Geology and Hydrocarbon Systems in the Western Black Sea

GEORGI GEORGIEV

Sofi a University ‘St. Kl. Ohridski’, Department of Geology, Palaeontology & Fossil Fuels, 15 Tzar Osvoboditel Blvd., 1504 Sofi a, Bulgaria (E-mail: [email protected])

Received 07 February 2011; revised typescript received 16 November 2011; accepted 27 December 2011

Abstract: Th is paper presents the author’s integrated regional studies during the last decade. Th e main purpose is to present an overall understanding of the geological structure, sedimentary basins and hydrocarbon systems of the whole Western Black Sea Zone (WBSZ). Th is study is based on original data from boreholes, seismic and gravity-magnetic surveys and hydrocarbon accumulations. Many geophysical borehole data obtained for WBSZ during the last 3–4 decades were interpreted mostly at a national level using diff erent approaches, terminology and nomenclature for the same or similar lithostratigraphic and tectonic units. Th erefore, a unifi ed approach to interpretation of borehole-seismic data and correlation of stratigraphic, sedimentological and tectonic units has a key importance for overall clarifi cation of the deep geological structure and the hydrocarbon challenges. A set of regional geological cross-sections along good quality basic seismic lines and basic boreholes was constructed. A detailed tectonic map of the WBSZ has been compiled by integrated interpretation of seismic borehole and gravity- magnetic data. Th e defi nition of hydrocarbon systems and promising exploration trends is made by source rock assessment, Oil-Oil and Oil-Source rock correlations, analyses of the reservoir/seal pairs and the hydrocarbon migration and accumulation. Genetic correlations are based on many Rock-Eval, Gas Chromatography/Mass Spectrometry (GC- MS) and carbon isotope analyses. Th e complex geological structure of the WBSZ is defi ned by four groups of tectonic units: (1) Western Black Sea basin (WBSB) – its western zone with the Kamchia and the Histria westward wedging branches (sub-basins); (2) portions of the Moesian, Scythian and East European platforms; (3) fragments of the North Dobrogea, Eastern Balkan, Eastern Srednogorie and Strandzha orogens; (4) Burgas and Babadag basins. Four diff erent oil genetic types have been identifi ed. Th ree main hydrocarbon systems with economic potential are defi ned, they relate to: WBSB and its Histria and Kamchia branches, the East-Varna trough and the Bourgas basin. Conceptual models for hydrocarbon systems and their prospect exploration trends are constructed.

Key Words: Western Black Sea Zone, tectonic structure, hydrocarbon systems

Batı Karadeniz’in Jeolojisi ve Hidrokarbon Sistemleri

Özet: Bu makale yazarın son on senede yürüttüğü bölgesel çalışmaların sonuçlarını içerir. Makalenin amacı tüm Batı Karadeniz Zonu’nun (BKZ) jeolojik yapısını, sedimenter havzalarını ve hidrokarbon sistemlerini anlamaya çalışmaktır. Sonuçlar kuyulardan elde edilen verilere, sismik ve gravite-manyetik ölçümlerine ve hidrokarbon oluşumları hakkındaki bilgilere dayanır. Son 30–40 senede BKZ’da çok sayıda jeofizik kuyu verisi elde edilmiştir. Buna karşın bu veriler ülke bazında farklı yaklaşımlar, farklı terminolojiler kullanılarak, ve aynı litostratigrafik veya tektonik birimler için farklı isimlendirilmeler yapılarak değerlendirilmiştir. Bu değerlendirmelerde ve stratigrafik, sedimentolojik ve tektonik birimlerin korrelasyonunda ortak bir yaklaşım benimsenmesi, bölgenin derin jeoloji yapısının ve hidrokarbon özelliklerinin anlaşılmasında büyük önem taşır. Bu çalışma kapsamında kuyular ile denetlenmiş kaliteli sismik hatlar boyunca bölgesel jeoloji kesitleri yapılmıştır. BKZ’nun ayrıntılı bir tektonik haritası gravite, manyetik, sismik ve kuyu verileri baz alınarak hazırlanmıştır. Kaynak kaya analizi, petrol-kaynak kaya, rezervuar-kapan ilişkileri ve hidrokarbon göçü ve depolanmasına dayanılarak hidrokarbon sistemleri tanımlanmış ve ümit vadeden aramacılık yaklaşımları belirlenmiştir. BKZ’nun jeolojik yapısı dört tektonik unsur tarafından belirlenir: (1) Batı Karadeniz Havzası (BKH) ve onun batıya doğru dallanan Kamçiya ve Histriya alt havzaları; (2) Moesya, İskit ve Doğu Avrupa platformlarının bazı parçaları; (3) Kuzey Dobruca, Doğu Balkan, Doğu Srednogoriye ve Istranca orojenlerine ait parçalar; (4) Burgaz ve Babadağ havzaları. Dört farklı jenetik kökenli petrol tipi tanımlanmıştır. Ekonomik potansiyel taşıyan üç hidrokarbon sistemi belirlenmiştir, bunlar BKZ ve onun Histriya ve Kamçiya kolları, Doğu Varna çukuru ve Burgaz havzasıdır. Bu hidrokarbon sistemlerinin aranması ile ilgili modeler sunulmuştur.

Anahtar Sözcükler: Batı Karadeniz Zonu, tektonik yapı, hidrokarbon sistemleri

723 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

Introduction is based on unifi ed precise correlation of basic well Th e Western Black Sea Zone (WBSZ) comprises the sections and their integration in the geological entire Bulgarian and Romanian off shore sectors, interpretation of seismic and gravity-magnetic the western part of the Odessa Gulf from Ukrainian results. For this purpose a map of the main gravity and off shore, the westernmost part of the Turkish off shore magnetic anomalies has been compiled (Figure 2), sector, as well as adjacent onshore zones (Figure 1). based on data from Sava (1985), Dachev (1986), Sava et al. (1996, 2000), Morosanu & Sava (1998), Stavrev Hydrocarbon exploration in the WBSZ started & Gerovska (2000) and Starostenko et al. (2004), and during the 1970s and was mostly undertaken on the a set of regional sections crossing the whole WBSZ shelf area. As a result 15 oil and gas accumulations (Figure 2), which follow good quality basic seismic have so far been discovered (Figure 2). lines and pass through the basic borehole sections Many geophysical (gravity, magnetic and mainly have been constructed (Figures 3 & 4). Th e location seismic) and borehole data have been obtained and orientation of these lines were also defi ned in during the last 3–4 decades. However, these data were accordance with the distribution of the main gravity interpreted and generalized mostly at a national level. – magnetic anomalies. Th ese regional cross-sections Moreover, the four countries in the region have used have a key importance for the identifi cation of deep diff erent interpretation approaches, terminology and geological structure and the construction of a detailed nomenclature for same or similar litho-stratigraphic tectonic map for the whole WBSZ (Figure 5). or tectonic units and faults, crossing international Th e reconstruction of some important episodes borders. Th erefore, a unifi ed correlation of from the Mesozoic–Tertiary geological evolution stratigraphic, sedimentological and tectonic units of the WBSZ, in the frame of a greater Scythian- and a uniform approach to the interpretation of Black Sea – Caucasus-Pontides domain, is based on borehole seismic data is of key importance for the integration of our data (Emery & Georgiev 1993; clarifi cation of the deep geological structure and the Dachev & Georgiev 1995; Georgiev & Dabovski evolution of the entire region. 1997, 2001; Tari et al. 1997) and published regional Th is paper presents the results of author’s data (Okay et al. 1994; Robinson 1997; Finetti et al. integrated regional study, carried out during 1988; Nikishin et al. 2001; Stampfl i et al. 2001; Ziegler the last decade. Th e main purpose is to gain a et al. 2001). Th e general concept is an alternation of better understanding of the geological structure phases of Mesozoic and Tertiary back-arc rift ing and and evolution of the whole WBSZ, as well as its back-arc compression that controlled the evolution sedimentary basins and hydrocarbon systems. Th e of this region. main tasks are: (i) integration of regional research Th e hydrocarbon source complexes have been and exploration data (mainly borehole, seismic and evaluated by all available well, log and seismic data, gravity-magnetic) by unifi ed precise correlation using modern investigative methods and techniques, and interpretation; (ii) clarifi cation of tectonic such as Gas Chromatography-Mass Spectrometry structure and interrelations between tectonic units; analyses (GC and GC-MS), Rock Eval Pyrolysis, (iii) characterization of promising hydrocarbon Carbon isotope analyses (C12, C13) and vitrinite sedimentary basins and their evolution; and (iv) refl ectance analyses (Ro) of cuttings and core samples identifi cation and estimation of main petroleum from many wells. Some lithological and seismic systems and exploration trends. facies data have also been used for to recognize facies Th is study is based on a very large database that changes, thicknesses and burial depths towards the integrates almost all original basic data from: drilled Western Black Sea deepwater zone. exploratory wells, seismic and gravity-magnetic Th e genetic hydrocarbon links (Oil to Oil and Oil surveys and discovered hydrocarbon accumulations, to Source) were investigated by correlation of their as well many analytical and research results (Figure biomarker profi les (n-alkanes, triterpanes – m/z 2). 191, steranes – m/z 217, triaromatics – m/z 231 and Th e detailed study of the deep geological structure monoaromatics – m/z 253), which form the main and relationships between diff erent tectonic units pattern characteristics of the source material.

724 G. GEORGIEV

EAST EUROPEAN PLATFORM Odessa

Carpathians

North Dobrogea Azov Sea Odessa Gul f SCYTHIAN PLATFORM

South Crimea G r e a t C a u c a s u s Constanta Middle Black Sea high MOESIAN (Andrusov-Archangelsky ridge) PLATFORM I B l a c k S e a Balkan Bourgas Western Black Sea basin Eastern Black Sea basin Strandzha I

Study zone P o n t I d e s İstanbul a Marmara Sea

W Mid Black E mGal Moesian Western Black Sea basinSea high Eastern Black Sea basin mGal 240 platform (Andrusov ridge) 240 G Bg 180 180 120 120 60 60 0 0 0 0 Pz J+K sea water P+Tr Plio+Q Oli+Mio 10 10 continental crust K 2 +Pal+Eoc continental crust

osceanic to uboceanic crust 20 20

Moho

30 b 100 km 30 1-DSS HB 92-15 SHT - 5/81 km km

Figure 1. (a) Tectonic units in the Black Sea domain with location of study zone (aft er Rempel & Georgiev 2005); (b) Geological-seismic cross-section along line I-I (aft er Dachev & Georgiev 1995). Defi nition of hydrocarbon systems and promising basin that developed north of the Pontide magmatic exploration trends was made by: evaluation of source arc, itself formed by northward subduction of the rocks and their spreading, Oil to Oil and Oil to Neo-Tethys ocean that was initiated in the Albian Source correlations, analyses of reservoir/seal pairs (Tugolesov et al. 1985; Finetti et al. 1988; Görür 1988; and hydrocarbon migration directions, also taking Okay et al. 1994; Dachev & Georgiev 1995; Robinson into account the latest exploration and investigative et al. 1995; Banks & Robinson 1997; Nikishin et al. results (Robinson et al. 1996; Bega & Ionescu 2009; 2001, 2003). Th e Black Sea basin, in terms of crustal Khriachtchevskaia et al. 2009; Sachsenhofer et al. structure, consists of Western and Eastern rift -type 2009; Tari et al. 2009). sedimentary basins, separated by the Andrusov (or Mid-Black Sea) ridge (Figure 1). Both basins are diff erent with respect to time of opening, structure, Tectonics of Western Black Sea Zone stratigraphy and thickness of their sedimentary fi ll Th e Black Sea is considered by many authors to be a (Figure 1b). Late Cretaceous to Palaeogene back-arc extensional

725 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

28OE 30OE

U K R A I N E BEZIMENNOE

ODESSA

LEBADA WEST OLYMPIISKOE R O M A N I A PORTITA SINOE LEBADA EAST

PESCARUS COBALCESCU

Constanta DOINA VI

O ANA

B U LG A R I A

TJULENOVO

Varna

GALATA V SAMOTINO MORE LA-1

II 0 50 100 km III drilled deep exploratory wells discovered hydrocarbon fields: oil accumulations gas accumulations

O 42 gravity anomalies:

N positive O negative 42 2000 TURKEY magnetic anomalies: positive negative I 100 200 a lines of composed cross-sections; 50 1000 a - shown in paper

28OE 30OE Figure 2. Database map showing drilled exploratory wells, discovered hydrocarbon fi elds, gravity and magnetic anomalies and composed regional cross-sections.

726 G. GEORGIEV 1 0 4 2 5 3 N

1 0 4 2 5 3 N-NE f. Intra-Moesian 3 2 1 N Oli+Eoc T T T +Pz seismic line 92B107 3 2 1 1-2 J T T T +Pz Tjulenovo trough East Varna trough East Varna

M O E S I A N P L A T F O R M 2-3 f. Bliznac N 2 K Eoc 1 K 1-2 seismic line E92-42 Kamchia basin Outer zone Pal+Eoc

2 Ushakov trough

K +Pg BALKAN OROGEN

Inner zone Back-Balkan f. Back-Balkan

M O E S I A N P L A T F O R M lza f. Bliznac

seismic line Tx92-55

K v 1-2

13 J K+J

canyon Drujba 1-2 f. Sea Black West 3 2 Pli Oli Eoc Eoc Eoc Mio 2 K +Pal Kamchia basin seismic line BGK92-42 Outer buried zone border Bulgaria-Turkey 2-3 2 Oli Eoc Mio 3 Mio 1-2 Plio K +Pal

K +Pg B A L K A N O R G E

Inner uplifted zone

B O U R G A S B A S I N 1-2

20 km 20 km

2 f. Back-Balkan Eoc

K 2-3 Pal Mio Oli Eoc seismic line BV-9-92 Geological cross-sections I and II across WBSZ (for location see Figures 2 & 5). see Figures location WBSZ (for II across I and Geological cross-sections 10 10 a b 0 0 Geological cross-section I Geological cross-section II

Figure 3. Figure S

1 1 BOURGAS BASIN 0 4 0 4 2 2 5 5 3 3 WT(s) TWTT WT(s) TWTT S-SW

727 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

BOURGAS BASIN BALKAN OROGEN W E S T B L A C K S E A B A S I N (Rezovo-Limankoy segment) E W Inner Outer uplifted zone 0 buried zone 0 Pli Mio Mio 1 Oli 1 Eoc 2-3 K +Pal Oli 1-2 2 2 Eoc 2

3 Oli 3 t

(s) Pli 4 K +Pal 4 2 Mio

TWTT Alb+Apt(?) 5 Oli 5

6 6 Eoc West Black Sea faul 2-3 7 Eoc 1-2 7 0 10 20 km K2 +Pal 8 a Alb+Apt(?) Seismic line E9202-8 & 8A Seismic line B92-40+38 IIIG eological cross-section

W E S T B L A C K S E A B A S I N W E-SE Kamchia subbasin (Kamchia depression ) 0 0 Oli 1 1 Eoc 2-3

(s) 2 2 K1 hot Eoc 1-2 3 Pal Plio 3 J2 K1 val-J3 K2 TWTT 4 T- Pz Mio 4 5 b Oli 5 Eoc 2-3 6 Outer buried zone of BALKAN OROGEN K2+Pal 7 0 10 20 km Alb+Apt K1val+J3 K1 hot+bar 8 Seismic line BGK 92-71 Seismic line B 92-33 IVG eological cross-section

W M O E S I A N P L A T F O R M W E S T B L A C K S E A B A S I N E-SE East-Varna trough Ushakov trough East Moesian trough Polshkov ridge 0 0 1 K1+J3 K 2 Costal (Emine-Razelm) fault P 1 J1+J2 T1 Balchic fault Balchic C2 (s) 2 T3 C1 2

3 T2 3

TWTT T1+P N 4 East-Moesian fault 4

5 c Eoc 2-3 - Oli 5

Kaliakra (WBS) fault Eoc 1-2 K2+Pal 6 0 10 20 km Alb+Apt K1+J3 7 Seismic line TX 92-2 Seismic line E 92-9 Seismic line B 92-15 VG eological cross-section

M O E S I A N P L A T F O R M N O R T H D O B R O G E A O R O G E N W H I s t r I a s u b b a s in E Ovidiu Rapsodia 0 0

1 Plio+Q 1

(s) Lac aemfaultRazelm K1 (Neo) ncon. 2 Intra-Pontian U 2 ul-Rosu faul ul-Rosu K2+Pal J3 an TWTT 3 Alb Sandstone Marker-Lower Ponti 3

West Midia faul Midia West Mio2

4 t 4 Peceneaga- 5 d Oli + Mio1 (Maikop Fm equiv.) 5 0 5 10 km Camena f. 6

t Eoc K2 +Pal VIG eological cross-section 7

Figure 4. Geological cross-sections III, IV, V and VI across WBSZ (for location see Figures 2 & 5).

728 G. GEORGIEV

28OE 30OE

Odessa (West Crimea) fa

Karkinit basin

Niculitel nappe East Vilkovian - Vilkovian depression Macin nappe Zmeinian bulge

Tulchea nappe Peceneaga - Camena fault St. George fault A ult Pelikan fau Sulina high Gibkin swell lt Babadag asin Danube flexure slope St. George block B Heraclea fault Sacalin Portita fault Palazu (Capidava-Ovidiu) fault -Lebada f. Tulchea step Sinoe fault

border R Histria sub-basin -Razelm) fault Agigea fault omania-Ukraine

mine

Constanta VI Delfin Costal (E fa ult

44 Mangalia fault

Lacul-Rosu fault

44 Intra-Moezian fault

Kavarna fault

West Midia fault

Balchic f. border Bulgaria-Romania Aksakovo-Danube fault Aksakovo-Danube 2

Kitchevo f. East Moesian fault zone Varna 1 3 EAST MOESIAN TROUGH POLSHKOV RIDGE V Kamchia Bliznac fault

ult sub-basin C WESTERN BLACK SEA BASIN B b IV a WESTERN BLACK SEA BASIN II a (a) sub-basins MOESIAN PLATFORM

Bourgas asin C Kaliakra (West Black Sea) fa greenschists zone - Central Dobrogea unit Pala eozoic zone: (a) South Dobrogea unit ab (b) North Bulgarian Arch Ropotamo-LimankoyIII trend Wrench basins (troughs): ((a) T3 - J 1-2 ; b) T 3 REZOVO border Bulgaria-Turkey ab B SEGMENT 1. East Varna; 2. Tjulenovo; 3. Ushakov R N ezovo trend

42 D SCYTHIAN PLATFORM

EAST EUROPEAN PLATFORM O

42 OROGENS : 2000 (1) North Dobrogea (2) Eastern Balkan thrust-fold belt (a) inner uplifted zone (Balkan) Thrace (b) outer buried zone (Forebalkan) I (3) Eastern Srednogorie unit Basin 100 200 1000 50 (4) Strandzha 050100 km OTHER SEDIMENTARY BASINS

28OE 30 OE

Figure 5. Tectonic map of the Western Black Sea zone.

729 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

Th e Western Black Sea Basin (WBSB), underlain zone, comprising several units: (a) Green Schist zone by oceanic to sub-oceanic crust, started to open in the (Central Dobrogea unit), (b) Palaeozoic zone (South Cenomanian and the sedimentary cover is about 14– Dobrogea unit and North Bulgarian arch), (c) Late 16 km thick (Görür 1988; Okay et al. 1994; Robinson Triassic and Early–Middle Jurassic wrench/pull-apart et al. 1995; Banks & Robinson 1997; Nikishin et al. basins, (d) Southern and Eastern platform edges and 2001, 2003). Th e Eastern Black Sea Basin (EBSB), margins, (ii) Scythian platform– the westernmost underlain by a thinned continental crust, started to fragment, (iii) East European platform– a small part open during the Coniacian or at the beginning of the of the southernmost zone; (3) Orogens: (i) North Palaeogene and the thickness of the sedimentary fi ll Dobrogea thrust-fold belt (inverted North Dobrogea is about 12 km (Robinson 1997; Nikishin et al. 2001, rift zone), (ii) Eastern Balkan thrust-fold belt and 2003). Th e Andrusov ridge is formed by a continental its easternmost Rezovo segment: (a) Inner uplift ed crust, overlain by 5–6 km of sediments (Nikishin et zone (Balkan), (b) Outer buried zone (Forebalkan), al. 2001, 2003). (iii) Eastern Srednogorie, (iv) Strandzha, (4) Smaller Th e Western Black Sea region (WBSR) is located sedimentary basins: (i) Bourgas basin and (ii) on the European continental margin, and covers Babadag basin. parts of the northern periphery of the Alpine orogen and its foreland. Th e Mesozoic–Tertiary evolution of Western Black Sea Basin (WBSB) – Western Zone the region was governed by geodynamic processes in the northern Peri-Tethyan shelf system (Nikishin Th e deep structure of this zone was revealed by et al. 2001, 2003; Stampfl i et al. 2001). Th e southern seismic data and resulting geological interpretation margins of the Scythian and Moesian blocks were only. repeatedly aff ected by Mesozoic rift ing cycles, Th e WBSB is a typical rift basin with steep western interrupted and followed by compressional events, slopes and a deep fl at fl oor. Th e rift ing started during causing strong shortening of these two margins and the Aptian (Okay et al. 1994; Robinson et al. 1995; ultimately their overprinting by the Alpine orogen Banks & Robinson 1997; Nikishin et al. 2001, 2003) (Georgiev et al. 2001; Nikishin et al. 2001). and lasted until the beginning of the Middle Eocene, Th e main problem in reconstructing the evolution as can be seen from cross-sections III & V (Figure of the Western Black Sea Region (WBSR) in the 4). Th e Middle Eocene to Quaternary sedimentary frame of the greater Scythian-Black Sea – Caucasus- succession is relatively undeformеd and has a Pontides domain lies in the superposition of the thickness exceeding 3–3.5 km (Figures 4a–c & 9a). In sequences of Mesozoic and Cainozoic extensional some areas the syn-rift Middle to Upper Cretaceous and compressional deformation phases, during deposits also belong to this undeformed sequence, as which the interaction of diff erent tectonic units has it is in the middle of this zone, characterized by the repeatedly changed (Nikishin et al. 2001; Stampfl i et eastern part of cross-section IV. East of the Moesian al. 2001). platform edge the Mesozoic sediments occur at great Geologically the WBSZ has a rather complicated depth – below 4–5 km (Figure 4c). geological structure, consisting of the western portion Th e western zone of the WBSB has a complex of the WBSB and some parts of ancient platforms and variable structure. Its southern and northern (Moesian, Scythian and East European) and of Alpine parts have diff erent characteristics. In both parts orogenic units (Strandzha, Eastern Srednogorie, western slope of the basin is marked by a sheaf of Eastern Balkan, Fore-Balkan and North-Dobrogea). listric extensional faults with a dominant N–S trend, Th e tectonic map of the WBSZ compiled in this through which a fast stairs-type subsidence was study is given on Figure 5. Th e main tectonic units realized (Figure 4a, c). Th e presence of extensional in the WBSZ are: (1) Western Black Sea Basin – faults and blocks, rollover anticlines and tilted graben- western zone, with two westward wedging deep troughs in this slope indicates rift ing processes. Th ese branches: Kamchia sub-basin and Histria sub-basin; structural elements are unevenly distributed, linear (2) Platforms: (i) Moesian platform–the easternmost in form and parallel to the basin palaeo-slopes.

730 G. GEORGIEV

In the south, east of the basin western slope, a years. Many seismic and borehole results for the relatively fl at fl oor is developed (Figure 4a), while off shore zone have been obtained during the last 3 in the north the basin fl oor structure is complicated decades. All this information has allowed detailed by a narrow SW–N-trending intra-basin linear deciphering of the sedimentary succession and high, named the Polshkov ridge, which is seen in the structural characteristics of this basin (Figures 3, 4b, Cretaceous–Lower Palaeogene succession (Figure 5 & 9b). 4c). To the north the ridge gets closer to the East Many authors considered the Kamchia depression Moesian platform edge and merges with it. Between as a post-Early Eocene foredeep, based mainly on the the SE Moesian platform edge and the Polshkov position and geometry of its westernmost periphery ridge is a narrow syncline, called the East Moesian exposed onshore (Figure 5). However, results from trough (Figures 4a & 5). It contains Lower Palaeogene off shore seismic surveys show that the basin gradually and Aptian–Albian(?) sequences onlapping to the deepens and expands eastwards and merges with the west and east. East of the Polshkov ridge a gentle WBSB fl oor (Georgiev 2004) (Figure 4b). Hence, this monocline marked the transition to the WBSB fl oor. geometry defi nes the Kamchia elongated basin as Th e WBSB comprises two deep westward westward wedging branch of the WBSB. wedging branches: the Kamchia and the Histria sub- North Dobrogea rift branch Basin sedimentary fi ll comprises Middle Eocene basins, which limit the easternmost off shore portion to Quaternary deposits. Th e Eocene–Oligocene of the Moesian Platform to the south and north, sequenceBucharest represents the major sedimentary fi ll in respectively. Th ey are superimposed over ancient the western shallower periphery of the basin, while rift zones, developed during the Late Permian–Early the Neogene thickness increases notably towards the Triassic and Late Triassic (Figure 6). WBSB fl oor (Figures 3, 4b & 9b). Eastern Srednogorie- Th e KamchiaBalkan rift branch basin trends to the west just in Kamchia Sub-basin front of the Balkan thrust-fold belt. Its westernmost Th e westernmost periphery of this unit, called by periphery covers a small area onshore, where its width many authors the ‘Kamchia depression’, extends is about 10–15 km and the sedimentary thickness is onshore (Figure 5) where it has been explored by up to 1300–1400 m. But eastwards off shore the basin seismic survey and deep drilling for more than 60 gradually widens to 60–70 km and deepens to 7000a

250 E 300 E 350 E 250 E 300 E 350 E Carpathians exposed land Carpathians exposed land continental sediments continental & shallow marine daelt ic, coastal & shallow sediments Odessa marine sediments deeper marine sediments marine sediments Odessa Late Triassic wrench basins subduction zone North Dobrogea rift branch Early- Mid Jurassic wrench basin volcanics subduction zone Early Cimmerian thrust front

45 N North Dobrogea

45 N branch Bucharest Bucharest Constanta Constanta

Varna ü Varna

Eastern Srednogorie- Balkan rift branch Eastern Srednogorie- Balkan rift branch Kü re-South Crimea back arc basin Meliata-Crimea-Svanetia-K re-Karakaya system of oceanic back arc basins

ccretionary complex rc and a anic a Volc Pala- eo Tethys 0 100 200 km Pala- eo Tethys a 0 100 200 km 250 E 300 E 350 E b 250 E 300 E 350 E Figure 6. (a) Late Permian–Early Triassic palaeogeography and depositional environment; (b) Late Triassic–Early–Middle Jurassic palaeogeography and depositional environment.

731

North Dobrogea branch

Eastern Srednogorie- Balkan rift branch

b GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

m (Figures 4b & 5). Th e basin basement is marked by correlated with the three onshore North Dobrogea intra-Middle Eocene Illyrian unconformity (Figure 3) nappes (Sandulescu 1984; Seghedy 2001). and its structure is characterized by the geometry of Th e basin gradually widens and deepens towards the Upper Cretaceous carbonate sequence. the SE and merges with the WBSB fl oor (Figures 4d Tectonically this basin is superimposed on both & 5). the southern margin of the Moesian platform and the frontal zone of the Balkan thrust-fold belt (Dachev et al. 1988) (Figure 3). Th e northern basin slope dips Platforms steeply through listric faults in the southern Moesian Moesian Platform-Th e Easternmost Zone – Th e Platform margin. Th e southern basin slope is thrust- Moesian Platform forms the foreland of the Alpine folded (this is actually the buried Forebalkan unit thrust belt and is separated from the Scythian of the Balkan thrust-fold belt). A chain of local platform by the North Dobrogea orogenic belt on its thrust-folds, trending W–SE, is observed within the north-eastern margin (Figure 5). southern basement slope. So, the basement structural Baikalian consolidated basement and Phanerozoic geometry is extensional in the northern basin slope sedimentary cover form the structural architecture and compressional in its southern slope. of the Moesian Platform. Th e basement, exposing Initial formation of the Kamchia basin was coeval the so-called ‘Green Schist formation’, outcrops with the stacking of the Eastern Balkan thrust-belt onshore locally in the Central Dobrogea. None of the during the Illyrian northward compression in the boreholes in Northern Bulgaria reached it. early Middle Eocene (Georgiev & Dabovski 2001). Further basin development was controlled by: (i) Th e Phanerozoic sedimentary cover comprises the uplift and N–NE propagation of the Balkan three main structural sequences: Palaeozoic, Triassic thrust-fold belt and (ii) the opening and expansion and Jurassic–Tertiary, which refl ect the main tectonic of the WBSB. Th roughout this evolution the basin stages of platform evolution. Numerous Late Triassic depocentre migrated north due to the SW one-sided (Norian?–Rhaetian) folds in the Moesian Platform sourcing of sedimentary fi lling, controlled by the are interpreted as fault-bend folds involving various erosion of the uplift ed Balkan thrust-fold belt. Palaeozoic decollement levels (Tari et al. 1997). In a wider palaeotectonic scenario, this thrust-fold belt represents the frontal part of the Mediterranean Histria Sub-basin Cimmerides propagating into the European foreland Th is northern branch of the WBSB, called by many (Tari et al. 1997). Th e main structural confi gurations authors the ‘Histria depression’, is located off shore from Jurassic to Tertiary are clearly oblique to each from Romania (Morosanu 1996, 2007; Morosanu & other. Sava 1998; Seghedy 2001; Dinu et al. 2002, 2005). Th e results from off shore exploration during the Th e basin sedimentary fi ll comprises Oligocene last 30 years proved the platform extension in Black to Quaternary deposits (Figures 4d & 9c), hence Sea and deciphered its structure. Th e easternmost it is younger than the Southern Kamchia branch. part of the Moesian platform extends up to 120 km Oligocene and Pontian sequences dominate the off shore and occupies a large central part of the sedimentary succession. WBSZ. Th e platform is delimited by the Peceneaga- Camena fault and the North Dobrogea orogen to Th e NW-trending Histria basin was developed on the southern and middle nappes of the the north, by the Bliznak fault and the Kamchia North Dobrogea orogen and covers also the sub-basin to the south, and by the WBSB to the east northeasternmost part of the Moesian Platform (Figures 3–5). (Morosanu 1996, 2007; Dinu et al. 2002, 2005). In the south-eastern Moesian platform zone the According to Morosanu (1996, 2007) the off shore faults trend in two main directions (Figures 3–5): seismic data allow some over-thrusts to be traced, normal and reverse east-trending faults, and strike- separating three subunits (Figure 5), which can be slip north-trending faults, related to the WBSB

732 G. GEORGIEV

opening, Th ese two major fault systems create a basin size and geometry, inferred their wrench/pull- complex structure of vertically displaced blocks and apart nature. of small wrench/pull-apart basins. Four diff erent Th is off shore wrench stage occurs in the Late tectonic units occur within the eastern Moesian Triassic as a system of wrench/pull-apart basins Platform zone: (troughs), formed mainly by strike-slip movements along the bounding Balchik, Kaliakra and East Green Schist Zone (Central Dobrogea Unit) – Th is Moesian faults (Figures 3, 4c & 5). Th e basin unit, well-known as Central Dobrogea (Figure 5), development terminated towards the end of the comprises uplift ed basement blocks, in which the Early Cimmerian orogeny (Tari et al. 1997), which Upper Proterozoic ‘Green Schist formation’ cropping partly complicated their structure. Only the East- out on the surface or thinly overlain by thin Jurassic– Varna trough was reactivated in the Early–Middle Tertiary sequences. Th is unit extends into the Central Jurassic and a clastic-shale succession up to 300–500 Dobrogea horst onshore, bounded by the Peceneaga- m thick was deposited (Figures 3 & 4c). Some seismic Camena and the Palazu crustal reverse faults, and indications of slight thickening of the Lower–Middle it extends off shore bounded by the Emine-Razelm, Jurassic sequence can also be observed in the southern Lacul-Rosu and West Midia faults (Morosanu 1996, part of the Ushakov trough (Figure 4c). Th is Early– 2007). Mid Jurassic off shore wrench stage is synchronous with the rift stage onshore in the East Srednogorie- Balkan zone (Georgiev et al. 2001; Figure 6b). Hence, Palaeozoic Zone (the South Dobrogea Unit and the presence of the same thick Lower–Mid Jurassic the North Bulgarian Arch) – Th is zone (Figure 5) sequences in the Kamchia zone (southwards of represents a mosaic of relatively small, vertically Bliznac fault) is quite possible (Figures 3b, 4b & 5). displaced blocks. Its Upper Proterozoic crystalline basement is overlain by thick Palaeozoic sequences, which crop out onshore in some zones of the Southern and Eastern Platform Edges and Margins South Dobrogea. In the North Bulgarian arch and Well-defi ned platform edges and margins can be off shore the dislocated and vertically displaced recognised in the northern Bulgarian off shore sector. Palaeozoic blocks are overlain by Jurassic to Tertiary Th e southern and eastern platform edges are well sedimentary sequences. Lower Triassic, mainly shaped by narrow uplift ed strips with horst-like continental clastics are preserved in some blocks, structure in some fragments, in which the Lower– located onshore on the eastern slope of the North Middle Triassic levels are exposed on the pre-Jurassic Bulgarian arch (Kalinko 1976) and in the Romanian subcrop (Figures 3, 4c & 5). Part of the southern off shore sector (Morosanu 1996). platform margin is buried below the Tertiary sedimentary fi ll of the Kamchia basin. Th e eastern platform margin is aff ected by the East-Moesian Late Triassic and Early–Middle Jurassic Wrench/Pull- north-trending fault system, through which a stair- apart Basins – Th is unit comprises three relatively like subsidence manifests the transition to the WBSB. small troughs, namely the East Varna, Tyulenovo and Ushakov troughs (Figure 5). Identifi ed in the south-eastern platform off shore zone by seismic data Scythian Platform (Drannikov et al. 1979; Dachev et al. 1988), these Th e Scythian platform in the WBSZ is covered troughs are fi lled mainly by Upper Triassic clastics off shore by the Histria branch of the WBSB (Figure up to and exceeding 1500 m thick, as shown by the 5). According to Nikishin et al. (1998a, b, 2001) this drilled sections (Figure 3a). westernmost fragment of the Scythian Platform Results from off shore exploration (seismic and is a southern marginal step of the East European drilling) during the last 20 years proved the local Platform. Th e southern margin of the East European presence of thick Triassic and thickened Lower– craton is fl anked by the Scythian orogen, which was Middle Jurassic successions. Th is, together with consolidated during the Late Carboniferous–Early

733 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

Permian. Th e border between them is marked by a with its endmost Rezovo segment; Eastern Srednogorie narrow faulted zone, whose location and designation and Strandzha. are controversial (Morosanu 1996, 2007; Maystrenko et al. 2000; Dinu et al. 2005; Stovba et al. 2006; Khriachtchevskaia et al. 2009). North Dobrogea Th rust-fold Belt (Inverted North Dobrogea Rift Zone) – Th is NW–SE-trending orogen Th e Late Palaeozoic folded basement of the is generally considered to be a Mesozoic north Scythian Platform is covered by a Mesozoic–Tertiary verging fold- and thrust-belt (Seghedi 2001). It is sedimentary succession, rapidly increasing in delimitated to the NE and SW by the crustal-scale thickness and deepening to the Histria basin to the Heraclea and Peceneaga-Camena faults (Figure 5; south. Th us a southward deepening fl exural slope Sandulescu 1984; Morosanu 1996). Geologically the about 50–55 km wide can be diff erentiated (Figure North Dobrogea orogen has been variably interpreted 5), complicated by northward thrusting provoked by as an intracratonic fold belt, a short-lived failed rift , a the North Dobrogea orogeny. Th e Pelikan thrust fault Middle Cretaceous transpressional strike-slip belt, a in the pre-Palaeogene sedimentary sequences is the fragment of a former back-arc basin related to a north- main evidence of compressional tectonics (Morosanu dipping Triassic subduction zone (Seghedi 2001). 1996). Two parallel E–W-elongated highs (uplift ed Th e thrust-folded structural model, constructed blocks/swells) occur on this slope – the northern fi rst for the exposed onshore orogen portion, shows one comprises the Sulina high and Gubkin swell; a system of NE-verging high-angle imbricate thrust the southern one is located on the middle part of the sheets, involving Mesozoic sediments and Hercynian slope and includes the St. George block (Figure 5). basement. Th ey can be grouped into three nappes – the Macin, Niculitel and Tulcea ones, which are thrust northeastwards (Sandulescu 1984; Morosanu East European Platform 1996, 2007). Th e studied WBSZ includes a small part of the Recent geological investigations, synthesized by East European Platform (Figure 5), mainly the East Seghedi (2001), indicate that the North Dobrogea Vilkovian-Zmeinian bulge (Maystrenko et al. 2000; orogenic belt is a Late Permian–Early Triassic rift ed Stovba et al. 2006), consisting of the Zmeinien basin with maximum magmatic activity during and East Vilkovian rises. Th ey are separated by Middle Triassic, and was inverted during the Late the Vilkovian depression off shore. Th e platform Triassic and the Early Cretaceous orogenic phases. basement consists of Carboniferous and older rocks Th ese caused compressional reactivation of the and was aff ected by strong deformation and intensive syn-rift extensional faults, accompanied by the erosion. Th e thickness of the overlying Mesozoic– propagation of the dominantly NE-verging thrusts Neogene sedimentary succession increases rapidly (Seghedi 2001). Th e NE-directed compressional eastwards in the Vilkovian depression and southwards tectonics and movements ceased during the on the Danube fl exure slope. Th e Zmeinian rise is Albian, when the entire North Dobrogea structural extensively faulted. Th e structure of the Vilkovian assemblage was completed (Sandulescu 1984; depression is complicated by many reverse faults. Seghedi 2001; Nikishin et al. 2001). Recently some authors included this part of Th e orogen is well exposed in the central onshore East European Platform as a westernmost fragment zone of the North Dobrogea, but towards the ESE in of the Scythian Platform (Dinu et al. 2005; the coastal zone and off shore it is buried progressively Khriachtchevskaia et al. 2009). to greater depths beneath the Cretaceous Babadag basin and the post-Eocene Histria basin. Orogens Th e orogen system in the WBSZ includes the following Eastern Balkan Th rust-fold Belt and its Endmost units: North Dobrogea thrust-fold belt (inverted North Rezovo Segment – Th e Balkan orogen in Bulgaria is Dobrogea rift zone); Eastern Balkan thrust-fold belt a E–W-trending thrust fold belt traversing the whole

734 G. GEORGIEV

country and it represents a segment of the Alpine spatially superimposed rift ed basins, which are orogen in Eastern Europe (Boncev 1986). strongly deformed by multiphase north-verging Th e Balkan orogen consists of a stack of thrusting during the Early Cimmerian (Late Norian– dominantly north-verging thrust sheets that Hettangian), the Mid-Cimmerian (Middle Jurassic), developed during multiphase collisional events along the Late Cimmerian (Tithonian), the Austrian a long-lived convergent continental margin. Th e (Middle Cretaceous), the Laramian (Late Senonian) compression culminated toward the end of the Early and the Illyrian (early Middle Eocene) compressions. Cretaceous and in the early Middle Eocene (Emery Th e Lower–Middle Jurassic black shales and the & Georgiev 1993). Th ese thrust sheets contain a Upper Triassic fl ysch-like deposits in the Kotel strip range of rock sequences of diff erent provenance and were accepted as a sedimentary fi ll of the ESBRZ age. Accordingly, they can be subdivided into four (Georgiev et al. 2001). groups, namely basement, basement-cover, cover Th e Eastern Balkan thrust-fold belt has a diff erent and exotic nappes (Georgiev & Dabovski 1997). Th e strike and morphology in the coastal and off shore fi rst three groups comprise some sequences derived zones. Th e outer Forebalkan zone disappears as a from the European continental margin, including surface exposure and subsides beneath the Tertiary Proterozoic and Palaeozoic basement rocks and their sedimentary fi ll of the Kamchia basin. Off shore, the dominantly Mesozoic cover that consists of platform, East Balkan orogen fi rst turns towards the SE, then, marginal basin and island arc sequences. Th e exotic through strike-slip movement along the Western nappes are composed of Palaeozoic and Triassic low- Black Sea fault (or the Kaliakra wrench fault), shift s grade metasediments of probable slope to slope-base considerably to the south at about 15 km distance origin. (Figure 5). In this way the endmost Rezovo segment Th e Balkan orogen is made up by an inner of the East Balkan orogen was detached aft er the (southern) uplift ed overthrust zone (called the early Middle Eocene. Balkan or Stara Planina) and an outer (northern) Th e Rezovo Segment is a complex positive subsided thrust and folded zone (called Forebalkan). structure, representing the endmost SE extension Th e Eastern Balkan unit is rather diff erent from the of the off shore Balkan orogen. It consists of two Western one (Byrne et al. 1995). Th ey are separated by positive fault bounded trends: Rezovo and Ropotamo- the NE-trending Tvarditsa transverse strike-slip fault Limankoy (Figure 4a). Both trends correlate well system (Boncev 1958). Th e Tvarditsa fault system with the inner elevated and outer subsided zones is believed to separate two domains with diff erent of the EBTFB (Figures 3). Th e inner Rezovo trend is Cenozoic evolutions (Boncev 1986; Byrne et al. 1995; relatively raised. Th e outer Ropotamo-Limankoy trend Georgiev et al. 2001). Th e Western Balkan orogen is relatively subsided and manifests the structural is dominated by Palaeozoic and Lower Mesozoic transition towards the WBSB. exposures, whereas in the Eastern Balkan segment the Palaeogene and Upper Cretaceous sediments Easten Srednogorie – Th is onshore exposed unit is are widely exposed. Th is K -Pg series is underlain by 2 covered by Pg-N sediments of the Bourgas basin Lower Cretaceous, Jurassic and Triassic sediments (Figure 5). Th e Eastern Srednogorie mostly contains exposed in the narrow Kotel strip associated with the products of Late Cretaceous island arc magmatic frontal East Balkan thrust unit (Georgiev et al. 2001). activity (volcanics, volcaniclastics and intrusive Th e Lower–Middle Jurassic black shales are typical of bodies), locally intercalated by thin deep marine the Kotel strip. In some localities, these black shales sediments (Dabovski et al. 2009). Back-arc rift are closely associated with thick Upper Triassic sequences are preserved in the northern parts of the fl ysch-like deposits. unit. Th e pre-Upper Cretaceous Mesozoic rocks are Recently the Eastern Srednogorie-Balkan rift neither exposed nor have they been drilled in depth. zone (ESBRZ) has been defi ned by Georgiev et al. However, the presence of thick Triassic and Jurassic (2001). Th is zone limits the Moesian Platform to rift -related sequences is indicated by some seismic the south. Th e ESBRZ is characterized by several data (Byrne et al. 1995; Georgiev et al. 2001).

735 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

Strandzha – Th is orogen represents the eastern depocentre is situated in the Turkish off shore sector part of the Rhodope-Strandzha crystalline region, near the Bulgarian border, where over 4 km of extending across Southern Bulgaria, Turkish Th race sediments have been deposited (Figure 3a). and Northern Greece. Th e basin opened during the Middle Eocene on Th e stratigraphy of the Strandzha Orogen is the limb of the rollover anticline in the hanging wall of strikingly diff erent from that in other Bulgarian the Back-Balkan fault line (Doglioni et al. 1996). Th e zones and in the Turkish İstanbul Zone (Savov et North-western termination of the basin is controlled al. 1995). Its basement is composed of Precambrian by the E-trending right-lateral transfer zone of the high-grade and Palaeozoic low-grade metamorphic Back-Balkan fault line along which the rollover rocks. Th e sedimentary succession comprises ends. Th e Late Eocene extension is also supported Triassic clastics and platform carbonates and Lower– by occurrence of coal and black shale and marls of Middle Jurassic sandy-calcareous and shaly-silty this age fi lling the basin. Th e basin development took series (autochthonous), which are topped by slices place mainly during the Neogene and Quaternary of exotic nappes (allochthonous), consisting of (Figures 3a & 10b). Palaeozoic(?) low-grade metamorphic rocks and Triassic (Spathian-Norian) deep marine carbonates. Babadag Basin – Th is small sedimentary basin in Th e Triassic to Middle Jurassic sequences, as the Romanian coastal area covers the southern well as the crystalline basement, were folded and Macin nappe of the North Dobrogea orogen (Figure thrust-faulted during the Early and Mid Cimmerian 5; Seghedy 2001, fi gure 2a). It opened during the orogeny (Georgiev et al. 2001; Nikishin et al. 2001). Late Albian–Cenomanian as a half-graben to the Other authors have considered the earliest Mesozoic north of the genetically linked Peceneaga-Camena compressional deformations of the Strandzha Orogen fault (Sandulescu 1984) and developed in a back- to be Mid Jurassic (Okay & Tüysüz 1999) or post-Mid thrust front position (Morosanu 1996). Th e basin Jurassic (Banks 1997). sedimentary fi ll consists of Upper Albian to Lower Campanian sediments over 1600 m thick. Smaller Sedimentary Basins Bourgas Basin – Th is NW-oriented Tertiary basin is Hydrocarbon Systems located mainly off shore in the SW part of the WBSZ; Hydrocarbon Discoveries and Oil Genetic Types only a small part of its NW periphery is exposed onshore (Figure 5). Th e basin is explored mainly In total 15 hydrocarbon discoveries have been made by seismic data in the Bulgarian off shore. Th ere is in the WBSZ (Figure 7), including 6 gas, 2 gas- drilling in the Bulgarian onshore part (the Bourgas condensate and 7 oil or gas-oil fi elds. 9 fi elds are in the area) and in the Turkish off shore part: the Igneada Romanian off shore sector, 4 in the Bulgarian off shore and Karadeniz wells. sector and 2 in the Ukrainian off shore sector. Th e Th is basin has half-graben geometry, bounded Olimpiyskoe discovery was made by Ukraine, but to the east by the Back-Balkan fault. Th e basin is now belongs to Romania. Th e basic characteristics of bounded by the Eastern Srednogorie unit to the west the discovered fi elds are shown in Table 1. and north, and by the endmost Balkan unit and its Most of the fi elds have not been appraised yet due Rezovo segment to the northeast and east (Figures 3, to diff erent reasons, such as limited hydrocarbon 4a & 5). Its extension in the Turkish off shore sector reserve and lack of investments. remains obscure: most probably it has no connection All the discovered hydrocarbon accumulations with the one located south of the Strandzha Th race are in shallow-water shelves in less than 100 m Basin (Turgut et al. 1991; Okay et al. 2010). water depth (Figure 7). Th ey are related to diff erent Th e basin sedimentary fi ll contains Mid–Late sedimentary basins – 8 are in the Histria sub-basin; 2 Eocene, Oligocene and Neogene clastics and clay in the Vilkovian depression, interpreted as a western (Figure 10b). Seismic data indicate that the basin branch of the Karkinit basin; 3 in the Kamchia sub-

736 G. GEORGIEV

O 28OE 30 E U K R A I N E BEZIMENNOE Karkinit basin ODESSA

Babadag asin

B OLIMPYISKOE ROMANIA PORTITA LEBADA WEST SINOE LEBADA EAST Histria sub-basin PESCARUS COBALCESCU

Constanta DOINA

44ON ANA 44ON

TJULENOVO

Varna

BULGARIA East-Varnatrough GALATA KamchiaLA-1 susub-basin SAMOTINO MORE b-basin

Bourgas asin WESTERN BLACK SEA BASIN

B 42ON

TURKEY 42ON discovered hydrocarbon fields 2000 oil accumulations Thrace gas accumulations Basin 100 200 1000 050100 km 28OE 50 30OE

Figure 7. Map of sedimentary basins in WBSZ and discovered hydrocarbon fi elds. basin and the adjacent southern edge of the Moesian Hydrocarbon accumulations in the WBSZ were Platform. Th e Tyulenovo fi eld is very close to the East discovered within reservoirs of rather diff erent age. Varna trough, while the Olimpiyskoe fi eld is close to In this respect, their genetic correlations are of great the Histria basin. importance.

737 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA sandstone & siltstonesandstone apprising sandstone & carbonatessandstone production-end eocene siltstone apprising Palaeocene Upper Cretaceous Upper 1951 Valanginian carbonates producing 1987 Eocene sandstone apprising 1979 Albian sandstone producing 2001 Eocene & 1984 Eocene sandstone producing 2 Mbs 30 Bcf 30 Bcf 242 Bcf 200 Bcf >350 Bcf elds in WBSZ. 19.4 30 Mbs 24.0 25 Mbs 37.5 42 Mbs 36.3 40 Mbs 28.8 70 Mbs Gas Gas Gas Gas Gas Condensate Ana Gas 2007 Mio–Pliocene sandstone apprising LA-1 Gas- Condensate 1994 Valanginian carbonates shut-in Sinoe Oil Doina Gas 200 Bcf 1995 Mio–Pliocene sandstone apprising Galata Gas 55 Bcf 1993 Eocene & Portita Oil 35.4 Oligocene sandstone shut-in Odessa Gas 427 Bcf 1987 Palaeocene siltstone apprising Pescarus Oil-Gas 70 MbOE 1999 Cretaceous sandstone apprising Tjulenovo Oil Cobalescu Gas 1997 Pliocene sandstone apprising Lebada East Oil Bezimennoe Gas 140 Bcf 1997 Pala Lebada West Oil Olimpyiskoe Oil Samotino MoreSamotino Gas- Condensate 53.5 1986 Eocene Middle & siltstone sandstone apprising Basic characteristics of the discovered Hydrocarbon fi Hydrocarbon the discovered of Basic characteristics 7 8 9 5 4 6 1 2 3 № Field name Field Type API Reserves Discovery Year Reservoir Age Reservoir Lithology Status 13 14 15 10 11 12 Table 1. Table

738

G. GEORGIEV

Vtype IV III type III II type II I type I S26 S26 S25 S25 S24 S24 S22 S22 S21 S21 S19 S19 S16 S16 S12 S12 RS6 RS6 RS2 RS2 RS1 RS1 main crude oils in Western Black Sea Black zone. crude in Western main oils % 100 35.0 (b) b Albian reservoir Paleocene reservoir Lebada E oil (API 37.52) Valanginian reservoir Valanginian Olimpiiskoe oil (API 28.8) Lebada W oil (API 36.29) Eocene reservoir Middle Eocene reservoir Samotino More condensate (API 53.52) Tyulenovo oil (API 19.4) Tyulenovo Albian reservoir Eocene reservoir Eocene reservoir all oils in Histria sub-basin (Romania); sub-basin (Romania); in Histria all oils Oligocene reservoir (a) Sinoe oil (API 24.01) Portita oil (API 35.38) Lebada E oil (API 37.52) Lebada W oil (API 36.29) S26 S26 S26 S26 S25 S25 S25 S25 S24 S24 S24 S22 S24 S22 S22 S22 S21 S21 S21 S21 S19 S19 S19 S19 S16 S16 S16 S16 S12 S12 S12 S12 RS6 RS6 RS6 RS6 RS2 RS2 RS2 RS1 RS2 RS1 RS1 RS1 Oil to Oil Biomarker correlation (Sterane m/z 217): (Sterane correlation Oil Biomarker Oil to a Figure 8.

739

GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

SEAL RESERVOIR

+ + + + +

SOURCE

shows

GAS / OIL / GAS

(m)

Thickness

5 1000 250 - 900 0 600 100 up to 800 to up 0 300 200 - - - up to 350 to up - - - - - 0 300 60 150 0 >200 60 350 300 450

LITHOLOGY Time (my) Time

65 97 5.2 3.4

23.3

1.64 35.4

124.5 145.6

Formation Maykop Fm analogue Fm Maykop

Pontian STAGE Dacian Apt(?)-Alb Neocomian

Histria sub-basin. Lithology legend is shown sub-basin. Lithology Histria is shown legend

K 2 K SERIES OLIGOCENE PLIOCENE EOC K 1 K

(c)

MIO

PALEOGENE NEOGENE J CRETACEOUS SYSTEM

Q SEAL

VOIR RESER + + + +

+ + +

SOURCE shows

Kamchia sub-basin; Kamchia GAS / OIL / GAS

(m) (b) 0-250

0 500 300 Thickness - 500 0 0 >1000 700 - 0 >1000 100 1000 0 5 >1500 950 up to 800 to up - 0 >1000 100 ------> 200 60 120 50 >250 100 200

LITHOLOGY Time (my) Time

50 65 97

135 161

154 5.2

23.3 35,4 56.5

1.64

Fm Fm

Formation

Ruslar Fm Ruslar Avren Fm Avren

Obzor Fm Obzor

Dvoyniza

Kamchia Belene

Titcha / Kaspitchan / Titcha Fm

WBSB (deepwater zone); zone); (deepwater WBSB

Provadia

STAGE Eoc Eoc 2-3 1-2 Hau Valanginian

(a)

EOCENE SERIES PALEOC. K J K OLIGOCENE 2 1 3 J 2 I PLI MIO

PALEOGENE CRETACEOUS JURASSIC SYSTEM NEOGENE Q SEAL

VOIR RESER ?

+ + + + + + + + SOURCE

? shows

? GAS / OIL / GAS

(m)

Thickness up to 1500 to up 500-750 1500 900-1400 - up - - 0150 to 500 0650 100-550 100-850 150 200 abc

LITHOLOGY Time (my) Time

3.4 5.2 10.4 23.3 35.4 56.5 65 1.64

Formation Maykop Fm (or equivalent) (or Fm Maykop

Pontian 10. Figure on

STAGE

Dacian

Mio 3 Mio 1-2 features: Hydrocarbon with columns Lithostratigraphic

SERIES PLIOCENE MIOCENE OLIGOCENE K 1-2 K

PAL EOC

NEOGENE PALEOGENE CRETA SYSTEM Q Figure 9.

740 G. GEORGIEV

VOIR LITHOLOGY VOIR

(m) LITHOLOGY

(m)

shows

GAS / OIL

Thickness SYSTEM SERIES STAGE Formation (my) Time SOURCE RESER SEAL

GAS / OIL

STAGE

Thickness SYSTEM SERIES Formation (my) Time SOURCE RESER SEAL

150- 200

PLI+Q 5.2

up to 800

NEOGENE + Q 23.3 0 500 Mio + 10.4

- ?

200 500

OLIGOCENE + 35.4

PALEOGENE Eo 2-3 50- 200

50 1500

2 EOCENE Eo 1-2 65 030-

NEOGENE 2 150- 250 +

K 97 Mio Hau 0- >200 MIOCENE 135 Kirazli (?) Fm

+ up to 1000

CRETACEOUS

Tth-Vlg 450 750 16.5 ?

Kaspitchan Fm

3 154

J Provadia - 1 Ox+Km Fm 161 30 100 - 30 40 +? - +

0 600

2 Etropole 60- 150 Mio J Fm

JURASSIC Baj+Bth 173 up to 120 +? 23.3 +

350 500

OLIGOCENE

Danisment Fm 35.4

Shabla Fm

Nor +Rht 1

up to 900

800 3 ?

2-3

- T + 223.4

Eoc

0 500

PALEOGENE

Ravnets Fm

EOCENE

A S S I C

Hamitabat Fm + ? 50 up to 600

Crn

1 +

up to 800 ? +

Kaliakra Fm

T R I ? +

235 PAL

0 250 ? Pal+Eoc 65 + 150- 300 Fm ?

Lad

Tjulenovo 239.5 +

450 >1000

Ans Doyrentzi 241.1 ? Dobruja 1 100- 150 Fm CRETACEOUS

T +

Scy 245 ab brecci a sandstone, carbonate clayey shale conglomerate siltstone shale limestone carbonates marl coal volcanics andesites

Figure 10. Lithostratigraphic columns with Hydrocarbon features: (a) East-Varna trough basin; (b) Bourgas basin.

Th e genetic Oil to Oil correlation comprises crude the Bulgarian off shore sector. Th e correlation has oils from the following fi elds: Lebada East (Albian been made by obtained alkane, triterpane (m/z 191), reservoir), Lebada West (Eocene reservoir), Sinoe sterane (m/z 217), triaromatic steroids (m/z 231) and (Eocene reservoir), Portita (Oligocene reservoir) and monoaromatic steroids (m/z 253) profi les, which are Olimpiyskoe (Palaeocene reservoir) in the Romanian of good quality (Figure 8). off shore sector; Tyulenovo (Valanginian reservoir) Although the four oil fi elds in the Histria sub- and Samotino More (Middle Eocene reservoir) in basin (Labada East, Lebada West, Sinoe and Portita)

741 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

are in reservoirs of diff erent age and lithology, and potential source units across the whole WBSZ. Th ere the Sinoe oil is strongly biodegraded, all have very are several sequences with source features (Figure similar biomarker patterns by triterpanes (m/z 191) 9a): and steranes (m/z 217) (Figure 8a). Th e correlation Th e Oligocene–Lower Miocene Sequence (Maikop between them is very good, indicating the same source, Formation Equivalent) is considered to be a primary i.e. the same genetic type. All four oils contain traces hydrocarbon source within the Tertiary succession. of Oleanane (Georgiev 2000), usually interpreted as Th is sequence is defi ned in the Ukrainian and the an indicator for a Tertiary source. Romanian off shore as a major gas/oil prone source Th e triterpane (m/z 191) and sterane (m/z 217) rock (kerogen type II and II–III) of regional extent. correlations between all crude oils in the WBSZ Th e nature of the kerogen in the deepwater zone clearly identifi ed four diff erent genetic types of oils is unknown and may be oil or oil/gas prone. Such with distinctive diff erences between them (Figure discoveries as Olimpiyski, Lebada and Pescarus 8b). Th ey are as follows: I– Olimpiyski type, II– (Figure 7) proved the oil potential of the Oligocene– Lebada type, III– Tyulenovo type and IV– Samotino Lower Miocene shales. More type. Th e Palaeocene–Eocene shale intervals are Th e diff erence between the fi rst two types considered as a secondary hydrocarbon source. Th ey (Olimpiyski and Lebada) is not so evident; they may can contain deepwater and lacustrine shales, each of be related with facies and maturity changes of the which has potential as source rocks. same source. Th e Triterpanes (m/z 191) correlation Th e Middle Miocene–Pliocene sequence, which is shows very slight similarities between the fourth rich in diatomaceous shales, is also considered as a Samotino More condensate type and the third secondary hydrocarbon source. Th e diatomaceous Tyulenovo type: only in the Tyulenovo oil there are shales demonstrate high micro-porosity, with over traces of Oleanan. 50% gas saturation in the Limankoy wells, but they have a very low permeability (Sefunç et al. 2000). Hydrocarbon Source Estimation Th ey are immature up to depth of 3500 m. Hence, their high gas saturation indicates the presence ‘in Estimation of the hydrocarbon generation potential situ’ of biogenic gas. Some Pliocene gas discoveries is made for each of the main sedimentary basins in in the Romanian off shore sector (Cobalescu, Doyna, the WBSZ (Figures 7, 9 & 10). Ana) are also biogenic. Several gas-hydrate accumulations are recognised Western Black Sea Basin – Th e Tertiary sediments by seismic data within the Tertiary succession of the within the WBSB were considered to be the principal WBSB. Th e gas in these fi elds is also considered as potential source complex a long time ago (Geodekjan biogenic in origin. et al. 1982, 1984). Th e results from some recently According to the maturity results from the drilled wells in the Romanian and Turkish off shore Rapsodia and Doina wells in the Romanian off shore sectors (Limankoy, Cobalcescu, Ovidiu, Rapsodia, sector and the Limankoy wells in the Turkish Delfi n & Olimpiyski – Figure 2), however, allowed a off shore sector the burial depth for marginal more precise source rock assessment to be made. organic maturation (Ro– 0.55%) and the onset Th e Lower Miocene and Oligocene sedimentary of oil generation should be more than 3500 m. sequences (Maykop Fm or equivalent) were drilled However there are some ‘warmer zones’ in the WBSB from only a few wells. Th erefore the estimation of (Duchkov & Kazantsev 1985; Kutas et al. 1998), in source complexes is mainly based on seismic data which modelling results show that the ‘oil window’ and extrapolation wells in the Kamchia and the begins at depths of about 2500 m. Histria sub-basins, and in the Danube fl exure slope to the north (Figures 5 & 7). Kamchia Sub-basin – Th e sedimentary fi ll of the Th e deepwater part of the WBSB contains the Kamchia sub-basin (Figure 9b) comprises the thickest Tertiary sequences, some of which are succession above the Illyrian unconformity in the

742 G. GEORGIEV

Middle Eocene. It contains two main sequences: enrichment indicates marginal to good source Mid-Upper Eocene–Oligocene and Neogene, of potential. Th e organic matter is dominated by 1000–1500 m total thickness nearshore and up to degraded humic kerogen type II–III (gas-oil-prone) about 6000 m in the transition to the deepwater with probably oxidised vitrinite composition. Th e zone of the WBSB. Th e basin basement comprises Pyrolysis Potential yield (S2) reaches values from over thick Lower–Middle Eocene, Palaeocene and Upper 1000 ppm up to 10000 ppm, indicating a fair to good Cretaceous sequences, which are intensively thrust source potential. Th e organic matter is immature, as and folded on the southern basin slope and listric- the values of Ro (0.31–0.39%), Tmax (422–438°C) faulted on the northern slope. and spore coloration (2.5–4) indicate. Th e Oligocene sequence (Ruslar Formation) is Overall, the Avren Formation is able to generate considered to be a primary hydrocarbon source. only biogenic gas. But towards the WBSB the shale Th is sequence mainly comprises shale and claystone, content, TOC, burial depth and maturity become occasionally grading to siltstone, with a total higher and the oil/gas source potential increases, thickness increasing northwards from 100–400 m on respectively. the southern basin slope to more than 1000 –1500 m Th e Upper Cretaceous–Lower Eocene sequence in the basin axial zone and eastwards to the WBSB. It is also considered to be a secondary hydrocarbon is an equivalent of the Maykop Formation, which is source. It contains some intervals with dark marl and the basic source unit in the larger Black Sea-Caspian shale present (Byala and Dvoynitza formations); their domain. total thickness is about 100–200 m. Th ey are enriched Th e organic matter content is good to very good with organic matter type II and II–III up to 2–2.5%. (>1%). Th e amorphous kerogen type II dominates. Th e Gas Chromatography/Mass Spectrometry (GC- Th e Pyrolysis Hydrogen index (HI) ranges from MS) data indicate early mature humic kerogen. Th e 30–50 to over 300, which indicates mainly degraded predominance of normal alkanes between n-C27 and humic organic composition. Th e dull-orange to n-C34 appears to be consistent with this interpretation. brown fl uorescence is due to the low level of maturity, Th e analyzed samples have high values of S2 (300– so the kerogen is interpreted as primarily gas-prone, 5000 ppm) and moderate values of HI (40–200). It is although some oil generative opportunities are also quite possible that these mature stage intervals may possible. Th e Pyrolysis Potential yields (S2) are fair be a source of liquid hydrocarbons, as the biomarker to good; the values are oft en over 2000 ppm, ranging correlation between them and the Samotino More up to 6000–8000 ppm. At the drilled depth intervals condensate showed. the formation is immature (0.27–0.35% Ro) and can Th e Neogene sequence is thin near shore (< 400 generate only biogenic gas. m), but towards the WBSB it thickens to 2000 m. Overall, the Ruslar Formation has fair to good In its succession there is a considerable amount of gas source potential, although considerably greater mudstones, enriched with organic matter type II burial depth would be required for it to be realized. (TOC 1.3–2%). Th e HI values are high (340–400 ppm) and Pyrolysis Potential yields (S2) are fair to Th e Mid-Upper Eocene sequence (Avren Formation) good (4810–7140 ppm). All these characteristics is considered to be a secondary hydrocarbon source. suggest a good source potential at the burial depth It comprises alternating shale, mudstones, siltstones appropriate for maturation. and sandstones, with thin limestone beds and conglomerates at the base. Northwards and eastwards the facies becomes more shaly and deepwater. Th e Histria Sub-basin – Th e source rock assessment in total formation thickness increases from 950 m near the Histria sub-basin is accomplished by many well, shore to more than 1500 m towards the WBSB. seismic and analytical data as well in conformity with TOC in shale is moderate to good (0.6–1.85%). the complicated facies architecture of the sedimentary Th e highest TOC values are measured at the top fi ll (Ionescu 2002; Ionescu et al. 2002). and base of the formation intervals. Th is organic

743 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

Oligocene calcareous shales (equivalent of the PI (<0.1) and the reduced thicknesses, indicate very Maykop Formation) with TOC values near and negligible hydrocarbon (gas) potential. above 0.90% and up to 1500 m thick, appear to be the primary hydrocarbon source in the basin (Figure 9c). Th e Hydrogen index (HI) values indicate mixed gas/ East-Varna Trough Basin – Very valuable new oil prone kerogen, in which a dominant component information for the basin source potential was obtained from the Galata 1 well, drilled in 1993, is the vitrinite, but the proportion of sapropel is which fi rst provided information about the presence also good. A marine depositional environment of thicker Lower–Middle Jurassic sequences in the is indicated by the relatively high abundance of Bulgarian off shore sector (Figure 10a). Th ey are diverse dinofl agellate cysts. Th e S2 values indicate a comparable with the same lithostratigraphic intervals transitional poor/moderate potential for hydrocarbon onshore, known as Etropole Formation and Provadia generation. Th e organic matter is immature (<0.5% Formation. Note that the thick shales of the Etropole Ro) and only biogenic gas can be generated. But Formation, across the Southern Moesian Platform at a greater depth (> 3500 m) and maturity these margin, are the main source for most of the oil and gas sediments can generate a considerable volume of oil fi elds discovered in Northern Bulgaria (Georgiev & and gas. Dabovski 1997; Georgiev 1998, 2000; Georgiev et al. Th e Pliocene (Pontian) mudstones are considered 2001). Hence, the presence of the same shale unit in to be a secondary hydrocarbon source. Th e TOC the East-Varna trough basin is a very positive result, content is moderate – 0.66–0.67%. Th e HI values especially considering their increasing thickness indicate that gas-prone kerogen appears to be north of Galata well, as indicated by the seismic data. dominant. Th e S2 values show poor to moderate Th e primary hydrocarbon sources are the potential for hydrocarbon generation. Th e main Etropole and Provadia formations, according to the kerogen components are vitrinite and, to a smaller parameters from the Rock-Eval analyses, log records extent, sapropel. Th e presence of C sterane 30 and biomarker correlations. biomarkers and the predominance of C27ααα steranes indicates marine depositional conditions. Th e Th e Etropole Formation (Bajocian–Bathonian in n-alkane distributions and palynomorph assemblage age) is about 70 m thick in the Galata section, but to are consistent with this interpretation. At this the central basin zone its thickness reaches 100–120 immature stage (<0.5% Ro) only biogenic gas can m and more, as indicated by the seismic data. Shales be generated, but at a greater depth (> 3500 m) and in the lower and the upper parts of the formation have diff erent source characteristics. higher maturity oil generation is also possible. Th e upper interval has a better source potential. Th e Neocomian sequence in the Egreta and Lebada Th e TOC content is up to 1.5%. Th e organic matter sedimentary successions is 500–700 m thick, but the is transitional mature to early mature according thickness of possible source intervals is much smaller to the vitrinite refl ectance, Tmax and biomarker (Ionescu 2002; Ionescu et al. 2002). Th e measured compositions. Gas/oil prone kerogen type II–III TOC values are 1.16–1.89%. Th e HI values indicate dominates. Th e Pyrolysis Potential yield (S2) is a mixed gas and oil prone kerogen. But the visual higher than those of the lower interval. kerogen analysis shows that all three components – sapropel, vitrinite and inertinite are signifi cant. Th e lower interval is poorer in TOC content – Several degraded palynomorphs and palynodebris up to 0.5–0.8%. Th e vitrinite refl ectance (Ro) and and the pristane/phytane ratio indicate that oxidation the Tmax value show a transitional mature to early has occurred. Hence the kerogen type is II–III and mature stage. Th e organic matter has diff erent III (mainly gas prone). Th e organic matter appears biomarker compositions, which indicates mainly gas to be at peak maturity (0.75% Ro) for hydrocarbon prone kerogen type III. generation. But S2 values show a transitional poor/ Overall the source potential of the Etropole moderate to moderate potential for hydrocarbon Formation is fair to good. Th e proven genetic link generation, which together with the low values of between Tyulenovo oil and Etropole Formation

744 G. GEORGIEV Valanginian reservoir Valanginian 3 Tjulenovo oil well Galata 1 well Galata 1 well Galata 1 well Galata 1 J1 Source rock well Galata 1 J3 Source rock (Provadia Fm) J2 Source rock (Etropole Fm - lower part) T Source rock (Rusinovdel Fm) J2 Source rock (Etropole Fm - upper part) for Tyulenovo oil type. oil Tyulenovo for b (b) for Lebada oil type; Lebada oil for (a) Lebada E field Lebada W field Lebada E oil Lebada W field Albian reservoir Lebada W oil Eocene reservoir Oligocene Source rock Neocomian Source rock S26 S26 S26 S26 S26 S25 S25 S24 S22 S24 S22 S22 S22 S22 S21 S21 S21 S21 S21 S19 S19 S19 S18 S18 Miocene (Pontian) Source rock S16 S16 S16 S16 S16 S12 S12 S12 S12 S12 RS6 RS6 RS6 RS6 RS6 RS2 RS2 RS2 RS2 RS2 RS1 RS1 RS1 RS1 RS1 Oil to Source Biomarker correlation (Sterane m/z 217): (Sterane correlation Biomarker Source Oil to a Figure 11.

745 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

(Figure 11b) is a very important fact, which cannot Th e Lower–Middle Miocene predominantly clastic be disregarded (Georgiev 2007). sequence (Kirazlı Formation), about 1700 m thick Th e Provadia Formation (Oxfordian– in the Igneada well, contains numerous lignite coal Kimmeridgian) only occurs in the southeasternmost beds with 3 m maximum individual thickness. Th e number of drilled coal beds in the Kara Deniz well zone of the Moesian Platform. Its thickness is not is 28. So, this rich coal content could produce a more than 100–110 m. Rich oil saturation in carbonate considerable amount of gas. breccia, overlying the Provadia Formation, has been established by some onshore boreholes near Varna. Th e organic enrichment of shale is fair to good – the Oils to Sources Correlations TOC ranges from 0.5% to 1.5%. Kerogen is oil/gas Th ere is no unifi ed perception among the authors prone - type II or II–III, the maturity is transitional who studied the genesis of the main crude oils in the to early mature. Th e genetic correlation between WBSZ before 1990. Most of them used traditional Tyulenovo oil and the Provadia Fm is very good bulk methods, such as physical characteristics, (Figure 11b) (Georgiev 2007), so the oil generative compositional fractionation, element concentrations potential of the Provadia Fm is beyond doubt. and ratios. However, the recent investigations have shown that non-genetic processes (as biodegradation, thermal maturation, water washing and migration) Bourgas Basin – In the Bulgarian off shore part of this aff ect such characteristics dramatically (Curiale 1994; basin there are no drilled wells, but in its Turkish part Peters & Cassa 1994). two wells were drilled in 1971 – Igneada and Kara Th e presented results focus on the genetic Deniz. Th e presented hydrocarbon source analysis correlation between the discovered types of crude for this basin is based on the following: (i) lower oils in the Bulgarian and Romanian off shore sectors basin sequences cropping out onshore in the Bourgas and their possible sources, based on molecular area (Radev et al. 1994; Velev et al. 1994); (ii) some characteristics (biomarkers profi les) and stable available information from Igneada and Kara Deniz carbon isotope ratios (C12/C13). All correlation results wells and (iii) good quality seismic data, acquired in have been interpreted in the context of the regional 1992/1993. geology. At least two probable source intervals can be Th e genesis of the Lebada oil type II has been identifi ed in the sedimentary succession (Figure studied by biomarker correlations with all possible 10b): source intervals in the Histria basin, which are Miocene–Pontian, Oligocene and Neocomian. Th e Th e Upper Eocene–Oligocene shale sequence has a correlation of the Lebada oils with the Oligocene total thickness of 300–500 m. Th e burial depths are source rocks by triterpanes (m/z 191) and steranes 800–1200 m. Th e TOC is up to 1.5–2%, the Pyrolysis (m/z 217) patterns is the best one (Figure 11a). Its yield (PY or S2) – 7 kg/t and the Hydrogen Index (HI) correlation with Miocene (Pontian) source rocks is about 300. Th e lacustrine lignite coals and shallow is also good, even though some diff erences in the to marginal marine shale (Danişment Formation), triterpane patterns (m/z 191) can be observed. Th e drilled in the Turkish zone, showed good source presence of Oleanane traces in the Miocene pattern parameters: PY up to 41.6 kg/t and HI up to 387. cannot be accepted as strong evidence for genetic In outcrop the coals and shale (Ravnets Formation) relations with the two oils. Th e diff erences between are immature (0.35% Ro), but their burial depth the Lebada oils and the Neocomian source rocks do off shore increases up to 2200–3000 m in the basin not suggest any genetic relationship. Hence the genetic depocentre, located around the Bulgarian/Turkish links of the Lebada oil type with Oligocene source rocks border. Hence, an increasing organic maturity up to look most likely. early oil generation can be expected, proven by basin Th e genesis of the Tyulenovo oil of type III has modelling. Overall, this source unit is mainly gas- remained obscure more than 45 years since its prone and can generate mainly biogenic gas. discovery in 1951. But the Galata 1 off shore well,

746 G. GEORGIEV

28 OE 30 OE

Karkinit

Basin

Histria sub-basin

Constanta

ure 3

Fig 1 b

Fig 12a Varna ure East-Varnatrough

Kamchia sub-basin

WESTERN BLACK SEA BASIN

050100 km

a hsydrocarbon ystem in WBSB b ((a) immature; b) mature a hsydrocarbon ystem in East-Varna trough ure b ((a) immature; b) mature N Bourgas asin O Fig 12c hsydrocarbon ystem in Bourgas B asin 42 a b ((a) immature; b) mature

O B hydrocarbon migration direction 42 2000 hpydrocarbon lay trend for g as

hpydrocarbon lay trend for og il & as 100 200 1000 d((iscovered fields: a) oil; b) gas 50 ab 28 OE 30 OE

Figure 12. Map of Hydrocarbon systems and Exploration trends in Western Black Sea zone.

747 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

MOESIAN PLA TFORM W E S T B L A C K S E A B A S I N W-NW East - Moesian trough Polshkov ridge E-SE 0 0

1 1 T2 2 T1 +Pz 2

3 3 East-Moesian fault Ng 4 4

TWTT (s) 5 Oli 5 a Eoc 6 6 K2 +Pal hydrocarbon kitchen Alb+Apt 7 maturation boundary 7 K+J13 migration direction for HC hydrocarbon accumulation zone (play) 01020 km

E a s t V a r n a t r o u g h T y u l e n o v o f i e l d S-SW Galata N-NE 0 0 N

Pg 1 K2+Pal 1 K1hot K1v+J3 2 2 (s) J1-2

TWTT 3 3 T3

4 4 T2 b T1 Hydrocarbon kitchen 5 Pz Maturation boundary 0 10 20 km Migration direction for HC

B O U R G A S B A S I N BALKAN OROGE N W Rezovo ridge E 0 0 Plio

Mio ? 1 1 Eoc 2- 3 Oli Eoc 1- 2

TWTT (s) K2 2 c + Pal 2

hydrocarbon kitchen maturation boundary 3 migration direction for HC hydrocarbon accumulation zone (play) 0 10 20 km

Figure 13. Conceptual models for hydrocarbon systems and exploration trends in: (a) Western Black Sea basin, (b) East-Varna trough, (c) Bourgas basin (location of profi les is shown on Figure 12).

748 G. GEORGIEV

drilled in 1993, has suggested an excellent opportunity Histria basins; (2) the East-Varna trough basin and to solve this problem. All possible source intervals (3) the Bourgas basin. Th e fi rst two hydrocarbon in the Galata section, related to the Upper Jurassic, systems are proven by the discovered hydrocarbon Middle Jurassic, Lower Jurassic and Upper Triassic fi elds in the Histria and the Kamchia sub-basins, the succession have been analyzed with GC-MS, Rock third one is prognostic. 12 13 Eval and stable carbon isotope (C /C ) analyses. Th e conceptual models for the hydrocarbon A very good biomarker correlation of the systems and their basic characteristics are shown Tyulenovo oil with the Upper Jurassic (Provadia on Figures 12 & 13. Th e estimation of the prospect Formation) and the Middle Jurassic (the Etropole exploration trends for gas and oil-gas is based on Formation – the upper part) source intervals was kerogen type and maturity. established (Georgiev 2007), although the genetic Th e West Black Sea basin hydrocarbon system links between the Tyulenovo oil and the Provadia (Figures 12 & 13a) is related mainly to presence and Formation looks slightly better (Figure 11b). Th e large distribution of the primary Oligocene–Lower 12 13 stable carbon isotope (C /C ) correlation between Miocene source complex (the Maykop Formation the Tyulenovo oil (27.5‰) and the Middle Jurassic or equivalent). Its huge generation potential was interval (27.73‰) looks very good. Hence the most proved by many economic gas-oil discoveries in probable source for the Valanginian oil accumulation the North Caucasus-South Caspian region as well in the Tyulenovo fi eld is the Provadia Formation (J ) 3 as the discoveries in the Histria and the Kamchia and the upper part of the Etropole Formation (J ). 2 sub-basins. Additional hydrocarbon sources in some We consider that the hydrocarbon output from zones may be provided by the Middle Miocene– the Etropole and Provadia formations obviously Pliocene shale, Pliocene (Pontian) mudstones in the combines during the migration, especially Histria sub-basin and the Palaeocene–Eocene shale considering that the two formations are separated in the southern zone of the Kamchia sub-basin. in the sedimentary succession by very thin and permeable clastic-carbonate sediments (Figure 10a). Th e mature zone of this system comprises burial depth of under 3000 m for the primary Oligocene– Th e Samotino More condensate (type IV), Lower Miocene source complex. Hence, the active accumulated in a Middle Eocene reservoir, contains hydrocarbon generation pod (kitchen) is located in a distinctive distribution of molecular biomarkers, the deep-water zone of the WBSB. which is of considerable assistance for correlation with the oil stain occurrences and the possible source Th e basic hydrocarbon migration is vertically rocks. and laterally westwards to higher elevated tectonic units and structures, or the reservoir targets and the Th e good genetic correlation of the Samotino entrapping zones related to the channel systems (fans, More condensate and the Lower Eocene oil stains deltaic bottom-set beds of the Danube and Kamchia with the Lower Eocene (the Dvoynitsa Formation) pro-delta). and the Palaeocene–Upper Cretaceous (the Byala Formation) source intervals in the sedimentary Th e major hydrocarbon accumulation zones or succession of the Kamchia sub-basin (Figure 9b) play trends are as follows: the southern fl exural slope of the Scythian Platform; the northern slope of the deserves special consideration. Mainly the C27–C29 distributions indicate good genetic links between Histria sub-basin; the East Moesian marginal fault them. zone and the Polshkov ridge; the northern and the southern slopes of the Kamchia sub-basin and the tilted blocks in the Ropotamo-Limankoy trend of the Hydrocarbon Systems and Exploration Trends Rezovo segment. Th ree main hydrocarbon systems with economic Th e East-Varna trough hydrocarbon system potential were recognised in the Western Black Sea (Figures 12 & 13b) is considered mainly due to the off shore zone (Figure 12). Th ey are related respectively presence of the Middle and Upper Jurassic source to the following: (1) the Western Black Sea basin rocks (the Provadia and Etropole formations) with a and its two western branches – the Kamchia and the modest generative potential in the restricted wrench

749 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

basin. However the economic signifi cance of this and East-European platforms; (3) fragments from source is proved by the discovered Tyulenovo oil fi eld the North Dobrogea, Eastern Balkan, Eastern (Georgiev 2007). Srednogorie and Strandzha orogens; (4) smaller Th e source assemblage is transitional to early Burgas and Babadag basins. mature, hence the active hydrocarbon generation Th e Kamchia southern branch of the WBSB, is launched. Th e hydrocarbon migration is directed called the ‘Kamchia depression’, is superimposed on laterally eastwards and laterally-vertically to the N– both the southern margin of the Moesian Platform NE. and the frontal zone of the Balkan thrust-fold belt. Its Th e main accumulation zone is related to narrow sedimentary fi ll comprises Middle–Upper Eocene, horst blocks, separating the three wrench basins, Oligocene and Neogene clastic and clay successions. where the Upper Jurassic–Valanginian carbonates Th e basin origin and development is controlled by the and the Lower Triassic clastics can be the main following: (i) the uplift ing and N–NE propagation of exploration targets (Georgiev 2007). the Balkan thrust-fold belt and (ii) the opening and Th e Bourgas basin hydrocarbon system (Figures 12 expansion of the WBSB. Th e basin gradually deepens & 13c) is immature and not yet explored by drilling. and expands eastwards and merges with the WBSB Th e main reason for its defi nition is the presence fl oor. of numerous coal-bearing beds and organically Th e Histria northern branch of the WBSB, called enriched shale in the Upper Eocene–Oligocene and the ‘Histria depression’, is superimposed on the the Lower–Middle Miocene sequences, which can be southern and middle nappes of the North Dobrogea sources for a coal-generated biogenic gas. Th e mature orogen and covers also the northeasternmost part hydrocarbon generation is likely to be launched in of the Moesian Platform. Oligocene and Pontian the deepest and very restricted basin depocentre, sequences dominate the sedimentary succession. Th e located on the Bulgarian/Turkish border, as the basin basin gradually expands and deepens towards the SE modelling shows. and merges with the WBSB fl oor. Some positive swell-like structures in the western Th e Kamchia and North Dobrogea branches of basin slope, mapped by seismic data beneath and the WBSB have been already settled during the Late above the basin bottom unconformity, as well as Permian–Early Triassic and the Late Triassic–Early- the uplift ed Rezovo trend (horst), are promising Mid Jurassic rift ing (Figure 6). zones for hydrocarbon entrapment, especially if we consider the shallow depths. Two tectonic units are defi ned in the deep-water part of the WBSZ: the Polshkov ridge and the East Moesian trough. Conclusions Four diff erent types of tectonic units occur within Th e WBSZ has a complex geological structure and the eastern Moesian Platform zone, they are: the evolution, engendered by complex interrelations Green Schist zone (the Central Dobrogea unit); the between diff erent tectonic units and events. A Palaeozoic zone (the South Dobrogea unit and the detailed tectonic map for the whole Western Black North Bulgarian arch); the System of Late Triassic Seas zone was compiled using an integrated unifi ed and the Early–Middle Jurassic wrench/pull-apart approach, interpretation and synthesis of seismic basins and the Southern and Eastern platform edges and borehole data along many regional cross-lines, and margins. Th e following three relatively small also considering the distribution of gravity-magnetic troughs are defi ned: the East Varna, Tyulenovo and anomalies (Figure 5). Ushakov ones. Th ey are formed mainly by strike-slip Th e complex geological structure of the WBSZ is movements along the bounding Balchik, Kaliakra defi ned by four groups of tectonic units: (1) Western and East Moesian faults in the Late Triassic. Only Black Sea basin (WBSB) – its western zone with the the East-Varna trough was reactivated during the Kamchia and the Histria westward wedging branches Early–Middle Jurassic, coeval with the rift stage in (sub-basins); (2) portions of the Moesian, Scythian the East Srednogorie-Balkan zone. Hence, presence

750 G. GEORGIEV

of the same thick Lower–Mid Jurassic sequences in hydrocarbon systems and their exploration trends the Kamchia zone is quite possible. are constructed (Figures 12 & 13). Th e Eastern Balkan thrust and fold belt changes the Major hydrocarbon accumulation zones in the strike and the morphology in the coastal and off shore WBSB hydrocarbon system are: the southern fl exural zones – fi rst it turns towards SE, then it shift s through slope of the Scythian Platform; the northern slope strike-slip movement along the Western Black Sea of the Histria sub-basin; the East Moesian marginal fault (or the Kaliakra wrench fault) considerably to fault zone and the Polshkov ridge; the northern and the south at about 15 km distance. Th us the endmost the southern slopes of the Kamchia sub-basin and Rezovo segment was detached aft er the early Middle the tilted blocks in the Ropotamo-Limankoy trend of Eocene. Th e Rezovo segment comprises two positive the Rezovo segment. fault-bounded trends: the Rezovo (relatively elevated) Th e main accumulation zone in the East Varna and Ropotamo-Limankoy (relatively subsided) ones. trough hydrocarbon system is related to narrow horst Th ey correlate well with the inner elevated and the blocks, separating the three wrench basins, where outer subsided zones of the Balkan orogen. the Upper Jurassic–Valanginian carbonates and the Th e oil-gas potential of the WBSZ is proved by the Lower Triassic clastics can be the main exploration 15 hydrocarbon fi elds discovered: 7 are mainly oil: 2 targets. are gas-condensate and 6 are gas ones. In the Bourgas basin hydrocarbon system some Four diff erent oil genetic types were indentifi ed positive swell-like structures in the western basin by biomarkers correlations. Th ey are: the Olimpiyski, slope as well as the elevated Rezovo trend (horst) are Lebada, Tyulenovo and Samotino More types (Figure promising hydrocarbon entrapment zones. 8). Among all defi ned exploration trends, those Th e opportunities for hydrocarbon generation related to the transition zone between the Moesian are estimated for all main sedimentary basins in the Platform and the WBSB as well a those related to the WBSZ (Figures 7, 9 & 10). southern edge of the Scythian Platform are of the highest potential. Th e main risk factor is the presence Th e oil accumulations in the Histria basin are of good reservoirs and more volumetric traps. related to the Oligocene source (Figure 11a). Th e likeliest sources of the Valanginian oil accumulation in the Tyulenovo fi eld are the Provadia Fm (J3) and Acknowledgements the upper part of Etropole Formation (J ), in the East- 2 Gratitude is expressed to British Gas Exploration & Varna trough (Figures 11b & 13b). Th e good genetic Production Ltd. and to OMV Exploration GmbH correlation of the Samotino More condensate in the for very fruitful collaboration. Many thanks also to Kamchia basin with source intervals in the basin Reinhard Sachsenhofer and Achim Bechtel (Leoben basement (Dvoynitsa Formation, Lower Eocene and University, Austria) for performing the most of Rock- Byala Formation, Paleocene–Upper Cretaceous) Eval and geochemical analyses. Th e author is grateful deserves special consideration. Th e origin of the to Ion Morosanu, Sergey Stovba and Cornelio Dinu for Olimpiyski oil type remains obscure. constructive discussions. Critical reviews of an earlier Th ree main hydrocarbon systems with economic version of this paper by Christo Dabovski and another potential are recognised. Th ey relate to: the WBSB anonymous reviewer are greatly appreciated. Special with its two western branches, the East-Varna trough thanks are extended to Aral I. Okay for his editorial and the Burgas basin. Conceptual models for the eff orts.

References

Banks, C.J. 1997. Basins and thrust belts of the Balkan coast of the Banks, C.J. & Robinson, A.G. 1997. Mesozoic strike-slip back-arc Black Sea. In: Robinson, A.G. (ed), Regional and Petroleum basins of the Western Black Sea region. In: Robinson, A.G. Geology of the Black Sea and Surrounding Region. AAPG (ed), Regional and Petroleum Geology of the Black Sea and Memoir 68, 115–128. Surrounding Region. AAPG Memoir 68, 53–62.

751 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

Bega, Z. & Ionescu, G. 2009. Neogene structural styles of the NW Emery, M. & Georgiev, G. 1993. Tectonic evolution and hydrocarbon Black Sea region, off shore Romania. Th e Leading Edge 28, potential of the Southern Moesian platform and Balkan- 1082–1089. Forebalkan regions of Northern Bulgaria. AAPG Bulletin 77, Boncev, E. 1958. Über einen zu den Balkaniden Schrāggelegenen 1620–1621 [Abstract]. horizontal veschiebungsgürtel. Bulletin de l‘Institut de la Finetti, I., Bricchi, G., Del Ben, A., Pipan, M. & Xuan, Z. 1988. Geologie pres de l‘Academie des sciences de Bulgarie VI, 3–17 [in Geophysical study of the Black Sea area. Bullettino di Geofi sica Bulgarian with German abstract]. Teorica ed Applicata 30, 197–324. Boncev, E. 1986. Th e Balkanides – Geotectonic Position and Geodekjan, A., Trotsjuk, V., Monahov, I. & Berlin, J. 1982. A Development. Publication House of BAS, Sofi a, Geologica quantitative evaluation of the scale of oil-gas formation in Balcanica, Series Operum Singulorium 1 [in Bulgarian with the Mesozoic and Cenozoic deposits of the Bulgarian Black English summary]. Sea sector (northern part). Geologica Balcanica 12, 3–19 [in Byrne, P., Georgiev, G., Todorov, I. & Marinov, E. 1995. Structure, Russian]. depositional setting and petroleum geology of Eastern Geodekjan, A., Trotsjuk, V. & Monahov, I. (eds) 1984. Oil and Balkanides, Bulgaria. 5th Conference of EAGE, Glasgow – Gas Genetic Studies of the Bulgarian Black Sea Sector. Sofi a, Abstract Book 2, EAPG Division, p. E-021 [extended abstract]. Publishing House of BAS [in Russian]. Curiale, J.A. 1994. Correlation of oils and source rocks – a Georgiev, G. 1998. Hydrocarbon plays and their prospects in conceptual and historical perspective. In: Th e Petroleum System Bulgaria. In: EAGE 60th Conference and Technical Exhibition – – From Source to Trap. AAPG Memoir 60, 251–260. Leipzig, Petroleum Division, Extended Abstracts 2, P-514. Dabovski, C., Kamenov, B., Sinnyovski, D., Vasilev, E., Georgiev, G. 2000. Oil-oil and oil-source correlation for the major Dimitrova, E. & Bayraktarov, I. 2009. Upper Cretaceous crude oils in Bulgaria. Annuaire de L’université de Sofi a ‘St. geology. In: Zagorchev, I., Dabovski, C. & Nikolov, T. (eds), Kliment Ohridski’, Faculté de Géologie et Géographie, Livre 1 – Geology of Bulgaria II: 5 Mesozoic Geology, 303–589. Géologie 92, 39–60. Dachev, C. 1986. Structure and evolution of the transition zone Georgiev, G. 2004. Geological structure of Western Black Sea between Moesian Platform and Black Sea depression. In: region. In: EAGE 66th Conference & Exhibition, 7–10 June 2004 Khrischev, Kh.G. (ed), Contributions to Bulgarian Geology Paris-France, Extended Abstracts (CD-ROM). (27th International Geological Congress, Moscow, 1984), Technika, Sofi a, 68–76 [in Russian]. Georgiev, G. 2007. Challenge for hydrocarbon exploration in Western Black Sea – case study for origin of Tyulenovo oil Dachev, C. & Georgiev, G. 1995. Rift tectonics problems in the fi eld. In: EAGE 69th Conference & Exhibition, 11–14 June 2005 Western Black Sea basin. IGCP Project No. 369 ‘Comparative London-England, Extended Abstracts (CD-ROM), P-255. Evolution of Peri-Tethyan Rift Basins’, 2nd Annual Meeting, Mamaya-Romania, Abstracts Volume. Georgiev, G., Bechtel, A., Sachsenhofer, R. & Gratzer, R. 2001. Petroleum play-concept for main oil/gas fi elds in the Southern Dachev, C., Stanev, V. & Bokov, P. 1988. Structure of the Bulgarian Moesian platform (Bulgaria). In: EAGE 63rd Conference & Black Sea area. Bullettino di Geofi sica Teorica ed Applicata 30, Technical Exhibition, Amsterdam-Th e Netherlands, Extended 79–107. Abstracts (CD-ROM), P-512. Dinu, C., Wong, H.K. & Tambrea, D. 2002. Stratigraphic and Georgiev, G. & Dabovski, C. 1997. Alpine structure and petroleum tectonic syntheses of the Romanian Black Sea shelf and geology of Bulgaria. Geology and Mineral Resources 8/9, 3–7 correlation with major land structures. In: Dinu, C. & [in Bulgarian]. Mocanu, V. (eds), Geology and Tectonics of the Romanian Black Sea Shelf and Its Hydrocarbon Potential 2, 101–117. Georgiev, G. & Dabovski C. 2001. Mesozoic and early Cenozoic evolution of the South Moesian platform margin. In: 7th Dinu, C., Wong, H.K., Tambrea, D. & Matenco, L. 2005. Zonenshain International Conference on Plate Tectonics, 30–31 Stratigraphic and structural characteristics of the Romanian October, 2001, Abstracts. Moscow, Scientifi c Peace, 239–240. Black Sea shelf. Tectonophysics 410, 417–435. Georgiev, G., Dabovski, C. & Stanisheva-Vassileva, G. 2001. Doglioni, C., Busatta, C., Bolis, G., Marianini, L. & Zanella, East Srednogorie-Balkan rift zone. In: Ziegler, P.A., Cavazza, M. 1996. Structural evolution of the Eastern Balkans (Bulgaria). W., Robertson, A.H.F. & Crasquin-Soleau, S. (eds), Peri- Marine and Petroleum Geology 13, 225–251. Tethys Memoir 6: Peri-Tethyan Rift /Wrench Basins and Passive Drannikov, E.N., Ognjanov, R.A., Jakimov, J.K. & Monachov, Margins. Memoires du Museum National d`Histoire Naturelle I.B. 1979. Some features in the geological structure of the north 186, 259–293. zone in Bulgarian Black Sea shelf in relations with oil-gas Görür, N. 1988. Timing of opening of the Black Sea basin. bearing. Comptes Rendus de L’academie Bulgare des Sciences 32, Tectonophysics 147, 247–262. 497–500 [in Russian]. Ionescu, G. 2002. Facies architecture and sequence stratigraphy of Duchkov, A.D. & Kazantsev, S.A. 1985. Heat fl ow through the the Black Sea off shore Romania. In: Dinu, C. & Mocanu, V. bottom of the Black Sea western part. Geology and Geophysics (eds), Geology and Tectonics of the Romanian Black Sea Shelf 8, 21–34 [in Russian]. and its Hydrocarbon Potential 2, 43–51.

752 G. GEORGIEV

Ionescu, G., Sisman, M. & Cataraiani, R. 2002. Source and Nikishin, A., Korotaev, M., Ershov, A. & Brunet, M. 2003. Th e reservoir rocks and trapping mechanisms on the Romanian Black Sea basin: tectonic history and Neogene–Quaternary Black Sea shelf. In: Dinu, C. & Mocanu, V. (eds), Geology and rapid subsidence modelling. Sedimentary Geology 156, 149– Tectonics of the Romanian Black Sea shelf and its Hydrocarbon 168. Potential 2, 67–83. Okay, A.I., Özcan, E., Cavazza, W., Okay, N. & Less, G. 2010. Kalinko, M. (ed). 1976. Geology and Oil-gas Bearing of North Basement types, Lower Eocene series, Upper Eocene Bulgaria. Nedra, Moscow. olistostromes and the initiation of the southern Th race basin, Khriachtchevskaia, O., Stovba, S. & Popadyuk, I. 2009. NW Turkey. Turkish Journal of Earth Sciences 19, 1–25. Hydrocarbon prospects in the Western Black Sea of Ukraine. Okay, A.I., Şengör, A.M.C. & Görür, N. 1994. Kinematic history of Th e Leading Edge 28, 1024–1029. the opening of the Black Sea and its eff ect on the surrounding Kutas, R.I., Kobolev, V.P. & Tsvyashchenko, V.A. 1998. Heat regions. Geology 22, 267–270. fl ow and geothermal model of the Black Sea depression. Okay, A.I. & Tüysüz, O. 1999. Tethyan sutures of northern Turkey. Tectonophysics 291, 91–100. In: Durand, B., Jolivet L., Horváth, F. & Séranne, M. (eds), Maystrenko, Yu.P., Stovba, S.M. & Sovich, T.A. 2000. Structure of Th e Mediterranean Basins: Tertiary Extension Within the Alpine the Zmeinian-East-Vilkovian uplift zone, Black Sea, Ukraine, Orogen. Geological Society, London, Special Publications 156, from regional seismic refl ection profi les. EUROPROBE, INTAS 475–515. Georift , Eurobridge and Caucasus Workshop, 12-16 October, Peters, K.E. & Cassa, M.R. 1994. Applied source rock geochemistry. 2000 Yalta (Gurzuf) Crimea, Abstracts, Geophysical Journal 4, In: Th e Petroleum System – From Source to Trap. AAPG Memoir 22. 60, 93–119. Morosanu, I. 1996. Tectonic setting of the Romanian off shore area Radev, M., Leu, W. & Todorov, I. 1994. Geological structure and at the pre-Albian level. In: Wessely, G. & Liebl, W. (eds), development of South Bulgarian Black Sea shelf. Th e Petroleum Oil and Gas in Alpidic Th rust Belts and Basins of Central and Geology and Hydrocarbon Potential of the Black Sea Area, Eastern Europe. EAGE Special Publication, 315–323. Varna, Abstract Volume. Morosanu, I. 2007. Romanian Continental Plateau of the Black Sea Rempel, H. & Georgiev, G. 2005. Hydrocarbon potential of the / Tectonic-sedimentary Evolution and Hydrocarbon Potential. Black Sea region. Ukrainian Geologist 4, 20–27. Oscar Print, Bucuresti. Robinson, A.G. (ed) 1997. Regional and Petroleum Geology of the Morosanu, I. & Sava, C. 1998. Geological framework of the Romanian Black Sea shelf by the integration of seismic Black Sea and Surrounding Regions. AAPG Memoir 68. interpretation with gravity and magnetic 2-D modelling. Robinson, A., Rudat, J., Banks, C. & Wiles, R. 1996. Petroleum Special Publication, 3rd International Conference on Petroleum Geology of the Black Sea. Marine and Petroleum Geology 13, Geology and Hydrocarbon Potential of the Black and Caspian 195–223. Seas Area. Neptun-Constanta, Romania. Geo-Eco-Marina 3, Robinson, A., Spadini, G., Cloetingh, S. & Rudat, J. 1995. 168–174. Stratigraphic evolution of the Black Sea: inferences from basin Nikishin, A.M., Cloetingh, S., Bolotov, S.N., Baraboschkin, modelling. Marine and Petroleum Geology 12, 821–835. E.Yu., Kopeavich, L.F., Nazarevich, B.P., Panov, D.I., Brunet, Sachsenhofer, R.F., Stummer, B., Georgiev, G., Dellmour, R., M-F., Ershov, A.V., Ilina, V.V., Kosova, S.S. & Stephenson, R.A. 1998a. Scythian Platform: chronostratigraphy and Bechtel, A., Gratzer, R. & Ćorić, S. 2009. Depositional polyphase stages of tectonic history. In: Crasquin-Soleau, S. environment and source potential of the Oligocene Ruslar & Barrier, E. (eds), Peri-Tethys Memoir 3: Stratigraphy and Formation (Kamchia Depression; Western Black Sea). Marine Evolution of Peri-Tethyan Platforms. Memoires du Museum and Petroleum Geology 26, 57–84. National d`Histoire Naturelle 177, 151–162. Sandulescu, M. 1984. Th e Romanian Geotectonics. Editura tehnica, Nikishin, A.M., Cloetingh, S., Brunet, M-F., Stephenson, R.A., Bucharest [in Romanian]. Bolotov, S.N. & Ershov, A.V. 1998b. Scythian Platform, Sava, C.S. 1985. First results of broad off shore Gravity survey on Caucasus and Black Sea region: Mesozoic–Cenozoic tectonic the Black Sea Romanian shelf: Studii şi Cercetări de Geologie history and dynamics. In: Crasquin-Soleau, S. & Barrier, E. Geofi zică Geografi e, Seria Geofi zică 23, 29–33. (eds), Peri-Tethys Memoir 3: Stratigraphy and Evolution of Peri- Tethyan Platforms. Memoires du Museum National d`Histoire Sava, C.S., Dimitriu, R.G. & Morosanu, I.C. 2000. Western Black Naturelle 177, 163–176. Sea continental margin as inferred from 2D modeling of gravity and magnetic data. SEG/EAGE/RSG Bucharest 2000 Nikishin, A.M., Ziegler, P.A., Panov, D.I., Nazarevich, B.P., International Geophysical Conference & Exposition (Conference Brunet, M-F., Stephenson, R.A., Bolotov, S.N., Korotaev, Volume). Romanian Geophysics, Supplement 1, 106–109. M.V. & Tikhomirov, P.L. 2001. Mesozoic and Cainozoic evolution of the Scythian Platform-Black Sea-Caucasus Sava, C.S., Morosanu, I., Dimitriu, R.G., Hannich, D.K. & Sava, domain. In: Ziegler, P.A., Cavazza, W., Robertson, A.H.F. P.C. 1996. Intra-Moesian structural features emphasised by & Crasquin-Soleau, S. (eds), Peri-Tethys Memoir 6: Peri- gravity and magnetics off shore Constanta. Proceedings of the Tethyan Rift /Wrench Basins and Passive Margins. Memoires du First Congress of Balkan Geophysical Society, 23–27 September, Museum National d`Histoire Naturelle 186, 295–346. 1996, paper P-112.

753 GEOLOGY AND HYDROCARBON SYSTEMS, W BLACK SEA

Savov, S., Dabovski, C. & Budurov, K. 1995. New data on the Tari, G., Davies, J., Dellmour, R., Larratt, E., Novotny, B. & stratigraphy of the Tethyan Triassic in the Strandzha Mts., SE Kozhuharov, E. 2009. Play types and hydrocarbon potential Bulgaria. Geologica Balcanica 25, 33–42. of the deepwater Black Sea, NE Bulgaria. Th e Leading Edge 28, 1076–1081. Sefunç, A., Aydemİr, V., Abalioğlu, I., Özçelİk, Y., Efeoğlu, T. & O’Connor, R.M. 2000. Tectono-stratigraphic control on the Tari, G., Dicea, O., Faulkerson, J., Georgiev, G., Popov, S., migration of hydrocarbons, Limankoy area, Western Black Sea. Stefanescu, M. & Weir, G. 1997. Cimmerian and Alpine Proceedings of 1st Turkish International Oil & Gas Congress and stratigraphy and structural evolution of the Moesian platform Exhibition, İstanbul 2000, Extended Abstracts. (Romania/Bulgaria). In: Robinson, A.G. (ed), Regional and Petroleum Geology of the Black Sea and Surrounding Regions. Seghedy, A. 2001. Th e North Dobrogea Cimmerian orogenic AAPG Memoir 68, 63–90. belt (Romania): a review. In: Ziegler, P.A., Cavazza, W., Tugolesov, D.A., Gorshkov, A.S., Meysner, L.B., Soloviov, V.V. & Robertson, A.H.F. & Crasquin-Soleau, S. (eds), Peri- Khakhalev, E.M. 1985. Tectonics of Mesozoic Sediments in the Tethys Memoir 6: Peri-Tethyan Rift /Wrench Basins and Passive Black Sea Basin. Nedra, Moscow [in Russian]. Margins. Memoires du Museum National d`Histoire Naturelle 186, 237–257. Turgut, S., Türkarslan, M.& Perİnçek, P. 1991. Evolution of the Stampfli, G.M., Mosar, J., Favre, P., Pillevuit, A. & Vnnay, J.C. Th race sedimentary basin and its hydrocarbon prospectivity. In: 2001. Permo–Mesozoic evolution of the western Tethys realm: Spencer, A.M. (ed), Generation, Accumulation and Production the Neo-Tethys–East Mediterranean basin connection. In: of Europe’s Hydrocarbons. EAGE Special Publication No. 1, Ziegler, P.A., Cavazza, W., Robertson, A.H.F. & Crasquin- Oxford University Press, 415–437. Soleau, S. (eds), Peri-Tethys Memoir 6: Peri-Tethyan Rift / Velev, V., Bendereva, K., Bankova, T. & Peeva, R. 1994. Structure Wrench Basins and Passive Margins. Memoires du Museum of the sedimentary fi ll of the Paleogene Bourgas depression National d`Histoire Naturelle 186, 51–108. and its hydrocarbon potential. Th e Petroleum Geology and Starostenko, V., Buryanov, V., Makarenko, I., Rusakov, O., Hydrocarbon Potential of the Black Sea Area, Varna, Abstract Stephenson, R., Nikishin, A., Georgiev, G., Gerasimov, Volume. M., Dimitriu, R., Legostaeva, O., Pchelarov, V. & Sava, C. Ziegler, P.A., Cloetingh, S., Guiraud, R. & Stampfli, G.M. 2001. 2004. Topography of the crust-mantle boundary beneath the Peri-Tethyan platforms: constraints on dynamics of rift ing and Black Sea basin. Tectonophysics 381, 211–233. basin inversion. In: Ziegler, P.A., Cavazza, W., Robertson, Stavrev, P. & Gerovska, D. 2000. Magnetic fi eld transforms with A.H.F. & Crasquin-Soleau, S. (eds), Peri-Tethys Memoir low sensitivity to the direction of source magnetization and 6: Peri-Tethyan Rift /Wrench Basins and Passive Margins. high centricity. Geophysical Prospecting 48, 317–340. Memoires du Museum National d`Histoire Naturelle 186, 9–50. Stovba, S.N., Khriachtchevskaia, O.I. & Popadyuk, I.V. 2006. Directions of exploration works within Ukrainian off shore. Problems of Oil and Gas Industry: Collection of Scientifi c Works 4, Kyiv, 41–51 [in Ukrainian].

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