; software, L.M., G.K.; validation, G.K., L.M.; formal analysis, G.K., L.M.; investigation, G.K., L.M., V.K., A.A.; resources, L.M., V.K.; data curation, G.K., L.M., V.K., A.A.; writing—original draft preparation, G.K.; writing—review and editing, G.K., L.M., V.K., A.A.; visualization, G.K., L.M.; supervision, V.K., A.A.; project administration V.K., A.A.; funding acquisition, L.M., V.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors are grateful to Andreas Kostis for his significant contribution to the fieldwork. Jean-Jacques Cornée and Fotini Pomoni-Papaioannou are warmly thanked for her kind assistance during the sedimentary facies analysis and her constructive suggestions regarding the interpreted depositional paleoenvironments. Four anonymous reviewers are deeply acknowledged for useful and constructive comments on the manuscript, and Victoria Li (Assistant Editor) is also thanked for her editorial handling.

**Conflicts of Interest:** The authors declare no conflict of interest.
