**1. Introduction**

Marine biogenic carbonates are among the most important archives of Earth's history reflecting past changes in ocean chemistry, water mass circulation and the evolution of life [1–10]. Particularly during the Cretaceous, changes in climate, oceanographic circulation, sea level, tectonic and volcanic activity contributed to triggering widespread crisis events, which resulted in world-wide anoxic episodes [11–17], drastic facies variations and biotic changes [18–23], and diffusion of bauxite deposits [24,25], among others. Such complex events were reflected on biotic and non-biotic constituents of the carbonate platforms and on the architecture of their depositional systems.

Within the Mediterranean Tethys (Neo-Tethys) Ocean, the Mesozoic-Paleogene Eras witnessed the development of vast carbonate platform belts that fringed its margins and passed into deep marine equivalents, completing thus the marginal succession in the deeper parts of the basin [26–29]. These successions, including the slope deposits in the platform margins, provide an excellent example of the evolution of depositional sequences ranging from the platform through rimmed carbonate-shelf to pelagic depositional settings, and further characterize long intervals in the rifting and subsidence history of past peri-Tethyan continents. Among them, the carbonate platform to basin transition can give rise to fragmentary distribution of marginal slope successions as fault-bounded units [30–32]. Such confined carbonate units are also considered to be aquifer systems, hydraulically independent of their siliciclastic cover, and therefore, can build prolific reservoirs and become potential exploration targets for oil and gas throughout the entire Mediterranean basin.

Within the Ionian Zone, both the Early Jurassic neritic limestones and dolomites, and the Late Cretaceous to Eocene re-sedimented carbonates (calcareous turbidites and coarser breccia) are considered the main reservoir successions and exploration targets for oil and gas in western Greece [33–35], central and southern Adriatic offshore Italy [36,37], and onshore Albania [35,38–40]. The Jurassic karstified platform carbonate facies represent an excellent analogue for the carbonate reservoirs, where the sea-level drawdown and subaerial exposure of the carbonate reef enhanced the facies reservoir quality. Moreover, the calciturbidites can be of great economic importance and they can serve as reservoir rocks [39,41,42], due to their high porosity and bulk density values, which can be additionally enhanced by the development of chert nodules [43]. However, the nature and distribution of these deposits along with their depositional mechanism processes and environmental conditions, especially in western Greece, are still poorly constrained. The high heterogeneity related to fabric, texture, fractures that usually characterize such kind of reservoirs has been highlighted by several authors [42,44–51]. Understanding heterogeneity of carbonate reservoir helps predicting reservoir petrophysical and geomechanical behaviors [52–54], which in turn play a crucial role in their exploration, production, and development [55–57]. In this regard, high-resolution sedimentological outcrop data are essential, because they help to fill the gap with respect to subsurface data, which assists in refining reservoir models. Petrographic constraints based on facies analysis provide sedimentological features and micro-textural characteristics, which are the key-link between the rock depositional/diagenetic history and its physical properties.

In the present study, we introduce a complete record of the marginal successions in the western (Ionian basin) segment of the southern Tethys, which consists of the Early Jurassic to Eocene carbonate platform and slope to basin successions from the Epirus region (Figure 1). This integrated study aims to define the Mesozoic-Paleogene depositional history, based on lithostratigraphic characteristics and reservoir petrophysical behaviors in the central Ionian domain, a major hydrocarbon prolific basin in western Greece. This was accomplished by detailed sedimentological and geomechanical/petrophysical analyses of the carbonate succession, in conjunction with a synthetic paleogeographic reconstruction. Beyond the hydrocarbon prospectivity, this work has further implications for regional geology, since it contributes to describing the evolution of these carbonates, and to a better understanding of the Ionian zone (pre-, syn- and post-rift stages; [34,58]) in western Greece, a region with crucial economic and strategic importance.

**Figure 1.** Geological map of the external Hellenides in NW Greece (modified from [34,35]) illustrating the principal tectonostratigraphic zones: Pre-Apulian, Ionian, Gavrovo, Pindos. The red box shows the study area, the northwestern part of the Epirus region, where the separation of Ionian zone to external, middle, and internal sub-basins along with the regional locations of the study sections to be also indicated. Legend interpretations are presented in the inset.
