Jardin Seismic

Subsalt Depth Seismic Imaging and Structural Interpretation in Dumre Area, Albania

A. Jardin1, F. Roure1 and L. Nikolla2

1 IFP Energies nouvelles, 1-4 avenue de Bois-Prau, 92852 Rueil-Malmaison - France2 National Agency of Natural Resources, Blloku Vasil Shanto, Tirana - Albania

e-mail: [email protected] - [email protected] - [email protected]

Rsum Imagerie sismique profondeur sub-salifre et interprtation structurale dans la rgionde Dumre, Albanie Le challenge de lexploration ptrolire par la sismique rflexion dans les zonesde montagne est dobtenir des coupes capables de bien imager les structures gologiques profondessitues sous une couverture sdimentaire plisse. Cette srie de sdiments peut tre trs complexe etengendrer ainsi de fortes variations latrales et verticales de vitesses sismiques. Dans ces rgions, limagesismique est souvent de mauvaise qualit et lvaluation des profondeurs o se situent les rservoirsdhydrocarbures potentiels reste dlicate et imprcise. Ces difficults proviennent dune propagationcomplexe de lnergie sismique travers les lentilles dformantes des couches superficielles. Unensemble de traitements sismiques, en particulier les migrations en profondeur applicables aprs et avantsommation, a t dvelopp pour obtenir une estimation plus prcise de la gomtrie des couches et pouramliorer, dans de tels contextes gologiques, la construction du modle structural en profondeur.Lapplication conjointe de techniques combinant raisonnement gologique et imagerie sismique enprofondeur peut amliorer la comprhension des dformations structurales et rduire les incertitudes surle modle en profondeur en dcrivant le plus prcisment possible la gomtrie et les vitesses descouches gologiques. Nous avons tabli une mthode interprtative et itrative pour effectuer cetteimagerie sismique et fournir ainsi les images adquates pour aider le gologue interprtateur dansllaboration dun modle structural plus raliste. Nous avons appliqu cette mthode dans la rgion deDumre, en Albanie. Cette rgion, qui fait partie du bassin Ionien situ dans la ceinture de chevauchementalbanaise, se caractrise par une gologie complexe (prsence de sel et tectonique compressive) et uneimagerie sismique difficile. Le principal objectif de cette tude tait de comprendre lchec dun forageprofond et de proposer une nouvelle localisation pour la fermeture possible dun rservoir carbonat,situ sous un diapir de sel affleurant en surface. Cette tude est prsente ici pour illustrer lesamliorations obtenues par lapplication dune dmarche intgre impliquant fortement gologues etgophysiciens et sappuyant sur des outils de traitements sismiques disponibles mais requrant une priseen compte plus importante des donnes gologiques.

Abstract Subsalt Depth Seismic Imaging and Structural Interpretation in Dumre Area, Albania The challenge of seismic exploration in fold and thrust belt settings is to optimize the depth seismicimages of the deep structural objectives beneath a complex overburden that may show strong horizontaland vertical velocity variations. In such areas, the seismic image is frequently of poor quality and thedepth models of deep layers is often false due to the perturbed propagation of seismic energy through thedeforming lens of the overlying layers. A range of seismic processing tools, including post-stack andpre-stack depth migrations, are appropriate to predict the accurate geometry of deep target structuresand to improve the building of a depth structural model.

Oil & Gas Science and Technology Rev. IFP Energies nouvelles, Vol. 66 (2011), No. 6, pp. 911-929Copyright 2011, IFP Energies nouvellesDOI: 10.2516/ogst/2011100

European Hydrocarbon ResourcesRessources ptrolires en Europe

Do s s i e r

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Oil & Gas Science and Technology Rev. IFP Energies nouvelles, Vol. 66 (2011), No. 6

INTRODUCTION

Seismic structural interpretation in complex areas such asfaulted, folded and thrust zones is particularly difficult whenit is based on conventional time seismic sections. These timeimages often show a distorted representation of the real sub-surface geometry even after the application of time migrationprocessing. This is due to complex propagation of the seis-mic signal and energy through the structured overburden.The distortion of the seismic signal can also be affected bythe presence of large lateral velocity variations generated bylithological layer characteristics (Jardin et al., 2005).

In such setting, the objectives of the seismic tools is toproduce seismic images to see the invisible beneath foothillsstructures (Gray et al., 2002), that is to say, firstly to per-form the displacement of misplaced reflections and thefocussing of diffractions generated by faults and discontinu-ities and then, to be able to accurately estimate the depthlocation of the potential reservoirs. Presently, to reach theseobjectives, the most sophisticated processing technique ispre-stack depth migration. But this technique, which is nowavailable for 2D and 3D applications on pre-stack seismicgathers, remains costly and requires difficult interpretationwork in particular to estimate the depth velocity model, espe-cially for onshore exploration (Albertin et al., 2002). To copewith these limitations, post-stack depth migration techniquesoffer an alternative processing flow even if the assumptionsof the simplified theory of the seismic stack based on thecommon mid point geometry and multiple fold coverage arecommonly violated in complex structured areas. Nevertheless,if the main objective of the seismic data is to obtain seismicimages which can help the interpreter to establish reliable andrealistic structural models of the subsurface, post-stack depthmigration is still a quick and efficient processing tool whichcan be easily applied to test the different hypotheses pro-posed by the interpreter (Cabrera et al., 1998). To obtain reli-able results, both seismic processing and interpretation tasksmust be done iteratively and require strong integrationbetween the geologist and the geophysicist. We have appliedthis workflow to several case studies, in various foothillsareas worldwide (Jardin and Collaku, 2004; Jardin et al.,

2005; Bche et al., 2005). The workflow has been applied tothe Dumre block which is characterized by a complex geolog-ical environment involving a salt diapir and thrust structuresand the results are presented here to illustrate the benefits ofthe workflow for oil and gas exploration.

The regional geology and hydrocarbon exploration ofAlbania will be reviewed to provide the context for thegeological setting of the Dumre area, which is in the Ionianbasin in the central part of Albanian thrust belt. The Dumrearea is structurally complex which is characterized by a largescale salt body outcropping at surface. The depth seismicimaging workflow is described in detail including the depthseismic images associated with depth velocity models.Finally an improved structural interpretation is proposedbased on these seismic images, which leads to new potentiallyprospective drilling targets in this region.

1 GEOLOGICAL BACKGROUND

1.1 General Information on Albanides

1.1.1 Regional Geology

Albania forms part of the Dinaric-Albanic-Helenic arc of theAlpine orogen, the formation of which occurred mainly dur-ing the Tertiary and gives the country its mountainous relief.From a geological point of view, Albania is a typical forelandfold and thrust belt system: the mountain front and the fore-land fold belt of Albanides include all the main elements ofthrust systems worldwide, including drag folds on thrustfaults, backthrusting, passive roof duplexes and trianglezones. Structurally, the mountains are divided into theInternal and the External Albanides. Figure 1 shows thestructural map of central and southern Albania.

The Internal or Inner Albanides consist partly of ophiolites.The External Albanides are divided in different zones sepa-rated by thrusts and characterized by Triassic to Eocenecarbonate facies and Oligocene to Pliocene silico-clastics.The zones are, from east to west, the Kruja platform zone andfurther to the west, detached on top of Triassic evaporites, the

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A strong combination of geological reasoning and depth seismic imaging processing can improve theunderstanding of the deep geological structures by reducing the uncertainties in depth geometrical andvelocity model estimation. We propose an interpretative and iterative approach to the post stack depthmigration method to guide the interpreter in the development of a reliable subsurface model. We haveapplied this approach during an exploration study in the Dumre area, located in the Ionian Basin(Albania) which is a complex fold and thrust belt. The main objectives of this study were to understandthe failure of a former exploration well and to propose a new location for the potential closure of thecarbonate structure. This subsalt imaging study aims at illustrating the improvements obtained byapplication of this integrated seismic imaging method especially in the evaluation of a subthrust prospectin a tectonically complex belt setting.

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A Jardin et al. / Subsalt Depth Seismic Imaging and Structural Interpretation in Dumre Area, Albania 913

Figure 1

Structural map of central and southern Albania with the location of the Dumre block and main oil and gas fields around this area (modifiedfrom Roure et al., 2004).

MACEDONIA

GREE

CE

ADRI

ATIC

SEA

SHKO

DER P

EJE LIN

EAME

NT

ALBANIANALPS

BURRELI

BASIN

TIRANA

DEPRESSION

PERIADRIATIC

DEPRESSIONKRUJA

ZONE

KORCABASIN

BERATI BELT

KURVELESHI BELT

CIKA BELT

APULIAN ZONE

1 BALLAJ-DIVJAKA 2 PEKISHTI 3 KDCOVA4 KOLONJA-BUBULLIMA5 MARINZA-PATOS6 SEMAN-POVEICA7 ERAKULA

8 BALSHI-HEKAU 9 CAKRANI10 GORISHTI-KOCULI11 DRASHUVICE12 FINIQ-KRANE13 DELVINA14 VISQKA

KEY TO OIL AND GAS FIELDS

0 40 km

IONIAN ZONE

Synorogenic deposits

Sazani zone

Ionian zone

Kruja zoneKrasta-Cukali zone

Inner Albanides

Major thrustsGas discovery

Gas field

Gas condensate field

Oil field

Gas seep

Oil seep

Approx. locationDumre permit

1

2

34

56

7

89

10

14

11

12 13

v v

vv v

v v

v

KRASTA

- CUKALI

ZONE

DUMRE SALT

VLOR

A-EL

BASA

N

TRAN

SFER

ZONE

IONIANZONE

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Berati belt, the Kurveleshi belt and the Cika belt which formtogether the Ionian zone and consist of pelagic limestones(Roure et al., 2004) (Fig. 1). The westernmost cape of theAlbanian coast is formed by the Sazani zone, which is aneastverging thrust involving the Apulian platform (Nikolla etal., 2002).

Major strike lineaments are present controlling thesedimentary infill and the tectonic style in the Albanides. TheVlora-Elbasan lineament roughly delineates the northernboundaries of the Cika belt, Kurveleshi belt and Berati beltand forms also the southern boundary of the Peri-AdriaticDepression (Fig. 1).

1.1.2 Oil and Gas Exploration

The northern part of the Ionian Zone is characterized by theoccurrence of numerous productive oil, gas and gas condensatefields, as well as yet untested prospects. Presently, theCretaceous-Eocene carbonate sequence sealed by theOligocene flysch is the main target of hydrocarbon explorationin the Albanian thrust belt. Some of the fields located incarbonates, are involved into the tectonic deformation wherethe Tertiary overburden is present, notably along the Vlora-Elbasan transfer zone which separates the Peri-Adriaticdepression in the north from the Ionian basin to the south,and extending from the External Albanides in the west, to theInner Albanides in the east (Fig. 1). The interplay betweenevaporites and carbonates can significantly control the sealcharacteristics of the reservoir and influence the deformationand the diagenetic and thermal conductivity processes. Themountainous relief provides excellent outcrop information onthe carbonate reservoir rocks and the marl and clay top seal.Due to the large scale overthrusting, new hydrocarbonprospects are expected in the subthrust domain. Many drywells were drilled however on promising exploration targetsprobably due to difficulties in interpretation of seismicimages in the time domain alone, and to insufficient geologicalcontrol from deep wells (Gjermani and Dhima, 2009).

1.2 Major Lithostratigraphic Units Around the Dumre Block

The Dumre block is located at the boundary between thePeri-Adriatic depression and the presently inverted Ionianbasin, along the major Vlora-Elbasan lineament (Fig. 1). Thesynthetic stratigraphic columns of both Ionian and adjacentKruja zones are presented in Figure 2. The stratigraphiccolumn of the Ionian zone is composed of Upper Triassicevaporites, Upper Triassic to Eocene carbonates, Oligoceneflysch, lower Miocene to Serravallian premolasse andTortonian to Pliocene molasse (Fig. 2).

Because the northern and southern External Albanidesdiffer greatly and because the Dumre block is located at theboundary between these two provinces, we shall describe

here two reference cross-sections, one immediately north ofDumre, and the other one just to the south (Fig. 3) (Roure etal., 2004).

1.2.1 Major Units North of the Dumre Block

From west to east, two distinct tectonostratigraphic domainsof major interest for petroleum exploration can be distin-guished (Fig. 3, cross-section A-A).The Peri-Adriatic DepressionIn the west, the Peri-Adriatic depression is characterized by athick synflexural or synkinematic terrigenous sequence andincludes both Oligocene flysch and Miocene-Pliocenemolasse series. Despite the fact that its pre-orogenicMesozoic and Eocene substratum has not yet been drilled, itis assumed to be of basinal type. It is probably very similar tothe Ionian basin in the south or to the offshore Adriaticprovince. However, unlike the Ionian basin, no detachment isknown to occur here in the basal Triassic sequence, indicat-ing a major lateral facies change with a progressive disap-pearance of the evaporites from the south (Ionian basin) tothe north (Peri-Adriatic depression). Nevertheless, both theOligocene flysch and the Neogene shales of the Peri-Adriaticdepression were locally activated as major detachment levelsat the front of the External Albanides. Exploration for biogenicgas is being actively pursued in the clastic reservoirs of theMio-Pliocene sequence.

The Kruja Platform DuplexesDuplexes of the Kruja carbonate platform have been recognisedby drilling and constitute two additional surface trends, i.e.the Makareshi trend and the Daiti Mountains, the major andoutermost relief involving Mesozoic carbonates in north-central Albania. Outcrops and well data indicate that theKruja duplexes are probably detached from their pre-Cretaceoussubstratum along intra-Cretaceous shale horizons, as noJurassic or Triassic rocks have ever been found in thesestructures. Cretaceous and Paleocene-Eocene sequencesrelate to shallow-water platform carbonates, with frequenthiatuses, flooding, erosional surfaces and even bauxitic hori-zons. Transgressive deep-water Oligocene turbiditic flyschsequences attest to the progressive flexing of the formerKruja platform as it entered into the Albanides foredeep inPaleogene times, soon after the obduction of the ophiolites inthe inner parts of the belt. Unconformable Tortonian toPliocene horizons were locally trapped in piggyback basinsthat overlay the Kruja duplexes, recording a progressive tec-tonic emplacement of these thin-skinned structures duringNeogene times. Although no oil has yet been produced fromthe Cretaceous carbonate reservoirs in this region, numerousoil seeps are known at the base of the Neogene unconformityand are probably sourced from a deeper but active petroleumsystem with a possible mature source rock in the Cretaceousseries.

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1.2.2 The Ionian Basin Units, South of the Dumre BlockSouth of the Vlora-Elbasen lineament, the Ionian basin ispresently dissected by four major thrust units, i.e. the Cika-Tragjasi belt, the Kurveleshi-Ftera belt, the Mali Gjere andthe Berati belts (Fig. 1). Unlike in the northern External

Albanides province, the Triassic evaporites constitute themajor detachment level in this region. Jurassic shales(Posidonia Schist) and Oligocene flysch sequences act alsolocally as secondary disharmonic levels (Fig. 3, cross-sectionB-B). The Triassic salt has also generated huge diapirs,


Figure 2

Synthetic stratigraphic column of the Ionian zone and the adjacent Kruja zone.

Burdigal ian

Post - Tortonian

Tortonian

Tortonian

Messinian

Pliocene

Serravalian

SerravalianBurdigalian

BurdigalianOligocene

Oligocene

Eocene

Unknown

PaleoceneUpper Cretaceous

Lower Oligocene

Lower Oligocene

Lower Cretaceous

Jurassic

EocenePaleoceneUpper Cretaceous

Upper Triassic

Mol

asse

Flys

chSa

lt?Pl

atfo

rm

Ionianzone

Krujazone

Miocene to Pliocene - Quaternary

Oligocene flysch Triassic evaporites

Jurassic - Cretaceouscarbonates (Ionian zone)

Paleocene - Eocene pelagic carbonatesCretaceous platformcarbonates (Kruja zone)

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which are now most frequently unrooted, being transportedpiggyback on the basal detachment together with theallochthon (Velaj and Xhufi, 1995). The Ionian zone inAlbania is characterized by great thickness and facieschanges in the Upper Triassic to Liassic series, indicating theoccurrence of tilted blocks (late Triassic to Liassic dolomiteor platform carbonate) and confined sub-basins (organic richToarcian Posidonia Schist). Two dark chert horizons arewidespread in the Dogger and Upper Jurassic sequences(Roure et al., 2004). Together with two intra-Cretaceousphosphatic horizons, respectively Turonian and early Senonianin age, these series constitute a set of parallel regional markerbeds that lie unconformably above the contrasted architectureof underlying horsts (white limestones) and grabens (darkshales) of the synrift sequence (Fig. 3).

Slumps and carbonate turbidites are frequent in the UpperCretaceous-Paleocene, whereas fine grained white to pinkpelagic carbonates (scaglia) constitute most of the Eocenesequence. Open fractures frequently improve the reservoirquality of these Upper Cretaceous to Eocene carbonates and

the overlying Oligocene flysch sequence constitutes a verygood seal. Potential source rocks are known both in UpperTriassic or Liassic bituminous dolomites and in the ToarcianPosidonia Schist.

1.2.3 Synorogenic Tertiary Sequences and Timing of the Deformation in the External Albanides

Synflexural deposits become progressively younger westward,with the first flysch being Upper Cretaceous to Eocene in agein the east (Krasta units) and no older than Oligocene in theIonian and Kruja domains. Additional flexural deposits weredeposited during the Neogene in the Periadriatic depressionand on top of the Sazani area (Roure et al., 2004). Similarly,the onset of the compressional deformation is oldest in theeast, i.e. late Jurassic obduction of the Mirdita ophiolite andbecomes progressively younger in the west, with the Plioceneinfill of the Peri-Adriatic depression being largely back-thrusted over the Kruja duplexes. Most deformations in theIonian basin and the thrust emplacement of the Kruja

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Figure 3

Structural sketch around the Dumre block and depth interpretation of the Vlora-Dumre transfer zone (modified from Roure et al., 2004).

0 40 km

Triassic evaporite

Triassic evaporiteTriassic evaporite No evaporite ??

IONIAN units(basinal)

No intra-Triassicdetachment

Cretaceousslope

Potential Triassicdetachmentsnot activaded

Periadriaticbasin

Periadriaticbasin

Intra-Tertiarydetachments



Periadriaticautochthon

Tortonianunconformity

DaitiOut-ofsequencethrust KRUJA

(platform)

Intra-Cretaceousdetachment

SAZANI(platform)

IONIAN inverted

units

Hypothesis without pre-existing basement structure Hypothesis with a pre-existing basement fault

A A'

B B'

C C C' C' Lateral ramps

Transferboundary

SazaniIONIAN

ZONE

PERI-ADRIATICDEPRESSION

A

C

C'

A'

BB'

Majorlineament

K

vv

vv

v vv

IONIAN invertedunits

Figs.5&6

Figs.7&8

INNER ALBANIDES

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duplexes are nevertheless best interpreted as resulting froma continuum of deformation, lasting from Burdigalian toPliocene time. Diachronous unconformities at the base of thepiggyback basins and progressive tilts of the synkinematicsediments effectively record ongoing deformation (Roure etal., 2004). Restoration of balanced cross-sections to their pre-orogenic, palinspastic geometries helps in estimating theamount of Neogene shortening but only accurate forwardmodelling techniques provide additional constraints on kine-matics, such as the velocities along individual thrust faults(Roure et al., 2004).

1.3 Structural Styles of the External Albanides

Although most of the present-day architecture of the ExternalAlbanides and their foreland relates to Neogene compressions,some pre-existing extensional structures are still preservedin the autochthon, or were passively transported in theallochthon.

1.3.1 Pre-existing Jurassic Extensional Structures

Lower Jurassic extensional structures, i.e. normal faults andtilted blocks inherited from the Tethyan rifting, are especiallywell preserved in the Ionian units. During the inversion of thebasin, only those faults whose spacing and orientation wereconsistent with the Neogene maximum principal stress(sigma 1) were reactivated. The majority of the Jurassic nor-mal faults, which trend at a high angle with the Neogenethrust fronts, were detached from their pre-Triassic substra-tum along the Triassic evaporites or the Toarcian blackshales, and subsequently, passively transported in the upperplate. As a result, the present-day structural grain of theIonian zone is largely inherited from the Jurassic one, butsurface cover structures are normally disconnected from theirinitial basement roots. The frequent occurrence of volcanicsor metamorphic rocks in the diapirs or along major thrustfronts in the Ionian zone indicates that most Jurassic faultsinvolved the basement, prior to being truncated during theNeogene episodes of deformation. Alternatively, basementshort-cuts could have taken place during the inversion tectonicsnear the crest of former tilted blocks, and would account forthe remobilisation of basement highs during Neogene times.

1.3.2 Decoupling Horizons and Thin-skinned Structures

Four major detachment levels have a regional distributionand account for most thinskinned structures observed, i.e. thethrust sheets, antiformal stacks of duplexes and trianglezones. They comprise: the Triassic evaporites, which constitute the basal detach-

ment beneath the Ionian allochthon; the Toarcian Posidonia Schist, which constitutes a secondary,

shallower potential detachment in the Ionian basin, whenpresent;

the hypothetical intra-Cretaceous evaporites of the Krujaplatform, which comprises only Cretaceous and youngerhorizons, being entirely detached from its former infra-Cretaceous substratum;

the Oligocene flysch and the Neogene shaly interbeds,which constitute the only potential detachments at thedeformational front in the Periadriatic depression.

1.3.3 The Vlora-Elbasan Transfer Zone and Changesin the Detachment Levels in the Vicinityof the Dumre Block

The deformational front is abruptly displaced westwardbetween the Peri-Adriatic depression in the north and theIonian basin in the south, whereas the front of the InternalAlbanides remains approximately linear farther east, preclud-ing any lateral change in the overall amounts of Neogeneshortening between the northern and southern parts of theExternal Albanides (Roure et al., 2004) (Fig. 1). This rapidjump or curvature of the External Albanides thrust frontimplies the existence of a transfer zone, which parallels themajor NE-trending Vlora-Elbasan or Lushnje-Elbasan linea-ment. Although no thrust or tear faults trend in the samedirection at the surface between the Ionian units and Peri-Adriatic basin, this structure has a clear regional meaning andprobably results from the reactivation of a major paleogeo-graphic boundary. When comparing this peculiar structuralpattern with results of analogue models of thrust propagationin anisopachous stratigraphic sequences, two distinct butcomplementary hypotheses can be proposed to account forthe changes observed in the thrust architecture, with a steepand relatively narrow accretionary wedge in the north (Krujaduplexes and Peri-Adriatic depression) and a wide butsmoother wedge in the south (Ionian basin) (Roure et al.,2004). The first parameter that can induce such a change inthe geometry of the thrust front is the depth to the basaldetachment. For the same amount of shortening, the deeperthe detachment is, the wider the thrust units are. The secondparameter relates to the coupling (friction) along the basaldetachment. Thrusts propagate faster and further in plastichorizons, i.e. evaporites, than in brittle materials, i.e. shaleand sand interbeds. From observation of the subsurface data,it appears that the basal detachment does remain shallow inthe Oligocene flysch and even in the Neogene clastics in thenorth beneath the Peri-Adriatic depression, whereas it isdeeper, in the Triassic evaporites, beneath the Ionian basin.Thus, the lineament probably represents a major intra-Triassic structure, with the disappearance of the Triassicevaporites northward beneath the Peri-Adriatic depression.

1.3.4 Triassic Evaporite Formations

The presence of large evaporite outcrops at Dumre and atother various outcrops in Albania (Gline, Kardhiq, Vermik,etc.) gives evidence of deposits of Triassic evaporites which


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constitute the substratum of the Albanide foreland fold andthrust belt. All the outcrops result from evaporite mobilitycaused by the stress regime as the folds and thrusts devel-oped. They occur where there is low lithostatic pressure intectonically active areas where longitudinal faults intersecttransverse faults. These faults occurred during the riftingstage and were inverted during the compressional stage. Theevaporites of the Ionian zone outcrop both along the regionaltectonic faults (e.g. in Dumre) and local structures (e.g. inDelvina). These formations constitute a perfect detachmentlevel. Prospective carbonate structures for hydrocarbonexploration are evidenced under the evaporites throughseismic profiles acquired in this region (Velaj, 2001).

2 DEPTH SEISMIC IMAGING STUDY

2.1 Depth Imaging Workflow in Foldand Thrust Belt Setting

2.1.1 Limits of Time Processing and Vertical DepthConversion of Seismic Lines

Fold and thrust belt areas are characterized by folded andfaulted sedimentary layers of great complexity, associatedwith highly variable topography. Seismic exploration in suchsettings requires modified acquisition and processing tech-niques, due to strong lateral and vertical velocity variations.The resulting seismic information is usually displayed eitheras a time stack section or a time-migrated section. Timemigration processing is used to focus the seismic energy dif-fracted by faults and to place dipping structural events in theright time location. However, in the case of a highly struc-tured subsurface, it results in seismic images which can be farfrom the geological reality. Depth conversion of seismic dataprovides more realistic images of the structural features.Simple time to depth conversion of the seismic sectionrequires the input of mean vertical velocity values related tothe characteristics of the subsurface layers. The vertical timeto depth conversion technique transforms time informationinto depth values but assumes no lateral velocity variations.When these variations are strong, depth migration processingis required to obtain a reliable seismic image (Robein, 2003).Depth migration can be viewed as a two step procedure. Thefirst step is to focus the diffracted energy, as for any migra-tion processing. The second step is to convert the seismicdata from the time domain to the depth domain. In the case ofa complex structural setting, the pre-stack depth migrationtechnique provides the best seismic images in terms of geom-etry but presents the disadvantage of being both costly andtime-consuming. Post-stack depth migration can be a feasiblealternative if the conventional stack sections can reasonablybe considered to represent zero-offset sections or if coherentseismic reflectors remain visible. Both pre-stack and post-stack

depth migration techniques directly provide depth seismicimages, but require the construction of a detailed structuraldepth model describing the geometry of the geological layersand the intra-bed velocity variations in the depth domain.This depth velocity model is especially uncertain in struc-tured zones and can be computed with advanced pre-stackinversion techniques. When pre-stack seismic data areunavailable, it is difficult to immediately define a reliabledepth velocity model from stacked seismic data only. Post-stack depth migration application therefore becomes an inter-pretative processing technique, requiring several iterationsbetween the seismic interpreters and processors.

2.1.2 Interpretative Post Stack Depth Migration

The depth structural model must describe the main geometricand lithological features of the subsurface. In salt and thrustbelt setting, the determination of a structural depth modelfrom post stack seismic data must be the result of combina-tion of both geological knowledge and of seismic techniquesrequiring a structured approach (Fig. 4): the interpretativepost stack depth migration workflow which aims at deter-mining a coherent structural depth model from seismic andgeological data.

Initial Interpretation StepsThe first step of the method is to delineate the target areaand to identify the structural objectives of the study. Thisobjective could be the recognition of hydrocarbon struc-tural traps, the improvement of appraisal well targets orimproved understanding of tectonic styles. In foothillsareas, the importance of velocity model determinationwithin near-surface layers for deep seismic image improve-ment is well known (Muller et al., 2000). Field studies andregional geological maps can yield important informationon surface dips and lithologies. Information from wells,when available, also contributes to the depth velocitymodel of the drilled layers: depth values used for seismiccalibration, velocity and lithology values from logs. Thecomparison of already processed seismic time images pro-vides an identification of seismic shadow areas, an estima-tion of the structural sensitivity to post stack time migra-tion velocities and an evaluation of the influence ofnear-surface layers on the propagation of the seismic signal(signal to noise ratio, consistency of seismic static correc-tions, etc.). Conventional time structural interpretation iscarried out in order to obtain an initial structural depthmodel, by map migration of seismic horizons or by simplelayer-cake conversion of the interpretation. Based on theseinitial processing and interpretation results, the subsurfaceis divided into zones related both to structural complexity(from light to heavily faulted zones) and to seismic imag-ing assumptions (from moderate to strong lateral velocityvariations).

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Iterative Depth Migration StepsA step by step method of depth velocity model building isthen applied, working from the previously delineated simplezones towards more complex zones. After the developmentof a reliable model for the complex overburden, post-stackdepth migration processing is applied using this depth model.For deep and structurally complex target formations, whendepth and velocity calibrations from well data are absent, the

strategy for structural model evaluation is essentially basedon seismic depth migration processing. Because velocityvariations and structural features can only be deduced fromseismic data and geological reasoning at this stage, analysisof the sensitivity of seismic images to the different structuralassumptions is necessary. Various velocity variations fordeep layers are simulated in order to compare the computeddepth images and to quantify the uncertainties related to the


Figure 4

Interpretative seismic depth imaging workflow.

Depth velocity modelsInitial information

Geology

Geophysics

Depth seismic image

Next study

Depth structural model+

Complex overburden

Possible structural models

Structurally simple zones

Structurally complex zones

Field work & outcrop study

Geological backgroundknowledge

Well logs(depths, dips, velocities)

Seismic signal analysis

Seismic velocity analysis

Pre-stack seismic

Structural geology

Time seismic sections

Processingand

interpretation tasks

Objectives

Time to depth

Deep layers

Structural target

Stack seismic checking

Time to depth conversion

Velocity sensitivity analysis

Structural seismicinterpretation

Forward ray-tracingmodeling

Depth migration

Well calibration(depth)

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velocity parameters. Thus the selection criteria for choosing ageologically and seismically consistent model are given byan interpretative comparison between the different depthimages. For each zone, iterations between geological inter-pretations and seismic processing techniques ensure a coher-ent structural depth model. We retain the best seismic imagesof the main structural features such as fault zones and anoptimal depth model is chosen after consistency control ofthe time and depth images obtained by forward modellingand consistency control between the depth velocity modeland its associated depth seismic image. The improvement ofthe seismic energy focussing by comparison with timemigrated images is an additional selection criterion.

Final Structural InterpretationFinally, the computed seismic depth images are interpreted,resulting in the best geological and geophysical depth struc-tural model that can be obtained from this post-stack seismicstudy. In addition, the results could be subsequently used asan initial velocity model for pre-stack seismic imaging pro-cessing such as travel time inversion and pre-stack depthmigration (Bche et al., 2005).

2.2 Application to Dumre Seismic Data

2.2.1 Initial Interpretation

The Dumre evaporite represents a salt dome with a surfacearea of 210 km2, which outcrops with an elliptical shape withaxes of 20 10 km (Fig. 5). In depth, the evaporite body hasa diapiric shape with a thickness of 6.1 km and a westwardoverthrust displacement of more than 20-30 km. Its volumeis over 1 500 km3. Numerous seismic surveys have beenrecorded in this area amounting to hundreds of kilometres ofdata. Figure 6 shows the time migrated section AAP-06-96(line 6) as an example of time images obtained after conven-tional time processing. The salt layer is recognized both bythe lack of seismic reflections inside the diapir and by thepresence of stronger seismic amplitudes at the base of theevaporite body. Below the evaporite, seismic reflections areinterpreted as the prospective carbonate subthrust platformbut its precise geometry cannot be easily determined due tothe lack of continuity of these seismic events. The time mapof the evaporite base has been estimated and combined withthe geological surface map where Dumre diapir boundariesare well recognized (Fig. 5). The features of this diapir rec-ognized through field outcrop studies could provide a betterunderstanding of the geometry of the underlying Mesozoiccarbonates of the Ionian allochthon. Many wells have beendrilled in the past within the Dumre block or in its vicinity butprovide information only on the velocities of the Oligoceneflysch and Neogene. In 1995, the Dumre-7 well drilledthrough the salt diapir, the parautochthonous Oligocene flyschseries being ultimately reached beneath the allochthonous salt

at a depth exceeding 6 km (Fig. 7). This well is located in thecentre of the study area, close to the intersection of linesAAP-11-96 and AAP-06-96. Unfortunately, the well wasabandoned before reaching the carbonates. Vertical seismicprofile data was recorded in the Dumre-7 well but this datawas recorded when the well was still within the salt body, notat the Total Depth (TD). Therefore, these data were only ableto predict the depth to the base of the salt, precluding anydirect computation of the depth to the carbonates (Fig. 7).Using a mean velocity value of 5 km/s, the depth range ofevaporites could be estimated from 5 500 to 7 500 m in thisarea. The Dumre-7 well was drilled at the location of theassumed minimum depth for the base of evaporites. The initialmodel was built based on this initial interpretation of the saltdiapir features and the recognition of the overburden layers.

2.2.2 Depth Migration

Although reliable velocity information and accurate geometricalfeatures of the infra-salt series are still lacking, a calibrationof the seismic on the Dumre-7 well was a pre-requisite forthe depth migration. Only the seismic lines of the 2D surveyof 60 km2 size acquired in 1996 have been reprocessed bydepth migration (Fig. 5). Therefore, two lines, line AAP-11-96 and line AAP-06-96, the most recent ones crossing theDumre-7 well and oriented N-S and E-W were processedfirst (Fig. 5). In order to evaluate the ability of depth migra-tion techniques to decipher the complex architecture of theIonian structures beneath the Dumre salt, six other E-Wtrending lines and four other N-S trending lines have beenreprocessed later, according to the velocity values definedfrom the two first lines.

Good seismic imaging and confident time to depthconversion are particularly difficult in the External Albanidesbecause of the strong lateral velocity variations: from 2 500-2 800 m/s in the Miocene-Pliocene clastics to 5 000-5 500 m/sfor the evaporite and carbonate layers, resulting in distortedtime seismic sections as it is shown in Figure 6, where thedifferences between non migrated and migrated time imagesare clearly visible. Therefore, lateral velocity changes in theoverburden, between the low-velocity molasse deposits in thewest and north and high-velocity salt in the south and eastresult in a complex seismic image, juxtaposing pull-downand pull-up effects in the time sections (Fig. 6). For this casestudy, a layer-cake velocity model was built integrating allvelocity and depth data from shallow and deep wells, andmodified velocity analysis based on the selected criteriaexplained in the previous section. Lateral velocity variationswere modelled using a lateral constant gradient for velocityparameterization in several layers when lithological bound-aries were difficult to accurately track. Control of the mainstructural features was performed by integrating regionalgeological knowledge. In Figure 8, the depth velocity modelestimated for the line AAP-06-1996 is shown as an exampleof velocity models built for all the lines. In the upper part of

920

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Figure 5

Geological map of the Dumre block, with the location of wells and seismic profiles used in this study. The lines indicate the isochronouslines of evaporite base from time seismic interpretation.

19 50

Messinian

Tortonian

Burdigalian

Aquitanian(undifferenciated)

Up. Oligocene

Mid. Oligocene

Permo - Triassic

Subcrop map (in time)= Basal surfaceof the evaporate

Low. Oligocene

Geological mapof the Dumre Block

41 0041 00

40 5040 50

19 50 20 00

Scale: 1:100 0000 1 2 3 4 5 km

20 00

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the sections, depth structural models were built using avail-able regional well data and the geological map of shallowformations (Pekisht 51, Paper-1, Peqini-2 in the western partof the study area and Dumre-7, in the central part, Fig. 5):these data were essential in ensuring the consistency and thereliability of depth migration results (Tab. 1). Therefore, it

was necessary to accurately describe the contours of the flysch/salt boundary in the structural depth model prior to furtherattempts at depth migration processing. Iterative migrationtests have shown that the geometrical limits between the lowvelocity sediments (Neogene molasse, Oligocene flysch) andthe high velocity salt body were critical to obtain a correct

922

Figure 6

Time seismic images for the line AAP-06-96, before (upper) and after (below) time migration processing application.

Pekhist Dumre - 7

Pekhist

Tim

e (s)

Tim

e (s)

Dumre - 7

2 km

2 km

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depth seismic image. Simple zero-offset seismic modellinghas demonstrated that the stack seismic response of sedimentsbelow the salt directly relates to the geometry of the salt/flysch velocity boundary. This lithological velocity boundaryeffectively accounts for strong deviations of the seismic raypropagation.

For the deeper carbonate units, depth model constructionwas based solely on geological conceptual models and geo-physical interpretation, as no well control was available forthese depth intervals (apart from the Triassic-Oligoceneboundary intersected by the Dumre-7 well). Thus, determina-tion of an accurate seismic image was necessarily limited dueto the poor control on geological assumptions. Only onevelocity boundary was introduced into the depth model, toaccount for a potential velocity contrast (assumed to beanabrupt decrease of the velocity) between the salt base anddeeper reflections, in association with moderate lateral veloc-ity variations. The quality of the lower part of the sections(i.e. below 6 km), can be estimated by consistency checkingof the seismic events: focusing of diffractions, continuityimprovement of seismic reflections, easy interpretation ofmigrated reflections firstly in the time domain, and then inthe depth domain. After validation of the velocity model forthe two lines crossing the deep well Dumre-7, post stackdepth migration has been applied to the other lines using thelateral and vertical velocity variations estimated along thesetwo lines. Comparison between adjacent 2D seismic lineswas then used to check the 3D consistency of velocity models.

2.2.3 Structural Interpretation

The 3D geometry of the structural model is now better con-strained. The new depth migrated lines were successful inproviding more realistic 1:1 scale 2D images of the structure.This model can be built by direct interpretation of these seis-mic images combined with the structural knowledge fromtime migrated seismic images. To illustrate this interpreta-tion, the post-stack depth migrated section corresponding tothe time migrated section AAP-06-96 is displayed in Figure 9and can be compared with the time images displayed inFigure 6.

Infra-Salt ArchitectureThe base of the salt is well calibrated in the Dumre-7 welland can be traced laterally, in most directions. However, dueto the lateral velocity changes, the geometry in the time sec-tions is poorly constrained and is better constrained on thedepth migrated lines. In addition, intra-salt out-of-sequencestructures are evident on most of the lines and connect super-ficially to a major thrust surface which brings the Oligocene


Figure 7

Summary schematic log of the deep Dumre-7 well.

Oligocene

Oligocene

Triassic

Flysch

Flysch

0

200

61026119 (TD)

Eva

porit

e

TABLE 1

Estimation of layer velocity variations from regional well data

Formations Layer velocity range (m/s)Post-Messinian deposits 2 400 - 2 700

Neogene molasse 3 500 - 3 800

Olygocene flysch 4 000 - 4 200

Upper Triassic salt 5 200 - 5 600

Ionian carbonates 5 000 - 5 700

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flysch on top of the Neogene molasse and has probably beenencountered in some of the shallow Pekisht wells to the west.

Top of Autochthonous Carbonate SeriesIn most of the lines, deep reflections related to the autochtho-nous carbonate series can be traced from the foreland in thestack sections. However, only the top marker of the autochthonis finally preserved in the migrated lines, at about 7.5 km(4.5 or 5 s TWT), the deepest part of the seismic recordsbeing largely destroyed during the seismic migrations (timeand depth processing).Intra-Flysch Detachment and Cut off of the IonianCarbonatesAs evidenced in the regional seismic and structural transects,the Dumre block is located above a major transfer zone (i.e.the Vlora-Elbasan lineament), where the basal detachment ofthe Ionian allochthonous units jumps progressively from anintra-Triassic horizon in the south-east, toward shallowerintra-Oligocene detachments in the north-west (Fig. 10). Asthe surface trend of this regional feature is approximatelyNE-SW, the cut-off of the Ionian allochthonous carbonatesis assumed to extend in the subsurface, parallel to the north-ernmost surficial contour of the carbonates, at an unknownbut probably discrete distance farther to the north-west (i.e.

forelandward). The western part of the lines AAP-04-96,AAP-06-96 and AAP-07-96 images an east-dipping reflectorbetween the top of the sub-horizontal autochthonous carbon-ates and the gently folded basal surface of the Neogenemolasse (Fig. 9). We have interpreted this reflector as theintra-flysch detachment, which extends westward into theshallow Lushne anticline which involves the Neogene series.In contrast, this detachment is progressively deepening east-ward, thus accounting for the involvement of the Mesozoiccarbonates in the sub-salt allochthon, and a progressive tiltingand rotation of overlying Tertiary horizons. The same intra-flysch detachment is also imaged on the north-south linesAAP-11-96 and AAP-12-96 (Fig. 11). However, on otherlines, the precise location of the cut off of the allochthonouscarbonate on the basal ramp remains questionable. Weassume that this lack of continuity of the reflections resultsfrom a drastic change in the dipping attitude of the carbon-ates, vertical or overturned beds of the frontal flank of theramp anticline thus ensuring a lateral closure for the prospect.

Architecture of the Ionian Carbonate AllochthonNumerous reflections can still be seen below the basal saltsurface and are interpreted as the top of the Ionian carbonates(Fig. 9 and 11). The thickness of the intervening Oligoceneflysch horizons seems quite anisopachous and may change

924

Figure 8

Depth velocity model estimated for the line AAP-06-96 describing the lateral and vertical velocity variations.

5 313

5 125

4 938

4 750

4 563

4 375

4 188

4 000

3 813

3 625

3 438

3 250

3 063

2 875

2 688

2 500

Velocity (m/s)

Depth1km

(Scale 1:1)

1.00

0

2.00Lateral velocity gradient

from 3 500 m/s to 5 500 m/s

Lateral velocity gradientfrom 4 000 m/s to 5 000 m/s

5 500 m/s

5 000 m/s

Target zone

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.0

2 500 m/s

3 500 m/s

4 000 m/s

ogst100046_Jardin 7/12/11 12:35 Page 924

greatly, depending on the structural interpretations. The depthcontour map of the top of the Ionian carbonates was based onthis interpretation (Fig. 12). Nonetheless, as indicated by theattitude of the basal salt contact, there is no structural closureat the present location of the Dumre-7 well, between lines 5and 6.

2.2.4 Definition of a New Prospect

The new depth migrated lines which provide better constraintson the 3D geometry of the structural closure, show that thisstructure extends to the northwest of the Dumre-7 well, theapex of the structure being now expected at a depth of


Figure 9

Depth migrated seismic image for the line AAP-06-96, without (upper) and with (below) interpretation.

W E

Pekhist

Dep

th (k

m)D

epth

(km)

Dumre - 7

2 km

2 km

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Figure 10

Schematic diagram outlining the geodynamic evolution of the Dumre block.

?

Dumre StructureStructural evolution cartoon

Direction of transport

Present

Seismic data

End Miocene

Burdigalien

Miocene to Pliocene IV

Oligocene flysch

Triassic evaporites

Paleocene - Eocene pelagic carbonates

Jurassic - Cretaceouscarbonates (Ionian zone)

Cretaceous platformcarbonates (Kruja zone)

ogst100046_Jardin 7/12/11 12:35 Page 926

approximately 5.5 km, around the intersection between linesAAP-04-96 and AAP-12-96 (noted 4 and 12 in Fig. 12). Astructural closure and a new prospect (an oblique anticlinestructure developed along the Vlora-Elbasan transfer zone) istherefore well defined westward of the previous location

(Fig. 12). The prospect documented here resembles a classicramp anticline, with small antithetic southeast-verging back-thrusts developing at the rear of the structure. Structural clo-sure is evidenced both to the south and the east, but remainsconjectural toward the north and the west. Hopefully, the


W

4.5

7

4.5

7

E

N S

Dep

th (k

m)D

epth

(km)

4.5

7

4.5

7

AAP-12-96

AAP-12-96

AAP-04-96

AAP-04-96

(Scale 1:1)1 km

2 km

Figure 11

Depth migrated seismic images interpreted to define the new Dumre prospect: W-E line AAP-04-96 (upper) and N-S line AAP-12-96(below).

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Figure 12

Contour map (isobaths in metres) of the Dumre prospect at the top of the Ionian carbonates.

5 300 6 300 7 300 8 300 8 700 m

0 1 2 3 4 5 (km)

carbonates will have a northwest-dipping attitude betweenthe anticline axis and their cut-off along the basal detach-ment, thus insuring the lateral closure of the structure. Thelack of continuity of the reflection is likely due to the steeplydipping attitude of the carbonates along the outer flank of the

structure. The shape of the prospect is outlined in the subcropstructural map and the depth to the objective is expected atless than 6 km (i.e. less than in the Dumre well, with asmaller column of salt to be drilled). In order to more accu-rately define the shape and volume of the prospect, the main

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concern would be to identify the northern and western cut-offsof the Ionian carbonates on the basal detachment. Ultimately,the lack of lateral continuity of the reflections between thetop of the structure and the thrust plane is assumed to relateto a very rapid change in the carbonates, with the develop-ment of a vertical or even overturned flank at the front of thestructure.

CONCLUSIONS

Application of poststack depth seismic migration on 2Dseismic lines in Dumre area has provided improved seismicimages resulting in a more reliable 3D structural interpreta-tion. Results led to a better understanding of the failure of theolder exploration wells and to propose a new prospect. Theconclusions of this integrated approach of depth seismicimaging indicate that the potential closure of the infra-saltIonian carbonates is located to the north-west of Dumre-7well. Nevertheless good quality seismic images of deepstructures below the salt diapir and thrust sheets remainschallenging in some areas even if poststack depth migrationis applied where required in case of strong lateral velocityvariations exist. A further improvement of the seismicimages could still be obtained in the Dumre block, by inte-grating additional geological information and by initiatingprestack depth migrations of the seismic sections, using thegeological model built by poststack depth migration.

This study has shown that an interpretative approach ofdepth seismic imaging can provide a better estimation of thegeological structures as it is required for better oil and gaspotential evaluation through advanced basin modelling stud-ies in complex areas (Vilasi, 2009; Thibaut, 2009). An inte-grated team of geologists and geophysicists was essential forcombining geological knowledge and depth seismic imagingmethods to build the required velocity models and interpretthe depth migrated seismic images.

ACKNOWLEDGMENTS

We acknowledge the National Agency of Natural Resourcesof Albania for permission to publish the data, Jean Letouzeyand Emily Albouy formerly from IFP Energies Nouvelles forcontributing to the study. Thanks also to Anglo-Albanian OilCompany and Premier Oil for access to well and seismicdata.

REFERENCES

Albertin U., Kapoor J., Randall R., Smith M., Brown G., SouflerisC., Whitfield P., Dewey F., Farnsworth J., Grubitz G., Kemme M.(2002) The Time for Depth Imaging, Oilfield Review 2.Beche M., Kirkwood D., Jardin A., Roure F. (2005) 2D Depth seis-mic imaging in the Gasp Belt, a structurally complex fold andthrust belt in the Northern Appalachians, Qubec, Canada, inThrusts Belts and Foreland Basins, Lacombe O., Lav J., Roure F.,Vergs J. (eds), Springer, Berlin, pp. 75-90.Cabrera J., Celis V., Amalricua S., Araujo M. (1998) Experiences indepth imaging of Venezuelan data, Lead. Edge 17, 5, 1743-1747.Gjermani I., Dhima S. (2009) Using the Seismic methods in theIonian Zone where overthrust are present, 5th Congress of BalkanGeophysical Society, Belgrade, Serbia, May 10-16.Jardin A., Chaker R., Krzywiec P. (2005) Understanding seismicpropagation through triangle zones, in Thrusts Belts and ForelandBasins, Lacombe O., Lav J., Roure F., Vergs J. (eds), Springer,Berlin, pp. 63-74.Jardin A., Collaku A. (2004) Interpretative poststack depth imagingfor seismic velocity model estimation in central Albania, AAPGEuropean Region Conference, Prague, Czech Republic, October10-13.Muller J., Ricarte P., Forgues E. (2000) An adapted 2D processingsequence for foothills data through synthetic examples, SEG AnnualMeeting, Calgary, Canada, August 6-11.Nikolla L., Kuneshka M., Ademi F., Bejtaj K. (2002) Developmentand structural styles of Albanides, www.akbn.gov.al.Robein E. (2003) Velocities, Time-imaging and Depth-imaging inReflection Seismics. Principles and Methods, EAGE publications.Roure F., Nazaj S., Mushka K., Fili I., Cadet J.P., Bonneau M.(2004) Kinematic evolution and petroleum systems: an appraisal ofthe External Albanides, in Thrust Tectonics and HydrocarbonSystems, AAPG Memoir 82, McKlay K.R. (ed), 24, 474-493.Thibaut M., Jardin A., Sulzer C., Faille I. (2009) ISBA3D:Innovative solutions for 3D basin modelling in complex areas, 71stEAGE Annual Conference, Amsterdam, Nederland, Abstract W047,4 pages.Velaj T., Xhufi C. (1995) The evaporite effect in the tectonic styleof the internal Ionian subzone of the Albanides, 57th EAGE AnnualConference, Glasgow, Scotland, 29 May-2 June, Abstract P574.Velaj T. (2001) Evaporites in Albania and their impact on thethrusting processes, J. Balkan Geophys. Soc. 4, 1, 9-18.Vilasi N., Malandain J., Barrier L., Callot J.P., Guilhaumou N.,Lacombe O., Muska K., Roure F., Swennen R. (2009) From outcropand petrographic studies to basin-scale fluid flow modelling: the useof the Albanian natural laboratory for carbonate reservoir characteri-zation, Tectonophysics 474, 1, 367-392.

Final manuscript received in March 2011Published online in September 2011


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