Hydrocarbon potential of the middle Eocene-middle Miocene ...users.uoa.gr/~pavlakis/KontopPaper02.pdf · Hydrocarbon potential of the middle Eocene-middle Miocene Mesohellenic piggy-back

Hydrocarbon potential of the middle Eocene-middle MioceneMesohellenic piggy-back basin (central Greece): A case study

Nikolaos Kontopoulosa,*, Teresa Fokianoub, Abraham Zelilidisa, Christos Alexiadisb,Nikolaos Rigakisb

aUniversity of Patras, Department of Geology, Patras, 26110, GreecebPublic Petroleum Corporation of Greece, Exploration and Exploitation of Hydrocarbons (DEP-EKY), 199 Ki®ssias Ave, Athens, 15124, Greece

Received 27 November 1998; received in revised form 15 July 1999; accepted 19 July 1999

Abstract

Two depocentres, >4200 m and >3200 m thick, have been recognized in the Mesohellenic piggy-back basin of middle Eoceneto middle Miocene age, where submarine fans have accumulated unconformably over an ophiolite complex. The hydrocarbon

potential is indicated by the presence of kerogen types II/III with minor amounts of type I; the evidence is mostly for wet gasand gas, with minor oil. Source rocks are the middle Eocene to lower Oligocene Krania and Eptachori formations, of up to2000 m total thickness, reaching maturation during the early Miocene. The source rocks consist of outer fan and basin plaindeposits. They are conformably overlain by the lower member (late Oligocene) of the up to 2600 m thick Pentalophos

Formation, which consists mostly of thick submarine sandstone lobes. Possible stratigraphically trapped reservoirs include thelower member of the Pentalophos Formation, which overlies source rocks, as well as limestones tectonically intercalated withinthe ophiolite complex, underlying the source rocks. Traps may have formed also on the western side of an internal thrust

(Theotokos Thrust), which in¯uenced the evolution of the depocentres. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Hydrocarbon potential; Piggy-back basin; Kerogen; Turbidites; Greece

1. Introduction

The Mesohellenic basin is a middle Tertiary inter-

montane marine basin developed within the Hellenide

orogen. The basin strikes NNW and the Greek sector

is 130 km long and <40 km wide, with a small north-

ern part of the basin in Albania (Fig. 1). The under-

lying rocks include limestones and ophiolites that

formed in an early Mesozoic small ocean basin

(Pindos basin). The ophiolites were emplaced eastward

onto the Pelagonian continental margin in the middle

Jurassic (Robertson, 1994) and the Pindos basin ex-

perienced Cretaceous to Eocene compression due to

the convergence of the Apulian continental margin

with the Pelagonian microcontinent, with supracrustal

rocks thrust westward over the Apulian foreland(Doutsos, Pe-Piper, Boronkay & Koukouvelas, 1993).The Mesohellenic basin is located near the basementsuture between these two continental blocks and devel-oped from the late Eocene as a piggy-back basin alongthe eastern ¯anks of a giant pop-up structure boundedby the Eptachori thrust, an east-verging back thrust(Doutsos, Koukouvelas, Zelilidis & Kontopoulos,1994).

The Mesohellenic basin shows a tectonically con-trolled variation in basin evolution along its axis(Doutsos et al., 1994; Zelilidis & Kontopoulos, 1997;Zelilidis, Kontopoulos, Avramidis & Bouzos, 1997).The presence of two small indentors at the southernand northern terminations of the basin induced a tec-tonic escape towards the central part of the basin untilthe middle Miocene. Sedimentation along the length ofthe basin was relatively uniform during the earlyOligocene, but became more variable during the late

Marine and Petroleum Geology 16 (1999) 811±824

0264-8172/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0264-8172(99 )00031 -8

* Corresponding author. Tel.: +30-61-996272; fax: +30-61-

996272.

Oligocene±early Miocene and was accompanied bydi�erent subsidence (Zelilidis, 1997).

Four formations were deposited from late Eocene toearly Miocene times, the Krania, Eptachori,Pentalophos and Tsotyli formations, and have beenstudied in outcrop by Zelilidis et al. (1997). The lower-

most deposits in the basin are the upper EoceneKrania Formation, characterized by small submarinefans. The formation outcrops in a small area at thewestern margin of the Mesohellenic basin, related toactivity on the east-vergent Krania Thrust (Figs. 1, 2and 3S1). During latest Eocene time, the synchronous

Fig. 1. Geological map showing surface facies distribution in the Mesohellenic piggy-back basin. S1, S2 are sections in Fig. 3; A±E are sections

in Fig. 2.

N. Kontopoulos et al. / Marine and Petroleum Geology 16 (1999) 811±824812

activity on the Krania and Eptachori thrusts in¯u-enced the evolution of the Krania Formation andformed a linked system with a piggy-back basin (west-wards) and a foredeep (eastward). Due to movementon the Eptachori Thrust, the lower to middleOligocene submarine fan deposits of the EptachoriFormation were deposited in a foredeep setting (Figs.1, 2 and 3). During latest Eocene to early Oligocenetimes, thick deltaic deposits accumulated between theKrania and Eptachori formations, while thick fan del-tas formed between the ophiolitic basement and theEptachori formation. The Eptachori formation passesupwards into the late Oligocene-early MiocenePentalophos Formation (Figs. 1, 2 and 3). The latter isoverlain by the early Miocene Tsotyli Formation. Inthe Pentalophos Formation, which is subdivided intotwo members, di�erent depositional environments wereformed along the basin axis (Zelilidis, 1997). The cen-tral part of the basin is characterized by the continu-

ous accumulation of submarine fans whereas

trapezoidal-type fan-deltas accumulated in the

southern part of the basin (Zelilidis & Kontopoulos,

1996). Thick shelf sediments were deposited north of

these fan-deltas (Figs. 1, 2 and 3). Throughout depo-

sition of the four formations, the Mesohellenic basin

was bounded on the west by a fault-controlled active

margin characterized by the absence of a wide shelf.

From outcrop studies (Zelilidis et al., 1997), the sub-

marine fans of the Mesohellenic basin deposits appear

to belong to turbidite system II of Mutti (1985), in

which (1) the basin is small and tectonically controlled;

(2) relatively small volumes of coarse-grained sedi-

ments, chie¯y fan-delta deposits, were accumulated

along the narrow and irregular shelves and, (3) the

di�erent turbidite stages within the same formation in-

dicate a rapid lowering of sea level, followed by a gra-

dual re-establishment of shelfal deposition. The lobe

Fig. 2. Stratigraphy of the Krania, Eptachori and Pentalophos Formations of the Mesohellenic basin. Columns A, C and E based on outcrops,

whereas columns B and D are based on seismic data and given only for correlation. For location of each column see Fig. 1. The age determi-

nation of studied deposits is based on nannofossils.

N. Kontopoulos et al. / Marine and Petroleum Geology 16 (1999) 811±824 813

deposits are likely to represent suprafan lobes (classi®-cation of Shanmugam and Moiola, 1991).

According to Zelilidis et al. (1997), the boundariesbetween the Eocene±Oligocene Krania and Eptachoriformations and the Oligocene±Miocene Eptachori andPentalophos formations record sea-level lawstands ofglobal eustatic origin. The Mesohellenic basin sedi-ments have the characteristics of lowstand fan depositswithout coastal onlap. The two ®ning upwardcyclothems that are recognised in the upper member ofthe Pentalophos formation and interpreted as allocyc-lic, represent sea-level ¯uctuations related to regressionand transgression events. On the other hand, coarsen-ing upward cycles of the studied sandstone fan lobessuggest basinward lobe progradation.

The aim of this work is to correlate data such as thedistribution of depositional environments interpretedfrom outcrop studies, geochemical analyses and tec-tonic regime with the seismic subsurface data (basincon®guration) in order to evaluate the hydrocarbonpotential of the basin.

2. Age determination

From nannofossils and from stratigraphic corre-

lation of the studied deposits the following ages werefound for the four formations (Fig. 4):

1. Krania Formation: Middle to late Eocene (NP16-NP19).

2. Deltaic deposits: Middle to late Eocene (NP16-NP19).

3. Fan-delta deposits: Unfossiliferous, but probablymiddle to late Eocene.

4. Eptachori Formation: Latest Eocene to earlyOligocene (NP20-NP21). Both deltaic and fan-deltadeposits constitute the base of the Eptachori for-mation and are thus probably synchronous.

5. Turbidites of the lower member of PentalophosFormation: Early Oligocene to late Oligocene(NP21-NP24).

6. Fan-delta deposits of the Pentalophos Formation:probably Oligocene. Both turbidites and fan-deltadeposits accumulated above the EptachoriFormation and so the fan-delta deposits are inter-preted as contemporaneous with the turbiditedeposits.

7. Turbidites of the upper member of PentalophosFormation: Late Oligocene to early Miocene(NP25-NN4).

8. Shelf deposits of the upper member of PentalophosFormation: Early Miocene (Aquitanian, according

Fig. 3. Schematic tectonosedimentary cross-sections based on seismic and ®eld data. For location see Fig. 1.


Fig.4.Synthetic

stratigraphic

columnsofsurface

outcrops(B1±B4)andtheirfacies

interpretation(C

1±C4)withcorrelationbasedonmodi®cationofFig.2from

Zelilidis

etal.(1997),new

out-

cropdata,andnew

agedeterminations.Seism

icfacies

interpretation(A

1±A2)basedoncorrelationorextrapolatingseismic

facies

tooutcrop.Seism

icfacies

1±9are

discussed

indetailin

text.


to Mavridis, Matarangas, Tsaila-Monopolis &Mostler, 1979). The shelf deposits, from outcropsand seismic lines, rest unconformably on the lowermember of Pentalophos Formation and on part ofthe upper member of Pentalophos Formation andfor this reason seem to have formed at the sametime as the upper part of the upper member ofPentalophos Formation.

9. Tsotyli Formation: Early Miocene (late Aquitanianto Burdigalian, according to Mavridis et al., 1979).

3. Seismic stratigraphy and seismic facies interpretation

3.1. Seismic stratigraphy

Twenty seismic sections have been used to establishthe seismic stratigraphy of the Mesohellenic piggy-back basin (nine across the basin and 11 along thebasin); seismic sections used in this paper are shownon Fig. 1. The seismic facies interpretation is based oncorrelation or extrapolation of seismic facies to out-

Fig. 5. Seismic pro®le subparallel to the basin axis, on which most of the seismic facies interpretation was based. For location see Fig. 1.

Fig. 6. Seismic pro®le across the northern depocentre. For location see Fig. 1. In the east large areas (indicated by vertical bars) have reverse

acoustic velocities in Eptachori formation and in the lower part of the Pentalophos formation.


crop, where detailed sedimentological studies havebeen summarized by Zelilidis et al. (1997).

The Eptachori and Pentalophos formations werede®ned by comparison with surface outcrops on theintersections on seismic section of Fig. 5 and the seis-mic sections of Fig. 6. Subsequently seismic correlationwas extended to the other dip sections (Figs. 7 and 8),based partly on seismic character with some controlfrom outcrops. Although intervening strike lines are

broken by faults, they generally con®rm the corre-lation.

3.2. Seismic facies

The following nine seismic facies (Fig. 4, columnA1) were recognized from the four studied formations(from base to top) and are interpreted from theiracoustic character and correlation with outcrops.

Fig. 7. Seismic line in the southern depocentre. For location see Fig. 1.

Fig. 8. Large sandstone lobe in the upper system of Pentalophos formation. For location see Fig. 1.


Seismic facies 1. A relatively transparent facies withspaced sub-parallel strong re¯ectors in theEptachori Formation is interpreted as ®ne-grainedturbidites alternating with sparse sands (Fig. 6).Seismic facies 2. High-amplitude channeling re¯ec-tors in the lower part of the lower member ofPentalophos Formation represent ``third order''channels with stacked ``second order'' channels(Fig. 5) [classi®cation of channel bounding surfacesis according the terminology of Allen (1983)].Seismic facies 3. Less continuous lower amplitudere¯ections in the upper part of the lower member ofPentalophos Formation seem to be lobe deposits(Fig. 5).Seismic facies 4. High-amplitude channeling re¯ec-tors at the base of the upper member ofPentalophos Formation are suggested to be con-glomerates, on the basis of correlation with outcrop(Fig. 8).Seismic facies 5. Strong subparallel re¯ectors in thelower 300 m of the upper member of PentalophosFormation are interpreted as sandy mid-fan lobes(Fig. 5).Seismic facies 6. Imore transparent re¯ectors rep-resent lobe-fringe turbidites in the middle 600 m ofthe upper member of Pentalophos Formation (Fig.5).Seismic facies 7. Strong subparallel re¯ectors couldbe basin plain/lower fan ®ne-grained turbidites forthe upper 640 m of the upper member ofPentalophos Formation (Fig. 5).Seismic facies 8. Strong re¯ectors in many seismicsections, such as in Fig. 6, might correspond toshelf sediments.Seismic facies 9. Continuous strong re¯ectors at thebase of the Tsotyli Formation correspond to sandylobes with some conglomeratic beds in outcrop(Fig. 6).

The correlation of seismic facies 4±9 from seismicpro®les to outcrop is straightforward and these faciesdo not show signi®cant changes in thickness. However,in the Eptachori Formation and the lower member ofthe Pentalophos Formation (seismic facies 1±3), thereare major changes in thickness from outcrops at thebasin margin to the seismic pro®les in the central partof the basin.

Beneath seismic facies 1 in Fig. 6, seismic re¯ectorsshow a subsurface fan-delta that thins to northeastwith a progressive decrease in dip and changes inacoustic character. This fan-delta package has thesame relationship as the late Eocene Krania deltaicdeposits in the seismic section of Fig. 6 to theEptachori turbidites (Fig. 4A1, C1).

The Eptachori shaly turbidites appear to be 250 mthick in outcrop (Fig. 4B1). The distal part of the

Eptachori shales is visible only in seismic pro®les (Fig.6) where they thicken to 500±700 m. This thick featureis not a sandy lobe: the seismic facies is quite di�erent.It may be involved in some overpressured zone or gasaccumulation, as velocity reversals are common in thisinterval.

The identi®cation of the base of the lower memberof Pentalophos Formation (Fig. 6) is based on seismiccharacter correlation with the section of Fig. 8, wherethe facies interpetation is quite clear and consistentwith the outcrop pattern (Fig. 4B1). The lower mem-ber is about 800 m thick and consists of a lowermuddy sequence (with channel sands at the base) andan upper sequence of sandy lobes. Projecting the baseand top of the member from the surface (Fig. 4B1) isconsistent with both thickness and facies.

3.3. Surface and subsurface correlation

An internal unconformity, recognized in the ®eldwithin the Pentalophos Formation, either between tur-bidites and turbidites (in the northern part of thebasin) or between shelf sediments and turbidites (in thesouth), is also recognized in the seismic pro®les (Fig.7). It is biostratigraphically constrained to the base ofthe Miocene. This unconformity may be related to thechange of basin con®guration due to the activity of theTheotokos Thrust within the southeastern part of thebasin (Fig. 7) in the latest Oligocene.

The mapped occurrence of surface (?) earlyOligocene fan-delta deposits (Fig. 4B3) along the basinaxis and of early Miocene shelf deposits (Fig. 4B4)passing to turbidites (inner and then to outer submar-ine fans) near the southern basin margin is also con-®rmed by seismic pro®les (Fig. 7). The shelf depositsare more than 900 m thick (Figs. 2B and 4A2). Deltaicdeposits that accumulated between the Krania andEptachori formations (Fig. 4B2) also seem to be pre-sent in the seismic pro®le (Fig. 7). These deposits arerelated to a major westwards-directed normal fault,also recognized in the ®eld. An internal unconformitywithin the deltaic deposits, which was recognized inthe ®eld, is not shown by the seismic pro®le (Fig. 7).

Fan-deltas at the base of Eptachori Formation andover ophiolitic basement in some areas of the westernmargins have up to 300 m exposed thickness (Fig.4B1), but from seismic pro®les (Fig. 6) these fan-deltasare up to 3200 m thick (Figs. 2B and 4A1) and arealso developed in areas without surface exposure. Theinternal unconformities between fan-deltas were alsocon®rmed on seismic pro®les.

Nannofossils show a late Eocene age of the deltaicdeposits that accumulated at the base of the EptachoriFormation and over the Krania Formation. Takinginto account ®eld and seismic data, a synchronousevolution of deltaic and fan-deltaic deposits is


suggested during the late Eocene. These two, synchro-nous but di�erent depositional environments are dueeither to pre-existing basement and/or to di�erent tec-tonic activity of the Eptachori Thrust. Activity on theEptachori Thrust is thus dated as late Eocene.

The Eptachori Formation has been mapped in the®eld as inner submarine fan deposits (Fig. 4C1).Seismic pro®les show that these deposits pass laterallyto basin plain deposits. Large stacked lobe depositsthat have been recognized in the ®eld within thePentalophos Formation (Fig. 4C1) are also recognizedin the seismic pro®les.

3.4. Paleogeographic evolution

Seismic data show that there are two structuraldepocentres in the basin (southern and northern), re-

spectively >4200 m and >3200 m thick (Fig. 9). Bothdepocentres are bounded westward by the upperEocene Eptachori Thrust and are restricted eastwardsby the internal late Oligocene Theotokos Thrust (Fig.1).

In the southern depocentre (Figs. 2C and 3S1), sedi-ments of the Krania±Eptachori and Pentalophos for-mations were deposited (Fig. 7). At outcrop, the upperEocene Krania and the lower Oligocene Eptachori for-mations (up to 2000 m thick) are characterized by sub-marine fans that consist of inner fan (channel andinterchannel deposits) and outer fan (sandstone lobesand lobe-fringe) deposits (Zelilidis et al., 1997). Thelower Oligocene to lower Miocene PentalophosFormation (up to 2800 m thick) is composed of twomembers; a thick lower member of lower to upperOligocene outer fan sandstone lobes (up to 2600 m inthickness) (Figs. 2D and 4C1) and an upper thickmember of lower Miocene shelf deposits (up to 900 min thickness). The change of depositional environmentwithin the Pentalophos Formation is related to activityon the Theotokos Thrust and the resulting change inbasin con®guration.

The sediment source of the Eptachori formation wasthe westward situated piggy-back Krania basin withthick deltaic deposits, rich in organic matter. Thesource for the lower member of the PentalophosFormation was the older Krania and Eptachori for-mations to the west. The source for the upper memberwas the fan-deltas to the east.

The northern depocentre consists of the Eptachoriand Pentalophos formations, with only submarine fanfacies (Figs. 2A and 3S2). At the base of the EptachoriFormation, thick fan deltas formed (up to 3200 m inthickness) that pass laterally to submarine fans (up to3100 m in thickness). The upper Oligocene to lowerMiocene upper member of the PentalophosFormation, up to 1600 m thick lies unconformablyover the lower Oligocene lower member (up to 1600 mthick) (Fig. 6). Seismic pro®les indicate large lobes

Fig. 9. Isopachs (in metres) of total thickness of clastic deposits over

the ophiolite basement.

Table 1

Microscopic analysis of selected samplesa

A/A S Formation Kerogen Anal org. mat. Maturity

Am H W I TAI Ro Tmax A Tmax B

1 8041 Krania 0 2 97 1 ± 0.43 440 429

2 8058 0 25 73 2 1+to 2ÿ 0.36 432 426

3 8036 0 1 99 0 1+to 2ÿ 0.53 423 408

4 9766 Eptachori 0 0 100 0 1+to 2ÿ 0.59 428 411

5 9783 0 3 96 1 1+to 2ÿ 0.41 444 436

6 8906 Pentalophos 0 2 98 0 1+ 0.34 397 363

7 9750 0 9 91 0 ± 0.52 431 430

a Am=amorphous material, H=exinite, W=vitrinite, I=inertinite, TAI=spore color index, Ro=vitrinite re¯ectance (%), Tmax A and B=

maximum temperature of pyrolysis (8C) from two di�erent laboratories.


Fig. 10. Diagrams showing character of organic matter in Mesohellenic basin: (a) maximum temperature (Tmax)/hydrogen index (HI); (b) pro-

duction index (PI)/Tmax; (c) total organic carbon (TOC)/oil potential (PP); (d) oxygen index (OI)/HI; (e) vitrinite re¯ectance (%) (Ro)/Tmax, and

(f) PP/HI. Ro was determined optically and Tmax, HI, PI, TOC, PP and OI by pyrolysis. Triangle symbols, marked by the symbol � in Table 2,

correspond to black samples, and circle symbols correspond to the samples in Table 2.


Table 2

Pyrolysis of the 25 more representative samples from the studied basin depositsa

A/A S Formation Age Tmax S1 S2 S3 PI S2/S3 PC TOC PP HI OI

1 8038 Limestone Cretaceous 445 1.28 2.66 0.36 0.32 7.38 0.32 1.56 3.94 170 23

2 8042 (basement) 440 0.45 0.07 0.52 0.87 0.13 0.04 0.15 0.52 46 346

3 9763 477 0.25 0.58 0.12 0.30 4.83 0.06 1.00 0.83 58 12

4 776� Krania L. Eocene 420 0.32 43.56 10.21 0.01 4.26 3.65 12.99 43.88 335 78

4 776 440 0.00 0.76 0.67 0.00 1.13 0.06 2.00 0.76 38 33

5 8041 429 0.13 3.37 0.84 0.04 4.01 0.29 4.32 3.50 78 19

6 8058�� 426 0.02 0.73 0.40 0.03 1.82 0.06 0.91 0.75 80 43

7 8018 E. Oligocene 416 11.84 75.19 9.13 0.14 8.23 7.25 27.81 87.03 270 32

8 8036 408 29.08 106.39 5.96 0.21 17.85 11.28 20.01 135.47 531 29

9 9765 Eptachori 0 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0 0

10 9766 411 17.80 102.88 11.36 0.15 9.05 10.05 54.16 120.68 189 20

11 9783 436 0.08 4.25 4.56 0.02 0.93 0.36 11.59 4.33 36 39

12 8066� L. Oligocene 401 0.82 58.08 9.04 0.01 6.42 4.90 15.67 58.90 370 57

12 8066 430 0.10 5.36 1.62 0.02 3.30 0.45 7.20 5.46 74 22

13 8069� 385 8.20 172.84 5.53 0.05 31.25 15.08 46.68 181.04 370 11

13 8069 434 0.01 0.77 0.68 0.01 1.13 0.06 1.97 0.78 39 34

14 8071� 370 41.89 216.42 5.74 0.16 37.70 21.52 53.53 258.31 404 10

14 8071 437 0.05 10.61 7.07 0.00 1.50 0.88 30.26 10.66 35 23

15 9750 430 0.02 0.24 0.14 0.08 1.71 0.02 0.60 0.26 40 23

16 9780 Pentalophos 437 0.01 0.26 0.06 0.04 4.33 0.02 0.52 0.27 50 11

17 9831� 376 9.95 74.24 11.76 0.12 6.31 7.01 20.41 84.19 363 57

17 9831 430 0.21 3.41 1.65 0.06 2.06 0.30 4.23 3.62 80 39

18 8906� E. Miocene 363 40.01 141.86 8.96 0.22 15.83 15.15 55.93 181.87 253 16

18 8906 482 0.20 5.42 29.85 0.04 0.18 0.46 34.58 5.62 15 86

19 9727 435 0.06 0.69 0.09 0.08 7.66 0.06 1.20 0.75 57 7

a Tmax=temperature on the S2 peak, S1=free hydrocarbonates (mg/g), S2=hydrocarbonates from pyrolysis, S3=CO2 from pyrolysis (mg/g),

PI=production index=(S1/S1+S2), TOC=total organic carbon (%), PP=oil potential (S1+S2), HI=hydrogen index (S2�TOC/100),

OI=oxygen index (S3�TOC/100. �Black samples, ��Deltaic sample.

Fig. 10 (continued)


(5 km long and up to 100 m thick) (Fig. 8) and someareas with seismic velocity reversals possibly due togas accumulation or to overpressure (Fig. 6).

4. Geochemical analysis

4.1. Methods

Geochemical analyses were made of more than 100samples collected from outcrops; results from 25 of themost representative samples are presented in Fig. 10,and Tables 1 and 2. These analyses were used to esti-mate the kerogen type, maturation of source rocks andhydrocarbon potential. The quality and maturation oforganic matter were determined by optical determi-nation of vitrinite re¯ectance (Ro) and pyrolysis formaximum temperature (Tmax), oxygen index (OI),hydrogen index (HI), production index (PI), total or-

ganic carbon (TOC) and oil potential (PP). In order topredict the maturation models of the basin an isodepthmap of the contact between sediments and ophiolitebasement was drawn on the basis of seismic data (Fig.9).

4.2. Geochemical data

Geochemical analyses of sediments from the KraniaFormation (the lowermost deposits that can act as po-tential source rocks) show that they are rich in organicmatter (TOC) (in 60 samples the organic matter ¯uctu-ated from 0.2 to 1.46%, with samples rich in organicmatter from ®ne-grained outer fan deposits). Oil po-tential (PP) is fair and kerogen is of type III, able togenerate wet gas and gas hydrocarbons (Table 2, Fig.10a,d). Microscopic analysis shows that the kerogenpresent consists mostly of vitrinite (Table 1),suggesting a terrestrial origin (Teichmuller, 1986). The

Fig. 11. Time Temperature Index (TTI) plot for (A) northern and (B) southern depocentres. Gas window was not appeared in the diagram. For

location see Fig. 5.


Eptachori and Pentalophos formation samples are alsorich in organic matter (in 73 samples the organic mat-ter ranged from 0.4% to 5%). Kerogen is of type II,with poor to fair oil potential, and type III, with abil-ity for wet gas and gas production. According toPeters' (1986) model, the S2/S3 ratio of these samplesindicates an ability for gas generation and in rare casesfor oil generation.

Fig. 10 uses the 25 most representative samples(from more than 100 collected) and shows that somehorizons have a high hydrocarbon potential able togenerate oil (Fig. 10c). A large number of horizons arerich in organic matter with a rich to very rich hydro-carbon potential and ability for gas and wet gas gener-ation (Fig. 10)

All of the samples are immature (Ro < 0.6%) (Fig.10a,f). Modelling of the time temperature index (TTI)using the method of Waples (1980) was attempted.Regional thermal gradients were estimated by Fytikasand Kolias (1979) for the basement as 3.58C/100 m,for clastic deposits in the southern part as 2.88C/100 mand in the northern part as 2.58C/100 m. In bothdepocentres the modelling suggests that in the earlyMiocene the Krania and Eptachori formations depos-its reached late maturation and that the lower memberof the Pentalophos Formation reached early matu-ration (Fig. 11).

5. Discussion

The geochemical data indicate that the Mesohellenicbasin contains organic matter in shales suitable for thegeneration of gas and wet gas. Such shales occur inthe Krania and Eptahori Formations in the southerndepocentre and in the Eptahori Formation in thenorthern depocentre. These shales were rapidly buriedin the Oligocene and early Miocene, reaching oil win-dow conditions (Fig. 11).

Many of the turbidite sandstones have about 15%porosity, reaching 25% in some samples. Interbeddedshales provide potential topseals. The most promisingreservoir rocks are outer fan sandstone lobes, classi®edas suprafan lobes (Shanmugam and Moiola, 1991),that pass up-dip into stacked channel sand bodies withgood lateral and vertical communication (Fig. 6).These features constitute excellent reservoir facies(Shanmugam and Moiola, 1991). The geometry of thelobes and channels and the internal unconformities(Fig. 8) present excellent stratigraphic traps (eitherdepositional or unconformity). Basement limestonestectonically intercalated with ophiolites also have morespeculative potential as reservoirs. Some rollover anti-clines developed early in the basin evolution alonggrowth faults of the Theotokos thrust (Fig. 5) havecreated local structural traps. Thus both structural and

stratigraphic traps were available early in the basinevolution and may have trapped principally gas fromthe early maturation of organic matter (biogenic gasdue to kerogen type III). Hydrocarbon migration mayhave preceded the later stages of cementation of sand-stones seen in outcrop. Therefore, it is suggested thatthe Mesohellenic basin may have signi®cant gas poten-tial.

6. Conclusion

In the Mesohelleic piggy-back basin, we have stu-died surface outcrops for recognition and mapping ofdepositional environments and for geochemical analy-sis and we have studied subsurface seismic stratigra-phy. Integration of these data sets allows surfacestratigraphy to be applied to the subsurface. Twodepocentres, >4200 m and >3200 m respectively,accumulated submarine fan sandstone and shales fromlate Eocene to early Miocene in four formations. Thelowermost deposits (late Eocene±late Oligocene) withinthe two depocentres reached late maturation in theearly Miocene. The hydrocarbon potential is indicatedfrom the presence of kerogen type II/III and the tim-ing of development of sources (outer fan and basinplain ®ne-grained turbidites), traps (thick sandstonelobes, limestones within the ophiolite complex and thewestern side on an internal thrust) and seals (overlying®ne-grained turbidites). The evidence is mostly for wetgas and gas with minor oil.

Acknowledgements

This work was supported by the Public PetroleumCorporation (PPC), to whom the authors are greatlyindebted for permission to carry out this work, andfurnishing much important data needed. We also wishto thank D.J.W. Piper (Bedford Institute ofOceanography, Canada) for his helpful commentsabout the manuscript. The comments and suggestionsprovided by the editor, D.G. Roberts, and anonymousreferees are gratefully ackowledged.

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