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Depositional and Diagenetic Patterns and Petroleum Potential of Cretaceous Carbonate Platforms, Ionian Islands, Greece*

By

Peter A. Scholle1 and Marios Patsoules2

 

Search and Discovery Article #40271 (2008)

Posted January 29, 2008

 

*Adapted from oral presentation at AAPG and AAPG European Region Energy Conference, Athens, Greece, November 18-21, 2007.

 

1New Mexico Bureau of Geology, NM Tech, Socorro, NM ([email protected])

2Hellenic Petroleum S.A, Maroussi, Greece

 

Abstract 

Isolated Cretaceous (Aptian-Maastrichtian) carbonate platforms are found in the Preapulian Zone on several of the Ionian Islands in western Greece, including Zakinthos, Kephalonia, Lefkada, and Paxos. Each island has exposures of one or more thick carbonate platform complexes (hundreds of meters to >1000 m in thickness) or has indicators of nearby platforms that are not presently exposed. Platform interior facies consist of micritic (chalky) mudstones to wackestones and pass laterally towards platform margins though muddy rudist patch reef facies and rudist wackestone to grainstone deposits. Shelf margins are dominated by massive rudist reefs and coarse, well-rounded rudist grainstones. Slope and toe-of-slope facies consist largely of reworked shelf limestone clasts in debris flows, slides, slumps, and turbidites. 

Diagenesis of these deposits is dominated by synsedimentary marine and later meteoric processes. Primary low-Mg calcite marine cements largely occluded porosity in shelf edge carbonates. Meteoric leaching, associated either with pre-Eocene erosion or with Pliocene and later exposure, had more positive effects on porosity, creating extensive secondary pores (locally >20%) in shelf, shelf margin and slope facies. The secondary porosity includes molds of formerly aragonitic grains, vugs, solution-enlarged fractures, and chalky microporosity and, coupled with preserved primary inter- and intragranular porosity, yields excellent potential reservoirs. Although not as spectacularly karstic as its Mexican Cretaceous counterparts in the Golden Lane and Poza Rica fields, the extensive leaching of the Greek platforms should make them an attractive exploration target to the east of the Ionian Islands where such platforms are preserved below thrust sheets that could provide adequate seals.

 

 

uAbstract

uFramework

uFacies

  uShelf margin

  uPlatform interior

  uSlope/basin

uDiagenesis

uAnalog

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFramework

uFacies

  uShelf margin

  uPlatform interior

  uSlope/basin

uDiagenesis

uAnalog

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFramework

uFacies

  uShelf margin

  uPlatform interior

  uSlope/basin

uDiagenesis

uAnalog

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFramework

uFacies

  uShelf margin

  uPlatform interior

  uSlope/basin

uDiagenesis

uAnalog

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFramework

uFacies

  uShelf margin

  uPlatform interior

  uSlope/basin

uDiagenesis

uAnalog

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFramework

uFacies

  uShelf margin

  uPlatform interior

  uSlope/basin

uDiagenesis

uAnalog

uConclusions

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFramework

uFacies

  uShelf margin

  uPlatform interior

  uSlope/basin

uDiagenesis

uAnalog

uConclusions

uReferences

 

Geologic Framework 

Figure 1. Ionian Islands study area.

Figure 2. Ionian Islands with Cretaceous outcrops.

Figure 3. Late Cretaceous facies distribution, Zakynthos (adapted from Accordi and Carbone, 1992).

Figure 4. Evolution of carbonate platforms on Zakynthos and Kefallonia (adapated from Accordi and Carbone, 1992).

Figure 5. Composite Cretacous platform model.

 

Facies 

Shelf Margin 

Figure 6. Coarse rudist-dominated reef, Nauvayo Road, Zakynthos.

Figure 7. Coarse, rounded, shelf margin rudist grainstone, Ksenopoulon, Kefallinia.

Figure 8. Marine cements in shelf margin rudist facies, Enos area, Kefallinia.

 

Platform Interior  

Figure 9. Rudists in peloidal mudstone of shelf-interior facies, Lithakia quarry, Zakynthos.

Figure 10. Rudist patch reef with muddy matrix, Argostoli Bay, Kefallinia.

 

Slope and Basin  

Figure 11. Lower slope to basin facies depositional structures, Paxos and Kefallinia.

Figure 12. Debris flow megabreccia and overlying turbidites, Missorachi, Paxos.

Figure 13. Truncated cements and grains in slope debris, Gaios harbor, Paxos.
Cretaceous, probably Maastrichtian, north side of Gaios harbor. Intraclast with truncated marine cements. Plane-polarized light; long dimension = 6 mm.

Figure 14. Shelf intraclasts in a slope debris flow deposit, Gaios harbor, Paxos.
Upper Cretaceous (Maastrichtian) limestone. Intraclasts also are common in shelf-margin and slope limestones. These reworked grains include marine-cemented rudist reef material as well as finer-grained, back-reef and/or upper slope carbonate fragments. All have been reworked into basinal deposits by turbidity currents and debris flows. Intraclastic slope deposits (and slope-derived basinal limestones) commonly have a more diverse or polymict assemblage of grain lithologies than do most coastal or open shelf deposits. Plane-polarized light; long dimension = 16 mm.

 

Diagenesis: Marine Cementation and Meteoric Leaching and Cementation 

Figure 15. Marine-cemented rudist rubble in slope debris flow, Gaios harbor, Paxos.
Upper Cretaceous (Maastrichtian?) limestone. A higher magnification view of an intraclast in a deep-water debris flow. It contains shelf-margin rudist biosparite with complete occlusion of porosity by extremely early marine cements. Both the rudist fragments and marine cements were truncated during the formation of this clast. Examination of such deeper water intraclasts can provide considerable information about nearby platforms and their early diagenetic history--information commonly of great value to explorationists. Plane-polarized light; long dimension = 11 mm.

Figure 16. Erosional Cretaceous to mid.-Eocene unconformity, Lithakia quarry, Zakynthos (from M. Kati, personal communication, 2007).

Figure 17. Weathered Maastrichtian limestone beneath unconformity, Lithakia quarry, Zakynthos Rubbly top Cretaceous. 

Figure 18. Eocene/Oligocene channel on Cretaceous platform, Agia Marina, Zakynthos.

Figure 19. Primary and secondary porosity in rudist grainstone, Upper Cretaceous, Agalas quarry, near Keri, Zakynthos.
Rudist fragment limestone (plane-polarized light; long dimension = 10 mm.).

Figure 20. Primary and secondary porosity in rudist grainstone, Upper Cretaceous, road to Porto Vromi (2 km from port), Zakynthos.
Rounded but very poorly sorted rudist biosparite to packed rudist biomicrite. Abundant, well rounded rudist fragments plus subordinate corals and benthic forams. Very poorly sorted, and in patches of zones surrounded by biofragmental matrix.  Numerous micrite envelopes and spectacular leaching to produce 30+ percent moldic porosity.  Thin, blocky, nonferroan calcite cements both inside and outside molds. Long dimension = 5.75 mm.

Figure 21. Poorly sorted grainstone with moldic porosity, west wall of Lithakia quarry, near Keri, Zakynthos.
Cretaceous, Maastrichtian, rubbly limestone from 1-2 meters below unconformity. Porous, rounded but poorly sorted rudist biosparite. Abundant rudists, possible other mollusks, and benthic forams (including large forms) dominate; minor echinoid detritus.  Highly fragmented and moderately well rounded but poor size sorting. Micrite envelopes common.  Blocky to bladed calcite spar fringe cement; development then of massive leaching with superb moldic porosity. All calcite is non-ferroan. Long dimension = 16 mm.

Figure 22. Chalky porosity in platform wackestone, Maries, Zakynthos.
Upper Cretaceous chalky rudist limestone, Maries-Alykes road. Packed rudist biopelmicrite. Large, virtually intact , extensively bored, rudists and rudist fragments set in a granular bioclastic pelletal micrite matrix.  Also some echinoid fragments.  Extensive leaching has partially dissolved rudist walls and has generated micromoldic secondary porosity in matrix. Even some earlier cements have been leached to yield odd "skeletal" fabrics. Long dimension = 3.5 mm.

Figure 23. Vuggy, channel porosity in shelf wackestone, Louha quarry, Zakynthos.
Upper Cretaceous limestone. The irregular shape of this large, porous fracture indicates that some solution-enlargement occurred along the fracture, thus creating what is termed “channel porosity” in the Choquette and Pray (1970) classification. Plane-polarized light; long dimension = 5.1 mm.

Figure 24. Carbonate debris flow with leached aragonitic clasts.

Figure 25. Vuggy porosity in basinal turbidite, Porto Katsiki, Lefkas.
Upper Cretaceous polymict intraclasts in a turbiditic limestone in the slope facies of a rudist platform. Rudist fragments are associated with torn-up finer-grained limestones. Long dimension = 12.5 mm.

 

Oil-Productive Analog 

The Mid-Cretaceous Golden Lane Atoll of Mexico: 1908-2001 cumulative production > 3.9 billion barrels of oil equivalent.

 

Figure 26. Comparison of Late Cretaceous platform sizes.

Figure 27. Cross-section on western margin of El Abra platform.

Figure 28. Vuggy, karstic porosity in El Abra reef limestone.

Figure 29. Breccia in basinal Tamabra Formation (Poza Rica trend).

Figure 30. Vuggy porosity in Tambra debris flow (Poza Rica).

Figure 31. Vuggy, oil-filled Tamabra debris flow (Poza Rica).

Figure 32. Porosity types in Tamabra debris flow.

Figure 33. Diagrammatic cross-section of western Greece in ~ late Eocene time. (adapted from Jenkins, 1972).

 

Conclusions 

  1. Several Cretaceous carbonate platforms or atolls are exposed in the Ionian Islands of western Greece.

  2. Platform margin facies contain rudist reefs and associated high-energy rudist grainstones; platform interiors consist of chalky wackestones with isolated patch reefs; slope and proximal basin deposits are dominantly turbidites, slumps, and debris flows.

  3. Diagenetic patterns are relatively simple and regionally consistent. Only platform margins were heavily marine cemented; all facies underwent Early Tertiary meteoric diagenesis and secondary porosity development.

  4. Depositional/diagenetic patterns in the Cretaceous of Greece are similar to those of the Mexican El Abra oilfields. Where buried under Tertiary flysch or younger thrust sheets, the Cretaceous platforms of Greece, by analogy, could have substantial petroleum potential.

 

References 

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

Jenkins, D.A.L., 1972, Structural development of western Greece: AAPG Bulletin, v. 56, p. 128-149.

Choquette, P.W., and L.C. Pray, 1970, Geologic nomenclature and classification of porosity in sedimentary carbonates: AAPG Bulletin, v. 54, p. 207-250.

 

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