Setting and Stratigraphy 

Figures 1-3 

	Figure 1. Paleogeographic connection of the Preapulian Zone to the Italian Apulian platform (adapted from Accordi and Carbone, 1992).
	Figure 2.  Structural setting of the Ionian Islands.
	Figure 3. Stratigraphy of the Zakynthos and western Kefalonia platforms.

 

The Preapulian Zone crops out in the Ionian Islands, western Greece. The paleogeographic connection of the Preapulian Zone to the Italian Apulian platform is considered the “autochthonous” margin of the Apulian microplate in the Hellenides (Figure 1). 

Recent lithostratigraphic and sedimentological studies indicate that the evolution of the Preapulian zone, especially from the Upper Cretaceous throughout the Paleogene, is quite complicated and consists of at least two main sedimentary domains belonging to different tectono-sedimentary units, which today are found in tectonic proximity (Figures 2 and 3) (Accordi and Carbone, 1992; Kati, 1999; Scholle and Patsoules, 2001). 

This work focuses on the distribution and evolution of porosity in Eocene limestones on Zakynthos Island, a part of the Preapulian carbonate sequence, which has been a significant object of hydrocarbon investigation in the wider area of western Greece.

 

Eocene Depositional Facies 

Figures 4-9 

	Figure 4. Modified geological map of Zakynthos Island after IGME (1980) and Accordi and Carbone (1992). Legend: 1. Cretaceous carbonates, 2. Eocene carbonates, 3. Oligocene marly carbonates, 4. Miocene clastics and carbonates, 5. Pliocene-Quaternary alluvial deposits, 6. Ionian evaporites and breccias, 7. Anticline, 8. Fault, 9. Main thrust.
	Figure 5. Graded breccias deposited by debris flows at Aghios Nicolaos area.
	Figure 6. Medium-thickness low-density turbidites and some massive “banks” of high-density turbidites, in Aghioi Pantes quarry.
	Figure 7. Unconformity contact between megabreccias and pelagic/hemipelagic facies at Lagopodo quarry.
	Figure 8. Eocene pelagic/hemipelagic facies onlapping the Upper Cretaceous shelf facies at Lithakia quarry.
	Figure 9. Megabreccia sheet interlayered between pelagic/hemipelagic facies at Sgobos area.

 

Eocene limestones consist exclusively of slope facies. Thin-bedded, mainly pelagic foraminiferal mudstones and wackestones are interbedded with redeposited sediments that include a large variety of transported and reworked shallow-water carbonate materials. Debris flows, the most common resedimented deposits, consist primarily of numerous reefal blocks and bioclastic lithoclasts, originating from the platform-margin and shelf. Thin- to-thick turbiditic beds, mostly low-density and minor high-density flows, largely consist of resedimented bioclastic and lithoclastic material originating from the outer shelf and/or upper slope.

 

Porosity Types 

Megabreccia deposits:

	Lagopodo area, central Zakynthos;
	Kamaroti quarry, southern Zakynthos;
	Kakavakia area, southern Zakynthos.

   

     Figures 10-16 

	Figure 10. Extensively altered coral grain (original wall dissolved, intragranular pores filled with blocky calcite, micrite cement or internal sediment).
	Figure 11. Intergranular pore space lined with radial-fibrous marine cements.
	Figure 12. Preserved intragranular pore space in a gastropod.
	Figure 13. Dissolution of early fibrous cements in an intragranular pore space of a gastropod.
	Figure 14. Secondary moldic porosity and some preserved intergranular porosity.
	Figure 15. Secondary vuggy porosity (SEM photo).
	Figure 16. Late dissolution of marine radial-fibrous cements and internal sediment.

 

The coarser-grained, reefal debris flows principally have secondary moldic and vuggy porosity and only minor primary porosity.  Most of the intergranular and intragranular pore spaces are mainly filled with synsedimentary cements and internal sediment. The extensive moldic porosity originated from the dissolution of formerly aragonitic components, such as corals, bivalves, and gastropods; the recognition of which is facilitated by the formation of micritic envelopes. The vuggy porosity originated from dissolution of some early formed fibrous cement and/or fine-grained internal sediment within the intergranular pore spaces, and these vugs were obviously formed in a burial diagenetic stage.

 

Low- and high-density turbidites and modified grain flows:

	Aghioi Pantes quarry, central Zakynthos;
	Lithakia area, central Zakynthos.

  

     Figures 17-22 

	Figure 17. Preserved intragranular pore in an echinoid.
	Figure 18. Intragranular and minor intergranular porosity in modified grain flows.
	Figure 19. Moldic porosity after the dissolution of a gastropod.
	Figure 20. Late dissolution of sparry calcite cements in high-density turbidites.
	Figure 21. Secondary porosity from late dissolution and some preserved intragranular porosity.
	Figure 22. Syntaxial overgrowths and equant blocky calcite spar-filling primary pore spaces in turbiditic facies.

 

The finer-grained resedimented facies and also the pelagic sediments mainly have preserved primary porosity and minor secondary porosity. More specifically, the high-density turbidites have substantial intragranular porosity within the larger transported benthonic bioclasts and, at least locally, remarkable intergranular porosity. The latter mainly originated from the dissolution of scattered skeletal debris and/or matrix material with metastable initial compositions (mostly high-Mg calcite) comprising secondary solution-enlarged intergranular porosity. Additionally, small amounts of moldic porosity have also been developed, due to the dissolution of some transported, originally aragonitic, fine-grained components, mostly gastropods. The low-density turbidites and the pelagic facies have small amounts of preserved primary porosity. Some intergranular pore spaces have also been increased locally.

 

Petrophysical Data 

Figures 23-25 

	Figure 23. Frequency plots of A) porosity and B) permeability distributions.
	Figure 24. Average porosity and permeability of the various Eocene depositional facies.
	Figure 25. Correlation plot of porosity.

 

Porosity Evolution Patterns 

Figures 26-27 

	Figure 26. Paragenetic sequence of debris flows facies (blue bars indicate porosity-generating episodes; red bars reflect porosity-reducing events).
	Figure 27. Paragenetic sequence of turbiditic facies, modified grain flows and pelagic-hemipelagic sediments (bars as in the previous figure).

 

Detailed petrographic examination of the depositional and diagenetic textural characteristics, in combination with estimations of the basic petrophysical parameters, allowed recognition of two main paths in the porosity evolution.

 

Conclusions 

     1. Preserved Eocene limestones comprise exclusively slope facies, consisting of pelagic sediments interbedded with very coarse-grained reefal debris flows and bioclastic turbidites (mostly low-density flows). The various facies show different porosity types mainly due to their initial composition and the subsequent diagenetic alterations they suffered.

     2. Although the primary porosity of debris flows was largely destroyed by synsedimentary cementation, they gained extensive moldic and minor vuggy porosity during early dissolution.  Some of that secondary porosity, however, was occluded with later cements. The fine-grained low-density turbidites and pelagic sediments have small amounts of preserved primary porosity and even less secondary (mostly vuggy) porosity, because compaction associated with later cementation and recrystallization destroyed porosity almost completely.

     3. In contrast to the low porosity and very low permeability values characterizing these slope facies, the coarse-grained high-density turbidites have substantial "effective porosity" with extensive preserved primary porosity coupled with vuggy and minor moldic porosity that combine to give them good reservoir characteristics.

 

Acknowledgments 

Sincere thanks to Ι. Abatzis, N. Springer, Ν. Christensen, J. Ineson, P. Frykman, N. Stentoft, A. Rasmusen, and H. Lindgreen of the GEUS, Denmark, for their kind hospitality and for providing laboratory facilities and analytical instruments. Many thanks also to M. Patsoules, K. Nikolaou, and K. Georgiades of Hellenic Petroleum S.A. for their help in various aspects of the project.

 

References 

Accordi, G., and F. Carbone, 1992, Lithofacies map of the Hellenide Pre-Apulian zone (Ionian Islands, Greece): Cons. Nazion. Ricer., Spec. Publ., 27p.

Kati, M., 1999, Deposition, diagenesis and evolution of porosity of Eocene limestones of the Preapulian zone in Zakynthos Island: Ph.D. thesis, University of Athens, Athens, Greece, 305p.

Scholle, P.A., and M. Patsoules, 2001, Sedimentology and petroleum potential of Cretaceous limestones in the Preapulian Zone, Ionian Islands, Western Greece: AAPG Annual Convention 2001.