Latvia’s First Offshore Round – its Potentialand Perspectives

By S. KANEV, O. LAURITZEN and U. SCHMITZ*

ABSTRACTThe first Latvian Offshore Round, ope-ned in April 2001 and closing by January

2002, renders a variety of opportunities relatedto oil exploration and production from EarlyPaleozoic reservoirs.Principal reservoirs of the general area areMiddle Cambrian sandstones, they are expec-ted to hold most of the oil potential in th offeredlicenses. Additional opportunities were identi-fied by means of geological modelling, in theOrdovician and Silurian carbonate succession.These opportunities are related to indicated in-version structures, i.e. to their fracturing, andto seismically identified reefal build-ups.The terms of the Latvian Round are favourable,they obviously honour the magnitude of the ex-pected potential. Likewise, market conditionswere found to be favourable.

1INTRODUCTIONThe Latvian Minister of Economy an-nounced the country’s first offshore

licensing round [1] on April 19th 2001. Theround offer comprises two parts: one co-vers the tender of E&P licence rights, invol-ving 3 licences in the SW of Latvia’s offsho-re and comprising a total of 2,675 sqkm,with a closing date on January 25th 2002,whilst the other, with its closing date onOctober 31st 2001, offers pre-investigationrights, covering major parts of the Latvianshelf (Fig. 1). The differentiation into twoparts reflects the differing expectations ofLatvian authorities with respect to workprogrammes: The first is deemed to be co-vered adequately by seismic and drillingdata, allowing for a reasonable work pro-gramme offer, the second is expected to seefurther exploratory investigations beforeentering into the committment phase.From the early nineties onwards data com-pilations, interpretations and publications[2, 3, 4, 5] were made available to the indu-stry. In addition, upon round opening, se-lected data, particularly well data, weremade available via website / internet. Withrespect to E&P activities, Latvia and its

neighbours during that time also have sub-stantially contributed to attract interest inthe general area, i.e. in exploring for oilfrom Early Paleozoic reservoirs at shallowto moderate depths. Particularly the blockallocations to AMOCO/OPAB on both si-des of the Swedish/Latvian offshore bor-der, the round openings in Poland’s andLithuania’s onshore areas, the productionstart up in the Polish offshore B 3 structure,the negotiations for settling border dispu-tes in the offshore, and the commissioningof studies to assess oil and gas potentials,led to an increased interest in this oil pro-vince.Upon the announcement of the round ope-ning, the authors had revisited the Latvianareas on offer for their merits [6, 7], with fo-

cus on hydrocarbon potential, terms, costsand marketing. The underlying geologicalevaluations resulted in the modification ofconcepts, which are the (1) carbonate depo-sitional pattern, (2) structural style and (3)migration possibilities. As a consequence,this summary is aimed at presenting boththe geological concept modifications andtheir bearing on the E&P potential.

2OIL GEOLOGY2.1 Regional SettingThe Latvian offshore area is part of

the Baltic Syneclise (Figs. 1, 2, 3). Thismainly Early Paleozoic basin formed onthe western margin of the East EuropeanPlatform. Evolution analyses [8, 9] suggest

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0179-3187/01/III© 2001 URBAN-VERLAG Hamburg/Wien GmbH

* Dr. Sergej Kanev, Geological Survey of Latvia, VGD, Riga,Latvia; Dr.Ornulf Lauritzen, PGS Reservoir Consultants AS,Oslo, Norway; Dr. Ulrich Schmitz, LO&G Consultants, Es-sen, Germany (E-mail: ulrich.schmitz@dland.de).

Fig. 1 Baltic Sea region orientation map, with basin outline, Latvian round areas and infrastructure

that the Early Paleozoic setting mainly re-flects the interaction of the Laurentia andBaltica Plates (the latter, simplified, tur-ning into the East European Platform at alater stage). From Late Precambrian/Mid-Cambrian these plates separated, and theIapetus Ocean opened. Successively, con-vergent margins developed, and reinfor-ced convergence – accompanied by oroge-nic events (Taconian Orogeny) – obviouslyled to subduction during the Ordovician.At Late Ordovician through Early SilurianBaltica locally was emergent. Eventually,the Late Caledonian continent to continentcollision, at Late Silurian through EarlyDevonian, resulted in the suturing of Lau-rentia and Baltica.Evidently, the Late Caledonian Orogenyhas most affected the structural setting ofthe general area – the outstanding featurebeing the Liepaja-Saldus Ridge which cross-cuts Latvia’s on- and offshore territories ina SW–NE direction. Locally, this ridgeshows heavy faulting.A somewhat camouflaged, yet significantelement in the tectonic setting appears tobe the occurrence of inversion, correlatedwith the orogenic event. Examples of up-thrusting exist on the N–S running LebaHigh feature of Poland, and on the Liepa-ja-Saldus Ridge, both in the offshore (seeFig. 4) and in the onshore, where on the In-cukalns gas storage structure drilling pro-ved the repetition of section owing to up-thrusting.Generally, the basin is referred to as an in-tracratonic basin [9, 10]. Ulmishek [10], ha-ving evaluated in detail the various criteriamet – or not met, suggests that the BalticBasin had been produced as „a hybrid bet-ween an intracratonic basin and a passive

margin“. This comes closest to our model-ling [7] of the basin, being mainly derivedfrom the platformal character of carbonatedeposition at both Ordovician and Siluri-an times. Analysing the tectono-sedimen-tary setting of the two periods suggeststhat major parts of the Ordovician wereformed under conditions of a differentiatedcarbonate shelf, whilst most of the Siluriansuccession was formed under ramp condi-tions. Both settings are separated by a gap.Comparable settings, however at largerscale, were described by Murris [11] fromthe Arabian Platform.The platform covers major parts of the Bal-tic Sea, including the Swedish islands ofGotland and Oland, and as far as onshoreareas are concerned, Estonia, Latvia andLithuania, together with the KaliningradOblast and northernmost Poland. Transiti-on to the southwesterly deeper water re-gimes takes place along the Teysseire-Torn-quist Lineament.To illustrate the model, the essential datahave been compiled on Figs. 2 and 3, em-ploying various sources [6, 10, 12, 13, 14].Fig. 2 shows the generalized and schematicOrdovician facies distribution, togetherwith the generalized Ordovician stratigra-phy of Latvia, correlated with the sea levelchanges, as adapted from Leighton & Kola-ta [9]. The outlined facies entities (1 to 7)mainly correspond with the palaeogeogra-phic features referred to by Ulmishek in hiscompilation [10], which are the North Esto-nian Depression (1), the South EstonianUplift (2), the Jelgava Depression (3) andthe Lower Neman Uplift (5). The NorthPolish Uplift (6) and the Natanga Depressi-on (7) derive from Geodekyan [12], identi-fied through thickness mapping, whilst the

Mid Lithuanian Unit (4) has been adoptedfrom Lithuanian data [13].Altogether, the data suggest the setting of adifferentiated shelf, having existed at sealevel high stand, with source shale sedi-mentation predominantly in the low areas,and with reefal build-ups correlated parti-cularly with high areas. As to the occurren-ce of source rocks, the Estonian kukersiteare the most outstanding ones, whilst as tocarbonate build-ups, the productive Got-land reefal features are the spectacularones, together with those described by Ul-mishek [10] from the Lower Neman Upliftand possibly those build-up clusters iden-tified on seismic [15] in the Latvian offsho-re, east of Gotland ( see Fig. 2). The settingtypically also hosts oolitic-bioclastic carbo-nates, at Ashgill level.Fig. 3 depicts the Silurian facies belts, to-gether with Latvia’s Silurian stratigraphy,correlated with the curve of global sea levelchanges. Sources of information are as forthe Ordovician. The facies belts obviouslycorrelate with the basin’s palaeo-highs andlows: (1) the Latvian – Estonian Slope, theLithuanian Depression (2) and the SouthBaltic Depression (3). The latter two namesare taken from the referred to thicknessmapping [10], whilst we would emphasizethat Ulmishek’s ”Latvian Uplift“ featurewhich cuts into (2) is not considered a Silu-rian palaeo-feature (see also [12])and hasno support in facies mapping.Altogether, the data suggest a setting whichcontrasts with that of the Ordovician. Themap suggests the conditions of a ramp,with facies belts running almost parallel tothe strike of the slope, the latter being de-duced from thickness mapping [10]. Thestratigraphic succession suggests falling sea

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Fig. 2 Ordovician succession: facies belts, stratigraphy and sea levelchanges. For explanations see text

Fig. 3 Silurian succession: facies belts, stratigraphy and sea level changes.For explanations see text

level conditions for the major part of the sec-tion. Source shale sedimentation predomi-nates in the low areas, and reefal build-upsdeveloped obviously on both sides of thetransition from the outer to the middle faciesbelt. As to the occurrence of source shales,the Llandovery graptolitic shales, predom-inating in (2) and (3), are the most conspic-uous ones, whilst as to carbonate build-ups, the partly oil productive “reef zones“[10] of central Lithuania are the most obvi-ous ones, together with the bioherms andbiostromes that crop out on the island ofGotland [16]. Typically, otherwise the car-bonates have poor reservoir properties.

2.2 Oil Habitat2.2.1 Sealed ReservoirsMain reservoirs of the Baltic Sea oil provin-ce are the Middle Cambrian quartziticsandstones of the Deimena Fm. Reservoirproperties show a wide range, with porosi-ties measured from appr. 2% to some 25%,and permeabilities ranging from few mDto appr. 1 D. Porosity deterioriation increa-ses with depth of burial [17, 18], which me-ans that at the moderate burial depths rea-ched in Latvian areas, porosities are onlymoderately affected by diagenetic effects.The oil flow potential is moderate. The ran-ge is from a few bbl/d to up to some 3,500bbl/d, whereby the higher flow rates arepartly achieved from horizontally drilledsections and/or upon underbalanced dril-ling. In Latvia, the Kuldiga field had tem-porarily produced from this level.As for Ordovician carbonates, those ofAshgillian age are probably the most pro-mising ones in terms of reservoir characte-ristics. On the island of Gotland, Ashgillianreefal build-ups are oil productive. The oilflow potentials are low. Other Ashgill car-bonate reservoirs are the oolitic – bioclasticlimestones which were found oil-stainedin several Latvian onshore wells, with po-rosities up to 20%. Remarkably, the Latvianoffshore E 6 well yielded an influx of appr.20 bbl/d on test from a reservoir with 17%porosity and insignificant permeabilities,and also Lithuania’s Kybartai field, on theedge of the Lower Neman Uplift (see Secti-on 2.1), exemplifies oil production fromLate Ordovician carbonates, despite poorreservoir properties.Within the Silurian succession, evidencefor reservoirs is scarce. Only the Lithuani-an Kudirka area with its three pools yieldsevidence for oil productive reservoirs, rela-ted to the a.m. Ludlow reefal build-ups.Maximum porosities are 15%, maximumpermeabilities are 122 mD.

2.2.2 Mature Source RocksWithin the preserved section of the basin,three outstanding source rock levels areidentified: the Late Cambrian and Trema-docian alum shales, the Ordovician (Cara-doc) shales, and the Silurian (Llandovery)graptolitic shales [4, 17, 19]. Other sourcerocks exist, yet are less significant.

The alum shales‘ lateral equivalents attai-ned oil maturity in the North Polish part ofthe basin and in basinal areas adjoining tothe east. Upon intra-Early Paleozoic uplif-ting and erosion the Late Cambrian alumshales are absent in parts of the basin. Thisalso holds true for Latvia.The Ordovician source rocks are compri-sed in the carbonate section as shale layers.In the southern areas they have reached oilmaturity, whilst in Latvian areas (and furt-her north) they are immature to early ma-ture, except for potentially local kitchenareas [3] which are suggested to yield ap-propriate maturity, for subsidence reasons.The Silurian source rocks likewise are sho-wing maturity in the southern areas andimmaturity to marginal maturity in Latvi-an territories (and further north). Again,exceptions for local kitchens may exist.Analyses of the source rocks [4, 17, 19]seem to indicate that TOCs do not differmuch, i.e. are mainly in the order of 10 to20% and that facies are varying little, i.e.are typically kerogen type II. The yieldsupon pyrolysis exhibit larger ranges, fromsome 20 kg to some 70 kg HC per ton ofrock. However, the variation is suggestedto likely reflect different maturity stages[6].A particular critical factor, controlling theentrapment of oil, is in this basin the timeof oil generation and migration. Consen-sus amongst those who modelled the ma-turation history [4, 17, 19] is that the oilwindow was reached at appr. end Siluriantimes and that the main phase of generati-on (and/or subsequent expulsion / migra-tion) had started by Early, Mid or Late De-vonian.

2.2.3 Petroleum System and PlaysOil to source correlations [19] indicate nounequivocal allocation of the reservoiredoils to specific source rocks. The authors,

who investigated Lithuanian samples,suggest that the oils trapped derive fromseveral source rocks. Consequently, andalso for the reason that no difference in ti-ming as to oil generation from the differentsource rocks has been found, it appearsreasonable to base the basin modelling onan Early Paleozoic Petroleum System, withthe understanding that Late Cambrian andTremadoc, Late Ordovician and Early Silu-rian source rocks are the potential and like-ly contributors.Reservoirs which are sealed and which arein the reach of mature source rocks, are theones to render the productive plays. Theseare the Mid Cambrian Sandstone Play, theLate Ordovician Carbonate Play, the LateOrdovician Reef Play, and the Late SilurianReef Play. Vital elements of the plays appe-ar to be long range migration [3, 4, 17] and,for the Mid Cambrian Play, the presence offaults that juxtapose the stratigraphicallyhigher source rocks.

2.2.4 Fields and ReservesThe majority of the fields found since 1963is oil-bearing at Middle Cambrian level.This holds true for the North Poland ons-hore fields [17], the Polish offshore fields ,the Kaliningrad Oblast fields and the Lit-huanian fields [19]. Recoverable reservesrange from some 0.2 MM bbl oil to about 63MM bbl oil of the Kaliningrad D 6 offshorefield [1]. Obviously, however, the bulk ofthe more than 30 fields contain small reser-ves, the referred to D 6 field is outstandingin size.Ordovician fields in total produced some0.7 MM bbl oil on Gotland island [20],which represents the EUR from at least 12pools. The reserves of the referred to Lit-huanian Kybartai field are estimated atappr. 0.6 MM bbl oil [19].The only Silurian field, located in Lithua-nia (see Section 2.2.1) which shows up on

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Fig. 4 Latvian offshore seismic line across suggested inversion structure, with potential fracture zone("high curvature area")

production statistics, is estimated to con-tain appr. 2.0 MM bbl oil [19].

2.2.5 Results of Play RevisitingThe apparent limited potential of the area –as compared to its major neighbouring oilprovinces, the North Sea and the RussianBasins – mainly reflects (1) reservoir insuf-ficiencies at Orvovician and Silurian levels,(2) the lack of a substantial number of largestructures, and (3) the delicate relation bet-ween structuring and migration. In ourmodel, we suggest that– at Ordovician and Silurian times reser-

voir-prone facies developed along pa-laeo-highs and slopes and/or shelf bre-aks, with reef clusters, reef zones, or bio-clastic-ooidal carbonates,

– from end Silurian times onwards earlymature source rocks commenced char-ging the reefal stratigraphic traps (whilstpossibly larger amounts of HCs migra-ted updip without being trapped), uponfocussed migration,

– at the Late Caledonian Orogeny timesstructures, faults and fractures formed,involving Cambrian sandstones and Or-dovician and Silurian carbonates. Parti-cularly inverted, upthrusted sections,owing to potentially intense fracturingalong pronounced curvature areas of thestrata, may have undergone poro-permenhancement,

– at post-Middle to Late Devonian timestraps of the Early Paleozoic section wereavailable for oil entrapment, as migrati-on became re-oriented.

It is obvious that in terms of oil entrapmentthe most favourable areas are those whichwere palaeo-highs at an early stage andwhich remained high areas.

3ROUND PERSPECTIVES3.1 The PotentialsPotential estimates as related to pro-

spects offered through the round are, forgood reasons, not released. In the follo-wing it is solely attempted to check the si-zes of model fields against reality of theeastern Baltic Sea area.

3.2 E&P Licensing AreasThe main reservoir is expected to be theMiddle Cambrian sandstone, the reservoircapacity of which had been tested in the E 6well, located in the licence on offer. Struc-ture mapping as published [6] indicates thepresence of structural closures ranging bet-ween some 10 and 35 sqkm (2,500 and 8,500acres). Upon application of net paysknown from Middle Cambrian reservoirsit is estimated that recoverable reserves re-late to some 16.1 standard cum per cukm ofgross rock volume (Sm3/km3 GRV) – thisdoes not infer the need for fracturing sincethe reservoir depth is expected above thecritical depths for diagenetically inducedreservoir deterioriation. The model fieldsize results in an order of magnitude of 32

MM bbl oil. This is approximately the sizeof the producing B 3 field in the Polish offs-hore. The D 6 field size (see Section 2.2.4)would be reached by adding the potentialupside, i.e. another 32 MM bbl oil.Modelling a carbonate case, based on themodel of a fractured, primarily low porosi-ty reservoir, i.e. by using 6.4 Sm3/km3 GRV(see above), smaller reserves are arrived at.This is also due to the fact that the producti-ve area, mainly controlled by the curvatureof the reservoir (see Section 2.3), is smaller.It is noted that the estimates for the AMO-CO /OPAB structure straddling the Swe-dish/Latvian offshore border (see Section1) are considerably higher, according toone source [21] more than ten times higherthan the model field of above.

3.3 Pre-Investigation Licensing AreasThe Pre-Investigation Licensing areas fun-damentally host the same prospect typesas the E&P Licensing areas, particularly inthe southernmost offshore parts.In addition, seismically indicated Ordovi-cian reefal build-ups east of Gotland aresuggested to offer the opportunity of fin-ding oil. Based on parameters from the Si-lurian Kudirka reef (see Sections 2.2.1 &2.2.4), potential estimates indicate that eit-her the cluster of reefal build-ups, to betreated as one entity, or the largestbuild-up, south of the cluster, having beenidentified on several seismic lines [15],would be drilling targets, subject to furtherseismic [15].

3.4 The Round TermsLatvian round terms are, without doubt,favourable. The principal term elementsare highlighted on Table 1. They show therationale behind the terms setting: to at-tract licensees who would commit to work

programmes reflecting their assessment ofthe licenses‘ potentials. This holds true forthe E&P Licensing part, and even more sofor the Pre-Investigation Licensing part.Already from pre-development studies ofthe most prominent and most attractivediscoveries made in this part of the BalticSea, i.e. from the B 3 field of the Polish offs-hore, and from the D 6 structure of the Kali-ningrad Oblast offshore, it is known thatthe issue of economic viability is of para-mount significance. Critical factors in thisare particularly well rates and the size ofrecoverable reserves. Obviously, the Latvi-an round terms honour both these expe-riences and the respective predictions ofLatvian offshore potentials.

3.5 Marketing ConditionsIn Latvia, conditions to achieve competiti-ve prices at the wellhead – an issue identi-fied by van Meurs [22] as being essentialfor the success of any E&P venture – are ob-viously favourable. Reasons for this as-sessment are: (1) no market, volume or pri-ce regulations for oil (or gas) are in force, (2)licencees may fully and according to theirown conditions dispose of their share inproduction, and decide on exporting ormarketing locally, (3) profit repatriation isguaranteed, and no currency exchange re-strictions exist, (4) local marketing is at-tractive as local demand for energy is fore-casted to grow (despite the fact that energyconsumption did not grow in Latvia from1996 to 1999, i.e. went down from 6,759 toabout 6,400 ktce), (5) the infrastructure ofthe region (see Fig. 1), comprising Latvia’sVentspils oil terminal and Lithuania’sMazheikiai refinery, facilitates market re-ach, and (6) the expected oil quality fromthe offshore (about 38° API) would be satis-factory.

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Table 1 Key licensing terms of Latvia's 1st offshore round

3.6 Costs in E&PCosts in terms of finding costs or, finding,development and production costs, are notreadily available. This is mainly becausethe previous activities of the PETROBAL-TIC venture had been carried out underdifferent conditions and different econo-mic guidelines.Whilst the estimate of development andproduction costs, at this point in time, willcontain uncertainties, the assessment offinding costs may be less problematic. Theprogress achieved in the regional under-standing of the plays, the availability ofseismic data, including reprocessed datasets, and the knowledge of the reservoirparameters from the neighbouring areasare expected to render a fairly high successratio and to contribute to minimizing costs,i.e. unit costs (in US$ per barrel).

4CONCLUSIONSThe Latvian first offshore round evi-dently renders opportunities as far as

exploration for and production of oil fromEarly Paleozoic reservoirs is concerned.Through its liberal attitude concerningdata release, Latvia allowed for early eva-luations of the offer – including the reviewand revisiting of exploratory concepts.From the assessment of both technical andnon-technical E&P factors it is concludedthat– potentials, together with reservoir proper-

ties and well rates, are the most critical is-sue. Successful development and pro-duction in the Polish B3 offshore field,however, are suggested to render thecase history,

– the round terms, being favourable accor-ding to international standards, adequa-tely reflect the potential of the area,

– favourable marketing conditions, both cir-cumstancially and through actively ta-king measures, sustain the round offer,and

– costs, in terms of unit costs, are expected,as far as finding costs is concerned, to bepotentially low, reflecting past experien-ce and activities.

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Licensing in the Baltic Republics. PESGBNewsletter 2001, 8/9, 72 – 76 (2001).

[2] Brangulis, A.P., Kanev, S.V., Margulis, A.S. &Haselton, T.M.: Hydrocarbon Geology of theBaltic Republics and the adjacent Baltic Sea. In:Spencer, A.M. (Ed.): Generation, Accumula-tion and Production of Europe’s Hydrocar-bons II, 111 – 115 (1992).

[3] Freimanis, A., Margulis, L., Brangulis, A.,Kanev, S. & Pomerantseva, R.: Geology andhydrocarbon prospects of Latvia. OGJ 1993,6/12, 71 – 74 (1993).

[4] Kanev, S., Margulis, L., Bojesen-Koefoed, J.,Weil, W., Merta, H. & Zdanaviciute, O.: Oilsand hydrocarbon source rocks of the Balticsyneclise. OGJ 1994, 11/7, 69 – 73 (1994).

[5] Kanev, S. & Lauritzen, O.: Latvia – Petroleum

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[7] LO&G: Latvia Offshore. Hydrocarbon Poten-tial of Blocks 1, 2 and 3 offered in Latvia’s 1st

E&P Licensing Round. Non-excl. Report, 98 p.(2001).

[8] Ziegler, P.A.: Geological Atlas of Western andCentral Europe. SHELL / Geol. Soc., 239 p.(1990).

[9] Leighton, M.W. & Kolata, D.R.: Selected Inte-rior Cratonic Basins and their Place in theScheme of Global Tectonics. In: Leighton,M.W., Kolata, D.R., Oltz, D.F. & Eidel, J.J.(Eds.): Interior Cratonic Basins. AAPG Mem.51, 729 – 797 (1990).

[10] Ulmishek, G.: Geologic Evolution and Petro-leum Resources of the Baltic Basin. In:Leighton, M.W. et al. (Eds.), q.v., 603 – 632(1990).

[11] Murris, R.J.: Hydrocarbon Habitat of the Mid-dle East. In: Miall, L.D. (Ed.): Facts and Princi-ples of World Petroleum Occurrence. Cana-dian J. Petrol. Geol. Mem. 6, 765 – 800 (1980).

[12] Geodekyan, A. (Ed.): Geological structure andhydrocarbon prospects of the central part ofthe Baltic Sea. (in Russian). Nauka Publ., 110 p.(1976).

[13] Grigelis, A. & Kadunas, V.: Lithuania’s Geol-ogy Summary. Geol. Surv. Handout, 19 p.(1994).

[14] Zagorski, J.: Oil and Gas Exploration in Po-land. In: Popescu, B.M. (Ed.): Hydrocarbons ofEastern Central Europe. Springer Verlag, 175 –215 (1994).

[15] Kanev, S., Kushik, E. & Peregudov, Y.: A largeOrdovician Carbonate Buildup Offshore Lat-via. Latv. Geol. Vestis, 22 – 23 (2000).

[16] Kershaw, S.: Sedimentation control on growthof stromatoporoid reefs in the Silurian ofGotland, Sweden. J. Geol. Soc. London, 150,197 – 205 (1993).

[17] Schleicher, M., Köster, J., Kulke, H. & Weil, W.:Reservoir and Source Rock Characterisation ofthe Early Palaeozoic Interval in the PeribalticSyneclise, Northern Poland. J. Petrol. Geol., 21(1), 33 – 56 (1998).

[18] Sliaupa, S., Lashkova, L., Rasteniene, V. &Cyziene, J.: Factors controlling Quartz Cemen-tation of Cambrian Reservoir Sandstones ofLithuania. EAGE Ext. Abstracts, P608, 2 p.(2001).

[19] Zdanaviciute, O. & Bojesen-Koefoed, J.A.:Geochemistry of Lithuanian Oils and SourceRocks: a preliminary Assessment. J. Petrol.Geol., 20 (4), 381 – 402 (1997).

[20] Kulke, H.: Sweden. In: Kulke, H.(Ed.): Re-gional Petroleum Geology of the World I, 361 –364 (1994).

[21] Anonymous: Latvian oil discovery. Europ.Offshore Petrol. Newsletter, 21/8, p. 4 (1996).

[22] Van Meurs, A.P.: Governments cut takes tocompete as world acreage demand falls. OGJ1995, 24/4, 78 – 82 (1995).

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