A New View to the Spatial Distribution of the Paleogene Lithostratigraphic Units in the Offshore Part of the Dolna Kamchiya Basi

СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 80, кн. 2, 2019, с. 61–69 REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 80, part 2, 2019, p. 61–69

A new view to the spatial distribution of the Paleogene lithostratigraphic units in the offshore part of the Dolna Kamchiya basin based on seismic profile’s interpretation

Boris Valchev, Hristo Dimitrov

University of Mining and Geology “St. Ivan Rilski”, 1700 Sofia, Bulgaria; E-mails: [email protected], [email protected]

Един нов поглед върху пространственото разпространение на палеогенските литостратиграфски единици от акваториалната част на Долнокамчийския басейн на базата на интерпретация на сеизмични профили

Борис Вълчев, Христо Димитров

Минно-геоложки университет „Св. Иван Рилски“, София 1700

Резюме. Настоящата статия има за цел да хвърли светлина върху пространственото разпространение и връзките на палео генските литостратиграфски единици от акваториалната част на Долнокамчийския басейн. На базата на литостратиграфска ин терпретация на шест сеизмични профила и реинтерпретация на пет сондажни разреза са разпознати пет официални единици – Беленска свита (Палеоцен), Двойнишка свита с Армерски и Гебешки член (Долен–Среден Еоцен), Долночифлишки член на Авренската свита (Среден–Горен Еоцен) и Русларска свита (Олигоцен). Получени са нови данни за литологията, стратиграфски те и латералните взаимоотношения, дебелината и вътрешната структура на единиците.

Ключови думи: литостратиграфия, Палеоген, сеизмични профили, Долнокамчийски басейн.

Abstract. The present article aims to elucidate the spatial distribution and relationships of the Paleogene lithostratigraphic units in the offshore part of the Dolna Kamchiya basin. Five formal units were identified on the base of lithostratigraphic interpretation of six seismic profiles and reinterpretation of five boreholes – the Byala Formation (Paleocene), the Dvoynitsa Formation with the Armera and Gebesh Members (Lower–Middle Eocene), the Dolni Chiflik Member of the Avren Formation (Middle–Upper Eocene), and the Ruslar Formation (Oligocene). New data on the lithology, stratigraphic and lateral relationships, as well as the thicknes and internal structure of the units were obtained.

Keywords: lithostratigraphy, Paleogene, seismic profiles, Dolna Kamchiya basin.

Introduction analyses and palaeogeography (Dimitrov, Geor giev, 2005; Dimitrov, 2008), and sedimenthology The offshore part of the Dolna Kamchiya basin has (Stefanov, 2018). Thus, a large amount of data and been investigated for oil and gas since the beginning geologic interpretations has been collected. of the 60s of the 20th century and thus it is the lon The Dolna Kamchiya basin (DKB), also known gest studied petroleum area in Bulgaria. On the base as Dolna Kamchiya depression (DKD – Bokov et of geophysical and borehole data (the offshore drill al., 1987) or Dolna Kamchiya sub-basin (Geor ing was done in the interval 1984–1993), outlining giev, 2012), comprises a small onshore area in the the deep geologic structure of the basin, the geolog central Bulgarian Black Sea coast and extends off ical investigations have been focused on litho- and shore to the southeast into the Western Black Sea biostratigraphy (Juranov, 1991), seismostratigraphy basin (Figs 1, 2). A brief review on the geological (Georgiev et al., 2004; Dimitrov, Georgiev, 2011), setting was published by Dimitrov and Georgiev sequence stratigraphy (Dimitrov, 2007), lithofacies (2005), and Stefanov (2018).

61 The present article aims to elucidate the spa tial distribution and relationships of the Paleogene lithostratigraphic units in the offshore part of the Dolna Kamchiya basin on the base of lithostrati graphic interpretation of seismic profiles and bore hole sections. The interpretation of seismic profiles for lithosttatigraphic purposes was successfully ap plied (Valchev, Dimitrov, 2018) to the Paleogene in the onshore part of the basin.

Studied material

Six selected seismic profiles (Figs 2, 4–6) were Fig. 1. Tectonic subdivision of the territory of Bulgaria (af- lithostratigraphically interpreted on the base of cor ter Dabovski, Zagorchev, 2009) with location of the studied relation to five borehole lithologic logs (Figs 2, 3), area that were reinterpreted here. The primary lithologic Фиг. 1. Тектонска подялба на територията на България data were collected from the geological reports kept (по Dabovski, Zagorchev, 2009) с разположението на из- in the National Geological Fund and they concern следвания район the following boreholes: LA-3 (Staples, Pierpoint,

Fig. 2. Tectonic subdivision of the onshore part of Central East Bulgaria and adjacent offshore area (after Bokov et al., 1987; Georgiev, 2012 with modifications) with location of the studied seismic profiles and boreholes Tectonic units: 1, Moesian Platform, 2, Provadia syncline, 3, Dolna Kamchiya basin, 4, Western Black Sea basin, 5, Eastern Balkanides, 6, seismic profile, 7, borehole; Faults: 1, Venelin-Aksakovo, 2, Balchik, 3, Kaliakra, 4, Bliznatsi, 5, East Balkan frontal thrust Фиг. 2. Тектонска подялба на сухоземната част на централна Източна България и прилежащата ѝ акватория (по Bokov et al., 1987; Georgiev, 2012 с изменения) с разположение на изучените сеизмични профили и сондажи Тектонски единици: 1 – Мизийска платформа, 2 – Провадийска синклинала, 3 – Долнокамчийски басейн, 4 – Западночер номорски басейн, 5 – Източни Балканиди, 6 – сеизмичен профил, 7 – сондаж; Разломи: 1 – Венелин-Аксаковски, 2 – Бал чишки, 3 – Калиакренски (Западночерноморски), 4 – Близнашки, 5 – Източнобалкански челен навлак

62 Fig. 3. Lithological logs and lithostratigraphic interpretation of the borehole sections in the offshore part of the Dolna Kamchiya basin Abbreviations: By, Byala Formation; Dv, Dvoynitsa Formation (A, Armera Member, Gb, Gebesh Member); DCh, Dolni Chiflik Member of the Avren Formation; Ru, Ruslar Formation Фиг. 3. Литоложки колонки и литостратиграфска интерпретация на сондажните разрези от акваториалната част на Долнокамчийския басейн Съкращения: By – Беленска свита; Dv – Двойнишка свита (A – Армерски член, Gb – Гебешки член); DCh – Долночифлиш ки член на Авренската свита; Ru – Русларска свита

1995f1), LA-1 (Pettit, Pierpoint, 1994f2), R-1 Sa 1994f5). For additional correlation an onshore seis motino Sea (Jelev et al., 1989f3), R-1 Samotino mic profile, located along the sea coastline, was East (Bogatskaya et al., 1986f4), and LA-2 (Staples, used (Fig. 7).

1 Staples, G., N. Pierpoint. 1995f. Well LA IV/91-3. Final Well Lithostratigraphic units Report. Ministry of Energy, National Geological Fund, report LA IV- 346, 14 p. (unpublished). 2 Pettit, W., N. Pierpoint. 1994f. Well LA IV/91-1. Final Well The lithostratigraphic reinterpretation of the studied Report. Ministry of Energy, National Geological Fund, report LA IV- borehole lithological logs allowed identifying of five 283, 28 p. (unpublished). 3 Jelev, S., A. Vasilev, R. Veneva, S. Juranov, G. Kovachev, Y. Kovacheva, R. Koleva, N. Kostova, T. Manchev, G. Marinova, L. Mo G. Radev, S. Rangelov, V. Stanev. 1986f. Report on the Results of the nahova, K. Mutafova, G. Pavlova, G. Radev, R. Stanev, G. Stoyanova. Deep Prospecting Drilling of R-1 Samotino East Borehole. Ministry of 1989f. Report on the Results of the Deep Prospecting Drilling of R-1 Energy, National Geological Fund, report ІІІ-356, 280 p. (in Bulgarian, Samotino Sea Borehole. Ministry of Energy, National Geological Fund, unpublished). report ІІІ-366, 360 p. (in Bulgarian, unpublished). 5 Staples, G. 1994f. Well LA IV/91-2. Final Well Report. Ministry 4 Bogatskaya, G., M. Vakarelska, A. Vasilev, R. Veneva, D. Grib of Energy, National Geological Fund, report LA IV-242, 18 p. neva, S. Juranov, S. Jelev, Y. Kovacheva, T. Manchev, L. Monahova, (unpublished).

63 formal units (Fig. 3): the Byala Formation (Pale ded to massive conglomerates. The Gebesh Mem ocene), the Dvoynitsa Formation with the Armera ber is composed of thin-bedded alternation of sand and Gebesh Members (Lower–Middle Eocene), the stones, marls, siltstones and shales. In LA-3 borehole Dolni Chiflik Member of the Avren Formation (Mid only the Armera Member is represented and it over dle–Upper Eocene), and the Ruslar Formation (Oli lies the Lower Cretaceous base, as the boundary is gocene). A brief description of their lithology, age, an unconformity (Fig. 3a). The lower boundary with spatial distribution, and relationships is given bellow. the Byala Formation was discussed above. The two members show great varieties of their thickness, as they do not demonstrate an exact stratigraphic posi The Byala Formation tion in the Dvoynitsa Formation volume. The Gebesh Member was recorded at two or three levels in four The rocks of this unit were described as “whitish borehole logs (Fig. 3b–e) and its presence was identi limy marls” (Zlatarski, 1907), “Byala clayey marls” fied in all seismic profiles. The thinnest (100 m) it is (Bontchev, 1926), “limestone-marl formation” (Ju in R-1 Samotino Sea borehole (Fig. 3c) and the thick ranov, 1984). It was formalized by Juranov (1991; est (over 2000 m) – in LA-2 borehole (Fig. 3e). The see also Juranov, 1993a). In the offshore part of the Armera Member occurs usually at one (Fig. 3a, d, e) Dolna Kamchiya basin the unit is represented by dark or two levels (Fig. 3b, c). Its thickness varies between gray solid marls, established in two borehole sec 54 m in LA-2 borehole (Fig. 3e) and 902 m in R-1 tions – LA-1 and R-1 Samotino Sea (Fig. 3b, c). In Samotino Sea borehole (Fig. 3c). The total thickness the first one, it overlies the Upper Cretaceous base as of the formation is from 395 (Fig. 3a) to over 2000 m the boundary is an unconformity and the thickness (Fig. 3e). The lithostratigraphic interpretation of the is 65 m. In the second section the lower boundary seismic profiles (Figs 4–6) shows clearly the com was not penetrated and thus it could be assumed that plex and variable stratigraphic and lateral relation the thickness is more than 106 m. The upper bound ships of the two units and the variety of their thick ary is a sharp lithologic contact with the Dvoynitsa ness. The chronostratigraphic range of the Dvoynitsa Formation (represented by the Gebesh Member in Formation was determined as Lower–Middle Eocene LA-1 and by the Armera Member in R-1 Samotino on the base of rare planktonic foraminiferal remains Sea). In the southernmost boreholes R-1 Samotino (Juranov, 1991). East and LA-2 (Fig. 3d, e) the pre-Paleogene base was not penetrated and therefore this boundary was not studied. The Byala Formation was identified in The Dolni Chiflik Member of the Avren all seismic profiles (Figs 4–6) underlying the Gebesh Formation or Armera Member of the Dvoynitsa Formation and showing almost constant thickness. In the northern It was known as “Dolen Ciflik Seria” (Pollak, part of B92-16 seismic profile (Fig. 4) there is prob 1933), “marl-sandy complex” (Efremochkin et al., 6 7 ably a lateral transition between the Byala Forma 1974f ; Vavilova et al., 1978f ), “Dolni Chiflik For tion and the Komarevo Formation (Thanetian). The mation” (Cheshitev et al., 1994; Kânčev, 1995), and planktonic foraminiferal data obtained from R-1 formalized as the Dolni Chiflik Member by Jura Samotino Sea borehole (Juranov, 1991) determined nov (1993b). Generally, the unit comprises alterna Paleocene age for the formation. tion of sandy marls and clayey sandstones, as it is possible two distinct levels to be recognized. The lower one is composed predominantly of clayey The Dvoynitsa Formation sandstones and siltstones alternating with thin- bedded shales and marlstones. The upper level is The unit was introduced as formal one by Juranov represented mainly by sandy marls alternating with and Pimpirev (1989) in the coastal region of the East siltstones, sandstones and shales. The thickness of ern Balkanides. Later on, Vangelov and Sinnyovsky the lower level varies from 653 (Fig. 3e) to 789 m (2011) formalized three members of the Dvoynitsa Formation (the Armera, Gebesh and Goritsa Mem 6 Efremochkin, N. V., R. Y. Yovchev, V. P. Strepetov, R. I. Plot bers), and Valchev et al. (2018) elucidated the inter nikova, L. Bryukner, S. Dimovski, A. Agopyan, V. Parashkevova, M. nal structure of this unit in the onshore part of the Vavilova, M. Nikolova, Y. Aleksiev. 1974f. Report on the Results from Dolna Kamchiya basin by 3D lithological modelling the Subsurface Iodine-bearing Water Prospecting in Dolna Kamchiya Source with Available Reserve Calculation (up to October, 1, 1974). recognizing all the three members. The Gebesh and Ministry of Energy, National Geological Fund, report V-223, 1202 p. Armera Members were established in all five off (in Russian, unpublished). shore borehole sections, while the Goritsa Member 7 Vavilova, M., S. Dimovski, L. Bryukner, V. Parashkevova, M. Nikolova. 1978f. Report on the Results of the Deep Drilling in Dolna was not recorded. The Armera Member comprises Kamchiya Area Conducted in 1955–75. Ministry of Energy, National thick-bedded sandstones with interbeds of thick-bed Geological Fund, report ІІІ-294, 3171 p. (in Bulgarian, unpublished).

64 Fig. 4. Lithostratigraphic interpretation of seismic profiles SP7866 and B92-16 Ru, Mb.; Chiflik Dolni DCh, Mb.; Armera A, Mb.; Gebesh Gb, Fm.; Komarevo Kom, Fm.; Byala By, Abbreviations: s). (TWT, depth - time double to sections Geologic - geophysical 4–7: Figs Ruslar Fm.; B+D+Al, Beloslav, Dikilitash and Aladan Fms; N, Neogene cover Фиг. 4. Литотратиграфска интерпретация на сеизмични профили SP7866 и B92-16 член; Армерски – A член; Гебешки – Gb свита; Комаревска – Kom свита; Беленска – By Съкращения: s). (TWT, дълбочина време двойно по разрези Геолого - геофизични 4–7: Фиг. DCh – Долночифлишки член; Ru Русларска свита; B+D+Al Белославска, Дикилиташка и Аладънска N неогенска покривка

65 Fig. 5. Lithostratigraphic interpretation of seismic profiles BGK92-22 and 84581 Фиг. 5. Литотратиграфска интерпретация на сеизмични профили BGK92-22 и 84581

(Fig. 3b), while the upper one is from 180 (Fig. 3e) with thin-bedded interbeds of sandstones, silt to 500 m (Fig. 3b) thick. It should be noted that the stones and rare limestones. The last, as well as the core intervals of R-1 Samotino Sea borehole are sandstones, occur mainly at the lower levels of the very scarce and this fact did not allowed us to rec formation. Its thickness varies from 126 (Fig. 3e) ognize the two levels. The total thickness of the unit to 500 m (Fig. 3d). The boundary with the Dolni is from 836 (Fig. 3e) to 1289 m (Fig. 3b). The chro Chiflik member is a sharp lithologic contact. In the nostratigraphic range of the Dolni Chiflik Member entire area of its distribution the Ruslar Formation was determined on the base of planktonic foramini is covered with Neogene sediments. Fossil remains feral data (Juranov, 1991) as Middle–Upper Eocene. have not been established, therefore the age of the unit was determined as Oligocene on the base of its stratigraphic position. The Ruslar Formation It was introduced as “Ruslar sandstones” by Zlatar Discussion ski (1927) and formalized by Aladjova-Chrisčeva (1991). In the offshore part of the Dolna Kamchiya The data, obtained from the studied borehole sec basin the unit comprises predominantly claystones tions and seismic profiles, allowed us to outline the

66 79124 SP

и 92-71 BGK and SP79124

BGK92-71

Литотратиграфска интерпретация на сеизмични профили

. 6 Fig. 6. Lithostratigraphic interpretation of seismic profiles Фиг.

67 Fig. 7. Lithostratigraphic interpretation of the onshore seismic profile 5506-84 Фиг. 7. Литотратиграфска интерпретация на сухоземния сеизмичен профил 5506-84 spatial distribution of the five established Paleo in the central area of the southern board of the gene lithostratigraphic units. Thus, we could claim basin. that the Byala Formation is distributed in the entire Compared to the onshore sections, the Ruslar southern board of the basin, showing almost con Formation shows similar lithologic features and stant thickness. It is covered with the Armera or variations of the thickness. It is maximal in the cen Gebesh Member of the Dvoynitsa Formation in the tral offshore areas. entire area of its distribution. The last one was also The uneven distribution of the borehole sec recorded in the southern board of the basin, as it tions and seismic profiles, especially in the northern is difficult to outline exactly the northern boundary board of the offshore Dolna Kamchiya basin, is an of its distribution. The unit demonstrates complex obstacle for a more precise outlining of the spatial internal structure with varied thickness. The com relationships with the Paleogene lithostratigraphic plicated tectonic structure of the southernmost area units recorded from the northernmost areas of the (Fig. 4 – profile SP7866, Fig. 5 – profile BGK92- basin, and the probable lateral transitions to the 22, Fig. 6 – profile SP79124) does not allow calcu Komarevo, Beloslav, Dikilitash, Aladan and Avren lation of the exact thickness, but we could assume Formations, established in the onshore sections. that it is over 2000 m. Variation of the stratigraphic position and the thickness of the Armera and Gebe sh Members confirms the data, obtained from the Conclusions onshore boreholes and seismic profiles (see Fig. 7), and reveals the Dvoynitsa Formation as the most The lithostratigraphic interpretation of the seismic complex Paleogene lithostratigraphic unit in the profiles by correlation to the reinterpreted offshore Dolna Kamchiya basin. borehole logs, gave us an opportunity to clarify the The data, concerning the Dolni Chiflik Mem spatial distribution and relationships (stratigraphic ber of the Avren Formation, proved its broad and lateral) of the Paleogene lithostratigraphic units spatial distribution in the basin, but we could not and to add new data of their lithology and thickness. mark the lateral transition to the typical Avren This fact is a reliable base for further investiga Formation in the northern board of the Dolna tion of the deep geological structure of the offshore Kamchiya basin. The member covers the Armera part of the Dolna Kamchiya basin including 3D or Gebesh Members of the Dvoynitsa Formation lithostratigraphic, chronistratigrapnic and lithologi as the boundary is unconformity. The thickness cal modeling, as well as more precise palaeogeo shows considerable variations, as it is maximal graphic reconstructions.

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Постъпила на 27.09.2019 г., приета за печат на 06.11.2019 г. Отговорен редактор Димитър Синьовски