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Hydrocarbon potential assesment of the

slope deposits along the SW Dinaridescarbonate platform edgeS. Grandi, I. Kratkovi and I. Rusan

ORIGINAL SCIENTIFIC PAPER

The peri-platform carbonate clastics extend along the entire Dinarides carbonate platform as slopedepositsfrom Istria offshore in northwest to the offshore Dubrovnik in southeast. These sediments are good potentialreservoir rocks of regional extension. This is the first attempt to estimate their hydrocarbon potential.The mentioned peri-platform deposits extend over 550 - 600 km. Assessment of their reservoir capacity isbased on petroleum-geological interpretation of 12 seismic lines crossing transition zone of the Dinaridesplatform/Adriatic basin. In the studied zone, Tertiary clastics overlay peri-platform slope deposits as

regional cap rock while Ladinian-Carnian shales of the Vlasta-1 type represent underlying source rocks,potentially mature in depocenters.Recent commercial oil discovery in neighboring Rovesti structure at the base of Apulia carbonate platformadditionally inspired the authors to make this assessment in the slope zone of the Croatian part of theDinarides platform. It should be emphasized that our assessment will be in future, most likely corrected byadditional interpretation of more cross lines and selected parallel seismic lines.

Key words: Hydrocarbon potential assessment, Peri-platform clastics, Adriatic off-shore

1. INTRODUCTION

This assessment includes 12 in-terpreted cross sections trans-

verse to Dinarides carbonateplatform of Upper Triassic to Mid-dle Eocene age as well as a part ofrecent offshore Dinarides as a tec-tonic unit. Along with the interpre-tation of the mentioned crosssections, a review on characteris-tics and potential of Friuli and

Apulia platforms was performed.The mentioned studies and analy-sis of several wells drilled for ex-ploration of peri-platform clasticsthat spread along southwesternmargin of Dinarides carbonateplatform showed that they repre-sented possible reservoir rocks ofregional significance.

Petroleum-geological relationsdefined by exploration on theneighboring Italian side, in thearea of Friuli and Apulia plat-forms margins that were createdunder identical paleo-geographicconditions were also analyzed in aseparate chapter, thus it has beenconsidered useful to use data

from this area for the assessmentin Croatian part of the offshorearea.

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Fig. 1. This Figure presents regional position of peri-platform clastics belt as a mainobject of consideration. Also the distribution of other Dinarides explorationunits is presented (modified according to Grandi-1974). The position ofJuni Jadran-1 well in Monte Negro offshore, coastal area of Garganopeninsula and Rovesti-1 well in Bar-Brindisi offshore are also marked.

Sl. 1. Slika prikazuje regionalni poloaj pojasa periplatformnih klastita kao glavnogobjekta razmatranja. Prikazan je i raspored ostalih istranih jedinica Dinarida iJadranskog bazena (modificirano prema Grandi-1974). Naznaen je i poloajbuotine Juni Jadran-1 u crnogorskom podmorju, transko-venecijanskopodmorje, priobalje poluotoka Gargano i buotina Rovesti-1 u podmorju Bar-Brindizi.


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Figure 1 presents the re-gional position of peri-plat-form clastics belt as a mainsubject of the study. The dis-

tribution of other Dinaridesexploration units is also pre-sented (modified accordingto Grandi 1974). The po-sitions of Juni Jadran -1and Juni Jadran 3 wellsin Monte Negro offshore are

given along with Trieste-Ven-ice offshore, Gargano penin-sula offshore and position ofRovesti-1 well in Bar-Brindi-si offshore.

In this phase of prelimi-nary assessment, only 12

chosen cross-sections weretaken in consideration whilein further explorations, suc-cessive completing of crosssections grid including thelongitudinal seismiccross-sections is planned.

On Figure 2, there is loca-tion map of 12 interpretedseismic cross sections which

were used for calculation oftotal reserves of Dinaridesplatform slope peri-platformclastics. Also the positions

of wells that provided datafor this study are presented and regional faults tracks

which delineate the considered parts of Adriaticsedimentary basin.

Istra more 1 (IM-1), Istra more - 3 (IM-3) and JuniJadran 1 (JJ-1) exploration wells need to be men-tioned. They were not meant for testing of peri-platformclastics reservoir characteristics but those sediments

were partially drilled there. According to project docu-mentation they had a different scope; testing of top car-

bonates as cap rocs at the base of Tertiary clastics,4,5

which was the basis for hydrocarbon exploration in theAdriatic basin during earlier exploration period. How-ever, the latest reinterpretation of well and seismic datapointed to uplifted carbonates structures which are, dueto frequent emersion phases, less promising. On theother hand, deposits at structures flanks, and especiallycarbonate clastics as sediments developed by Dinaridescarbonate platform weathering, can have better charac-teristics of potential reservoir rocks of regional spread-ing. Collectors characteristics were especially confirmed

by laboratory research performed on cores from IM-3well in Istria offshore with determined porosity up to15% and permeability up to 45 mD.10 Gas show inOligocene clastics in JJ-1 well and in South Adriatic alsoindicated good possibility for hydrocarbon generation

and accumulation. However, systematic testing of totalperi-platform deposits reservoir potential within theircomplete spreading in Croatian offshore has not been

performed up to now in big interspace from Istria off-shore to South Adriatic.

In this work we have tried to estimate their total porevolume based on interpretation of 12 cross sectionsthrough peri-platform deposits. The following seismiccross-sections were interpreted:

I-14-89, PU-74-82, I-42-85, KV-86A-81, K-106-83,J-22-82, V-16-83, P-44A-82, L-16-82, D-68-83, M-67-82and Y-80-10 (locations in Figure 2).

Joint sedimentary body of peri-platform clastics hasmost often wedge to irregular shape. Within this initialphase, the total volume of this unit was estimated on rel-

atively small number of transverse cross-sections. To in-crease the accuracy of assessment, assessment on highernumber of interpreted transverse and longitudinalcross-sections regarding platform spreading isanticipated.

Along with interpretation of peri-platform clastics sedi-ment body sizes, the special attention was given to deter-mining sedimentation depressions within theirenvironment, where the development of potential sourcerocks exists which are capable for expulsion and migra-tion of hydrocarbons into the surrounding traps.

The following were separately described:1. Istrian platform slope;2. area southwest of Kvarner transverse fault,3. area southeast of olta-Komia-Pt.Pietra.Nera trans-

verse fault.

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S. GRANDI, I. KRATKOVI AND I. RUSAN HYDROCARBON POTENTIAL ASSESMENT OF THE SLOPE...

Fig. 2. Index map shows the position of 12 interpreted seismic cross-sections which wereused in calculation of total reserves of peri-platform clastics belt from Dinaridesplatform slope. Positions of wells from which the data were used in this paper arealso presented. Several regional faults which bound the considered parts of Adriaticsedimentation basin are also shown.

Sl. 2. Karta lokacija 12 interpretiranih seizmikih profila koji su koriteni za izraun ukupnihpotencijalnih rezervi u periplatformnim klastitima padine Dinaridske platforme. Prikazani su ipoloaji buotina koritenih u ovom radu, te trase regionalnih rasjeda koji omeujurazmatrane dijelove Jadranskog sedimentacijskog bazena.


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Those are parts of thestudied area that were fromUpper Lias paleo-tectoni-cally completely or partially

separated as separate sedi-mentary basins. However,main considered sedimenta-tion area represents a partsouthwest of the mainDinarides carbonate plat-form slope, where, after Up-per Lias pelagic andhemipelagic sedimentationtook place in Adriatic basin,

which resulted in sedimen-tation of Sciaglia-Bianconeformation deposits.Halokinetic structures like

KM-1 diapir, Jabukadiapirs, Palagrua diapirsand diapirs in Mljet islandoffshore regularly appearsouth of Kvarner transversefault along Dinarides car-

bonate platform edge. It issupposed that these diapirs

were initiated by reactiva-tion of deep faults that af-fected to the uplifting ofPermian-Triassic evaporitedeposits (Fig. 3).

The generalized geological

column, presented in Figure3 (indicated by heavy redline) shows total sedimen-tary sequence of Dinarides;from Triassic-Paleozoicclastic-evaporite sediments,carbonate-anhydrite sedi-ments of younger Mesozoicto Tertiary clastics. The epi-sode of separation of Dinarides from the Apuliaplatform is especiallymarked.

2. Istrian platformslope sectorStudying of petroleum-geo-logical potential started innorth Adriatic area andspread up to the most south-ern part of the offshore be-longing to Croatianterritorial waters.

Dinarides platform edge inthe Istrian offshore is clearlyexpressed and marked byperi-platform clastics show.

Significant gas fields likeIvana, Ika and other are lo-cated in this area, however

HYDROCARBON POTENTIAL ASSESMENT OF THE SLOPE... S. GRANDI, I. KRATKOVI AND I. RUSAN

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Fig. 3. This generalized geological column presents total Dinarides sedimentation sequencefrom Triassic-Paleozoic clastic-evaporites deposits, carbonate-anhydrite sediments ofyounger Mesozoic to Tertiary clastics. Separation episode of Dinarides from Apuliaplatform is especially marked.

Sl. 3. Na ovom generaliziranom geolokom stupu prikazana je ukupna sedimentacijska sekvencaDinarida od trijasko-paleozojskih klastino-evaporitnih naslaga, karbonatno-anhidritnihsedimenata mezozoika do tercijarnih klastita. Posebno je oznaena epizoda odvajanjaDinaridske od Apulijske platforme.

Fig. 4. Relatively narrow spreading of peri-platform clastics in top carbonates and Messinianbasement is presented. IM-3 well which includes only the lower part of slope depositsis on I-140 cross-section thus the prosperity of the upper part with clinoform pinch outis not known.

Sl. 4. Na profilu I-140 nalazi se buotina IM-3 koja je zahvatila samo donji dio naslaga pregiba te

nije poznata perspektivnost gornjeg dijela s klinoformnim isklinjenjima. utom je bojomoznaeno razmjerno usko rasprostranjenje periplatformnih klastita u ovom podruju. Oni seuvijek nalaze u krovini karbonata i u podini mesinskih sedimenata.


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up to now there were no oil shows. One of the possiblereasons is the lack of oil generating centers and also thefact that certain wells were not drilled on place of pinchout and closing of potential traps as presented in Fig. 4and Fig. 6.

Figure 4 shows seismic cross-section I-140 and its IM-3well which included only lower part of slope deposits;thus the potential of upper part with clinoform pinch outis not known. Yellow color marks relatively narrowspreading of peri-platform clastics in this area. They arealways situated in carbonate top and in the basement ofMessinian sediments.

Istrian platform slope, as well as the whole Dinaridesplatform, is paleo-tectonically conditioned. According to

Del Ben2 interpretation, thisis the case of deep faults

which spread up to ''MOHO''horizon. This is a lineament

which caused, southwest ofDugi Otok, intrusion of Tri-assic batholiths and Perm-ian-Triassic evaporites anddevelopment of structuraluplift of Palagrua Island.

In the area of Istrian slopeof carbonate platform, westof Rovinj-1 (Ro-1) well, Istramore-3 (IM-3) well wasdrilled at shot-point 1700 onseismic cross sectionI-140-85. By its smallerpart, the well included

peri-platform clastics at thedepth of 2 200 2 225 m(Fig. 5). Laboratory measur-ing data are significant andconfirmed good porosity of15% and permeability of 45mD. As on the remainingpart of the slope there wasno such measuring, thisdata is taken as referencedata for the whole explored

belt of peri-platformclastics.

Figure 5 shows porosity

type on IM 3 well, pre-sented in geological columnand on the photograph ofmicro facies, marked in blueand by symbol A. This is ef-fective porosity that can beidentified on well logging di-agrams on left part of the fig-ure. By the curve shape ithas spontaneous potential,neutron well logging, density

well logging and sonic welllogging. This interpretationis considered as a model for

the whole peri-platformclastics belt.

According to the presented interpretation of PU-74-82(Fig. 6) it is obvious that the drilling should have beenperformed closer to vertical projection of shallow gas

well Ivana-4A which was stopped in Plio-Quarternaryclastics at the depth of 925 m, while possible oil showsare much more likely to occur in peri-platform clastics ofPre-Oligocene age. The distance between IM-1 andIvana-4A is 6.5 km, which is probably too big distance.From the example of oil positive well Well-1 JI fromGargano peninsula it is possible to estimate that the opti-mal distance would be 3.0 km (Figl. 11). According to re-

gional exploration results it is known, however, thatseparation of Dinarides and Apulia platform began in

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Fig. 5. Detailed porosity on IM-3 well is presented in geological column and the photographof micro-facies where effective porosity is marked in blue color and by symbol A -effective porosity. This data is taken as orientation one for the whole belt ofperi-platform clastics.

Sl. 5. Tip poroziteta na buotini IM-3 prikazan je na geolokom stupu i fotografiji mikro-facijesa nakojem je naznaen plavom bojom i simbolom A. Rije je o efektivnom porozitetu koji semoe identificirati i na karotanim dijagramima na lijevom dijelu slike, po obliku krivuljaspontanog potencijala, neutronske karotae, karotae gustoe i zvune karotae. Ovainterpretacija uzeta je kao ogledna za cijeli pojas periplatformnih klastita.


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Upper Lias when peri-plat-form clastics were formed as

well as potential reservoirrocks.

Clear pinching out ofperi-platform clastics in di-rection of Istrian carbonateplatform is shown on pre-sented cross-sectionPU-74-82, Fig. 6. Pinchingout is expressed in ''upwardclinoform'' show of UpperCretaceous to Oligocene ageas possible petroleum geo-logical traps of structuralstratigraphic type that wereunfortunately not tested byexploration drilling.

As mentioned before, thedata from IM-3 well wereused as reference forcross-section processing incentral and south Adriatic,i.e. for the whole belt ofperi-platform clasticsspreading.9 Euxinic depo-centers were not recognizedas starting points of possibleexpulsions into describedclinoforms in southwest di-rection, i.e. toward northernpart of the Adriatic Basin.

Authors propose the inter-pretation of cross-sections

grid of the considered areato check the possibility ofthe existence of suchdepocenters and after thatthe reinterpretation of mi-

gration paths.

Figure 7 presents seismiccross-section I-42-85 whichincludes southern part ofIstrian platform where it iscrossed by ''Kvarner trans-

verse fault'' which spreadsfrom SM-1 to Barbara gasfield in Italian offshore.

This is the case of two par-allel faults with tectonictrough formed betweenthem. Considering the datafrom Susak more-1 (SM-1)

well, in the deeper part ofthis trough, Middle Triassiceuxinic deposits were devel-oped with TOC up to 4.5%.This trough is crossed by

J-1-J-7-83 seismic cross-section which presents theplatform slope.


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Fig. 6. Pinch out of peri-platform clastics in the direction of Istrian carbonate platform isclearly pointed out in the presented PU-74-82 cross-section. Pinch out is visible in"upward clinoforms" show of Upper Cretaceous to Oligocene age as possiblepetroleum-geological traps of structural-stratigraphic type which were, unfortunately,not tested by exploration drilling.

Sl. 6. Na prikazanom profilu PU-74-82 jasno se istie isklinjenje periplatformnih klastita u smjeruIstarske karbonatne platforme. Isklinjenje se oituje u pojavi ''uzlaznih klinoformi''gornjokredne do oligocenske starosti kao moguih naftnogeolokih zamki stratigrafskogtipa koje jo nikad nisu bile testirane istranim buenjem.

Fig. 7. Seismic cross-section I-42-85 covering southern part of Istrian platform where it iscrossed by "Kvarner transverse fault" which spreads from SM-1 to Barbara gas fieldin Italian offshore.

Sl. 7. Seizmiki profil I-42-85 obuhvaa juni dio istarske platforme na mjestu gdje ju presijecakvarnerski transverzalni rasjed koji se protee od buotine SM-1 do plinskog polja Barbarau talijanskom podmorju.


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3. Area Southeastof KvarnerTransverse Fault

Central Adriatic area is dif-ferent from Central Adriaticbasin. The difference is infact that here, southeast ofpaleo-tectonic line abar-SM-1-J-7 (Kvarner trans-

verse fault), Permian-Trias-sic evaporite deposits weredeveloped which causedhalokinetic movements andstructures forming whichhad impact on sedimenta-tion along Dinarides carbon-ate platform slope southeast

of the mentioned line. Thementioned tectonic line hasmost probably transcurrentcharacter which is reflectedon differential shift of west-ern from eastern wing. Thisfault with shift, according toits spreading is also possible

good migration corridor.On seismic cross-section

KV-86A-81, Fig. 8, there isconsiderable thickness ofover 1 000 m and width of10 km of peri-platform

clastics as potential reser-voir rocks. Possible trapsmake pinch outs that standout on this seismic cross-section.

Movements along the slopehave most probably inten-sively affected the sedimen-tation of slope deposits.Changes are clearly ex-pressed on cross-sectionK-105-82, D-10, D-8 and es-pecially on J-22-82 i.e. in

the area of diapirs of Jabukaisland (Fig. 10). On thiscross-section, structural de-pression between Jabukadiapir and Jelena-1 struc-tural uplift is pointed out.Considering the intense sub-sidence, Ladinian-Carnianand Carnian euxinic shaleas good and mature sourcerocks, are anticipated in thementioned depression.From these potential sourcerocks, it is possible to fore-

cast migration into traps onflanks of the mentionedJabuka and Jelena struc-

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Fig. 8. The considerable thickness of over 1 000 m and width of around 10 km ofperi-platform clastics as potential reservoir rocks stands out on KV-86A-81 seismiccross-section. The possible traps make pinch outs which are clearly visible on thisseismic cross-section.

Sl. 8. Na seizmikom profilu KV-86A-81 istie se znatna debljina od preko 1 000 m i irina od 10km periplatformnih klastita kao potencijalnih leinih stijena. Najznaajnije mogue zamkeine isklinjenja u smjeru sjeveroistoka.

Fig. 9. Seismic cross-section K-106-82 shows protrusion of evaporates at the very transitionof platform into Adriatic basin with younger Mesozoic pelagic deposits and Tertiaryclastics and peri-platform clastics in Messinian deposits base.

Sl. 9. Na seizmikom profilu K-106-82 prikazan je proboj evaporita na samom prijelazu platformeu Jadranski bazen s mlaim mezozojskim pelagikim naslagama i tercijarnim klastitima i

periplatformskim klastitima u podini mesinskih naslaga.


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tures. These traps are pro-posed as good plays forfurther exploration.

Seismic cross-sectionK-106-82, Fig. 9, shows pro-trusion of evaporites at the

very transition of platforminto Adriatic Basin with

younger Mesozoic pelagicdeposits and Tertiaryclastics and peri-platformclastics in Messiniandeposits base.

The mentioned Jelena-1well confirms strongemersion phase after UpperLias when Miocene sedi-ments discordantly overliedeposits of AmmoniticoRosso type, and the logicalassumption is that possiblereservoir and seal rockscould have been eroded anddegraded at the vertex ofthis structure. According tothe stated above, it is possi-

ble to conclude that mainpotential should be expectedat the northeastern flank ofJelena structure where it ispossible that hydrocarbonaccumulations are situated

in upward clinoforms asstructural-stratigraphictraps. Migration is here ex-pected from the deepdepocenter south of Jabukaisland where Ladinian-Carnian deposits were lowered ata very big depth i.e to the maturation level of potentialeuxinic deposits of ''Vlasta-Komia'' type. According toseismic interpretation, this is the case of depression

where E horizon is situated at the depth of 10 km.Seismic cross-section J-22-82 in Figure 10 clearly

shows relations between Jabuka island structures(evaporites) and Jelena-1 structure and depression be-tween them which is filled with potential source rocks.Depression depth, i.e. burial of potential source rocks of

Vlasta-1 type on the most part of the cross-section is suf-ficient for their maturation which makes this area prom-ising for further hydrocarbon exploration.

However it is necessary to mention that also Posidoniadeposits which were recognized in Jelena-1 well at 1 440m represent also potential source deposits just like in theIonian Basin area, where, in Kurveleshi zone they containup to 15% of TOC.

Lately, as well as in the Ionian Basin area, intense ex-ploration is performed south of the island of Vis andthese results shall be studied in the next period and in-cluded in this preliminary assessment of peri-platform

clastics pore volume. The important thing is that oilshow has been determined in Vlasta-1 well. Testing hasacquired 16.0 m3 of medium heavy oil while the

Ladinian-Carnian shale at this depth contained 4.2% ofTOC.

4. Area Southeast of olta Komia -Pt. Pietra Nera Transverse Fault

4.1 Area South of olta - Komia Fault to MljetIsland Offshore

Cross-sections V-16-83, Vls-Pat-82, L-16-82 and

Vlatka-1, Vlasta-1, Lastovo-1 and Maja-1 wells were cho-sen as typical for this area. Results are illustrated in Fig-ures 8, 9, 10 and 11. Cross-section V-16-83 was chosenas especially significant one. This cross-section crosseshalokinetic structure at shot point 1 650. This structureon its southern side separates sedimentation depression

which was filled with peri-platform clastics from south-western flank of Dinarides carbonate platform.

As on the previous seismic cross-section, cross-sectionV-16-82, Fig. 11 also shows diapiric evaporites protru-sion at platform edge and deep subsidence of potentialsource rocks of Vlasta-1 type. Especially visible arestructures at the southwestern part of depression wheretraps in upward clinoforms can be expected.

One of the possible interpretations is that along thisstructure there is a deep fault which makes path for hy-


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Fig. 10. Seismic cross-section J-22-82 clearly shows relations between Jabuka islandstructures (evaporites) and Jelena-1 structure and inter-depression with potentialsource rocks. Depression depth, i.e. burial of potential source rocks of Vlasta-1 typein most part of the profile is sufficient for their maturation which makes thiscross-section prosperous for further hydrocarbon exploration.

Sl. 10. Seizmiki profil J-22-82 jasno prikazuje odnose izmeu struktura otoka Jabuka (evaporiti) istrukture Jelena-1 te depresije izmeu njih ispunjene potencijalnim matinim stijenama.

Dubina depresije, odnosno zalijeganje potencijalno matinih stijena tipa Vlasta-1 na veemdjelu profila je dovoljna za njihovo sazrijevanje to ini ovo podruje perspektivnim zadaljnja istraivanja ugljikovodika.


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drocarbon migration from deep depression where burialof E horizon is supposed i.e. top of Ladinian-Carnianshale as potential source rocks at the depth of 3.5 sec-onds of two way time or approximately at 4 000 m. Thussubvertical hydrocarbon migration from theseLadinian-Carnian shale in peri-platform clasticsclinoforms is supposed and then their accumulation instratigraphic traps and clinoforms of upward type. Con-sidering the very close location of Vlasta-1 well, where oil

was discovered in the same Ladinian-Carnian euxinicshale and according to the mentioned interpretation ofpetroleum-geological relations, the mentioned trap wasproposed by the author as a very significant lead, i.e.

potential hydrocarbon accumulation.Paleo-tectonically significant cross-section V-16-83 is

within the domain of well known Pt. Pietra Nera faultnorth of Gargano peninsula. According to study of A. DelBen2 and later processing of deep CROP cross-sectionsof the mentioned locality near Pescara all the way to oltaintense transverse fault is marked. This fault is parallel

with Kvarner fault and can also have a function of goodmigration corridor (Fig. 1). As already noted on Fig. 13,migration from euxinic deposits of Vlasta-1 facies is sup-posed. Unfortunately neighboring Vlatka-1 well wasdrilled on paleo-structural uplift which was several timesin emersion and is prone to incision and

resedimentation, thus the forming of clastic sedimentsoccurred with potential traps on the flank of upliftedstructure. Due to the mentioned reason, the complex of

peri-platform clastics is pro-posed to be more prosper-ous. Favorable conditionsare forecasted also on the

flanks of the marked struc-ture (TP 700) whereclinoform sedimentationshapes are well developed aspossible traps.

4.2 Palagrua SlopeIn the deeper part of seismiccross-section P-44A-82 (Fig.12) top of Middle Triassichas been interpreted (''E1''horizon) with potentialsource rocks of ' 'Vlasta-Komia'' type of

Ladinian-Carnian age whichare buried in Patricija-1 wellarea at the depth up to twoseconds of two way time,

which should, according tomaturation diagram,6 besufficient for the maturation(late mature) of mentionedsource rocks. It should bementioned again that in

Vlasta-1 well 16 m3 of oilwere determined in men-tioned Triassic rocks at5 402 m.

The purpose of interpreta-tion of seismic cross-sectionP-44A-82, presented on Fig. 12, is to present more com-pletely the sedimentation basin with pelagic developmentof younger Mesozoic deposits southwest from Palagruastructural uplift. Potential source deposits ofLadinian-Carnian, southwest of the structure were low-ered over 5 sec. of two way time i.e. to maturation level.The migration into traps within peri-platform Creta-ceous-Paleogene clastics is supposed.

Platform-basin transition is, like in the other parts ofDinarides platform slope, conditioned by reactivated Tri-assic faults which brought to surface carbonate depositsnear Palagrua and Galiola Triassic and Jurassic whereolder Permian-Triassic evaporites are most probably inthe uncovered basement. Specific structural characteris-tic in this area, southeast of Vis island, is significant in-tensity of tectonic vertical shifts along Dinaridescarbonate platform slope and according to this, more in-tense development of sedimentation depressions as po-tential oil-generating centers (southwest of Palagrua-1(Pal-1) and Patricia-1 (Pat-1) wells or from Palagruaslope uplift). Dinarides carbonate platform was moved

by olta-Pt.Pietra Nera strike-slip fault and by parallelfault on the eastern part of Palagrua island in the direc-tion of Adriatic pelagic basin central part. Proofs for this

are seismic cross-sections P-40A-82 to P-2-82 southwestof Palagrua island and Gargano Mare E-1 well whichencompassed the pelagic deposits of younger Mesozoic.

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Fig. 11. The previous seismic cross-section V-16-82 shows diapiric evaporites protrusion atplatform edge and deep subsidence of potential source rocks of Vlasta-1 type.Especially visible are structures at the southwestern part of depression where traps in"upward" clinoforms can be expected.

Sl. 11. Kao i na prethodnom seizmikom profilu i na profilu V-16-83 istie se dijapirski probojevaporita na rubu platforme i duboka subsidencija potencijalno matinih stijena tipaVlasta-1. Posebno se istiu strukture na jugozapadnom dijelu depresije gdje se moguoekivati zamke u ''uzlaznim'' klinoformama.


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4.3 Vis Lastovo Mljet SectorSector south of the island of Visa, which nicely illustratesthe transition from platform into South Adriatic Basin is

visible on interpreted seismic cross-section L-16-82 (Fig.13).

Seismic cross-section L-16-82, Fig.13, includes a partof Dinarides platform on the transition into the AdriaticBasin. Transition is characterized by development ofslope deposits, i.e. peri-platform clastics as potential res-ervoir rocks of considerable thickness.

Intense eroding-abrading of Upper Cretaceous rudistlimestone is very typical northeast of Maja-1 well. Detri-tus of these sediments is resedimented in the direction of

Adriatic basin. Upward clinoform shapes were createdin these peri-platform clastics as potentially promisingpetroleum-geological traps, which are especially ex-pressed at shot point 800 - 900 of this cross-section.Traps are situated at the depth two seconds of two waytime, i.e. approximately at 3 500 m. In this study they

were studied as promising plays for further exploration.It is especially important that they are situated nearPalagrua east depocenter which can be considered as

potentially oil generativecenter for migration intomentioned peri-platformclastics. Reservoir charac-

teristics are supposed in thedeposits similar to the previ-ously described ones onIM-3 well which were cre-ated in identical paleo-geo-

graphic conditions. In theirtop, these deposits have

good seal sediments of youn-ger Tertiary age of consider-able thickness. Consideringthe favorable petroleum-geo-logical conditions, seismiccross-section V-16-83 andespecially cross-section

L-16-82 and maturation dia-gram (Fig. 14), indicate es-pecially favorable conditionsfor oil and gas generationand accumulation, thus thementioned area is consid-ered to be very promising

within the domain of statedcross-sections. According tothe mentioned above it isconsidered as the mostsuitable one for furtherpetroleum-geologicalexploration.

Fig. 14 shows maturationdiagram for the selectedpoint A which is close to shotpoint 1000 of seismiccross-section L-16-83 andconfirms that Ladinian-Carnian depositshave been for 60 million

years at the depth of over 7 500 m where their averagematurity of 0.7-1.0 (%Ro) can be expected.

According to the constructed maturity diagram for thispart of South Adriatic, deposits older than Norian andRhaetian at the depth of over 8 000 m should be situatedin the main maturation phase with 0.1 to 1% Ro. Two

ways time of 5.5 seconds undoubtedly proves thatLadinian-Carnian deposits are situated below that depth.Geothermal map also proves that the considered area issituated in favorable geothermal conditions for sourcerock maturation with temperatures up to 80 C (Fig. 15).

Fig. 15 shows regional map of geothermal relations(map of isotherms) where it is visible that in the area ofSouth Adriatic there are favorable conditions for matura-tion of potential source rocks such as Ladinian-Carnianeuxinic shale. The map indicates the potential migrationdirection toward northwest.

One of especially favorable conditions is the existenceof good potential source rocks such as the ones drilled in

Vlasta-1 well, where the thickness of Ladinian-Carnianshale is 1 360 m, TOC up to 4.2% with oil show at thedepth of 5 402 m, where 16 m3 of oil were acquired. This


NAFTA 61 (7-8)325-338 (2010) 333

Fig. 12. The purpose of P-44A-82 seismic cross-section interpretation is to present morecompletely the sedimentation basin with pelagic development of younger Mesozoicdeposits southwest of Palagrua structural uplift. Potential source deposits ofLadinian-Carnian southwest of the structure were lowered over 5 sec. of two way timeto maturation level. The migration into traps within peri-platform Cretaceous-Paleogene clastics is supposed from this depression.

Sl. 12. Namjena je interpretacije seizmikog profila P-44A-82 da prikae potpunije sedimentacijskibazen s pelagikim razvojem mlaih mezozojskih naslaga jugozapadno od strukturnogizdignua Palagrua. Potencijalno matine naslage ladinika-karnika jugozapadno odstrukture sputene su preko 5 sek. dvostrukog vremena tj. na nivo maturacije. Iz ovedepresije se pretpostavlja migracija u zamke unutar periplatformnih kredno-paleogenskihklastita.


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data is especially encourag-ing because the spreading ofeuxinic deposits of Vlasta-1type has been determined on

the wider area especially inthe direction of the designedPopovii-1 well in RavniKotari.

We propose the explorationdrilling on the position ofshot point 900 where up-

ward clinoforms were inter-preted as favorablepetroleum-geological trapsfrom the depocenters men-tioned in the previouscolumn.

4.4 Mljet Rt otra(Boko kotorski zaljev)Bitumen traces were recog-nized on Mirjana-1 well. Thedrilling was performed at the

vertex of uplifted structure.Interpretation showed thatalso here, due to frequentemersions, main collectorand seal rocks were de-stroyed by erosion.

Interpretation of cross sec-tions D-68-83 and M-67-82, Fig.16 showed in-tense erosion and resedimentation on

Dinarides carbonate platform margin andsedimentation down the platform slope. Thisis very steep and tectonically conditionedslope where tectonic leap is over 4 000 m and

where it is possible that the peak of paleo-tec-tonic structure is rotated in the direction of

Adriatic basin.

Similar procedure was proposed by Grandiand Veseli already in 2002.5 It was clearlystated in the mentioned work that the possiblereason for lack of oil discovery on Kate-1structure was the fact that this structure has

been, after initial development in Laramianphase and additional shaping in Pyreneanphase, rotated in counterclockwise direction.The mentioned rotation most probably moveda potential reservoir of this structure in the di-rection of Dugi Otok Basin 1.5 km. Kate-1 wellconfirmed only numerous oil traces instead ofcommercial reservoir. From the top of Eoceneforaminifera limestones which is situated at2 000 m up to final depth at 5 800 m, shows ofoil were determined which has most probablylaterally migrated from ''missed'' reservoir to-

ward vertex of Kate structure.

Potential traps are primarily expected atstructure flanks in peri-platform clastics

spreading zone which are marked on the map.According to the above described conditions,this is the case of great thickness and mass of

334 NAFTA 61 (7-8)325-338 (2010)


Fig. 13. Seismic cross-section L-16-82 includes part of Dinarides carbonate platform on thetransition into the Adriatic Basin. Transition is characterized by development of slopedeposits, i.e. peri-platform clastics as potential reservoir rocks of considerablethickness.

Sl. 13. Seizmiki profil L-16-82 obuhvaa dio Dinaridske karbonatne platforme na prijelazu uJadranski bazen. Prijelaz je karakteriziran razvojem pregibnih naslaga, odnosnoperiplatformnim klastitima kao potencijalnim leinim stijenama znaajne debljine.

Fig. 14. Maturation diagram for the selected point AA which is close toshot point 1000 of L-16-83 seismic cross-section confirms thatLadinian-Carnian deposits are situated at the depth of over 7500mwhere their average maturity of O.7-1.0 (%Ro) can be expected.

Sl. 14. Dijagram maturacije za odabranu toku A koja se nalazi u bliziniTP-1000 seizmikog profila L-16-83 potvruje da se ladiniko-karnike

naslage ve 60 milijuna godina nalaze na dubini veoj od 7 500 m gdjese moe oekivati njihova srednja zrelost od 0,7-1,0 (%Ro).


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peri-platform clastics which,according to seismiccross-section (TP 700-800)can have thickness even big-

ger than 2 000 m. This geo-logical model represents alsoan interesting explorationplay as on the neighboringJJ-3 well oil has been con-firmed and it can be con-cluded that also this is thecase of very promisingpetroleum-geological play(Fig. 17).

Cross-section Y-80-10, inFig. 17, includes pointed outstructure on carbonate plat-form margin. Medium heavy

oil show was determined onmarked JJ-1 well.

Significant quantities ofwet gas were confirmed onJJ-1 well which is situated15 km from JJ-3 well inOligocene peri-platformclastics (C4 - C7).

The described petro-leum-geological relations incentral Adriatic area and es-pecially of south Adriaticarea pose the need for thor-ough interpretation. It is nec-essary to adjust theexploration concept of cer-tain structures to new ac-quirements. That especiallyconcerns negative impacts ofemersion processes and it isimportant to determine com-plete evolution of certainstructural traps from initialstructuring phase to firstemersions or covering by sealrocks.

5. Preliminary hydrocarbon potentialassessment of peri-platform deposits

Based on 12 interpreted cross-sections of North, Centraland South Adriatic, a preliminary assessment of poten-tial reserves was made. It is supposed that this is only thefirst phase of this assessment which should be continued

by cross-sections interpretation.

Additional significance of peri-platform clastics petro-leum-geological potential was acquired by oil discoveryon Rovesti structure on the Italian side of Adriatic wherein 2007, commercial oil quantities were discovered.Transverse seismic cross lines where their cross-section

was marked were used for assessment of total volume of

reservoir rocks. Naturally that it is very difficult to as-sume that along the whole edge of carbonate platform thesame sediments were almost continuously saturated

with hydrocarbons but it can be assumed that there isbigger or smaller number of possible accumulations thatwould contain significant quantities. For potential reser-

voir it is also necessary or lateral pinching out of reser-voir rocks (clastics) toward impermeable deposits orfaults which serve as barrier. If we assume that thoseconditions exist, we can try to determine the total volumeof reservoir rock and assess regional potential ofhydrocarbon accumulation.

Using the average surface of reservoir rocks calculatedfrom transverse cross-section profile and longitudinalspreading of peri-platform sediments, the total volume

was approximately calculated. Considering the ex-pressed great uncertainty in the assessment of parame-ters for calculation, stochastic (Monte Carlo) simulation

was applied for acquiring the most realistic value possi-

ble and due to the same also the other parameters wereobserved in a very strict manner. Figure 18 shows MonteCarlo simulation results.


NAFTA 61 (7-8)325-338 (2010) 335

Fig. 15. Regional map of geothermal relations (map of isotherms) shows that in SouthAdriatic area there are favorable conditions for maturation of source rocks likeLadinian-Carnian euxinic shale. Potential migration direction toward northwest ismarked on the map.

Sl. 15. Regionalna karta geotermalnih odnosa (karta izotermi) prikazuje da u podruju JunogJadrana postoje povoljni uvjeti za sazrijevanje potencijalno matinih stijena kakovi suladinikokarniki euksinski ejlovi. Na karti je oznaen potencijalni smjer migracije premasjeverozapadu.


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From the figure it is visiblethat the total petroleum-geo-logical potential can be as-sessed from 2 3.3 billion

m3 of oil equivalent, i.e. 386 -524 million m3 conditionallyrecoverable reserves. Natu-rally these values are withoutexpressed geological riskand they show maximal val-ues that can be expected. Inthe attempt to assess the sizeof certain ''lead'', the risk of95 - 99% was calculated

which is the adopted valueconsidering poor percentageof exploration of the consid-ered play and unproved hy-

drocarbons. Thus the valueof 5.3-21.2 million m3 of con-ditional oil as an assumptionfor the range of certain ''lead''is in fact boundary value forundertaking exploration byexploration drilling regard-ing the associated price andrisk.

Determining explorationplays and their feasibility arestill far from realization asother geological parametersneed to be determined first.

6. Conclusion andrecommendationsPetroleum-geological poten-tial of certain area is usuallyassessed on the basis of con-ditions for hydrocarbon res-ervoirs and those are: (1)existence of source rocks, (2)existence of depocenters with

good geothermal conditionsfor TOC maturation, (3) con-ditions for migration into fa-

vorable traps, (4) existence ofreservoir rocks with goodcollector characteristics and(5) existence of seal depositsof regional spreading.

The existence of these basicpetroleum-geological condi-tions shall be describedmore precisely herebelow.

1. Source rocks. As we havepreviously mentioned,Ladinian-Carnian euxinicshale of Vlasta-Komia typedeposits were taken in con-

sideration as most importantsource rocks. in these depos-its in Vlasta-1 well, TOC up

336 NAFTA 61 (7-8)325-338 (2010)


Fig. 16. Interpretation of D-68-83 and M-67-82 cross-sections shows intense erosion andresedimentation on Dinarides carbonate platform margin and sedimentation down theplatform slope. This is very steep and tectonically conditioned slope where tectonicleap is over 4000m and where it is possible that the peak of paleo-tectonic structureis rotated in the direction of Adriatic basin.

Sl. 16. Interpretacija profila D-68-83 i M-67-82 svjedoi o intenzivnoj eroziji i resedimentaciji narubu Dinaridske karbonatne platforme i taloenju niz -pregib platforme. Radi se o vrlostrmom i tektonski uvjetovanom pregibu gdje je tektonski skok preko 4 000 m i gdje jemogue da je vrh paleotektonske strukture rotiran u smjeru Jadranskog bazena.

Fig. 17. Cross-section Y-80-10 includes pointed out structure on carbonate platform margin.As already mentioned, heavy oil was confirmed on the well drilled on this structure.

Sl. 17. Profil Y-80-10 obuhvaa istaknutu strukturu na rubu karbonatne platforme. Na oznaenojbuotini JJ-1 utvrena je pojava srednje teke nafte.


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to 4.2% was determined as well as medium heavy oilshow. These deposits, which in the mentioned well havethickness of 1 300 m and in the direction of Ravni Kotari,according to seismic cross-sections can reach thicknesseven up to 2 000 m. They were also interpreted in thearea of seismically confirmed depressions fromPalagrua to depressions in Mljet island offshore.

2. The existence of oil generating depocenters. Thementioned depressions were considered as generativedepocenters which were related to Kurveleshi trough in

Albania and are genetically related to oil fields in Vloraport area. In the present exploration phase, oil generativedepressions are not so expressed as in Central and North

Adriatic.

3. Distance of potential generative centers. Depocen-ters Palagrua and Mljet from 4.5 to 5,5 seconds ofdepths are situated at the distance that can provide goodconnection for hydrocarbon migration. The example of

discovery on the structure indicates the possibility thathydrocarbon accumulation was result of migration fromdepocenters of central part of South Adriatic Basin and

Albanian part of Ionian Basin which is well known for nu-merous source rock horizons. Especially known isKurveleshi belt where five horizons of Reth-Lias,Toarcian (Posidonia beds), Dogger (green beds),Oxfordian-Tithonian (''upper chert'') and argillaceous de-

posits of Albian age. The mentioned horizons are impor-tant also for prosperity of Croatian part of Adriatic basinconsidering the possibility of migration in direction ofLastovo and Palagrua islands.

4. Peri-platorm clastics as potential reservoir rocks.According to interpretation of seismic cross-sectionsL-16-82 and M-34-82 at the southwestern flank ofDinarides carbonate platform slope, good possibilitiesfor hydrocarbon accumulation into statigraphic traps

were anticipated. Mostly those are upward clinoformswhich pinch out opposite of platform slope or towardfault plane which has caused the creation of the slope.Porosity and permeability were graded according to labo-ratory data on IM-3 well core analyses as there are noother wells in peri-platform clastics with laboratory data.


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Fig. 18. shows the results of Monte Carlo simulationSl. 18. Rezultati Monte Carlo simulacije


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5. Seal rocks. According to seismic and well data, goodseal rocks are made of Tertiary deposits of argillaceous marl content with impermeable characteristics. Analogy

with Rovesti-1 well shows that those are Lower Miocene

deposits of Bisciario formation type. Seal rocks are situ-ated in Rovesti well at the depth of 970 m and accordingto interpretation of seismic cross-sections, somewhatlower depths can be expected in the Croatian part of

Adriatic.

References1. Aleotti, L., et al.: Integration of Well and Seismic Data for Studying Carbon-

ate Reservoir. Proceedings from the 55th meeting of the EAEG, StawangerNorvay, 1983.

2. Del Ben, A., et al.: Seismic and Heat Flow Study of the Southern Adriatic Ba-sin. Bulletino di Geofisica Teoretica ed aplicata .Vol. XXXVI .n141-144,March December .1994.

3. Grandi,S., et al.: Review of the hydrocarbon plays in the Croatian offshoreand Periadriatic areas. Nafta,7-8. 1974.

4. Grandi, S.: Some exploration results of the stratigraphic modeling of theyounger Mesozoic carbonate strata of the Outer Dinarides, III SimpozijDinaridske Asocijacije. Zagreb 1969.

5. Grandi, S., i Veseli, V.: Hydrocarbon potential of the Dugi Otok basin in theoffshore Croatia. Nafta 53 (6-7), 215-224 (2002).

6. Grandi, S., i Kolbah, S.: Znaaj najnovijeg otkria komercijalne nafte nastrukturi Rovesti u Junom Jadranu za hrvatski dio jadranskog bazena. Nafta60 (2), 83-86 (2009).

7. Kolbah, S.: The Croatian Data for the Global Heat-Flow Database. Newdevelopements in the Geothermal Measurements in Boreholes, Regional

Analyses of the Geothermal Data, Klein Koris-Berlin.

8. Nikolai, C. & Gambini, R.: Structural architecture of the Adria platform andbasin system. Boll.Soc.It. Spec.Isue No.7, 21-37,15 figs., 2007.

9. Petacca ,E.& Scandone, P.: Structural Architecture of the Central Apennines:Interpretation of the CROP 11 Seismic Profile from the Adriatic Coast toOrographic Divide. Tectonics, Vol.27, 2008.

10. Veseli,V.: Facijesi karbonatnih sedimenata mlaeg mezozoika i paleogena u

puinskim buotinama Sjevernog Jadrana. Disertacija,RGN fakultetSveuilita u Zagrebu, 306 str., 1999.

11. Zampieri,D.: Tertiary in the Trento platform Southern Alps, Italy. Tectonics14, 3.645-657, 1995.

Authors:

Sanjin Grandi, Retired graduated engineer of geology.Viktora Kovaia 14, 10010. Zagreb, Croatia.E-mail: [email protected]

Ivan Kratkovi, INA d.d., Zagreb, SD Naftaplin

Igor Rusan, INA d.d., Zagreb, SD Naftaplin

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