Petroleum geology of the Black Sea

Marine and Petroleum Geology, Vol. 13, No. 2, pp. 195-223, 1996

Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0264-8172/96 $15.00 + 0.00 ELSEVIER

0264-8172(95)00042-9

Petroleum geology of the Black Sea A. G. Robinson*, J. H. Rudat t, C. J. Banks* and R. L. F. Wiles BP Exploration Operating Company Ltd, 4/5 Long Walk, Stock/ey Park, Uxbr/dge, Middlesex UB11 IBP, UK Received21 October 1994; revised20 March 1995; accepted 12April 1995 The Black Sea comprises two extensional basins formed in a back-arc setting above the northward subducting Tethys Ocean, close to the southern margin of Eurasia. The two basins coalesced late in their post-rift phases in the Pliocene, forming the present single depocentre. The Western Black Sea was initiated in the Aptian, when a part of the Moesian Platform (now the Western Pontides of Turkey) began to rift and move away to the south-east. The Eastern Black Sea probably formed by separation of the Mid-Black Sea High from the Shatsky Ridge during the Palaeocene to Eocene. Subsequent to rifting, the basins were the sites of mainly deep water deposition; only during the Late Miocene was there a major sea-level fall, leading to the development of a relatively shallow lake. Most of the margins of the Black Sea have been extensively modified by Late Eocene to recent compression associated with closure of the Tethys Ocean. Gas chromatography-mass spectrometry and carbon isotope analysis of petroleum and rock extracts suggest that most petroleum occurrences around the Black Sea can be explained by generation from an oil-prone source rock of most probably Late Eocene age (although a wider age range is possible in the basin centres). Burial history modelling and source kitchen mapping indicate that this unit is currently generating both oil and gas in the post-rift basin. A Palaeozoic source rock may have generated gas condensate in the Gulf of Odessa. In Bulgarian waters, the main plays are associated with the development of an Eocene foreland basin (Kamchia Trough) and in extensional structures related to Western Black Sea rifting. The latter continue into the Romanian shelf where there is also potential in rollover anticlines due to gravity sliding of Neogene sediments. In the Gulf of Odessa gas condensate has been discovered in several compressional anticlines and there is potential in older extensional structures. Small gas and oil discoveries around the Sea of Azov point to further potential offshore around the Central Azov High. In offshore Russia and Georgia there are large culminations on the Shatsky Ridge, but these are mainly in deep water and may have poor reservoirs. There are small compressional structures off the northern Turkish coast related to the Pontide deformation; these may include Eocene turbidite reservoirs. The extensional fault blocks of the Andrusov Ridge (Mid-Black Sea High) are seen as having the best potential for large hydrocarbon volumes, but in 2200 m of water. Keywords: petroleum; Black Sea; extensional basins

The Black Sea is located north of Turkey and south of Ukraine and Russia, bordered to the west by Romania and Bulgaria and to the east by Georgia (Plate 1). It is linked to the Mediterranean by the Bosphorus, the Sea of Marmara and the Dardanelles and currently contains water of below normal salinity, a result of restricted exchange with oceans and of the large freshwater input from major rivers such as the Dnieper and the Danube. Much of the present basin floor is a flat abyssal plain lying at a depth of 2200 m. Before recent work focused on hydrocarbon exploration, knowledge of Black Sea geology was * Current address: JKX Oil and Gas plc, Eastgate Court, High Street, Guildford, Surrey GU1 3Dr, UK t Current address: GES, 701 Cherokee Drive, Suite G, Chattanooga, TN 37405-3303, USA :~ Current address: Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK

based mainly on regional grids of Russian geophysical data, compiled in a special volume of Bolletino di Geofisica Teorica ed Applicata (Finetti et al., 1988 and other papers), and on the drilling of a few DSDP holes (Ross, 1978) and on very regional plate tectonic scale studies (e.g. Letouzey et al., 1977; Dercourt et al., 1990). The Black Sea is bordered by two of the world's oldest petroleum provinces: to the north-east, the Indolo-Kuban Basin, the northern foreland basin to the Greater Caucasus; and to the north-west, the Eastern Carpathians and their foreland, the Moesian Platform. Exploration in the shelf areas of the Black Sea began only in the mid-1970s, in Romanian waters. Activity subsequently extended north into the adjacent Gulf of Odessa and into the Sea of Azov, and south into Bulgarian waters and, to a lesser extent, the shelf of north-west Turkey. No wells have yet been drilled in the Black Sea itself off the Russian coast or in Turkish

Marine and Petroleum Geology 1996 Volume 13 Number 2

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Petroleum geology of the Black Sea: A. G. Robinson et al.

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waters further east than the Ak~akoca-1 well, partly because of the rapid increase in water depth (Plate 1). This exploration effort has resulted in two producing petroleum fields: Lebada, which produces oil on the Romanian shelf, and Golitsyna, which produces gas condensate in the Gulf of Odessa (Ukraine). Lebada has recoverable reserves of about 85 mmbbl oil and a nearby discovery, Sinoie, may contain a similar amount. Golitsyna contains about 420 bcf gas and 2.6 mmbbl condensate; six further discoveries in the area raise the total gas reserves to about 1.5 tcf. Though the results of exploration of the Black Sea shelf since the 1970s have thus been fairly modest, the basin is currently the subject of renewed exploration interest. In the belief that drilling in water as deep as 2200 m is now possible, exploration seismic grids have been shot over parts of the Black Sea abyssal plain. In this paper, we present the first regional synthesis of the petroleum geology of the Black Sea. It is based on a database of more than 50 000 km of multichannel seismic (some reprocessed, some newly acquired in the south-east Black Sea); data from 28 offshore wells and from further onshore wells situated around the Black Sea; oil and condensate analyses of samples from seven discoveries in and around the Black Sea and from a marine seep; extensive field studies in all countries surrounding the Black Sea, with the exception of Georgia; published geological maps; and regional gravity and magnetic surveys. We begin by briefly describing the tectonic and stratigraphic evolution of

196

the Black Sea (Figure 1). We then consider the evidence for a working regional petroleum source system based on geochemical correlation between petroleum samples, and between these samples and extracts of potential source rocks exposed at outcrop around the margins of the Black Sea. Those areas of the Black Sea that have petroleum potential are then described individually. For each, we cover in more detail the geological setting, exploration activity, working and potential petroleum plays.

Regional geology and geochemistry Tectonostratigraphic evolution of the Black Sea The general geological setting of the Black Sea has been known for many years (Letouzey et al., 1977; Zonenshain and LePichon, 1986; Manetti et al., 1988; Dercourt et al., 1990; Okay et al., 1994). Lying towards the northern margin of the group of orogenic belts related to the closure of the Tethys Ocean, it is generally considered to be a result of back-arc extension associated with northward subduction of the Tethyan Ocean, now closed at a suture passing east-west through northern Turkey. Thus the basin is primarily extensional even though it is surrounded by compressive belts: the Pontides of northern Turkey (Bocaletti and Manetti, 1988); the Caucasus of Georgia and Russia (Borsuk and Sholpo, 1983; Gamkrelidze, 1986; Philip et al., 1989); the Crimea in Ukraine

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Figure 1 Tectonic elements of the Black Sea and surroundings. The Western and Eastern Black Sea basins are extensional in origin, the major extensional faults all lying seaward of the present Black Sea shelf margins. All of the present coastlines, except in the north-west and Georgia, result from Tertiary compressive deformation. Tectonic elements shown are all of Cretaceous or Tertiary age, except for Dobrogea and Strandzha Zones (both Jurassic)

(Karantsev, 1982) and the Balkanides of Bulgaria (Dachev et al., 1988). For this reason, the geology of areas surrounding the Black Sea is of only limited use in predicting what lies offshore. Although the Black Sea is today a single depocentre, deep reflection seismic studies have shown that it comprises two major extensional basins, probably of different ages, separated by a complex N W - S E trending high block (the Mid-Black Sea High) and flanked by other extensional high blocks such as the Shatsky Ridge (Figure 1 and Plates 2 - 4 ) . The Western Black Sea opened by the separation of the Western and Central Pontide continental strip from the Moesian Platform and Odessa Shelf, moving between two major transform faults as shown in Figure 1. (The transform on the south-west margin is conjectural, as it has since been overridden by compressive structures.) The Eastern Black Sea opened between the Shatsky Ridge and the Mid-Black Sea High by rotation about a pole west of Crimea. Its south-east margin has been obscured by the Eastern Pontide thrust belt. The Western Black Sea basin is thought to be floored by oceanic crust, and the Eastern Black Sea is highly thinned, possibly oceanic (Artyushkov, 1992; Belousov et al., 1988; Finetti et al., 1988). The two basins coalesced in their post-rift phases to form the current single depocentre. The frontal folds of the Tertiary compressive deformation lie in most

areas close to the present coastline. The development of the present Black Sea and its hydrocarbon potential are largely determined by Cretaceous and Tertiary events, and these are discussed at some length in the following. The earlier history, including Triassic and Jurassic back-arc extension and compression, are of less direct relevance and are summarized more briefly. Pro-Mid-Cretaceous. Pre-Triassic stratigraphy in most of the Black Sea region consists of Palaeozoic sediments as old as Cambrian and including Silurian to Lower Devonian shales, Devonian to Carboniferous carbonates and Upper Carboniferous clastics and coals (Pol'ster et al., 1976; Kerey, 1984; Dachev, 1988). These lie above variably deformed or metamorphic rocks (probably originally Proterozoic or Early Palaeozoic sediments) or on granitic metamorphic rocks. These sequences can be considered to be European (Hercynian) basement, whether labelled Scythian, Moesian or Pontide (where it is known as the Istanbul Series). The only occurrence of a Palaeozoic-Lower Triassic sequence in the Eastern Pontides is rather different, but can still be accepted as of European affinity rather than representing a fragment of Gondwanaland (Robinson et al., in press, a). There is little evidence for intense Hercynian deformation in the area.

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Petroleum geology of the Black Sea: A. G. Robinson et al. Permian sediments are locally recognized on the Moesian Platform and include red bed clastics and some limestones, evaporites and volcanics (Yanev, 1993). The Permian is otherwise a time of regional unconformity. The Triassic starts with red beds deposited on this regional unconformity and includes some aeolian sandstones on the north Turkish coast at ~akraz. This is followed by middle Triassic limestones, only locally preserved. This sequence probably records the establishment of a south-facing passive margin to Eurasia for a brief period in the Permian to Middle Triassic. It became an active margin in the Late Triassic when Tethys started to subduct northwards under the Pontides and the Black Sea region. Late Triassic rocks are flysch and volcanics, with ophiolite at Kiire in the Central Pontides - - interpreted to indicate the opening of a short-lived Triassic to Early Jurassic oceanic-floored back-arc basin in which a thick flysch sequence was deposited (Usta6mer, 1993; Usta6mer and Robertson, 1994; Yllmaz and Seng6r, 1985). Late Triassic extensional structures are also seen on and around the Moesian Platform. Flysch deposition continued through the Early Jurassic in the Pontides (Akg61 Formation), in Crimea (Tavric Series) and in Bulgaria and Romania (Lipachka and Nalbant Flysch). There followed a regional unconformity of Middle to Late Jurassic age resulting from the Cimmeride Orogeny (~eng6r, 1984; 1987;

~eng6r et al., 1988), a compressive and magmatic event related to some change in the subduction system, such as a possible microplate collision and the closure of the Triassic back-arc basin (Usta6mer and Robertson, 1994). Compressive structure of Middle to Late Jurassic age is most notable in the Strandzha Range in Bulgaria (Chatalov, 1990), North Dobrogea in Romania (Visarion et al., 1990) and in Crimea (Karantsev, 1982). Erosion of collision-related granites such as those in the Central Pontides (Yllmaz and Boztu~, 1988; ~eng6r et al., 1993) led to widespread deposition of a sequence of Middle Jurassic continental clastics in the Central Pontides (Btirntik Formation) and in Crimea, before widespread carbonate deposition ensued. In the southern Pontides, however, there are Bajocian to Bathonian volcaniclastic sediments presumably eroded from arc volcanoes, which pass conformably upwards into Upper Jurassic carbonates (Robinson et al., in press, a). Volcanics of the same age are found in the Greater Caucasus. Carbonate deposition became established in the Callovian and continued over the entire circum-Black Sea region during the Late Jurassic and Neocomian, much of it in platform facies (Plate 5). The southernmost occurrences are of slope to basinal facies, possibly deposited in forearc basins and there are some local basins in Bulgaria and the Caucasus. Evaporites were

Plate 2 Depth map to near-base of post-rift basin fill. The map is based on mapping of > 5 0 0 0 0 km of reflection seismic and shows the main structural features in the Black Sea. The post-rift fill of the Western Basin is considerably deeper than that of the Eastern Basin, but note that as the basins are not of the same age, the map shows a strongly diachronous surface, probably Late Cretaceous in the west and top-Eocene in the east

198

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LATEST JURASSIC ('rlthonlan) Palaeogeography and facies

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deposited in small depocentres off the coast of Romania and in Georgia.

Black Sea rifts. Rifting is recognized in the Western and Central Pontides in the mid-Cretaceous, based on facies and thickness variations in Aptian-Albian stratigraphy (Gfrtir, 1988) and on the structural geometry of the now inverted onshore basins (Plates 3 and 4). The syn-rift deposits (~a~layan and Ulus formations) include shallow- and deep-water sandstones, some massive, and olistostromes with large boulders of Devonian and Jurassic limestones (Plate 6). Based mainly on this onshore evidence, the age of rifting in the Western Black Sea is dated as Aptian, or locally Late Barremian. In the Eastern Pontides and in Bulgaria and Romania, the Cretaceous rift event is marked by a regional unconformity, probably a result of thermal doming (Robinson et al., in press, a). The postrift stratigraphy exposed in the Western Pontides starts with deposition of a deep-water pelagic red mudstone, micrite or 'ammonitico rosso' deposit (Kapanbo~azl Formation, Gfriir et al., 1993). This is followed either by volcanics or mudstone with volcaniclastic turbidites in a major mid- to late Cretaceous magmatic arc extending from Georgia through the Eastern Pontides, offshore from the Western Pontides (Yemi~liqay

200

Formation) to the Srednogorie zone in Bulgaria. This arc was the result of the renewed northward subduction of Tethys. Turbidites became increasingly quartz-rich, and then calcareous in the latest Cretaceous and Danian, as this phase of volcanism waned. In the Eastern Black Sea it is more difficult to determine the age of rifting; one previously published suggestion has been that the basin is Jurassic, based on inaccurate and variable seafloor heat flow measurements (Golmshtok et al., 1992). Stratigraphic breaks occur in the mid-Cretaceous and in the Palaeocene in the Eastern Pontides, but there is only one possible occurrence of a Cretaceous syn-rift sequence (Robinson et al., in press, a). Although some faulting in the Eastern Black Sea area may have occurred in the midCretaceous, a Palaeocene (post-Danian) age for rifting is supported by the presence of a largely complete Mesozoic to Lower Palaeocene stratigraphy in wells on the Shatsky Ridge in Georgia and by dredged sequences from the Archangelsky Ridge (Rudat and Macgregor, 1993). In both areas, the Upper Cretaceous sequences lie within the major high fault blocks and appear to be part of the pre-rift of the Eastern Black Sea, whereas all younger rocks are evidently part of the clastic post-rift. The extensional geometries seen offshore on seismic

Marine and Petroleum Geology 1996 Volume 13 Number 2

Petroleum geology of the Black Sea: A. G. Robinson et al. data appear to be highly asymmetrical. Basins started as half-grabens, and in those which have been extended sufficiently, the upper crustal strata on the hangingwall sides of major faults now appear in places to roll over and dip steeply towards the floor of the deep basins (e.g. Mid-Black Sea High, Plate 3). This may result from extension on a regional detachment surface, probably at mid-crustal level, creating rollover anticlines sometimes of enormous size and complexity such as the Mid-Black Sea High. In contrast, the footwall strata on the conjugate margins are largely undeformed or lie in simple back-rotated fault blocks (Shatsky Ridge, Plate 3, and also possibly the Polshkov High,

Plate 4). Tertiary post-rift sedimentation and compression. The late Palaeocene to middle Eocene was a time of passive infill of both basins with limestones deposited on the shelves, especially in the north-west, and clastic turbidites fed into the deep basins (Plate 7). The mid-basin highs were draped with a thin layer of probably pelagic marly sediments, with a distinctive seismic character. Compression started in the southern Pontides in the Late Cretaceous, in the Greater Caucasus possibly in the Palaeocene and became widespread regionally in the late Eocene. Most of the deformation in the Pontides and Caucasus appears to

MID CRETACEOUS (Alblan) Palaeogeography and facies

be of Late Eocene to Oligocene age, but earthquakes still occur along compressional faults today in northern Turkey (Alptekin et al., 1986). Half-grabens in the Pontides became inverted, with structural styles similar to analogue model results (Buchanan and McClay, 1991). In the southern zone of the Greater Caucasus, the deformation style is more like a typical detachment thrust belt, suggesting that the earlier extensional basins had not developed that far north. Minor inversion structures formed on the Romanian shelf and Gulf of Odessa and the Balkanide thrust belt developed, probably by inversion of a Triassic to Jurassic extensional basin. The spread of compression regionally and the formation of foreland basins such as the Indolo-Kuban basin and the Kamchia Trough in the late Eocene modified the extensional basin geometry and probably profoundly changed the patterns of sediment distribution and water circulation. Anoxic conditions suitable for the deposition of muds rich in organic carbon developed in all the basin deeps. The basins were filled with muds and basin floor clastics until the earliest Late Miocene when the basin suffered a major lowering of water level due to changes in regional drainage patterns associated with the growth of the Carpathians (Muratov et al., 1978; Ross, 1978; Schrader, 1978; Kojumdgieva, 1983). The depth of

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Plate 6 Palaeogeographical reconstruction, Mid-Cretaceous. The map shows the Western Black Sea starting to open. Sedimentation regionally is clastic, with olistostromes and thick sandstones in the rifts. There is also a regional supply of sand from the Russian Platform, distributed to the basinal areas as turbidites. Much of the Pontides were emergent, with arc volcanism starting in the east. The locations of forearc basins and subduction zone are conjectural. Lithology and other symbols as in Plate 5

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Petroleum geology of the Black Sea: A. G. Robinson et al.

EARLY- MIDDLE EOCENE Palaeogeography and facies

I

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Plate 7 Palaeogeographical reconstruction, Early-Middle Eocene. Both basins of the Black Sea are now open and filling with basin floor deposits. The major isolated ridges are being covered with pelagic drapes. Most of the shelf areas, especially in the west, are sites of shallow water carbonate deposition, but in many places clastics dominate, probably because of early phase compressive deformation. The Eastern Pontide magmatic arc is active once again. Collision of Anatolia with the Pontides starts with northward obduction of ophiolites. Lithology and other symbols as in Plate 5

water in the centre of the basin was reduced to a few hundred metres, with major associated incision of the basin margins and widespread deposition of fluvial strata over earlier shelf and even basinal areas (Robinson et al., in press, b). The water level rose rapidly through the Late Miocene to Pliocene. During the Quaternary, the Black Sea was frequently isolated from the Mediterranean and became a lake. Extreme subsidence and sedimentation ( > 2 . 5 k m ) in the Quaternary was probably due to loading by large volumes of sediment derived from glaciated catchment areas.

Regional source systems Source rocks. Rock-Eval pyrolysis and total organic carbon (TOC) analyses have been carried out on outcrop samples from the Pontides, Greater Caucasus and Crimea covering stratigraphic intervals from the Silurian to Oligocene (Tables 1-3). The results suggest that three main stratigraphic intervals may have or have had some source potential for oil - the Upper Devonian, Aptian-Albian and Upper Eocene. In addition, Upper Carboniferous coals are mined along the north Turkish coast (Zonguldak Formation) and are certainly capable of generating dry gas if mature. The Upper Devonian Yilanh Formation

202

of the Western Pontides is dominated by carbonates, but two samples of mid-mature mudstone analysed had P2 values of 3.4 and 4.1 kg/t. Both the Upper Devonian and Carboniferous are likely to extend offshore into the shelf areas of the Western Black Sea and do indeed reappear in similar facies on the largely undeformed southern margin of the Russian Platform. The AptianAlbian is a syn-rift deposit around the margins of the Western Black Sea (~a~ayan Formation), possibly even into Crimea, and includes black mudstones. Though some outcrop samples from the Western Pontides have reasonable pyrolysis yields (up to 3.2 kg/t), most tend to have rather limited source potential, with TOC values generally less than 1% and P2 less than 1 kg/t. Where it is exposed in the Greater Caucasus, the Upper Eocene shows by far the best source potential of any stratigraphic interval in the Black Sea region. (The unit here is described locally as 'Maykop'. The term appears to be used in part as a lithostratigraphic term, but is ascribed chronostratigraphic significance, usually denoting a mudstonedominated unit supposed to be earliest Oligocene to early Miocene in age. This chronostratigraphic Maykop is usually considered to be the source of the oils in the Indolo-Kuban basin, which was linked to the Eastern Black Sea until some time in the Oligocene. We have found source potential only in the lowermost part of the

M a r i n e a n d P e t r o l e u m G e o l o g y 1996 V o l u m e 13 N u m b e r 2

Petroleum geology of the Black Sea: A. G, Robinson unit described as Maykop which we have dated using nannoplankton as within the range Middle to Upper Eocene rather than Lower Oligocene. This source interval is therefore described as Upper Eocene in this paper.) Analytical data for two bulk samples are shown in Table 4 and Figure 2 shows a gas chromatogram from an extract. The rocks are oil-prone and have a source potential of around 8 kg/t. At Tuapse, the Upper

e t al.

Eocene is entirely hemipelagic distal turbidites with very little sand. In the Mzimta Valley (Greater Caucasus) the unit contains a greater proportion of sandy turbidites, but intervening mudstones still have source potential of between 3.3 and 6.7 kg/t. Samples from elsewhere in the Caucasus have P2 values as high as 31 kg/t. Apart from the intervals with source potential noted

Table 1 Source rock screening of samples from Western and Eastern Pontides Age

Formation

Sample number

Silurian Silurian Silurian Lower Devonian Lower Devonian Upper Devonian Upper Devonian Carboniferous Lower Jurassic Lower Jurassic Lower Jurassic Lower Jurassic Lower Jurassic Lower Jurassic Lower Jurassic Lower Jurassic Lower Jurassic Middle Jurassic Middle Jurassic Middle Jurassic U 3per Jurassic A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-AIbian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-AIbian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A }tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A }tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian A )tian-Albian Aptian-Albian Aptian-AIbian Aptian-Albian Aptian-Albian Aptian-Albian Aptian-Albian Aptian-Albian Aptian-Albian Aptian-Albian Santonian Santonian/Maastrichtian Campanian Maastrichtian

Findlkli Fmdlkh Fmdlkh Kartal Kartal Yilanh Yilanh Zonguldak NN NN NN NN NN NN NN NN Akg61 NN NN NN Inalt~ (;a~layan (;a~layan (;a~layan (;a~ laya n (~a~laya n (;a~layan (;a~layan (;a~jlayan (;a~layan (;a~layan (;a~layan (;a~layan (;a(~layan (;a~layan (;a~llayan (;a~layan (;a~laya n (;a~layan ~a~llayan (;a(~layan (;a~layan (;a(llayan (;a~layan (;a~layan (;a~layan (;a~layan (;a~layan (;a~layan (;a~layan ~a~layan (;a~layan (;a~layan (;a~jlayan (;a~layan (;a~layan Ulus Yemi~li(;ay G6rsL3kQ NN NN

T91.2 T91.4 T91.265 T91.332 T91.17 8 T91.14 T91.15 T91.44 T91.180 T91.184 T91.188 T91,189 T91.204 T91.215 T91.216 T91.368 T91.268 T91.76 T91.154 T91.117 T91.421 T91.61 T91.10 T91.281 T91.283 T91.285 T91.305 T91.314 T91.318 T91.320 T91.322 T91,324 T91.326 T91,328 T91.331 T91.333 T91.385 T91.386 T91.391 T91.393 T91.361 T91.330 T91.18 T91.36 T91.38 T91.427 T91.434 T91.353 T91.372 T91.373 T91.405 T91.456 T91.457 T91.8 T91.174 T91.334 T91.380 T91.408 T91.250 T91.260

P1 (kg/t)

P2 (kg/t)

TOC (%)

HI

Ro (%)

0,1 0.0 0.5 0.5 0.9 1.3 1.2 0.0 0.9 1.1 1.1 2.6 0.8 1.5 1,1 1,3 0,5 0.3 0.8 1.1 1.0 0.6 0.3 0.2 0.8 0.7 0.1 0.4 0.9 0.4 0.4 0.4 0.8 0.6 0.9 0.6 0.7 1.3 0.4 0.4 1.1 0.4 0.6 0.4 0.3 0.3 0.5 0.3 1.4 0.9 1.4 1.2 0.8 1.2 0.1 1.1 0.8 0.3 0.7 1.2 1.1

0.0 0.0 0.1 0.1 1.6 3.4 4.1 0.0 0.2 0.2 0.0 4.0 1.1 1.9 1.2 4.3 0.1 0.1 0.2 0.3 0.0 0.2 0.1 0.5 0.2 1.1 0.1 0.1 0.1 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.4 1.3 0.1 0.1 0.3 0.3 0.1 2.2 0.7 3.1 0.4 0.5 0.7 0.9 2.5 0.7 2.7 1.7 0.0 0.0 0.2 0.1 1.2 0.1 0.4

0.04 0.09 0.32 0.10 0.94 3.3 3.6 0.52 0.18 0.18 0 10.8 0.45 1.4 0.86 8.7 0.66 0.25 0,50 0,24 0,04 0.26 0.06 0.43 0.17 0.79 0.25 0.20 0.44 0.29 0.35 0.54 0.82 0.29 0.14 0.28 0.51 0.67 0.45 0,03 0.44 0.24 0.10 2.0 1,27 1.09 0.33 0.40 0.65 0,58 1.1 0.50 1.0 0.52 0.15 0.16 0.59 0.04 0.77 0.08 0.15

0 0 31 100 170 103 115 0 111 111 0 37 244 136 140 49 15 40 40 125 0 77 167 116 118 139 40 50 23 103 57 19 12 34 71 36 78 194 22 333 68 125 100 109 55 284 121 125 108 155 229 140 262 327 0 0 34 250 156 125 267

ID ID ID ID ID 0.64 0.70 0.73 1.36 1.13 0.82 0.49 0.67 0.85 0.89 1.69 2.08 0.50 0.86 0.54 ID 0.49 ID 0.52 ID 0.48 0.66 3.2 2.1 1.9 3.6 3.7 4.0 0.77 0.80 1.1 0.69 4.8 4.4 ID 4.3 0,60 0,67 0,55 0.80 0.42 0.43 0.36 0.64 0.56 0.53 0.70 0.48 0.46 4.0 1.6 1.8 ID 0.50 0.82 0.53

NN = Not named; ID = indeterminate

Marine and Petroleum

Geology

1996 Volume

13 N u m b e r

2

203

Petroleum geology of the Black Sea: A. G. Robinson et al. Table 2 Source rock screening of samples from Gorni Crimea Age

Sample number

Triassic/Jurassic Triassic/Jurassic Triassic/Jurassic Triassic/Jurassic Triassic/Jurassic Triassic/Jurassic Triassic/Jurassic Upper Jurassic Oxfordian Oxfordian Oxfordian Oxfordian Oxfordian Aptian Aptian Upper Aptian Upper Aptian Middle Albian Lower Oligocene ?Oligocene ?Oligocene

2.1 5.1 9.1 10.1 11.6 13.5 19.1 C63.30 C68.33 12.1 12.2 26.1 28.1 24.1 24.2 C42.22 C61.28 C62.29 C54.25 C59.27 17.1

P1 (kg/t)

P2 (kg/t)

TOC (%)

HI

Ro (%)

0.1 0.1 0.4 0.3 0.3 0.1 0.2 0.3 0.3 0.2 0.2 0.3 0.3 0.1 0.4 0.5 0.1 0.2 0.4 0.3 0.1

0.2 0.2 0.1 0.3 0.3 0.1 0.1 0.9 0.2 0.2 0.2 0.1 0.2 0.4 0.5 0.4 0.1 0.3 1.4 0.2 0.1

1.0 0.5 0.6 1.9 0.8 0.46 0.43 0.8 0.3 0.63 0.62 0.82 0.80 0.93 0.87 0.4 0.3 0.8 0.6 0.2 0.29

21 42 17 16 38 22 23 120 69 32 32 12 25 43 57 93 36 38 233 91 34

1.8 1.4 2.6 0.74 0.57 0.47 1.3 0.65 0.47 0.61 0.66 0.73 1.3 0.43 0.39 0.45 0.48 0.45 0.42 0.52 ID

Abbreviations as Table 1

here, there may be others in the post-rift stratigraphy which are not represented at outcrop or in the wells on the shelf areas. In the Greater Caucasus, onlap of the Upper Eocene to Oligocene onto folds shows that this unit was deposited during early compression, when the folds were growing under deep water. Later sediments deposited in the Black Sea after the mountain belt emerged do not outcrop or have shallow water equivalents onshore. During much of the Neogene, the Black Sea is likely to have been partly isolated from the Mediterranean as it is today and source rock deposition in the Miocene to Quaternary section is possible (though source rocks of this age may not be mature; see later). There may also be Tertiary source rocks older than the Upper Eocene. The pre-rift fault blocks of the Mid-Black Sea High (Andrusov and Archangelsky ridges) are covered by a thin drape of probable Late Palaeocene and younger age that is likely to have been deposited in deep water shortly after rifting of the Eastern Black Sea. If the bottom waters of the Black Sea had been anoxic at this time, this unit might also have source potential.

Oil and extract analyses: gas chromatography and gas chromatography-mass spectrometry. Figure 3 shows gas chromatograph traces of oil or condensate samples from the two producing petroleum fields in the Black Sea - - Lebada and Golitsyna; from four other small onshore oil discoveries - - Tyulenovo, Semyenovska, Serebrianska and Okumi; and for the marine seep close to the town of Rize (see Plate 1 for locations). Samples were also analysed by liquid chromatography/gas chromatography- mass spectrometry ( G C - M S ) to further characterize source and maturity (or by high resolution G C - M S with no prior separation if particularly small). The GC traces shown in Figure 3 appear very different due partly to differing maturities and degrees of biodegradation. The oil sample from the Upper Jurassic carbonate reservoir at Tyulenovo, Bulgaria has been highly biodegraded (API = 18.4 °) with n-alkanes

204

having been totally removed, so little information can be recovered by GC. The biomarker oleanane (derived from angiosperms; Thomas, 1990; Moldowan et al., 1993) is present in the G C - M S analysis, suggesting that the oil has come from a source rock of Barremian or younger age (probably Tertiary). Hopane and sterane aromatization reactions are complete and sterane aromatization reactions have just begun, indicating that the oil is of low maturity. The two oils from the Lebada Field, offshore Romania (Eocene and Albian reservoirs, API = 34.5 ° and 32.2 °, respectively) appear almost identical in terms of source type and maturity. Pristane/phytane is fairly low (about 1.5), suggesting that the source rock was marine algal/bacterial. Land plant input is nonetheless demonstrated by a large proportion of long chain n-alkanes, the presence of oleanane, a predominance of C29 abb-steranes and a relatively high hopane/sterane ratio. The G C - M S traces show regular hopane distributions and large proportions of rearranged steranes resulting from their reaction with clay surfaces, indicating source rock deposition in a clay-rich marine environment. Hopane and sterane isomerization reactions are complete and aromatization of monoaromatized steranes is almost complete. The oils are thus of medium maturity. The sample of condensate from the Upper Palaeocene carbonate reservoir of Golitsyna Field, Gulf of Odessa, is of very high maturity and contains no biomarkers. Pristane/phytane is, however, high (>3), suggesting a source rock deposited in a relatively oxygenated environment containing land plants. The ~ oils from Serebrianska in the Western Crimea and Semyenovska in the Eastern Crimea have both been biodegraded, the latter seriously so. Serebrianska oil has a relatively high pristane/phytane (2.3) and both contain oleanane, suggesting a significant land plant input. A large proportion of rearranged steranes points to source rock deposition in a clay-rich marine environment. Hopane and sterane isomerization and aromatization of monoaromatized steranes are complete, so the oils are of

Marine and Petroleum Geology 1996 Volume 13 Number 2

Petroleum geology of the Black Sea: A. G. Robinson et al.

moderate maturity. The oil from Okumi, Georgia, is very light (API = 42.9°). Pristane/phytane is moderate and characteristic of normal marine source rocks (1.9). The diasterane

content is high, indicating a significant clastic component, and the oil contains no oleanane. Another oil sample from a Georgian field - - Supsa - - was highly biodegraded, but did not contain abundant oleanane.

Table 3 Source rock screening of samples from the Greater Caucasus

Age

Sample number

Middle Jurassic Middle Callovian Middle Callovian Middle Callovian Middle Callovian Berriassian/Valangian Berriassian/Valanginian Berriassian/Valangian Berriassian/Barremian Upper Hauterivian Upper Hauterivian Upper Hauterivian Upper Hauterivian Hauterivian/Barremian Hauterivian/Barremian Lower Barremian Barremian Lower Aptian Aptian Aptian Upper Aptian Upper Aptian/Lower Albian Aptian/Albian ?Aptian/Albian Lower- Middle Albia n Albian Campanian Campanian Lower Palaeocene Upper Palaeocene ?Upper Palaeocene/Eocene Middle Eocene Middle Eocene Middle Eocene Middle Eocene Middle Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Middle-Upper Eocene Upper Eocene Upper Eocene Upper Eocene Upper Eocene Upper Eocene Upper Eocene Upper Eocene ?Upper Eocene ?Upper Eocene ?Upper Eocene Middle Eocene-Lower Oligocene Upper Eocene/Oligocene Upper Eocene/Oligocene Upper Eocene/Oligocene Upper Eocene/Oligocene Eocene Oligocene Oligocene Oligocene Oligocene Oligocene Oligocene

14A 36A 36B 37A 38A 34A 34B 34C 35A 31A 31B 31C 31D 32B 32C 30A 44A 29B 24A 24B 42A C38.15 29D C38.14 45A C11.5 25C 25D 26A 23A C40.17 5C 5D 5E 5F 5A 27A 27B 27C 27D 27E 21A 21B 21C 21D 22A C32.12 C40.18 5G 6A 9A 10A 10B C27.19 C27.20 C27.21 C40.16 4A 4B 4C 4D 39A 17A 17B 17C 18A 18B 18D

P1 (kg/t)

P2 (kg/t)

TOC (%)

HI

Ro (%)

0.3 0.5 0.4 0,2 0.5 0.4 0.2 0.2 0.2 0.4 0.3 0.1 0.1 0.3 0.2 0.5 0.2 0.4 0.8 0,4 0.4 0.2 0.2 0.1 0.8 0.2 0.2 0.2 0.2 0.6 0.1 0.8 0.7 0,8 1.3 0.7 0.7 0.3 0.3 0.3 0.3 0.2 0.3 0.3 1.3 0.2 0.1 0.8 0.6 0.5 0.5 0.2 0,1 0.2 0,4 0.2 0.4 0.3 0.3 0.2 0.4 0.1 0.3 0.3 0.2 0.2 0.2 0.1

0.0 0.7 1.2 0.8 1.9 0.5 0.4 0.5 0.5 1.0 0.7 0.9 0.3 0.1 0.9 0.6 0.2 2.3 2.5 1.0 1.1 0.2 1.6 0.2 2.2 1.2 0.4 0.3 0.4 3.3 0.1 4.2 3.5 7.9 15.2 3.7 15.0 6.8 13.3 12.8 16.1 0.1 2.0 1.1 31.1 0.6 0.4 4,1 4,0 3.0 1.1 0.6 0.8 0.1 8.3 7.8 1.6 6.7 4,0 3.7 3.3 0.4 0.8 0.3 0.7 0.1 0.1 0,9

0.19 0.49 1.2 0.98 1.7 0.49 0.36 0.40 0.37 0.89 0.62 0.76 0.60 0.22 0.65 0.54 0.36 1.8 2.8 0.92 1.3 0.5 1.4 0.3 1.3 0.8 0.38 0.37 0.54 1.2 0.1 1.6 1.4 2.8 4.4 1.4 3.9 2.7 3.5 3.5 3.9 0.10 0.76 0.55 8.4 0.69 0.4 1.4 1,4 1,2 0.64 0.52 0.61 0.0 3.0 2.3 0.9 2.2 1.5 1.4 1,3 0,74 1.0 0.38 0.94 0,23 0.17 0.80

0 143 100 82 113 102 111 125 135 112 113 118 50 45 138 111 56 127 89 109 87 44 116 61 164 143 105 81 74 277 83 268 243 283 3,2 268 390 254 386 361 411 100 263 200 368 87 108 295 286 252 172 115 131 250 279 333 184 303 276 261 260 54 76 79 74 43 59 113

ID 0.80 0.99 1.2 0.71 0.66 0.72 0.75 0.71 0.49 0.49 0.46 0.48 0.91 0.62 0.40 ID 0.34 0.48 0.45 0.51 1.5 0.34 1.2 0,43 0.40 0.46 0.57 0.56 0.47 0.43 0.43 0.43 0.41 0.38 0.42 0.38 0.35 0.33 0.37 0.35 ID 0.42 ID 0.47 0.43 0.39 0.44 0.44 0.48 0.37 0.47 0.54 ID 0.34 0.37 0.44 0.42 0.42 0.42 0.41 0.48 0.37 0.42 0.38 ID ID 0.38

Abbreviationsas Table 1

M a r i n e a n d P e t r o l e u m G e o l o g y 1996 V o l u m e 13 N u m b e r 2

205

Petroleum geology of the Black Sea: A. G. Robinson et al. Table 4 Analytical data for samples of Upper Eocene mudstone,, Greater Caucasus Sample C27.20 C27.21

fill 115 Iptlph 120

12511S 130

UpperEoceneextract(Caucasus)

Figure2 Gas chromatogram for extract of Upper Eocene mudstone, Tuapse, Greater Caucasus

Oil dredged from the seafloor off the north-eastern Turkish coast near Rize has been highly biodegraded and little can be made of the GC results. The G C - M S traces show abundant oleanane.

Oil and extract analyses: carbon isotopes. Carbon isotope analyses for the oils from around the Black Sea and for the extract from Upper Eocene mudstone from Tuapse are shown in the form of Galimov curves in Figure 4. There appears to be a clustering of curves in the middle, with the Okumi oil distinctly isotopically lighter (more negative) and the Supsa oil heavier than the main group. Analyses of the extract from the Upper Eocene mudstone overlap those of the oils in the main group. The Rize slick samples are slightly heavier, but this may be due to their high degree of biodegradation (bacteria preferentially remove isotopically light compounds). The presence of the highly mature Golitsyna condensate within the main group is something of an anomaly. Increasing maturity tends to drive the Galimov curves to the right, so that light oil expelled from the rock that sourced the Golitsyna condensate ought to have a more negative isotopic signature. Conclusions: ages of source rocks, source kitchen mapping and migration. The Galimov curves, GC and G C - M S data are consistent with the oils from Tyulenovo, Lebada, Serebrianska, Semyenovska and Rize all coming from a single and probably Upper Eocene source as exposed in the Greater Caucasus near Tuapse (see Table 3). All of these oils contain oleanane, suggesting a maximum age of Barremian, more probably Tertiary, and all are isotopically similar. The oils from Okumi and Supsa, in Georgia, clearly do not fit this pattern. Near Okumi, the only dominantly muddy portion of the stratigraphy is the Lower Jurassic, whereas at Supsa a Tertiary source younger than Eocene is more probable. The geochemistry of the Golitsyna (Odessa Shelf) condensate suggests that it too is probably not derived from an Upper Eocene source and there are geological reasons for suspecting that the source rock may be Upper Devonian. To map present day petroleum kitchens, burial history and thermal modelling of the Upper Eocene was carried out on selected shot points in the Eastern

206

P2 (kg/t) TOC (%) 8.3 7.8

3.0 2.3

HI

Ro (%)

TSE (%)

Pr/Phy

279 333

0.34 0.37

0.3 0.5

1.0 1.2

and Western Black Sea basins. These points were used to calibrate the depth map of the Top Eocene in terms of maturity. Figure 5 shows a representative burial history for a shot point in the centre of the Eastern Black Sea. Oil expulsion would have commenced in the centre of the basin in the Late Oligocene, gas expulsion in the Early Miocene. The central parts of both Eastern and Western basins are today generating gas, the marginal parts oil (Plate 8). Where mature, the Upper Eocene is buried beneath very substantial thicknesses of sediment characterized by parallel reflectors on seismic data and probably composed largely of mud. Significant vertical migration is therefore likely only where the post-rift is breached by faults. A possible example of vertical migration of this kind can be seen in the gas effects associated with vertical faults of small offset that occasionally cut the post-rift, particularly above the Andrusov Ridge culminations (see Plate 12). The distribution of petroleum occurrences around the margin of the basin itself suggests that lateral migration is dominant. The Lebada Field contains 34° API oil but the Upper Eocene (-?Lower Oligocene) is locally at a depth equivalent to only 2.5 s TWT and cannot be mature. Oil appears to have migrated at least 10 km in a westerly direction out of the basin, guided by a local structural or erosional trough. Even longer migration distances are probably implied for the Tyulenovo Field, currently just onshore near the Bulgarian coast and with no current obvious link to mature Upper Eocene source. The oil seep near Rize seems to be located near the point where the Eocene pinches out against the basin margin. This suggests lateral and then vertical migration to the sea bed.

Petroleum potential by sector Offshore Bulgaria The Bulgarian Black Sea includes the complex western end of the Western Black Sea basin (Figure 1 and Plate 4). Many of the features associated with Cretaceous extension on the margin of the basin have, however, been overprinted by the offshore extension of the Tertiary Balkanide fold belt (Dachev et al., 1988). Onshore, the Balkanides are a narrow east-west trending thrust belt with main vergence to the north, but at the coast the belt turns sharply to the south, with a probable strike-slip component. Back-thrusting on the south-west side of the offshore Balkanides establishes the north-east limit of the Burgas Basin as the deeper offshore part of the Srednogorie Zone (a zone of thick Cretaceous volcanics). Immediately north of the Balkanide thrust belt there is a small foreland basin - - the Kamchia Trough. This thins out northwards onto the Moesian Platform, which extends into Romanian waters and forms the north-west margin of the Western Black Sea.

Marine and Petroleum Geology 1996 Volume 13 Number 2

Petroleum geology of the Black Sea: A. G. Robinson et al.

Pr/Phy = 1.5 Reservoir Upper J urassic

115 I prlph 120

Reservoir Eocene Sandstone/

12511S 130

Tyulenovo

Lebada West 2250m

Pr/Phy = 1.7

Pr/Phy = 3.2

Reservoir Albian

Reservoir ? Upper Palaeocene Limestone

I15

Lebada West 2456m

Iprlp h 120

1125|1S

Golitsyna 2210m

Pr/Phy = 2.3

Reservoir Miocene Sandstone

Reservoir ? Upper Cretaceous Chalk

115 Iprlph 120

12$IIs

130

115 Ip~ph 120

Serebrianska 1820m

12511S

130

Semyenovska 239m

PdPhy = 1.9 Reservoir Cretaceous

II,, Okumi

,2,,,s Rize seep

F i g u r e 3 Gas c h r o m a t o g r a m s o f s a m p l e s o f oil f r o m Black Sea and n e a r b y o n s h o r e fields. Gas c h r o m a t o g r a m s are f o r s a t u r a t e f r a c t i o n s a p a r t f r o m t h o s e f o r t h e t w o Lebada s a m p l e s , w h i c h are o f w h o l e oils

Marine and Petroleum Geology 1996 Volume 13 Number 2

207

P e t r o l e u m g e o l o g y o f the Black Sea: A. G. R o b i n s o n et ai.

T Sr G $rn L

OK

SATURATES -

Sp

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Sp -OK -T Sm G Sr L ................ • O

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includes Palaeozoic sediments and Lower to Middle Triassic clastics and carbonates, locally affected by Late Triassic extensional faulting (seen especially at the Kaliakra-1 well on Plate 4) and truncated by the pre-Upper Jurassic (Cimmeride) unconformity (Dachev et al., 1988). The platform is regionally covered by Upper Jurassic to Lower Cretaceous shallow-water carbonates, which are the reservoir for the small onshore Tyulenovo Field (though this

Offshore Bulgaria has been the subject of recent exploration activity. The majority of currently unleased exploration acreage lies in waters deeper than 1000 m, where the tilted fault blocks and Tertiary submarine fans appear to be the only viable petroleum plays. However, numerous play types are possible in shallower waters.

Stratigraphy and structure. The Moesian Platform

0.

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Age (MillionYears) Figure 5 Burial history and petroleum expulsion model for the centre of the Eastern Black Sea. Basal heat flow history was calculated by assuming an oceanic heat flow of 250 mW m 2 60 Ma ago decayed exponentially to the present day value of 60 mW m 2 This present day value was calculated from seafloor heat flow measurements corrected for the rapid Quaternary burial; the corresponding figure is lower in the Western Black Sea (about 55 mW m-2). Other input to the model includes stratigraphy (ages, thermal conductivities and thicknesses); organic matter type and its kerogen maturation kinetics; and heat production in the sediments. The depth of the source rock in this model is a p p r o x i m a t e l y 8 km below the seafloor today. This is the greatest plausible depth calculated using the slowest likely interval velocities and the figure therefore gives the most gas-prone picture of the source kitchen. Note datum is seafloor

208

Marine and Petroleum Geology 1996 Volume 13 Number 2

Petroleum geology of the Black Sea: A. G. Robinson et al. probably relies on fractures rather than matrix porosity). The field is in a low-relief tilted fault block of mid-Cretaceous age. In the deep water off the present day shelf edge, there are large extensional rotated fault blocks that formed as the Western Black Sea opened in the M i d - L a t e Cretaceous (such as the Polshkov High, Figure 1 and Plate 4) and which probably contain a prerift Moesian Platform stratigraphy. Syn-rift AptianAlbian sandstones have been noted regionally and may onlap these fault blocks, The Kamchia Trough is an Early Tertiary foreland basin produced by loading of the Moesian Platform by the Balkanide thrust sheets. The basin is filled with thrusted and folded Eocene to Oligocene flysch, which thins and onlaps northwards onto the Palaeocene and Upper Cretaceous. In the Kamchia gas condensate field and in the Samotyno More-1 discovery (Plate 1 and Figure 6), production of gas and condensate comes from Lower Eocene turbidites in anticlinal traps. The Upper Eocene indicated to be the source rock on Figure 6 would be expected to be immature at that location, but possibly mature deeper below the thrust belt. The Burgas Basin contains a maximum of 1500 m of Tertiary sediments overlying thick Upper Cretaceous volcanics of the Srednogorie Zone. It is limited to the north-east by the steep faulted boundary of the Balkanide thrust belt. It has no known potential for petroleum. To the south-west, onshore, is the north-

east vergent, mainly Jurassic (Cimmeride), Strandzha thrust belt.

Petroleum plays. Numerous plays have been identified in the Bulgarian Black Sea. All probably rely on a connection with a mature Upper Eocene source rock in the post-rift of the Western Black Sea or below the Balkanide thrust belt. (The Tyulenovo oil is believed to be a highly biodegraded Eocene oil, having migrated a long distance from the Black Sea Basin, see Figure 3). 1. Extensional fault blocks f o r m e d during the Mid-Cretaceous Western Black Sea rifting: the Tyulenovo trend on the edge of the Moesian Platform is thought to be of this type. E o c e n e Oligocene shales are the main regional seal. The Polshkov High is a large tilted fault block (ca. 900 k m 2 areal closure) in 2000 m of water and may include Aptian-Albian syn-rift and Tertiary sands in drape traps, or various reservoirs in the pre-rift. 2. Anticlines in the Kamchia Trough with Eocene clastic reservoirs (e.g. Samotyno More). The southern margin of the Kamchia Trough has many north-east verging thrust structures in shallow and deep water. 3. Stratigraphic pinch-outs towards the northern margin of the Kamchia Trough. The updip pinch-out of the Eocene strata onto the Moesian

Plate 8 Depth map to Top Eocene showing areas presently expelling oil and gas, based on heat flow modelling in Figure 5. Error range in this calculation is not estimated, and may be large. Note datum is sealevel

Marine and Petroleum Geology 1996 Volume 13 Number 2

209

Petroleum geology of the Black Sea: A. G. Robinson et al. E 4-r-

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Petroleum geology of the Black Sea: A. G. Robinson et al. Platform was the likely trap for a recent shallow gas discovery. 4. Reef build-ups of Jurassic to Cretaceous age on the deeper offshore part of the Moesian Platform. Mounding in this interval is visible on seismic data. Reservoirs may include Campanian rudistid banks as exposed in north-west Turkey.

Offshore Romania Approximately 35000 km 2 o f the Black Sea lie in Romanian territorial waters, south and east of the Danube delta (Plate 1). In contrast with most other parts of the Black Sea, the shelf is wide at this point and around 20000 km 2 have water depths of less than 100 m. The Romanian Black Sea is a proved oil province - - the only one in the Black Sea to date. One field - - Lebada with reserves of 85 million barrels of oil - - is currently producing and a second discovery nearby Sinoie - - is under appraisal. The most important features of the Cretaceous and Tertiary geology of the shelf are related to its position on the northern margin of the Western Black Sea basin (Figure 1 and Plate 2) and are much less influenced by the nature of the preCretaceous geology as seen onshore - - the Moesian Platform in the south-west and North Dobrogean orogen in the north-east. The offshore is dominated by mid-Cretaceous extensional structures and their cover sediments, which contain the major hydrocarbons (Figures 7 and 8; Catuneanu, 1992). Romania is the basin margin conjugate to the Western Pontides and the Albian and older stratigraphy is similar. Offshore Romania has, however, barely been affected by Tertiary compression. -

-

Stratigraphy and structure. The basement in Romania and its Black Sea shelf is of two fundamentally different types separated by the PeceneagaCamena fault, a major pre-Cretaceous thrust and strike-slip fault which separates the North Dobrogean orogenic belt to the north-east from the Moesian Platform to the south-west (Sandulescu, 1978; Visarion et al., 1990). The stratigraphy in North Dobrogea ranges from Palaeozoic to Kimmeridgian, is extremely complex and apparently includes no potential reservoir intervals (Gradinaru, 1984). South-west of the

Peceneaga-Camena Fault, the Moesian Platform has a relatively undeformed sedimentary sequence of Palaeozoic to Neocomian age, but because of a regional southerly tilt, little of this cover is preserved in Romania (Vinogradov, 1988). The Upper Jurassic to Neocomian carbonates which extend over much of the offshore area have very low matrix porosity. In one local fault-controlled basin the Upper Jurassic includes a thick sequence of evaporites. The syn-rift sediments of Albian age found offshore have been the target for several exploration wells and are the main reservoir in the Lebada Field. They show rapid thickness and facies changes, mostly clearly related to faults, which makes the prediction of reservoir distribution difficult. By the Cenomanian, Western Black Sea rifting had ceased. The Late Cretaceous to Danian is dominated by chalky carbonates and marls draping extensional fault blocks and showing significant thickness variations across them. The Upper Palaeocene and Lower Eocene are then absent over the entire area. The Middle Eocene is dominantly marls and shallow water nummulitic limestones and is represented on seismic data by a characteristic package of high amplitude reflectors about 200 ms thick that can be traced across the whole Romanian shelf. Eocene sandy marls are the secondary reservoir (for gas condensate) in Lebada, though their permeability is poor. According to Romanian biostratigraphic dating, there is a major pre-Oligocene erosional unconformity that appears to represent the development of submarine canyons. The ?Upper Eocene to Oligocene onlaps a significantly eroded seafloor topography and the transgressive unit at its base is the most likely location for the source rock for the Lebada oil. The base of this unit could be as old as Upper Eocene and correlative with the regional Black Sea source rock. The Lower Miocene is absent on the Romanian shelf and the Oligocene is overlain by a thin Upper Miocene [This unit is described locally by the Paratethyan stage names, Sarmatian-Badenian. These are equivalent to the Langhian and most of the Serravallian (Steininger et al., 1988).] clastic sequence bounded top and bottom by erosional unconformities and probably deposited in fluvial to shallow marine environments during the Late Miocene lowstand (proved in DSDP wells 380 and 381;

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Ross, 1978; Schrader, 1978). This interval includes the most important reservoirs in the Carpathian foreland basin to the west. Above the Middle Miocene is a sediment wedge that thickens towards the Black Sea and that probably represents the next highstand. It is described locally as Pontian (Late Miocene), but some of the fauna that it contains are of Pliocene age and it seems likely that the highstand wedge is in part Lower Pliocene. This is overlain unconformably by further clastic wedges of Pliocene to Quaternary age.

Lebada Field. Lebada - - the only producing oilfield in the Black Sea - - was discovered in 1980 and is presently producing oil and gas from Albian sandstone under water injection at an aggregate rate of 10000 bopd. There are plans to produce gas condensate from the Eocene. The Lebada structure seems to be a tilted extensional fault block, but its geometry is unclear. It was highly modified by Eocene-Oligocene submarine erosion, which produced a major east-west trending submarine canyon south of Lebada - - the Istria Trough (Figure 7). The Albian sandstone onlaps a sequence of Lower Cretaceous to Triassic age, which is the offshore extension of the North Dobrogean zone and the effective basement. Upper Cretaceous muddy chalk provides a top seat. This is overlain unconformably by Eocene sandy marls, which contain gas condensate in the eastern portion of the field. The field appears to rely partly on lateral seal from Oligocene mudstones which fill the deeply cut canyons. The Albian sandstone reservoirs are of very variable thickness up to 300 m. The proportion of net sand is close to unity but reservoir quality is reduced dramatically by zones cemented by carbonates. The two parts of the field, Lebada East and West, are separated by such a tight zone and to the north-west, both accumulations are effectively sealed by carbonate cement. The average porosity of the reservoir is 20%, with permeabilities from 100 to 200 mD. The average well rate is about 1100 bopd, with a specific gravity of 34°API. The Eocene sandy marls are poor reservoirs with porosities around 20%, but permeability around 1 mD. The source of the oil is believed to be the transgressive Upper Eocene where this is deeply buried in the extension of the Istria Trough to the east and south.

212

Petroleum plays. There are several potential and proved plays in the Romanian offshore. All rely on connection with a mature Upper Eocene source rock to the east or south. 1. Transgressive Aptian-Albian shallow marine sandstones onlapping extensional fault blocks (Figure 7). Lebada is of this type, but is complex and partly reliant on stratigraphic and diagenetic seal. The principal problem with this play is probably the variability of syn-rift facies over short distances and the consequent difficulty in predicting reservoir distribution. The geometry of the fault blocks is also difficult to define. 2. Tilted extensional fault blocks with Moesian Platform pre-rift stratigraphy and a Cenomanian or later marl or mudstone seal (Figure 8). Critical to the success of this play is the presence of a viable reservoir in the pre-rift. The Delfin well tested a structure of this type and encountered tight Silurian below the Upper Cretaceous. 3. Rollover anticlines on the hangingwalls of Neogene growth faults close to the present shelf edge (Figure 9). Pliocene sandstones up to 70 m thick are known in the Ovidiu well and there is potential for stacked reservoirs, probably because of the proximity of the Danube delta. The main risks for this play would be the presence of biogenic or thermogenic gas rather than oil, and at present the poor seismic definition at reservoir level.

Gulf of Odessa The Gulf of Odessa includes around 50 000 km 2 of shelf located mainly in Ukrainian territorial waters. The sea is less than 100 m deep over much of this shelf area. The Gulf of Odessa is a proved wet gas province with one producing field (Golitsyna) and a further six discoveries awaiting or under development. Total discovered reserves are around 1.5 trillion cubic feet. Some seismic and well data and interpretation were supplied by Yuzhmorgeologiya Association, Gelendzhik (Plate 1). The area is located on the southern margin of the Scythian and Russian Platforms. Mesozoic sediments onlap gradually to the north and pinch out onto Palaeozoic or older basement beyond the northern limits of the available seismic. In the centre of the shelf is the Karkinit Trough (Figures 1 and 10), an elongate basin aligned approximately

Marine and Petroleum Geology 1996 Volume 13 Number 2

Petroleum geology of the Black Sea: A. G. Robinson et al. E N E - W S W in which the base of the Tertiary reaches a depth greater than 4 km and which links to the Karkinit Peninsula of Crimea, where there are many small gas fields (Karantsev, 1982). South of the Karkinit Trough is the Kalamit Ridge, where the top of the Cretaceous lies at depths of less than 1 km. To the south, the top of the Cretaceous dips to the south, eventually plunging down rapidly at the margin of the Western Black Sea Basin.

Stratigraphy and structure. The oldest rocks penetrated by the wells available are mudstones in the bottom of Golitsyna-2, described as Precambrian (but it is not known on what the dating is based). The entire Palaeozoic to Jurassic is then missing at this well, cut out either by a major unconformity or by a major extensional fault, so that Lower Cretaceous lies directly against the supposed Precambrian. The next oldest rocks known are dark mudstones, described as Triassic to Jurassic, penetrated in Odesskaya-1 in the west and by the two wells on the Kalamit Ridg6, Ilycheskaya-1 and Desantnaya-1. Lithologically, these appear similar to the Tavric Series exposed in Gorni Crimea, a sequence of highly deformed fine-grained turbidites which is structurally complex as a result of Cimmeride (early Middle Jurassic) compression. In Gorni Crimea, the Tavric is overlain unconformably by Middle Jurassic volcaniclastic sandstones and lavas and Upper Jurassic shallow-water limestones, but more regionally in Crimea, and also on the Kalamit Ridge, the MiddleUpper Jurassic is absent. The Lower Cretaceous occurs widely onshore in Crimea (Karantsev, 1982), and has been penetrated by five of the available offshore wells. It is clastic and predominantly muddy, but it includes gas-productive sandstones on the Karkinit Peninsula, and a sandstone several tens of metres thick was tested in Desantnaya-1, where it produced water at a rate of more than 4000 bpd. Albian and older stratigraphy is very poorly imaged on existing seismic data. As on the Romanian shelf, the Upper Cretaceous to Danian consists of chalk, which thickens into the Karkinit Trough. The section thins onto the Kalamit Ridge, partly due to Early Tertiary erosion, but also because of nondeposition during some parts of the Late Cretaceous. The chalks are considered as secondary reservoir targets, but despite having been extensively tested, they have never flowed at commercial rates.

The Upper Palaeocene is mainly limestone and includes the main reservoir in the Gulf of Odessa. (The productive limestone is described as 'Lower Palaeocene'. Onshore in the Crimea, however, the change from chalk to shallow-water grainstone takes place at the end of the Danian, which is locally included m the Cretaceous. It is likely that all of the Palaeocene identified as such in the wells, including the reservoir limestones, is Upper Palaeocene.) The interval is up to about 200 m thick and is restricted to the Karkinit Trough, being absent over the Kalamit Ridge (Figure 10). The Lower and Upper Eocene consist of marls and mudstone, with a Middle Eocene limestone in between. The Eocene as a whole displays a huge thickness variation related to the Kalamit High-Karkinit Trough and contains several erosional unconformities that suggest growth of the Kalamit High during that time. The Oligocene is mainly muddy, but contains some sandy units that constitute the second most important proved reservoir in the area. The Oligocene and Miocene both appear to onlap undeformed onto the Kalamit Ridge without any of the unconformities that characterize the Eocene. The Pliocene to recent sediment package cuts down into the Miocene and shows thickening towards the Western Black Sea Basin. There are important folds on two scales in the Gulf of Odessa. Smaller scale asymmetrical folds with wavelengths of a few kilometres and amplitudes of a few hundred milliseconds are clearly related to northvergent compressional faults, possibly reversing earlier extensional faults. Strata as young as Miocene are folded. These anticlines have been the major exploration targets onshore in the Karkinit Peninsula and offshore in and around the Karkinit Trough, the Golitsyna trend being a good example. Such small compressional folds, however, cause rather minor modification of the prominent large-scale features, the Kalamit Ridge and the Karkinit Trough, the origin of which is uncertain.

Petroleum plays. The Gulf of Odessa contains one known working play and others which are yet to be proved offshore: 1. Palaeocene grainstones in compressional anticlines formed during the Oligocene to Miocene, sealed by Eocene mudstones/marls. Well flow-rates may be

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Petroleum geology of the Black Sea: A. G. Robinson et al. > 3 0 × 10 6 ft3/d. Many such structures have been tested on and offshore, and many contain gas. 2. Aptian-Albian sandstone reservoirs sealed by Upper Cretaceous marls. Lower Cretaceous sandstones are gas- and oil-productive onshore, and have been tested in Golitsyna 2 and 3 (up to about 30 x 10 6 ft3/d gas) and Desantnaya-1 (4178 bpd water). Potential traps may not be restricted to Tertiary compressional structures, but to explore for possible syn-rift sandstones it would be necessary to image them properly on reflection seismic. This has not yet been achieved. 3. Oligocene sands are secondary reservoirs (e.g. South Golitsyna) but flow-rates are lower (a few × 10 6 ft3/d) and there are sand production difficulties. Upper Cretaceous chalks are considered potential reservoirs, but flow-rates tend to be low (a few × 10 6 ft3/d). The source of the hydrocarbons in the Gulf of Odessa is problematic. The oil from the Cretaceous at Serebrianska on the Karkinit Peninsula fits with the general character of Upper Eocene source rocks (Figures 3 and 4), but it is difficult to see how this can be within the migration range of any mature Eocene source kitchen. It might be necessary to invoke longrange migration from the Indolo-Kuban Trough, or from the Black Sea before the uplift of the Crimean mountains, perhaps in the Miocene. The source rock for the Golitsyna condensate may be deeper in the stratigraphy: the Saratskaya-1 well north of the Danube delta drilled a thick Devonian sequence. The Middle and Upper Devonian (limestones and evaporites) had abundant oil shows and the Lower Devonian is mudstone. Upper Devonian mudstones in the Pontides have some source potential (Table 1).

Sea of A z o v The Sea of Azov is a very shallow (mainly < 10 m) sea covering part of the northern Caucasus foreland basin, the offshore extension of the oil and gas producing Indolo-Kuban Basin of Russia (Figure 1 and Plates 1 and 9). Subsurface data (two seismic lines and several wells) and some mapping were supplied by Yuzhmorgeologia Association, Gelendzhik. The south flank of the basin is steep and represents the northern front of the Caucasus thrust belt which plunges to make a saddle at the Straits of Ketch, continuing to the west into the mountains of Gorni Crimea. The north flank of the basin has a shallow dip to the south, characteristic of load-related foreland basins. However, there is a major structural ridge, the Central Azov Ridge, interrupting this regional dip. The foreland basin is probably Eocene to Oligocene in age. There is also evidence of pre-Tertiary (Jurassic?-Cretaceous) extensional faulting at the northern margin of the Central Azov Ridge, recognized by abrupt changes in stratigraphic thickness of the Cretaceous, and onlaps onto the southern flank of the Ridge, which is cored by Triassic-Lower Jurassic flysch and volcanics. The main established hydrocarbon plays (mostly onshore) are in various Oligocene to Pliocene sandstones derived from the north and seen seismically at some levels to prograde to the south. Most of the oil discovered on the Kerch and Taman Peninsulas is

biodegraded and occurs in shallow anticlines cored by mud diapirs (such as the Semyenovska field, oil analysis, Figure 3) and can be linked to an Upper Eocene source (the 'Maykop', probably from the south flank of the Indolo-Kuban Basin rather than from the Black Sea). There is also at least one gas discovery in Upper Cretaceous limestones at depths below the Taman Peninsula. The discoveries on the north flank of the Indolo-Kuban basin are gas and are in subtle fault-controlled traps on trends that probably extend offshore. The gas may derive from the same Upper Eocene source buried as deep as 8 km in the basin axis. There is also a regional trend onshore of gas discoveries in Lower Cretaceous sandstones (as also on the Karkinit Peninsula of west Crimea; Karantsev, 1982). This trend may continue into the Azov Sea and is an exploration target in extensional or compressive (inversion?) structures on the north or south flanks of the Central Azov Ridge. At least one well demonstrates the presence of sands at this stratigraphic level. The Jurassic 'reef' play suggested in Plate 9 would be very deep, and is completely speculative.

Shaksky Ridge The Shatsky Ridge is a linear W N W - E S E trending extensional high running from offshore Crimea, through the Russian sector of the Black Sea to meet the coast in Georgia. It is tilted to the north, into the Tuapse Trough - - the southern foreland basin of the Caucasus - - and also plunges regionally to the WNW. The south-west margin of the ridge is a major normal fault zone with several terraces (and not a thrust front as interpreted by Finetti et al., 1988), taking the base of the Tertiary post-rift rapidly down to the oceanic(?) depths of about 13 km in the Eastern Black Sea Basin (Figure 1 and Plates 1-3 and 10). Subsurface data including wells onshore Georgia were supplied by Yuzhmorgeologiya Association, Gelendzhik. There are three large regional culminations on the Ridge: Ochamchire, straddling the Georgian coast north of the Rioni Basin, Gudauta, offshore western Georgia and a large unnamed culmination near the (undefined) Russian-Ukrainian border. There are also some smaller tilt block structures at the WNW end of the ridge and numerous compressive anticlines in the frontal folds of the Caucasus near the coast. The Ochamchire High has been drilled onshore and some oil discovered in Lower Cretaceous and Upper Cretaceous limestones, both with poor reservoir quality. The top of the upper limestone reservoir (approximately Danian) is an erosive unconformity and is overlain by a thin elastic seal of Eocene age. The Upper Jurassic-Lower Cretaceous shallow-water limestone facies can be traced onshore in the Mzimta Gorge, but further west onshore they pass laterally into slope or basinal facies (Plate 5), apparently confining this play to offshore Georgia. There are a few metres of turbidite sandstone in the Albian-Aptian onshore near Tuapse and this may become a reservoir objective on the Shatsky Ridge. No good quality reservoirs are recognized at any other stratigraphic level - - the Middle Jurassic consists of thick volcanics and overlies Lower Jurassic shales. A Cretaceous reservoir in the Shatsky Ridge could be sourced from the Upper Eocene in either the

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Petroleum geology of the Black Sea: A. G. Robinson et al. Eastern Black Sea Basin or the Tuapse Trough. However, the oil sample from the Okumi well on the Ochamchire High (Figures 3 and 4) is markedly different from the Upper Eocene sourced oils. It is a high maturity oil derived from a clastic shallow marine algal/bacterial source - - possibly Lower Jurassic or mid-Cretaceous mudstones [none of the other Jurassic to Cretaceous formations sampled (Table 3) having shown any source potential].

Eastern Turkish Shelf. The southern shelf of the Eastern Black Sea east of the Mid-Black Sea High is a narrow strip of compressive structures, the frontal zone of the Eastern Pontides and the Adjaria-Trialet Zone, which are dominated by thick Cretaceous to Eocene arc volcanics and granites. In this area, there is no discernable rift margin platform. Little is known about the stratigraphy below the thick sequence of Upper Cretaceous volcanics. The nearest outcrops of preCretaceous are the volcanics of probable Triassic age and Jurassic volcaniclastics south of Artvin (Robinson et al., in press, a). These and other formations lying south or east of the magmatic arc probably have limited relevance to Black Sea geology. On the north-west side of the arc near Trabzon there are some marbles which may be a metamorphosed equivalent of the regional Upper Jurassic-Lower Cretaceous limestones. Close to the coast there is a seaward-dipping belt of latest Cretaceous to Palaeocene thin-bedded limestones and Lower Eocene clastics, followed by more volcanics in the later Eocene. Not until the Oligocene does the section become predominantly clastic. Wells drilled onshore in the Rioni Basin (Georgia) have porous and oil-productive sands in the Miocene and Pliocene, but flow-rates are extremely poor, probably because the sediments are derived from the erosion of volcanics (but also probably for technical reasons). The Upper Eocene source interval is present and the Rize oil seep lies above its updip pinch-out. There is, however, some doubt as to whether the oils in the shallow section in the Rioni Basin are sourced from this interval, or from a young unit (see under Regional source systems). The most prospective structures offshore are compressive anticlines involving Eocene to Miocene stratigraphy north of Trabzon. The folds are aligned en echelon S W - N E , and the major ridges plunge to the north-east so that significant closures are rare. All lie in deep water. Mid-Black Sea High The Mid-Black Sea High is a N W - S E trending high separating the Western and Eastern Black Sea basins, a remnant shoulder of the rifting events which created the two basins (Figure 1 and Plate 2). The Archangelsky Ridge starts just off the Turkish shoreline (Plate 11), where it is a prominent bathymetric feature and plunges north-westward. The Andrusov Ridge becomes the predominant feature to the north-west as the Archangelsky Ridge plunges away (Plate 12). The top of the Archangelsky Ridge has been affected by Tertiary compression near the coast and includes a number of small thrust anticlines. The Andrusov Ridge, however, shows no evidence of compression, but is compartmentalized by major extensional faults that run approximately N E - S W , at a high angle to the

218

trend of the Mid-Black Sea High (which appears as an unfaulted rollover on most of the N E - S W 'dip' seismic lines; Plate 12). The blocks between these faults are very large with areal closures and structural relief comparable with the largest of the structurally similar fields in the northern North Sea (Plates 2 and 13). All lie in water depths close to 2200 m.

Stratigraphy. The pre-rift in the tilted fault blocks of the Andrusov Ridge is characterized by tilted parallel reflectors (dark green, labelled R in Plate 13). No wells can be tied into this unit. At its base, the deepest continuous reflectors have a distinctly high amplitude. Mapping of seismic interval velocities within the green interval suggests that it is dominated by clastic rather than carbonate rocks. The thin veneer of sediment covering the ridge is the earliest post-rift deposit, most likely a pelagic drape. Successively above and onlapping the ridge are the post-rift passive fill strata including supposed Upper Eocene source rocks, labelled S in Plate 13. (There is no syn-rift sediment on the ridge related to this event.) The geometry of the Andrusov and Shatsky ridges and the intervening Eastern Black Sea Basin suggests that the ridges have rifted apart about a pole of rotation located somewhere near Crimea (see Plate 2). The ages of the pre-rift stratigraphy in the two ought therefore to be the same. The Shatsky Ridge stratigraphy is known from wells in Georgia and also from outcrops along the Russian coast (where the sequence is brought to the surface by thrusting in the Caucasus). It includes a complete sequence of Lower Jurassic mudstones, Middle Jurassic volcanics, Late Jurassic to Neocomian limestones, Aptian clastics and Upper Cretaceous to Lower Palaeocene chalks. The rifting event is thus likely to be post-Early Palaeocene. Further evidence of the age of the latest pre-rift interval comes from dredge sampling of the Archangelsky Ridge, where it outcrops on the sea floor (Rudat and Macgregor, 1993). The dredged sequence is of similar thickness to the green seismic interval and is also floored by a set of acoustically bright reflectors. The dredge showed that the bright reflectors are Lower Barremian to Upper Hauterivian platform limestones and that the unit correlated with the green interval is an Upper Cretaceous sequence unconformably overlain by Eocene to Miocene strata. On the basis of onshore exposures, the section below the platform carbonates is probably composed of Middle Jurassic volcaniclastics. In places there is some divergence of reflectors in the lower part of the green interval - - perhaps evidence for minor mid-Cretaceous syn-rift sediment (probably sandstone) representing the Western Black Sea rift event. Petroleum plays. 1. Extensional tilted fault blocks in the Andrusov Ridge with a pre-(Palaeocene) rift reservoir: this play includes at least three regional closures, each composed of several individual fault blocks and with closures of the order of 1000 km 2. Within this unit, reservoirs are most likely to be developed within the Aptian-Albian (towards the base of the green interval). The Aptian-Albian contains sandstones

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Petroleum geology of the Black Sea: A. G. Robinson et al. in the Western Pontides and Caucasus and possibly also Crimea. Sand would have come from the Russian Platform to the north and, by analogy with the Caucasus, would probably be in deep-water facies. Source rock is likely to be present in the Upper Eocene onlapping the fault blocks and should be generating oil and gas today. These would tend to migrate towards the Mid-Black Sea High from parts of both Western and Eastern Black Sea basins. The drape over the fault blocks may also be a source rock. Further source intervals are possible in the pre-rift, but would be generating gas. Critical to the success of this play are the uncertain development of reservoir and the necessarily tortuous migration pathway from post-rift into pre-rift sequences. 2. Drapes over the Andrusov Ridge closures, with post-rift reservoirs: samples obtained from nearby mud volcanoes in the Western Black Sea contained quartz arenites of Oligocene age with porosity as high as 28%. Sandstones may be interbedded with mudstones throughout the post-rift; their development is most likely during the Late Miocene lowstand when the shelf areas were strongly incised. The presence of petroleum associated with the Andrusov Ridge is conclusively demonstrated by velocitY pull-downs affecting the shallow section on seismic lines vertically above the major culminations (see Plates 12 and 13, labelled G) and by the presence of wet gas seepage on the seafloor detected by high percussion core fluorescence and the presence of characteristic biomarkers (Rudat and Macgregor, 1993).

Western Turkish Shelf West of the Mid-Black Sea High, the narrow shelf of the Western Black Sea includes the offshore extension of the Western Pontides thrust belt. This brief evaluation is based very largely on the one newly acquired seismic line shown, with geological interpretation, in Plate 14. The most obvious prospective structures close to the shore are compressional anticlines formed on out of the basin thrusts that characterize the northern parts of inverted Aptian-Albian half-graben onshore. Ak~akoca-1 was drilled on such a compressional anticline and discovered gas in Eocene clastics. Further offshore, compressive deformation dies out and does not affect the extensional structures formed as the Western Black Sea rifted. These are, however, located in deep water. The large amount of erosion on the shelf that can be observed on seismic and the presence of the thrust belt onshore suggests that there should be numerous basin floor fans throughout the post-rift sequence. Seismic data show numerous downlapping geometries, particularly in the Middle Eocene, which onlap back towards the south. These too are located in deep water (Plate 14). The Upper Cretaceous comprises thick volcanics of the subduction arc, obscuring most of the deeper formations. Possible reservoir intervals exposed onshore in the Western Pontides are of mainly poor quality. Nonetheless, Upper Palaeocene to Eocene sediments (Plate 7) are mainly siliciclastic turbidites (Atba~l and Kusuri formations) and the lower part of the Kusuri Formation tends to include very coarse debris flows and slumps. Plays associated with Pontide compressional

:)22

structures can possibly not be charged by mature Upper Eocene source rock in the Western Black Sea and could also be sourced from the Palaeozoic source. The most likely candidate is the Upper Carboniferous coal (which may have sourced the Ak~akoca gas).

Conclusions The Black Sea region has a long and complex history determined by its position above a subduction zone at which the Tethyan Ocean was being consumed northwards, probably from Late Triassic time to its final closure in the mid-Tertiary. The back-arc was subjected to alternating extension and compression, the present Black Sea being the result of the latest phase of extension (Mid-Cretaceous to Eocene), modified by the ensuing and final phase of compression. This history has resulted in a wide variety of structural and sedimentary environments and facies, and also in widespread and repeated volcanism. Hydrocarbon habitats are widely distributed and varied, and although discoveries to date have been modest, a large potential remains. The presence of a major regional source (Upper Eocene) known to be generating oil and gas in large amounts, is a significant positive factor in the assessment of any part of the basin.

Acknowledgements We are grateful to BP Exploration for permission to publish and to numerous ex-colleagues: Andrew Wright, Paul Batey, Jackie Bannon, David Roberts, Guy Flanagan, Phil Hirst, Martin Illingworth, Martin Riviere, David Parker, Mair6ad Rutherford and Dave Gurney. We benefited from logistical assistance and scientific comment from staff at Istanbul Technical University, TPAO (Ankara), Yuzhmorgeologiya (Gelendzhik), Chernomorneftegas (Simferopol) and Petromar (Constanta). The following institutes and companies supplied data: TPAO (seismic and well data), Yuzhmorgeologiya (regional seismic lines, well data, structure maps and other interpreted data), Chernomorneftegas (petroleum samples), Rompetrol (regional seismic and well data), Petromar (oil samples).

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