To date exploration in the Bulgarian Black Sea has been

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1 SPECIAL The Black SECTION: Sea The region Black Sea region Play types and hydrocarbon potential of the deepwater Black Sea, NE Bulgaria GABOR TARI, JULIA DAVIES, RUDOLF DELLMOUR, and ELIZA LARRATT, OMV Development and Production BERNHARD NOVOTNY, OMV, Cairo, Egypt EMANUIL KOZHUHAROV, Jes E, Sofi a, Bulgaria To date exploration in the Bulgarian Black Sea has been largely restricted to the shelf and therefore the deepwater area remains untested in spite of significant hydrocarbon potential. The presence of a deepwater petroleum system is proved by several DHIs, such as gas chimneys and gas-related velocity sags on high-quality 3D seismic data. At least six different deepwater play types have been identified by OMV in the Bulgarian offshore. Most are related to the prominent syn-rift Polshkov High either in intra-tertiary structural closures in the compactional anticline above (or within) the Mesozoic syn-rift or early post-rift sequence. The deepest exploration target is the Polshkov High itself, where the trap is defined by several fault blocks in which the assumed Jurassic and/or Lower Cretaceous carbonate platform sequence may have a reservoir facies suitable for deepwater gas production. The target of this play is analogous to the ones targeted over the mid-black Sea High (Andrusov Ridge) offshore Turkey or over the Tetyaev High offshore Ukraine. Figure 1. Simplified onshore geology of Bulgaria redrafted after Georgiev et al. in relation to the western Black Sea. Exploration history Tyulenovo Field was found in 1951, right along the coastline of the Bulgarian Black Sea in an elongated faulted anticline Figure 2. Structure map on top of the pre-rift strata, offshore NE Bulgaria. Orange areas indicate structural closures of the Polshkov High. Red polygon indicates the area of the D seismic survey. (Location on Figure 1.) 1076 The Leading Edge September 2009

2 The Black Sea region northeast of Varna (Figure 1). Reservoirs of gas and heavy oil (19 API) are found in karstified and fractured Valanginian dolomitic limestones at a depth of m, with some minor amounts of gas in Oligocene sandstones. Some 20 exploration wells have been drilled on the offshore Bulgarian shelf, with three discoveries. In 1986, the first offshore well in the Kamchia segment of the Bulgarian offshore, Samotino More-1, made a sub-commercial discovery (flowing 17.6 MMCFGD and some condensate from Eocene turbidites). Another significant milestone was the 1995 Galata-1 discovery (Figure 1) drilled by the partnership of Texaco, Enterprise, and OMV on an anticlinal structure along the southern edge of the offshore Moesian Platform. The well tested gas at a rate of 34 MMCFGD from a 25-m interval of Eocene sandstones. The appraisal well, Galata-2, tested 40 MMCFGD from a 23-m interval of Paleocene algal limestones and Maastrichtian sandstones. OMV, in 2003, was the first company to acquire significant 2D seismic data sets in the northern part of the Bulgarian Black Sea to extend specifically the existing vintage seismic data on the shelf into the deepwater basin. As the result of the new data, a major Miocene deepwater fan system was defined, the Varna Fan. This fan system was interpreted to be sourced from the long-lived Kamchia river system. At the early 2D stage of the exploration program, the Varna Fan was believed to be the primary target on the block and therefore the subsequent 3D survey was specifically designed to cover it. The results of the 600 km 2 3D seismic survey acquired in 2006, however, did not confirm the expectations as to the size and predicted reservoir quality of the Varna Fan. The 3D data proved the presence of an active petroleum system and several pre-miocene clastic reservoir intervals, but made it clear that future exploration should be focused further offshore from the Varna Fan on the Polshkov High (Figure 2). Geological setting, stratigraphy and structure The Varna Deep Sea Block is in the Bulgarian segment of the Western Black Sea Basin. Structurally, the margin is in a lower plate position, in relation to the opening of the basin during mid-cretaceous times. The paleo-shelf edge of the basin is marked by a major syn-rift hinge zone trending NE- SW. The basinward flank of the hinge zone formed a major submarine escarpment throughout much of the basin s history, separating the shallow-water stratigraphy from that of the deepwater (Figure 3). The numerous erosional unconformities documented onshore and/or on the shelf do not exist in the deepwater as sedimentation was uninterrupted since early post-rift times. The only exception to this is the Messinian unconformity corresponding to a drawdown event which subaerially exposed most of the Black Sea Basin. Note that the stratigraphy described here is somewhat speculative and primarily based on seismic facies (Figure 4) due to the lack of well data in the deep water. Whereas the style of most of the Miocene to Recent sedimentation was dominated by mass transport complexes Figure 3. Conceptual chronostratigraphy of deepwater area offshore NE Bulgaria. No wells have been drilled in the deepwater Black Sea to constrain the stratigraphy depicted here. derived from the nearby shelf, a dramatic change occurred in the sedimentary source at around the Oligocene/Miocene boundary. During the Paleogene, the Kamchia foredeep basin developed to the north of the Balkans folded belt. Therefore, mostly during the Eocene, several distal basin-floor fan units deposited here alternating with slope fan channel systems. The foredeep unconformity is tentatively placed within the Paleocene as its precise dating is problematic in the deep water. The post-rift Cretaceous sequence landward from the Polshkov High is assumed to made up of bathyal shales with only a few Senonian turbidite units. The Albian/Cenomanian syn-rift succession should have more clastics in the actively growing half-grabens of the overall Polshkov High. September 2009 The Leading Edge 1077

3 T h e B l a c k S e a r e g i o n Figure 4. Typical 2D seismic example from deepwater Bulgarian Black Sea. The pre-rift strata are most probably preserved outboard from the hinge zone, but eroded down to various depths along the crest of the Polshkov High. By extrapolating the stratigraphy known from the shelf, Upper Jurassic carbonate rocks could be expected beneath the breakup unconformity, developed in various neritic-to-bathyal facies belts. The Middle Jurassic, corresponding to another rifting episode, is interpreted to have a clastic sequence. As to potential structural traps in the deepwater area, the most prominent structural closure can be found on the top pre-rift succession. The NW SE Polshkov High actually is composed of separate smaller closures, the southernmost being the largest (about 500 km2). The edge of the paleo-shelf edge is very well defined based on the regional 2D seismic grid, some km basinward from the present shelf edge. The escarpment-looking steep surface delimiting the paleoshelf edge is actually a major listric normal fault which can be easily traced along strike. Actually, the map-view expression of the breakaway line of the major normal fault suggests the presence of two slightly concave listric faults to the SE. The easternmost is interpreted to be responsible for the formation of the Polshkov High during syn-rift extension of the Western Black Sea Basin. Sometime during the late syn-rift stage, an ENE hinge zone of a rift shoulder(?) collapsed and faulted down with a map-view counterclockwise rotation of about 20. This new structural interpretation of the Polskhov High is very important in the sense that it predicts the exact same pre-rift stratigraphy, as potential reservoir, as known from the 1078 The Leading Edge present-day shelf and also from Tyulenovo Field (i.e., karstified and fractured Valanginian dolomitic limestones). Play types At least six different deepwater play types have been identified in the block (Figure 5). One of these, the Varna Fan play (play 1) was defined by OMV, based on 2D data, very early in the deepwater exploration of NE Bulgaria. The trap for this play is provided by the unconformity surface of the Messinian event, which, similarly to the Mediterranean Basin, corresponds to a period of exposure and erosion in the Black Sea Basin. Based on the 2D data, the clastics for the reservoir were expected to be derived from the Kamchia Trough in the SW; however, the subsequent 3D survey showed that the Varna Fan is sourced from the shelf from a NW direction. This finding significantly increased the reservoir quality risk associated with this play. Farther out in the deep water, the traps of plays 2 and 3 are associated with the compactional anticline above the prominent Polshkov High. The four-way closure within the Tertiary succession is mappable from about the top Oligocene downwards and becomes significant (i.e., with a vertical closure of more than 100 m) within the lower part of the Paleogene sequence. The reservoir sequence is a channel system, sourced from the Kamchia Trough, forming slope fans within the Maykop series. The slope fan reservoir units transition to basin-floor fans in the Eocene and Paleocene as distal turbidites of the Kamchia foredeep basin in the SW. September 2009

4 The Black Sea region Figure 5. Play types in the deepwater area of NE Bulgaria. Several examples of fans pinch out against the paleo-escarpment of the hinge zone within the Paleogene defining play 4. These fans are assumed to be fairly coarse-grained clastics with limited horizontal extent. From a risking standpoint, the critical question is the presence of an efficient updip seal. It is difficult, inboard from the Polshkov High, to determine the exact position of the breakup unconformity on the 2D seismic grid. However, post-rift Cretaceous seismic units onlap against the overall syn-rift high, regardless of their tectonostratigraphic position. These onlapping basin floor fans define play 5, largely on the northwestern flank of the Polshkov High. The deepest target is the Polshkov High itself, which represents a large, back-rotated syn-rift fault block. In play 6, the target is the Mesozoic pre-rift succession preserved along the elongated apex of the structural high. The trap is defined by several fault blocks in which the assumed Jurassic carbonate platform sequence may have a facies (for example, neritic carbonates with oolites) suitable for gas production. The target of this play is identical to the ones targeted by the industry lately over the mid-black Sea High (Andrusov Ridge) offshore Turkey or over the Tetyaev High offshore Ukraine. Deepwater petroleum system Regarding the source and generation, basin modeling indicates that the Lower Miocene-Oligocene Maykop sequence is presently in the oil-generation window. Deeper source rock intervals, such as the Eocene, are partly or entirely in the gasgeneration window. Therefore gas is predicted as the dominant hydrocarbon phase in the deeper play types. The 600 km 2 3D survey was also processed by dgb to produce a chimney cube specifically addressing hydrocarbon presence by detecting DHIs and vertical migration via gas chimneys. There are indeed several gas chimneys charging intra-tertiary packages (Figure 6). Uncalibrated AVO work also underlines the presence of shallow gas accumulations. Detailed velocity work done for PSDM processing of the 3D cube also points to gas-charged units, a finding consistent with the frequently observed velocity push-downs in the time domain. Moreover, BSRs are quite frequent on the seismic and numerous shallow gas accumulations are seen on the 2D data in the deepwater part of the block. September 2009 The Leading Edge 1079

5 T h e B l a c k S e a r e g i o n Figure 7. Attribute expression of the Upper Miocene clastics based on 3D seismic data. (Approximate location on Figure 2.) Figure 6. Example of gas chimneys, based on a chimney cube generated from the 3D data set: (a) conventional seismic display and (b) anomalies from the chimney cube added. Migration into the Oligocene sequence is assumed to occur by mostly lateral migration from the Maykop source rock sequence; the Eocene and deeper stratigraphic units were charged by dominantly vertical migration along syn-rift faults and their reactivated continuation into the post-rift sequence. The Pliocene and younger reservoirs are questionable, as to their quality, as they are dominated by several mass transport complexes (debris flows) derived from the shelf area (Figure 7). However, the Paleogene sequence has a different seismic signature on the 3D data (Figure 8) which show channel systems which could be traced back updip from the Varna Deep Sea Block onto the paleoshelf to the Kamchia foredeep basin in the W and SW. Beneath the Oligocene slope fan complexes, Eocene sheet sands of distal basin-floor fans are predicted based on the 3D seismic data (Figure 9). These distal turbidites of the Kamchia foredeep basin have well studied outcrop analogs along the coastline The Leading Edge Figure 8. Attribute expression of the Oligocene clastics based on 3D seismic data. (Approximate location on Figure 2.) Figure 9. Attribute expression of the Eocene clastics based on 3D seismic data. (Approximate location on Figure 2.) September 2009

6 The Black Sea region The deeper reservoir sequences are difficult to predict. Some ponding of deepwater clastics is predicted during the early post-rift Cretaceous landward from the Polshkov High. Beneath them, the Albian/Cenomanian syn-rift succession should have more clastics in the actively growing half-grabens of the overall Polshkov High. The pre-rift strata are eroded to various stratigraphic depths along the crest of the Polshkov High, but assumed to have a pre-middle Cretaceous Mesozoic sequence. Whether it is a Lower Cretaceous clastic or an Upper to Middle Jurassic carbonate sequence remains to be seen after new 3D seismic data become available. Reservoir quality is difficult to predict as borehole data with Mesozoic penetrations drilled on the shelf may not be directly applicable to the Polshkov High due to the different burial history and paleogeographic position. Traps are well defined on the 3D survey targeting the Varna Fan and turned out to be smaller than expected beneath the complex Messinian erosional unconformity. However, Polshkov High offers a very large structure with significant vertical and map-view closure, just outboard from the existing 3D survey. The overall amplitude of the vertical closure is about 500 m on the top pre-rift level, not considering individual syn-rift fault blocks, and it decreases below 50 m by the top of the Oligocene in the compactional anticline above Polshkov High. The proper depth conversion of the new 3D data will be critical to determine the extent of the closure and therefore to define the size of the traps at different stratigraphic levels. Based on the chimney cube generated from the 3D cube, there are efficient seal intervals within the Upper Tertiary serving as ultimate seals. Intraformational shale sequences as top seals are also common based on their seismic signature. Suggested reading. Geophysical study of the Black Sea: by Finnetti et al. (Bollettino di Geofisica Teoretica ed Applicata, 1988). Kinematic history of the opening of the Black Sea and its effect on the surrounding regionsî by Okay et al., (Geology, 1994). Regional and Petroleum Geology of the Black Sea and Surronding Region, edited by Robinson (AAPG Memoir, 68, 1997). East Srednogorie-Balkan Rift Zone by Georgiev et al. (Peri-Tethys Memoir 6, 2001). Acknowledgments: Many thanks to the Ministry of Environment and Waters of Bulgaria for their support of OMV s exploration efforts and for their permission to publish this paper. Llew Vincent provided critical help with the detailed velocity work done for PSDM processing of the 3D cube. We also acknowledge dgb for producing a very useful chimney cube. Finally, we are grateful to Georgi Georgiev for his insights to the regional geology of the Western Black Sea. Corresponding author: gabor.tari@omv.com Conclusions We believe that the Bulgarian segment of the deepwater Black Sea is severely underexplored even though it has very promising exploration aspects. The next technical steps should focus on the Polshkov High which is associated with several different play types. After acquiring a new 3D survey, it may be possible to find a deepwater drilling location over Polshkov High where several prospect segments of independent play types can be stacked together to maximize the chance of an economic discovery. The primary target is the Mesozoic sequence with very large recoverable gas resources (i.e., > 10 TCF). The various Paleogene targets above Polshkov High are considered secondary, but could provide condensate or oil. In conclusion, the Bulgarian segment of the deepwater Black Sea remains untested even though it has very significant exploration potential. However, as the result of the first deepwater drilling campaign in the Black Sea, starting in 2010, we predict an exploration breakthrough in this last deepwater frontier basin of the world. September 2009 The Leading Edge 1081