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Geological Quarterly, 2015, 59 (3): 479–490DOI: http://dx.doi.org/10.7306/gq.1219

The unconventional hydrocarbon resources of Greece

Ananias TSIRAMBIDES1, *

1 School of Geology, Aristotle University of Thessaloniki, Department of Mineralogy-Petrology-Economic Geology, Egnatias131, 54124 Thessaloniki, Greece

Tsirambides, A., 2015. The unconventional hydrocarbon resources of Greece. Geological Quarterly, 59 (3): 479–490, doi:

10.7306/gq.1219Intensive exploration of probable conventional hydrocarbon reservoirs in Greece is taking place, through the interpretation ofseismic profiles and of abundant surface geological data. The unconventional hydrocarbon potential of the country is un-known, as detailed inves ti gations are lacking. The most im portant rock formations which may contain shale gas are found inthe land and offshore basins of northeastern, north-central, and western Greece. A re-evaluation of the data from all bore-holes is needed, on the basis of new in formation, with the aim of identi fying possible reserves of un conventional hydrocar-bons retained in highly compacted fine-grained strata. Methane hydrates have been de tected in the submarine AnaximanderMountains, east of Rhodes Island. They cover an area of about 46 km2 and the volume of methane is es timated at 2.6–6.4 tril-lion m3. The low content of the Greek lignites in gaseous hydrocarbons and the widespread tectonics in the Hellenic Penin-sula are the main factors which prevented large gas accumulations in its 14 main coal deposits. However, additionalresearch is needed to eval uate the coal-bed gas potential of the country.

Key words: shale gas, gas hydrates, coal bed gas, Greece.

INTRODUCTION

Greece, in relation to many other countries of sim ilar size,possesses very abundant re serves of energy mineral raw ma-terials, such as lignite. However, the fu ture of lignite in Greeceis un certain. Further ex ploration is needed, new and improvedmining methods must be applied, beneficiation methods mustbe implemented, and un derground min ing must be initiated(Koukouzas et al., 1997; Tsirambides and Filippidis, 2012).

The probable and proven re serves of almost all the hydro-carbon resources of Greece are unknown, as detailed investi-gations (e.g., boreholes, measurements, analyses, etc.) arelacking. The USGS (U.S. Geological Survey), the IODP (Inte-grated Ocean Drilling Pro ject), the Geophysical Institute of France (FIGR), and the Norwegian seismic survey companiesTGS-Nor and PGS have made in the last 40 years numerousexplorations and drilled many boreholes in the international wa-ters of the eastern Mediter ranean (and hence within the GreekExclusive Economic Zone – EEZ). According to their publishedreports, the oil reserves of Greece may be in the order of tens ofbillions of barrels, and its natural gas re serves of tens of tril lionsof m3 (Tsirambides and Filippidis, 2012).

The total value of the fossil fuel reserves of Greece is €1,417 billion, of which € 268 billion belong to the lignites whichwere exploited for decades to produce only electric ity. The esti-

mated oil reserves are 10 billion barrels, with a current gross

value of € 719 billion, and the natural gas re serves are 3.5 tril-lion m3 with a cur rent gross value of € 430 billion (Tsirambidesand Filippidis, 2012).

Nowadays, reserves of conventional oil and gas, which canbe pro duced cheaply, are lim ited. This is why many of the ma jor oil com panies invest in what we commonly call “uncon ventionalresources”. Large re serves of such hydrocarbons, re tained inlayered and highly-compacted de posits, exist in many coun-tries. The main types are: tight gas, coal-bed methane, shalegas, shale oil, heavy oil/tar sands, and methane (gas) hydrates.

European shale gas re sources are estimated at 16 tril lionm3, com pared to 20 trillion m3 for the US. However, it will takemore time for explora tion to find out how much can be ex tractedat a reasonable cost. Europe could start pro ducing gas fromshale in the next 3–4 years.

Shale gas deposits are possible in all ar eas where shales or other fine-grained and highly-com pacted strata exist be low thegroundwater level, with significant extent and organic matter content >1%. During their geological history, they were formedunder conditions corresponding to the gas window (generally todepths over 5,000 m).

The sedimentation of dark, organic-rich deposits in basins ismainly controlled by cli ma tic changes and changes in the car-bonate-compensation depth. In addition, basin morphology, in-put of terrestrial organic material, and local volcanic activity areother significant factors controlling the deposition of these sedi-ments. Jurassic to Lower Cretaceous black, organic-rich peliticdeposits are considered targets for unconventional hydrocar-bon exploration in the Outer Carpathians and ad jacent part of

the Alps. These anoxic or poorly-oxygenated de posits (averagetotal organic carbon [TOC] is 2.5 wt.%) were laid down in indi-

* E-mail: [email protected]

Received: November 4, 2014; accepted: February 16, 2015; firstpublished online: March 26, 2015



vid ual bas ins over a per iod of 30 to 50 Myr, and their thick nessreached hundreds of metres (Œl¹czka et al., 2014).

The most important Juras sic to Neogene rock formationswhich may contain shale gas are found in the land and offshorebasins of northeastern (e.g., Evros Delta-Orestias, Lemnos Is-land, Kavala-Prinos-Nestos Delta, Epanomi-Thermaikos Gulf),

north-central (e.g., Mesohellenic), and western Greece (e.g.,Diapondia islands, Preveza, Zakynthos Is land, Peloponnesusbasins). Gas hydrates are found in the submarine AnaximanderMoun tains, east of Rhodes Is land. They cover an area of about46 km2 and the vol ume of methane is 2.6–6.4 trillion m3. Withapproximately 43 active open-cast coal mines, Greece holdsthe 2nd po sition in the EU and 11th worldwide in the productionof coal (only lignite). The exten sive tecton ics across the wholeHellenic Peninsula and the low content in gaseous hydrocar-bons of the Greek lignites are the main fac tors which preventedlarge gas accumulations in the coal deposits. Additional re-search to evaluate the coal-bed gas po tential is needed(Papanicolaou et al., 2005; Lykousis et al., 2009; Tsirambidesand Filippidis, 2012).

GEOTECTONIC SETTING OF GREECE

The Hellenic Peninsula consists of three orogenic belts:1 – the Cim merian inter nal belt, cre ated in pre-Late Jurassic

time as a result of the col lision of northwards drifted Cim -merian continental fragments with Eurasia;

2 – the Al pine orogenic belt, created in the Creta-ceous –Paleogene after the Neo-Tethyan subduction be-neath the Cimmerian-Eur asian plate and the col lision of the Apulian microplate with this plate;

3 – the Mesogean orogenic belt along the External Hellenic Arc, which re sulted from the underplating of the Meso-gean- African plate beneath the Alpine-Cim merian-Eur -asian plate in the Mio cene–Pliocene.

Paleogene Al pine col lisions formed the nappe pile of theHellenic orogen. Late orogenic extension fol lowed the litho -sphere thick ening and caused the collapse and crustal thin ningwith exhuma tion of the lower crustal units as meta morphic corecomplexes in Crete, southern Peloponnesus, and theCyclades. In addition, a series of tectonic windows in conti nen-tal Greece (e.g., the windows of Olympus, Ossa, Rizomata andKrania) wer e formed, con sist ing of Me sozoic and Paleogeneneritic carbo nates (Mountrakis, 2006).

Northern Greece is dom inated by the Alpine orogenic evo-lution, the ma jor events of which involve (Papanikolaou, 1980):

1. The pre-Alpine zones (e.g., Rhodope and Serbomace -donian) of Pa leozoic, which comprise the internal part of the

Hellenic Arc and there fore are called the Inter nal Hellenides.They consist of high-grade metamorphic rocks (e.g., migmati-tes, gneisses, amphibolites and marbles);

2. The palaeo-Alpine orogenic characteristics are represen -ted by large segments of the Tethyan oceanic crust of Late Ju-rassic–Early Cretaceous age (mainly ophiolites and pelagic de-posits), which were overthrusted on to pre-Alpine and Al pinerocks;

3. The Alpine zones (e.g., Ionian, Gavrovo, Pindos, Sub- Pelagonian, Pelagonian, Axios, Circum Rhodope) which com -prise the ex ter nal part of the Hellenic Arc and there fore arecalled the Ex ternal Hellenides. They are composed of Me so-zoic and Ce nozoic rock units (e.g., flysch, neritic and pelagiclimestones, dolomites and their low metamorphic de riv atives,such as cr ystalline limestones, schists, phyllites and quart -

zites). The Alpine orog eny started in the Late Eocene, culmi-nated during the Oligocene and Mio cene, and continues today;

4. The post-Alpine for ma tions, which consist of sedimentaryand volcanic rocks of Mio cene, Plio cene and Quaternary age,rest un conformably on Alpine rocks. The extended molasse de-posits, occurring across the Hellenides, are con sidered to be re -

lated to Alpine back-arc bas ins, rather than to post-Alpine str uc-tures. These bas ins are shown in Figure 1 (IGMEM, 2012).

Northern Greece is a complex of grabens, basins, andhorsts, ar ranged in a primary NW–SE trend. Intermountainoushigh-level grabens (e.g., Ptolemais-Florina) or low-level gra -bens (e.g., Mygdonia) and large coastal basins and plains (e.g.,

Axios-Thermaikos, Serres-Strymon) were filled with Paleogeneto Quaternary sedimentary deposits (Psilovikos et al., 1987).Sediment thicknesses vary depending on palaeotopography,lo cal erosion events, and local tec tonics.

Western Greece is dom inated by the Exter nal Hellenidesfold-and-thrust belt, namely the pre-Apulian (Paxi), Ionian andGavrovo zones. From the Tri as sic to Late Creta ceous, westernGreece was part of the Apulian continental block on the south-ern passive margin of the Tethys Ocean. Little is known aboutthe pre-Mesozoic evolution of western Greece. Mesozoic rocksare rarely exposed at the sur face. The oldest known lithologycomprises Triassic evaporites, which are heavily tectonizedand dominated by anhydrite, gypsum, and ha lite in some bore-holes. The thick ness of the evaporites, the lithology underneaththem, and the tectonic movements in western Greece are cru -cial factors in the hydrocarbon explora tion within this re gion(Marnelis et al., 2007; Karakitsios, 2013).

The Ionian Zone is bounded by the pre-Apulian Zone in thewest, and by the Gavrovo Zone in the east. In the Early Mio-cene, ma jor compressional events affected the previous stra-tigraphy and structural style, due to the westwards merging of the Ionian Zone. In the Pliocene and Qua ternary, continued re-gional compression resulted in uplift of the Ionian Zone. De -

tailed examination of many boreholes which penetrated theIonian Zone show that the thick ness of its Me sozoic rock for -mations varies from 1.2 to 3.8 km in the north, from 1.1 to 3.5km in the centre, and from 1.5 to 3.9 km in the south. However,data from seismic sections show that the thicknesses of theseMesozoic formations may reach 8 km (Zelilidis et al., 2003;Mavromatidis, 2009).

CONVENTIONAL HYDROCARBONS

Today, no suffi cient data exists to substan tiate the presenceof proven oil reserves in Greece, except that of Thassos, thedaily output of which has been limited to 2,000 barrels. Hypo-

thet ically, however, we can accept the presence of oil and natu-ral gas, since all the neigh bouring countries of Greece in theeastern Mediterranean host substan tial oil and gas fields, andhave exploited these resources for decades. The last con fir ma-tion of this hypoth esis is the agreement in 2010 on the delim ita -tion of the EEZ between Israel and Cyprus.

Encouraging evidence for the existence of oil are in Kata-kolo Peloponnesus, in the huge basin of the Ionian Sea and inthe two basins SW (Syrti Basin) and SE (Herodotus Basin) of Crete. Also, in all the Ionian islands, on the western main landcoast, in the basin of Grevena, Gulf of Thermaikos, Chalkidiki,in Stavros and Maronia Rhodope, Tavri Evros, in the islets of Babouras Thassos and Zourafa Samothrace, and in the islandsof Lemnos, Lesvos, Ikaria, Skopelos, and SE Cyclades(Tsirambides and Filippidis, 2012).

480 Ananias Tsirambides



Except for the exploitation of gas from Thassos, which is al -most depleted, other large reserves may be found in the IonianSea and in the two basins SW (Syrti Basin) and SE (HerodotusBasin) of Crete. Smaller gas deposits have been located inBabouras Thassos and Epanomi Thessaloniki. The former En -ergy Direc tor of the Cyprus Republic, S. Kasinis (pers. comm.,2012), estimates that 70% of the world gas re serves exist in the

huge basin of the Mediterranean Sea and the surround ing coun-tries. At least 5% of it be longs to Greece (Tsirambides andFilippidis, 2012).

Accretionary prisms throughout the world (e.g., Barbados,Makran, Irrawaddy-Andaman, West Timor Trough, etc.) are in-dicative of giant hydrocarbon deposits. A large number of accretionary prisms are encountered within the Mediter raneanRidge (MR), the prin cipal structure in the east ern Med iter ra-nean Sea. Hence, nowadays, the existence of large to gi ant hy-drocarbon deposits is the sub ject of intense exploratory re-search within this ridge (Foscolos et al., 2012). The MR stre-tches from the Calabrian Rise in the west to the Flor ence Risein the east, a dis tance of about 1,500 km. The ridge widthranges from 150 to 300 km, and it has an arcuate southward

convex shape, al most parallel to the Hellenic Arc. The av er agewater depth on the MR is about 2.1–2.2 km, varying between1,200 m and more than 3,000 m. The ridge rises 1–2 km abovethe sea-floor, but because of its large width, the general sloperarely exceeds 20° (Le Pichon et al., 1982).

Underneath Crete, in the region where the Afr i can plate issubducting, there are many active mud flow volca noes, pock-

marks, and pipenecks, which have been emitting methane for thousands of years. As a re sult, a thorough investi gation couldreveal giant oil fields. Geochemical analysis of emitted methanebubbles from active mud-flow volcanoes indicates that their ori-gin is thermogenic. This im plies that pyrolysis (ther mal crack-ing) of hydrocarbons takes place at depth where tem peraturesare 160 to 180°C. Based on a geother mal gra dient of 33C/1,000 m, pyrolysis should take place at the lower layers of the sea-bottom de pos its with a thick ness of at least 5,000 m.Hence, working petroleum systems southern of Crete may beencountered at such depths (Foscolos et al., 2012).

In the Ionian and Paxi zones, a significant pres ence of methane gases has been lo cated, mainly of biogenic origin.These gases are hosted in res ervoirs with coarse-grained Plio-

The unconventional hydrocarbon resources of Greece 481

Fig. 1. Sedimentary basins of Greece (in light green colour)under exploration; in yellow colour areas with insignificant hydrocarbon potential (IGMEM, 2012)



cene deposits, with poros ity of 22–30%. Other catagenic gasesare trapped in reservoirs with flysch or carbonate deposits of Eocene and Me sozoic age (Karakitsios, 2013).

A great number of oil shows on surface are known in west -ern Greece, mainly in the Ionian Zone, but also in the pre- Apulian and Gavrovo zones (Fig. 2). Most of the wells drilled inthe area have also identified oil shows. Their presence indi-cates that oil gen er ation and mi gration have certainly takenplace. Fur ther more, by their study, the petroleum systems of western Greece can be con strained. Most surface oil shows arebiodegraded (Rigakis et al., 2007).

Last year’s extensive exploration, especially offshore from

Greece, gave encouraging results. The important hydrocarbonsystems of Greece are being in ves tigated in detail through the

interpretation of seismic profiles and the abundant surface geo-logical data. In December 2014, consortium companies weregranted permis sion by the Greek par lia ment for hydrocar bonexploration and exploitation in the areas (YPEKA, 2012b):

a – gulf of Patraikos (west) (off shore) with reserves of about 200 million barrels;b – Katakolo (offshore) with reserves of about 7 mil lionbarrels;c – Ioannina (onshore) with reserves of about 80 mil lionbarrels.

Seismic research data acquisition of non-exclusive usewithin the marine ar eas of the Ionian Sea and south of Crete Is -land were held by the Norwegian Pe troleum Geo-Ser vices(PGS; YPEKA, 2011a, b, 2012a). The research was completed


Fig. 2. Geological sketch map of western Greece, indicating the Alpine zones with their flysch, the post-Alpine de posits, the boreholes, and the principal oil show loca tions (Karakitsios, 2013)



in April 2013 and the data process ing in Decem ber 2013. TheNo tice from the Government of the Greek Republic for grant ingand us ing li censes for the prospection, exploration, and produc-tion of hydro car bons was published in the offi cial jour nal of theEU in Novem ber 2014. The sec ond round of conces sions of thetwenty marine blocks ends in July 2015 (YPEKA, 2014).

UNCONVENTIONAL HYDROCARBONS

SHALE GAS

In Greece, promising oil and gas shale reservoirs ex ist,which must be added to its hydrocarbon potential. However, thepossible traps need verification, predominantly by modernseismic methods.

The Ionian and pre-Apulian (Paxi) zones of western Greececontain fine-grained source rocks, found within the gas window,from which shale gas may be produced. The shale layers areintercalated within the subsurface evaporites of the Ionian (Tri-

assic) and pre-Apulian (Triassic–Early Jurassic) zones, respec-tively. The Gavrovo and Ionian zones of western Greece con-tain significant underground im ma ture – or close to the earlymat uration stage – source rocks, with black shales rich in or-ganic con tent (Fig. 3). Some of them have suf ficient thick -nesses and are located be low 1,500 m depth. They can be arti -

ficially converted by in situ heating (pyrolysis), generally to350–500°C so that most of the oil can be gen erated from thecontained kerogen. The promising areas where shale gasesand oils exist need systematic and de tailed seismic in vestiga-tion (Kafousia et al., 2011; Karakitsios, 2013).

In west ern Greece, si liceous facies are widely associatedwith organic carbon-rich deposits. The Pre-Apulian Zone con -sists mostly of Tri assic to Miocene mixed neritic-pelagic car -bonates. Hydrocarbon source rocks include pelagic depositsrich in ma rine organic material, although terrigenous or ganicmat ter is also found in the clastic depos its. The Ionian Zoneconsists of sedimentary rocks, ranging from Triassic evaporitesto Jurassic–Upper Eocene carbonates and minor cherts andshales, overlain by Oligocene flysch. Organic-rich intervals oc-cur within Tri as sic evaporites and Jurassic–Cretaceous pelagic


Fig. 3. Geological sketch map of central Greece where sites of black shales are shown

Mainly they belong to the Meso zoic Era (modified from Kafousia et al., 2011); Ionian Zone sites:1 – Gotzikas, 2 – Chionistra, 3 – Lithino, 4 – Petoussi, 5 – Toka, 6 – Poros; Olonos-Pindos sites:

7 – Sirako, 8 – Kalarites, 9 – Proussos, 10 – Panetoliko, 11 – Livartzi, 12 – Kastelli



clayey deposits. The Gavrovo Zone was a shallow-water plat-form during the Triassic to Late Eocene, from which no or ganicmat ter-rich deposits have so far been found (Karakitsios, 2013)

The most important lo cations with Jurassic to Neogeneformations which host conventional hydrocarbons are shownin Figure 1. The same lo cations may be considered promis ing

areas for the dis cov ery of shale oil and gas. The basic geolog i-cal parameters characterizing these areas are presented inTable 1.

NORTHEASTERN GREECE BASINS

Evros Delta-Orestias. The rock formations of thesesub-bas ins in Thrace Region consist mainly of siliciclastic de-posits (e.g., conglomerates, sandstones, siltstones) of Oligo-cene to Pleistocene times, with char acter is tics of shallow ma-rine and/or deltaic depositional environ ments. In the up per lay-ers of both sub-basins, lig nites and shales rich in organic mate-rial, meaning a change in the depositional environment to sa-line, are found. In both sub-bas ins, exploratory boreholes for hydrocarbon detec tion were carried out. The depth of the bore-holes ranges from 2,100 to 3,700 m. In all the boreholes,biogenic methane was de tected at shallow depths.

At greater depths (2,170 to 3,050 m) of the Orestias sub-ba-sin, gaseous hydrocarbons of catagenic or igin, trapped inOligocene sandstones, were detected. The upper limit of the oilwindow in this ba sin is located at 2,100 m depth. In the Evros

Delta sub-basin, the pres ence of methane of both biogenic andmetagenic origin was detected. It is located within Oligocene toPliocene lignite units. The geother mal gra dient in this basin,3C/100 m, is lower than that of the other sedimentary basins of eastern Greece (>5C/100 m), due to the extended magmaticintrusions of the Paleogene. The oil window is re stricted to the

in terval 1,200–2,400 m (Rigakis et al., 2001; IGMEM, 2012).In ad jacent Eu ropean Tur key (eastern Thrace), the exploi-

tation of shale gas is pending.Lemnos Island. This is a unique site in the north Aegean

Sea, with Paleogene formations. The creation of Lemnos Basinis placed in the Mid dle Eocene, when it received the first sed i-ments of this age. The depositional environment was initially ter -restrial and then shallow ma rine, with depos its of nummulit iclimestone, which evolved to pelagic. During the Late Eocene toEarly Oligocene, an ad vancing clastic system de veloped with de -posits of typical continental slope. The depos ited sediments ar ecomprised of thick-bedded sandstones and conglomerates, withinter ca lations of silts and clays in the form of turbidites. South of Lemnos, a borehole (LE-1, 2,250 m depth) was drilled and thedeposits drilled reached Mid dle Eocene (IGMEM, 2012).

Shale and clay samples from the Late Eocene to EarlyOligocene submarine fans, slope, and shelf of Lemnos Is landpresent poor to excellent source-rock gas potential. In partic u-lar, the shales can be classified prin cipally as secondary sourcerocks with great potential to gen erate gas. The TOC rangesfrom 0.01 to 5.04% with an average value of 1.12%. The or-ganic ma terial in most of the samples is composed of type III


T a b l e 1

Basic geological parameters characterizing the promising areas of Greece for the discovery of shale gas and shale oil*

Sedimentary

basinSource rocks age Lithology OM

type

TOC

[wt.%]

Burial

depth [m]

Source rocksnet thickness

[m]

Thermalmaturity

[Ro]HP

EvrosDelta-Orestias

Pliocene–PleistoceneOligocene–Miocene

shalessandstones-siltstones

n.d.n.d.

n.d.n.d.

2,7503,700

n.d.n.d.

n.d.n.d.

gasgas

LemnosIsland U. Eocene–L. Oligocene shales-clays III to IV 0.1–5.0 2,250 n.d. n.d. gas

Kavala-Prinos –Nestos Delta M.–U. Miocene claystones II to III 1.2–3.9 3,500 10 1.10 oil-gas

Epanomi–ThermaikosGulf

Eocene–Oligocene shales I to II n.d. 2,100 n.d. n.d. gas

MesohellenicBasin

Oligocene–Miocene:Eptachori–Pentalophos

Form.Eocene–Oligocene:Krania–Eptachori

Formation

sand stones with shale beds

sandstones with shalebeds

II to III

III

0.4–5.0

0.2–1.5

3,200

4,200

n.d.

n.d.

0.3-0.6

0.3-0.6

gas-oil

gas-oil

WesternGreece(Ionian Zone)

Pliocene–PleistoceneU. Miocene–L. Pliocene1

L. Oligocene–M. Miocene2

U. Oligocene–M. MioceneL.–U. CretaceousM.–U. Jurassic

L. JurassicTrias sic

sandstones with clay bedsmudstonesmudstones

mudstoneslimestones with shale beds

cherty claysmarls-marly lime stones

breccias with shale beds

I to IIII to III

III

I to III to III to III to II

I

n.d.0.2–9.2

0.6–32.5

n.d.0.9–5.01.0–3.3

1.0–19.10.5–16.1

2,000n.d.n.d.

n.d.n.d.n.d.n.d.n.d.

n.d.n.d.n.d.

n.d.10–6510–14010–150

<1

0.6–1.0n.d.n.d.

0.6–1.00.6–1.00.6–1.00.6–1.00.6–1.0

gasgas-oil

oil

gasgasgasgas

oil-gas

WesternGreecePre-Apulian(Paxi) Zone

Miocene–PlioceneU. Jurassic

L. Jurassic (4 horizons)

marls-clastic sedimentsmarly limestones

marls-marly lime stones

IIII to II

II to III

n.d.0.6–11.20.6 av.

1,850–3,2601,850–3,250

n.d.>200n.d.

1.0–2.0

0.6–1.0

0.6–2.0

oil

gas

gas-oil

*– the rel ative ref erences are presented in the text; OM – Organic Matter, TOC – Total Organic Carbon, HP – Hydrocarbon Prospectivity, n.d. –

no data, 1 – Zakynthos Is land, 2 – Diapondia islands



kerogen, which is suitable for gas production. Organic matter composed of type IV kerogen has been de tected in a few sam-ples. Only one sample is composed of type I kerogen, which issuitable for oil production. The shales and clays stud ied aremainly of the im ma ture oil stage. Additional geo chem ical mea -surements, such as spore-colour index and determination of

biomarker parameters, are required (Maravelis, 2009).Kavala-Prinos-Nestos Delta. The development of this ba -

sin started during the Mio cene, when terrestrial de pos its (e.g.,pebbles, sands, silts and clays) wer e deposited in a re strictedhyper-saline environment. Organic materials, intercalated withclastics and car bonates, followed. They were overlain by turbi-dites that cor respond to the hydrocarbon reservoirs, and theseby thick evaporite layers of the Messinian, due to the dry climateand isolation of the basin. The sedimentation continued withclastic marine deposits during the Pliocene-Holocene. The con-tinuous sedimentation and gradual subsidence of the basin re-sulted in a thick sediment formation of about 5.8 km. The deep-est parts of the basin r eached ad equate thermal matu rity for hy-drocarbon generation (Pollak, 1979; Proedrou, 1979, 1988;Georgakopoulos, 1998, 2000).

Marine claystones of the Middle to Upper Miocene, depos-ited un der highly reducing conditions and in ter rupted by hyper-saline episodes, are consid ered the source rocks of oil and gasin this ba sin (Fig. 4). The claystones are character ized by awaxy sapropelic oil-prone kerogen with terrestrially-derived or-ganic mat ter frequently dominant. The organic matter be longsto the types II and III of kerogen. TOC val ues vary from 1.20 to3.90%. Maturity measurements show vitrinite reflectance up to1.10 for depths up to 3,500 m (Proedrou and Papaconstan -tinou, 2004).

Since 1981, the exploitation of oil and gas has taken placehere. In the southern part of the taphrogenic basin (area of Prinos), the gas deposit is hosted in fine- to coarse-grainedsandstones and conglomerates alternating with clays and

marls. The deposit is inter calated between two evaporitic hori-zons, and is found at a depth of 1,660–1,720 m, with net thick-ness 10 m, porosity ranging from 10 to 24%, and transmit tanceup to 400 md. The in place re serves are estimated at 1 billionm3. The production until 2010 was 0.9 bil lion m3 of gas and120 mil lion barrels of oil. Gases are also detected in the oil wellsof the Prinos oil field. The natural gas of the Prinos sub-ba sin istoo mature, as it is concluded by its high H2S content and isoto-pic data. The gas in the south Kavala sub-basin is less ma turethan that of Prinos. The gases from both sites are of catagenicorigin (Rigakis et al., 2001; IGMEM, 2012).

Three deep wells, drilled in the Nestos Delta sub-basin for oil exploration, identified very promising gas horizons (from1,820 to 2,770 m) of catagenic origin within sand stone layers of Mio cene times. At shallow depths the gas is methane, obvi-

ously of biogenic or igin. Most of this gas is related to lignite hori-zons of significant thicknesses (Rigakis et al., 2001).

A detailed analysis of samples (especially fine-grainedsandstones and shales) of Miocene age, selected from bore -holes and cut tings (1,780–3,550 m depth) of the Nestos River Delta, showed that these depos its have been sub jected to lowtemperatures (65–110C) and that the gaseous and liq uid hy-dr ocarbons in them are lim ited. The low hydrocarbon con tent isdue to (Tsirambides and Soldatos, 1989):

– the limited presence of fine-grained clastic deposits,which are con sidered source rocks for the formation of hydrocarbons;

– the migration of these products to ad jacent reservoirswith clastic deposits of significant porosity (e.g., the

Kavala-Prinos Basin), because of the intense tec tonicactiv ity in the area during the Neo gene and Quater nary.

Epanomi-Thermaikos Gulf . In the Epanomi sub-basin,two exploration bore holes (depth 2,470–2,800 m) carried out onland detected a small gas field, which during the tests gave amaximum gas production of 19 × 106 cf/day and a small quan-

tity of light oil in deeper forma tions. The gas is trapped in Upper Jurassic to Lower Cretaceous lime stone with 1% average po-rosity. The chemical composition (wt.%) of this gas is (Roussos,1993):

The Epanomi gas is mainly of catagenic origin and may becharacter ized as wet gas, since it contains a significant percent -age of C2 + compounds (3.2–4.0%). The minimum re serves inplace are 500 × 106 m3 (Roussos, 1993). The deposit has adual or igin: one source may be considered the Eocene shalesof the ad jacent Posidi area (POS-1 borehole) about 40 km SEof Epanomi. The organic mat ter in these shales de rives from al -gae and higher plants, so this gas is consid ered of catagenic or-igin. A second source is considered to have ascended fromdeeper lev els, as shown by the high level of matu rity of or ganicmat ter con tained therein. This gas is of thermogenic origin. Thegas field of Epanomi is the first discov ery of such a gas de positin the Paleogene land basins of northern Greece. Its rel ativelysmall size is mainly due to the very low porosity of the res ervoir (Roussos, 1993; Rigakis et al., 2001).

In the Thermaikos Gulf sub-basin, two boreholes (depth1,180–3,500 m) were drilled and very signifi cant gas accumula-tions were de tected. At shallow depths, dry gas of biogenic ori-gin was identified in sand stone layers. This gas is related to lig-nite units of Miocene to Pleistocene age. At deeper layers, wet

gas of catagenic or igin was de tected and is related to Eocene toMiocene lignites. This categenic gas has a similar compositionand probably the same source as the Epanomi gas. The identi -fication of source rocks, the certif ication of maturity, and the po -tential of these rocks for the production of gaseous and liq uidhydrocarbons, as well as the fa vourable conditions of their mi-gration, directed the investigations to identify suitable traps inthe lower members of the molasse deposits of this basin. Thesupply of the gas deposit oc curs later ally in the deeper parts of the basin from Eocene source rocks (Rigakis et al., 2001).

NORTH-CENTRAL GREECE

Mesohellenic Basin (Fig. 3). This is a piggy-back basin of

the Hellenides Orogen, located in north- central Greece. It is 130km long and 40 km wide and trends NNW–SSE. It comprisestwo depocentres, the first one in the north (3,200 m thick), andthe sec ond in the south (4,200 m thick). The basin developedfrom the Late Eocene to Late Mio cene and is filled with coarse-to fine-grained deposits. The sedimentary sequences consistmainly of submarine fan (sandstone and shale) and deltaic de-posits, which accumulated unconformably over an ophiolitecomplex. The hydrocarbon potential is indicated by the pres -ence of kerogen types II/III with minor amounts of type I. The ev-idence is mostly for wet and dry gas, with minor oil. Sourcerocks are the Mid dle Eocene to Lower Oligocene Krania andEptachori for mations, of up to 2,000 m in total thickness, whichreached maturation during the Early Miocene. The source rocks


CO2 N2 C1 C2 C3 iC4 nC4 iC5 nC5 C6+

23 7.2 66.2 1.9 0.8 0.2 0.2 0.1 0.1 0.3



consist of outer fan and basin plain deposits. They are conform -ably overlain by the lower mem ber (Late Oligocene) of the up to2,600 m thick Pentalophos Formation, which consists mostly of thick submarine sandstone lobes. Possible stratigraphi-cally-trapped reservoirs include the lower member of thePentalophos For ma tion, which overlies source rocks, as well as

limestones tectonically intercalated within the ophiolite com-plex, under lying the source rocks. The main features of this ba -sin are the thermal immaturity of the con tained organic matter and the existence of limited potential source rocks, hav ingreached the very early stage of oil maturity (Kontopoulos etal., 1999; Rigakis et al., 2001; Avramidis and Zelilidis, 2007).

WESTERN GREECE BASINS

NW of Diapondia islands (north of the Borsh-Khardhiqitstrike-slip fault). This basin lies on the pre-Apulian Zone andwas formed following the Ionian thrust movements of 80 kmeastwards, close to the Albania coast. It is considered analogousto the Durres Ba sin of Al bania and is filled by depos its of great

thickness. The area potentially hosts two hydrocarbon systems:a Neogene one and, more speculatively, a Mesozoic one. TheNeogene hydrocarbon system consists of Tortonian shales thatmay comprise po tential source rocks, while Tortonian-Messiniansandstones may comprise the reservoirs. These may be sealedby Messinian evaporites (as it hap pens in the Durres Ba sin). TheMesozoic hydrocarbon system is mainly attributed to the Mid- Mesozoic hydrocarbon system in Greece and Albania, with com-parable source potential and maturation profiles. Structural trapsmay be present, while traps formed by compressive tectonic pro -cesses may also occur. Because of the continuous devel opmentof the Ionian thrust belt, the compressive tectonic re gime maylead to the de velopment of ma jor compressional anticlines(Maravelis et al., 2012).

Diapondia islands (Ionian thrust piggy-back basin).The Lower Oligocene to Middle Mio cene slope succes sion inthe west ern part of the Hel lenic Fold and Thrust Belt (FTB) onthe Diapondia islands (Ionian Sea) was studied to evaluate its

source rock potential for hydrocarbon discovery. Geochemicaldata in dicated that or ganic mat ter is present in sufficient abun -dance and in good enough quality in the drilled mudstones to beregarded as po ten tial source rock. The Rock-Eval II pyrolyticyields and the calculated val ues of hydrogen and oxygen indi-ces indicate that the organic mat ter is of type III kerogen. The

rock succes sion is immature with re spect to oil generation andhas not experienced high temperature during burial. However,its eastern downslope equiv alent deep-sea mudstone faciesshould be considered as good gas-prone source rock onshore,since they may have experienced higher thermal evo lution. Inaddition, they may have improved organic geochemical param -eters be cause the organic mat ter is not oxidized. TOC con tentis the primary parameter for source rock appraisal, with athreshold of 1% at the immature stage for potential sourcerocks. With average TOC val ues close to this lower limit(0.96%) and the occurrence of samples exceeding this thresh -old (up to 32.5%), the mudstones stud ied may be consideredpoten tial source rocks. Such TOC values indicate that sedimentemplacement occurred under conditions favourable for organicmatter preservation related to short distance transportation anddeposition in a rather low water depth (Maravelis et al., 2014).

Ionian thrust foreland basin (IFB). The basin, lying onthe pre-Apulian Zone, is asymmetri cal with maximum sub si-dence oc cur ring towards the thrust complex. Poten tial hydro-carbon source rocks in the IFB are mostly pelagic deposits richin marine organic material, although terrigenous organic mat -ter is found in the clastic deposits of Miocene to Pleisto ceneage. Hydrocarbons in this basin are present within the rock for-mations of:

– Pliocene, – Miocene, – Late Jurassic, – Middle Jurassic bituminous shales, – Tr iassic black shales (Karakitsios and Rigakis, 2007).

Porosity measurements were carried out on surface sam-ples of various forma tions in the IFB, and porosity deduced fromelectrical logs indicate moderate reservoir quality accompanied


Fig. 4. Kavala-Prinos hydrocarbon field with its two sub-basins (Kiomourtzi et al., 2008)



by low permeabilities. In pelagic facies rocks, po rosity and per-meability are even lower. The continuous development of theIonian thrust belt led to the de velopment of ma jor com -pressional, roll-over anticlines that may form large-scale struc-tural traps. In addition, tilted fault blocks may also lead to theoil/gas entrapment (Maravelis et al., 2012).

Preveza Basin. This basin was mostly formed in the pre- Apulian (Paxi) Zone and was in flu enced by the local strike- slipmovement of the Cephalonia trans form fault. The accumulateddeposits were strongly influenced by the Ionian thrust. Theycomprise the thick sedimentary formations of the Preveza Ba-sin which in crease in thickness southwards, and towards theCephalonia fault. Hydrocar bon source rocks in the Preveza Ba-sin may consti tute the Tortonian shales. Tortonian-Messiniansandstones are able to con stitute ideal reservoirs, sealed byMessinian evaporites. Struc tural traps derived from the Cepha -lonia transform fault activ ity may exist, while Pleisto cene strike- slip faults, ac commodated by thrusts and folds, may act asstructural traps too. Additionally, footwall and hanging walltraps, such as tightly-folded anticlines sealed by shale andanhydrite, related to folding and thrusting, may also oc cur (Maravelis et al., 2012).

Zakynthos Island. Organic geochemical analysis of Upper Miocene-Lower Pliocene mudstones from the southwesternedge of the Hellenic FTB (Zakynthos Is land) led to the estab-lishment of a system with gas eous source rock po tential. Thisstudy also enabled the analysis of their char acter is tics in termsof the total organic mat ter con tent, and its type, quality, and ma-turity level, to gether with the anal ysis of their generating poten-tial. The study region con tains rock for mations that are worthyof further con sid eration (TOC 0.2–9.2%). Rock-Eval values re-veal the oc cur rence of source rocks with fair to excel lent hydro-carbon-generating potential. The studied organic-rich faciesfrom one section are oil-prone, whereas the kerogen in all other sections is gas-prone. The GC-MS data, which is a direct indi-

cator of the types of hydrocarbons that can be gen er ated by thekerogen dur ing the matura tion pro cess, indicate that all the de -posits studied are gas-prone. Ma turity data suggest that thesection studied is thermally immature. Even though the rocksinves tigated have sufficient TOC con tent and SP values andcontain gas-prone organic mate rial, they have not attained ade-quate thermal matu rity. These fa cies might have ma tured suffi-ciently to enter the oil generation zone where they are moredeeply buried (Maravelis et al., 2015).

Peloponnesus basins. The area of Katakolo is built fromthe bedrock of Alpine strata of the Ionian and Gavrovo zones, ofPaleogene flysch, and of de pos its of the Neogene and Quater -nary. From bore holes car ried out on land to detect the pres enceof hydrocarbons, a small gas field was found (depth1,230–1,760 m) trapped in medium- to fine-grained sandsto -

nes. This gas will be able to meet local needs. In addition, shal-low biogenic gas ac cu mu lations in Neogene sandstones havealso been revealed. Ho rizons of lignites with high contents of or -ganic mat ter have been considered to be the source rock of thisgas. The oil reservoir is lo cated at a depth of 2,400–2,600 m(Kamberis et al., 2000; Rigakis et al., 2001).

The Pindos foreland basin in west ern Peloponnesus in-volves three narrow linear sub-basins (e.g., Tritea, Hrisovitsi,and Finikounda), the result of internal thrust ac tivity. This activ -ity produced intra-basinal base ment highs, which were cross- cut by transfer faults and created path ways for the transporta-tion of coarser material in the distal parts of these sub-bas ins.The transfer fault zones contain com plex struc tural ge om etriesthat could make them opti mal locations for structural hydro car-

bon traps and reservoirs. The Eocene-Oligocene depos its are

thicker and sand-rich on the downthrown sides of the trans fer faults. In these areas, sand stone reservoirs could be pres ent onthe mar ginal blocks or in deep troughs resulting from the ac cu-mulation of deep-water turbidites. Such tectonically active ar -eas consti tute prom is ing oil and gas reservoirs and traps(Konstantopoulos et al., 2013).

Gas eous hydrocarbons were detected in most boreholesoffshore of NW Peloponnesus. These gas con centra tions,mainly of methane, are hosted in the clastic horizons of the Up-per Neogene or in Me so zoic carbonates and are separated intotwo cat egories: dry and wet gas, possibly of mixed origins(biogenic and catagenic). The gas is found in deposits of thePlio-Pleistocene and is lo cated at shallow depths (<2,000 m).Here the geo thermal gradient var ies from 1.15C/100 m in theevaporites up to 2°C/100 m in the clastic and carbonate depos-its. The biogenic gases are associated with the presence of lig -nite horizons and they are too im mature for oil gen esis (Rigakiset al., 2001).

METHANE HYDRATES

While global esti mates vary considerably, the energy con-tent of methane occurring in hydrate form is immense, possiblyexceeding the com bined energy content of all other known fos -sil fu els. Mud volcanoes (MVs) are also known for their highmethane fluxes, ascending methane-rich fluids from the sea- floor, gas hydrates, and favourable environments for chemo -synthetic symbiotic fauna. Active MVs are associated, through -out the world, with hydrocar bon occurrences (Timor, SouthCaspian Sea, the Carib bean, Egypt and Cyprus). However, fu-ture production volumes are speculative because methane pro-duction from hydrates has not been doc umented beyondsmall-scale field experiments (Woodside et al., 1998; Aksu etal., 2009; Lykousis et al., 2009; Perissoratis et al., 2011; Fosco-

los et al., 2012).The occurrence of MVs, mud diapirs, and fluid seeps has

long been known in various different geological environments ofthe eastern Mediterranean and thus in the southern AegeanSea. They occur not only in regions of compressional stress insubduction zones (e.g., Calabrian Arc, Mediterranean Ridge,and other sub marine orogenic belts), but also on passive riftedmargins such as the Egyptian offshore and ar eas of mixed tec-tonic signature like the Anaximander Mountains. Here the mudvolcanism is related to faults with normal and transcurrent fault -ing com ponents. The accretionary prism of the Hellenic Arc(part of the Med iterranean Ridge) is an area where a large num-ber of mud volcanoes have been studied following their discov -ery in the late 1970s. MVs within the Greek Ex clusive EconomicZone (EEZ) have been emitting nat ural gas bubbles for morethan one mil lion years. The area cov ered by the gas hydrates isabout 200,000 km2 and the vol ume is roughly calculated at30 trillion m3, assuming an average thick ness of 150 m.Roughly, 1% of this volume is found in the form of methane hy-drates: that is 0.30 trillion m3. This amount should be mul tipliedby 170 m3 of nat ural gas/1 m3 of hydrate in order to equate it tothe conventional nat ural gas re serves. This im plies that in theGreek EEZ, 51 tril lion m3 of natural gas exists. This is equiv alentto 328 bil lion bar rels of oil, which is a huge amount (Foscolos etal., 2012). However, this as sump tion seems exaggerated.

The Anaximander Mountains are sit uated between the Hel -lenic and Cyprus Arcs (Fig. 5) at sea water depths of 1,500–2,500 m. Their set ting is clearly favour able for the oc cur -rence of mud volcanoes because of the presence of over-pres -

sured fluids and faults that act as conduits for the fluids to es -




cape (Ten Veen et al., 2004). Discov ered dur ing the Dutch AN AXIPROBE pro ject by multi-beam sur veying in 1995, sevenlarge mud volcanoes were sam pled in 1996 when the first gashydrate samples in the Mediter ra nean Sea were collected fromKula MV. By 2002, the Amster dam and Kazan MVs were alsodis covered to have hydrates. Between 2003 and 2005, theEC-funded pro ject ANAXIMANDER tar geted these mud vol ca-

noes in a study of gas hydrates and this was followed by theHERMES pro ject (2005–2009). Dur ing these pro jects, manygas-hydrate samples were collected and studied. Such a sam-ple from the Am ster dam mud vol cano showed the prev alenceof thermogenic light hydrocarbons, as in ferred from C1/C2+ ra -tios and isotopic data (Pape et al., 2010).


Fig. 5. Anaximander Mountains with the main mud volcanoes (MVs) (Lykousis et al., 2009)



The area cov ered by the gas hydrates at Anaximander Moun tains is about 46 km2 and the volume of methane is esti-mated at 2.6–6.4 tril lion m3. The sea bottom water temperatureis 13.7°C and the mean regional geother mal gradient 30C/km.Methane hydrates are present, usually deeper than 0.4 m fromthe seabed surface. Their texture re sembles compacted snow

and their external morphology resembles flakes or lumps or bigice crystals. Biostratigraphical analyses of mud and rock clastsindicate that they were derived from sedimentary rocks underly-ing the seabed (Up per Creta ceous lime stones, Paleocenesiliciclastic rocks, Eocene biogenic limestones, and Miocenemudstones; Lykousis et al., 2009).

COAL-BED GAS

Coal-bed gas is meth ane-rich gas occurring in self-sourcingreservoirs in un developed coal-beds and worked coal-seams.

Although the USA, Can ada, and Aus tralia are the only signifi-cant coal-bed gas pro ducers, coalmine methane is pro duced inanother 13 coal-producing countries. Coalmine gas productionhas a dual purpose: to re move methane as a safety measure,and to capture vented methane because of its severe impact asa greenhouse gas.

The quality character is tics of the 14 most important coal--basins of Greece, on an “as-received” basis, are: 9–66% mois-ture, 16-33% vol atile matter, 8–28% fixed carbon content,10–43% ash, 0.1–4% sulphur, and 4–15 MJ/kg calorific value.Methane content is <3%. The large vari ations of these con stitu-ents in the bas ins, as well as within each ba sin, are attrib uted tothe age, na ture of occurrence (multilayer deposits), environ-ment of deposition, and the geological setting (Papanicolaou etal., 2005; Tsirambides, 2005).

The low con tent in gaseous hydrocar bons and the wide-spread tecton ics across the whole Hellenic Peninsula are themain fac tors which prevented large gas ac cu mu lations in thecoal deposits. However, additional research is needed to evalu-ate the coal-bed gas po tential of the country.

CONCLUSIONS

The unconventional hydrocarbons potential of Greece isunknown as long as detailed inves tigations are lacking.

Exploration for conventional hydrocarbon reservoirs,through the inter pre tation of seismic profiles and sur face geo -logical data, will simultaneously provide the subsurface geome-tr y of the unconventional ones. Their exploration should followthat of conventional hydrocarbons.

Today, because the interest of many of the ma jor oil compa-nies is directed to unconventional sources of en ergy, Greeceneeds re-evaluation of the data from all bore holes carried out,on the basis of new information, with the aim to iden tify pos si ble

re serves of unconventional hydrocarbons retained in highlycompacted fine-grained deposits.In addition to the large fi nancial in terest in the use of these

unconventional sources of en ergy, there is currently no nationalor European Union legislative frame work which deals with themining issues and environmental impacts.

Acknowledgements. I am grate ful for the inspiring notes ofthe four referees (H. Kiersnowski, A. Georgakopoulos,V. Karakitsios, and J. Golonka), as well as those of the editorsand J. Zalasiewicz. Their detailed reviews and correc tions im-proved my manuscript greatly.

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