Nicholas Roussos, Fedon Marnelis
Public Petroleum Corp. of Greece
Athens
Hydrocarbon exploration in Greece dates to near the beginning of the 20th century (1903). Until 1960 this exploration effort was discontinuous and naturally limited to onshore areas.
Since 1960 the Ministry of Industry in conduction with the Institute of Geology and Mineral Exploration undertook an exploration program including geological, geophysical, and drilling activities, and companies like BP, Esso, Hunt, Texaco, Chevron, and Anschutz as permit holders drilled more than 60 wells in Greece's onshore and offshore areas.
Oceanic Exploration of Denver, exploring offshore, discovered South Kavala gas field and Prinos oil field in the North Aegean Sea during 1972-74.
Following this first ever commercial discovery of hydrocarbons in Greece, Public Petroleum Corp. of Greece (DEP SA, later to be DEP-EKY SA), the state's own exploration company, was created in 1975.
Geological and geophysical work and drilling conducted by DEP-EKY during the past 20 years led to the discovery of Katakolon oil field and Epanomi gas field. Numerous structural and stratigraphic trapping possibilities and good quality source rocks, reservoirs, and seals were also identified.
Greece's geological provinces subdivided into the so called "external" nonmetamorphic zones were developed mainly in Western Greece and the "internal" metamorphic zones in Eastern Greece, where later on post orogenic Tertiary basins formed (Fig. 1) (105782 bytes).
New opportunities for international oil companies to explore for hydrocarbons in Greece will emerge shortly. Parliament ratified a new petroleum law in January 1995, and DEP-EKY SA will undertake an international licensing round for offshore-onshore areas mainly in western Greece during second half 1995.
WESTERN GREECE: FOLD AND
THRUST BELT
Greece belongs to the Alpine orogenic system of the peri-Adriatic chains that includes the Dinarides, the Southern Alps, and the Appenines.
The External zones are part of a chain system derived from the compression of the sedimentary cover at the margin of the Apulian plate.
The structural history begins in the Mesozoic (Middle Triassic) when extensional fault systems originated different isopic zones, i.e. carbonate shallow water platforms (Gavrovo and Apulia zones) and pelagic deepwater basins (Pindos and Ionian zones).
The tectonic regime changed from extensional to a compressional one with the collision of the Apulian and Eurasia plates in the Latest Cretaceous time.
A continuing collision during the Tertiary affected the eastern margin of Apulia and later a Neogene subduction event affected basins left untouched by the collision.
According to the adopted model, folds and thrusts essentially originated by detachment of the sedimentary sequence, mainly from a level corresponding to the Triassic evaporates (Fig. 2) (66816 bytes).
Halokinesis following the compression could have generated narrow diapiric structures. The stratigraphy includes Triassic evaporates, Triassic to Eocene carbonates, Tertiary flysch, and Neogene marine and continental clastic sediments (Fig. 3) (72558 bytes).
Surface oil seeps and subsurface oil shows are abundant in Pre-Apulian, Ionian, and Gavrovo zones in western Greece. Potential source rocks for oil generation have been discovered in Ionian and Pre-Apulian zones. Calcareous shales of Lower Cretaceous age in the internal Ionian zone and Possidonian shales of Lower to Middle Jurassic age in the central and external Ionian zone have very good potential for oil generation. Good oil generation potential also exists in Middle Jurassic formations of the pre-Apulian zone (Fig. 4) (45048 bytes).
Oil that could not be attributed to the above source rocks is suspected to have an origin from Triassic formations that in Albania and Italy contain potential source rocks.
Traps include stratigraphic features in Miocene sands, Mesozoic or Eocene carbonates covered by flysch or Neogene clastics and salt diapirs. Potential for structural trapping is expected in the folded thrust belt area (anticlines, thrust faults, blind thrusts).
Deep structures below the main detachment level of Triassic evaporates not yet explored, are considered to have promising hydrocarbon potential.
Exploration of the present as well as future targets requires that we understand better the kinematic history, dynamics, and evolution of this particular thrust belt area by using new "mobilistic" models and more effective seismic techniques.
KATAKOLON OIL FIELD
The relatively limited hydrocarbon exploration that has taken place onshore and offshore in western Greece resulted in the discovery of offshore West Katakolon field.
This field is situated 3.5 km southwest of Katakolon peninsula off western Peloponessus. The producing horizon is the Eocene-Cretaceous carbonates of a paleostructure, unconformably covered by clastic Neogene sediments (Fig. 5) (94147 bytes).
The first deviated well confirmed presence of oil and sidetracked to a higher position tested gas and condensate from two zones. Another delineation well on the same structure tested oil again from two layers (flow rates 1,200-1,500 b/d each), confirming an oil zone of approximately 100 in thickness.
EASTERN GREECE:
THE TERTIARY BASINS
In eastern Greece exploration is oriented towards the post orogenic Paleogene and younger Neogene basins. The main tectonic regime that controlled their evolution was extensional. The stratigraphy includes Eocene reefal limestones, thick Eocene to Oligocene marine elastic sediments, and Neogene terrigenous deposits with extended Messinian evaporates (Fig. 6) (33535 bytes).
Potential source rocks for gas and oil generation have been discovered in Eocene and Miocene sediments (Fig. 4) (45048 bytes). Traps include rollover anticlines, faulted structures, and stratigraphic features in Eocene reefal limestones, Eocene Oligocene sandstones, and Neogene sands. Trapping potential also exists in fractured Mesozoic formations (Epanomi gas field).
Prinos oil field in the Northern Aegean Sea proves the substantial exploration potential of the Neogene evaporitic basins, and the recent discovery of Epanomi gas field has increased the opportunities for successful exploration in Paleogene basins.
PRINOS FIELD
Miocene sands, capped by thick salt and evaporite sections, form the reservoir for Prinos field.
Source rocks are considered to be marine shales of Upper Miocene age. The Prinos structure is a roll-over anticline bounded by sealing faults which dip towards the center of the basin. During the first 13 years of production more than 90 million bbl of oil have been produced.
PRINOS NORTH
Early in 1994 the Prinos North-2 well tested the Prinos North structure (Kavala Bay, North Aegean Sea) and flowed 3,560 b/d from two zones.
It is believed that Prinos North field, situated 2.5 km from Prinos field, can be exploited at low cost in conjunction with Prinos field, taking advantage of the existing facilities and infrastructure created for Prinos field in the early 1980s.
It is expected that production from Prinos North will enhance the declining production from Prinos field in the coming years.
EPANOMI GAS FIELD
Epanomi gas field in the Thessaloniki area was discovered in 1988 by the well Epanomi-1 (EP-1) at 2,605 in depth. In 1989 the Epanomi-2 (EP-2) well, drilled in a smaller feature of the same structure, gave a maximum production of 19 MMcfd of gas and small quantities of light oil. The structure is formed by the paleoerosional surface of Mesozoic limestones buried below Tertiary clastic sediments (Fig. 7) (139834 bytes).
Distal turbidite facies of Upper Eocene-Lower Oligocene age are the excellent cap rock of the field.
Source rocks were found at the lower part of the Eocene-Oligocene sequence in adjacent areas. The hydrocarbons migrated laterally into the reservoir from deeper parts of the Thermaikos basin.
The reservoir is composed of shallow platform carbonates of Upper Jurassic-Lower Cretaceous age with very low matrix porosity and locally of thin Eocene reefal limestones.
The limestones are highly fractured. Fractures, faulted zones, and karsts provide the essential effective porosity and permeability.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.
