Aytuna Consulting

International Petroleum Consulting

Geology of Turkey

There are seven onshore and four major offshore basins in Turkey. The onshore ones are called SE Turkey (Anatolia) Basin, Thrace Basin, Adana Basin, Tuz Gölü Basin, East Anatolia Basin (including several subbasins) and the onshore Black Sea Basin (Zonguldak and Sinop). The offshore basins can be named as; Black Sea, Marmara Sea, Aegean and Mediterranean Sea.

Topography of Turkey (click for larger image)

The most active onshore basins are, as far as the exploration and production concern; firstly SE Turkey (Anatolian) and Thrace, secondly Adana and Tuz Gölü Basins, whereas SE Turkey (Anatolian) Basin is known to be the oil prone and the Thrace is the gas prone ones. All the oil and gas fields are located in these two basins where the total production reaches 50M bbl for oil and 60MM SCFD for gas; however, in the Adana Basin there is one oil field (Bulgurdag) that still produces oil.

Geology of Turkey (click for larger image)
(Courtesy of General Directorate of Mineral Research and Exploration (MTA), Ankara, Turkey)

SE Turkey (Anatolian) Basin

The SE Turkey (Anatolian) Basin is the most active and oil prone basin in Turkey and it is located on the northern extension of the famous oil prone of the Arabian Plate, where the geology is very similar to Northern Iraq and Syria. However, in the SE Turkey Basin, the major reservoir and sources rocks are not as many as in the Arabian Plate. In SE Turkey, the major source rocks are Dadas Shale (Ordovician) and Late Cretaceous Carbonates (Karabogaz and Karababa-A) and the main reservoir rocks are Late Paleocene (Sinan & Garzan) and Late Cretaceous Mardin Carbonates (Karababa & Derdere) and the Paleozoic sandstone of Bedinan and Hazro.

The northern extension of the Arabian Plate is defined by the major subduction zones (the overthust belt). In the basin, rift faultings had been active in the Paleozoic time and most of the normal faults were reactivated during the Late Cretaceous compression, overthusts and trust faults were formed (related picture 1, related picture 2), also widespread strike-slip faults and more overthusts formed later during the Late Miocene shortening and the present shapes were gained (related picture 1, related picture 2). Most of the surface folds trends E-W direction which were formed during Late Miocene shortening and are generally sub parallel to the Late Cretaceous tectonism and both structural trends are superimposed on the each other (related picture).

The main oil reservoirs are made of limestones and dolomites of the Mardin Group (Derdere, Karababa-A/B/C and Karabogaz) of Late Cretaceous (related picture). The rate of dolomitizations and fractures play very important role for this group as far as the production is concerned. Usually, matrix porosity ranges between 1-4% and fracture porosities are 0.5-2% which tends to increase towards the trust belt zone in the north. In the reservoir rocks average permeability runs between 0.2-50md, all those parameters change with depth. The oil gravity in the overthust fields in the Cretaceous run between 24-36 API, however, on the gentle folded (the plains) area the oil gravity is between 23-11 API. There are several heavy oil fields in the area; West Raman (1961) and Kahta (1958) are the most famous ones where oil gravity ranges between 11-14 API. The main Paleozoic reservoirs are made of sandstones. It has been reported that the oil gravity in the Paleozoic in SE Turkey is more than 40 API where as in Bardes Deep-1 (1972) well the tested oil in Bedinan is around 50 API gravity.

There are about 100 oil fields discovered in SE Turkey Basin, mainly by TPAO, Mobil, Shell, AME and ARCO and the production is mainly from the Mardin Carbonates. Some fields are 40 years old and still produce. There are several oil and gas discoveries made in Paleozoic, such as in Deep Barbes-1 in the Bedinan Sandstone and in Hazro-1 in the Hazro Sandstone (related picture) by Shell and TPAO, respectively. The total oil production of the SE Turkey Basin is around 50M bopd (end of 2005) from the 73 fields. The field sizes (OIP) run between 20-2 million barrel ranges, average of 3 million barrels. The average recovery factor runs between 25-35%. GOR is very low; no commercial gas in the Mardin section, however, some gas in the Paleozoic has been tested. Most of the wells in basin have targeted to Mardin Levels or shallower reservoirs (Sinan and Garzan Formations). One can say that deep Paleozoic is more virgin than the Mardin Group in the basin however; some potential are exist both in the Mardin and Paleozoic reservoirs.

Mainly, there are three oil source rocks in the basin; Silurian Dadas Shale (north of Diyarbakir fields), Late Cretaceous Karabogaz Limestone (around Adiyaman) and Triassic-Jurassic Judi Group carbonates (around Batman area). The fourth sourced may be the combinations of the two. The Silurian source rocks generated less sulfur, mature and light oil and widely distributed on Districts X and XI in the basin.

Thrace Basin

The Thrace Basin is located on the European section of Turkey. This is the most important gas prone basin in Turkey; the total daily gas production is around 60MM SCFD by three companies, (TPAO, Thrace Basin Natural Gas Company and Zorlu Energy). The basin is made of Eocene through Oligocene-Miocene rocks (related picture). In the south, one can see the basement of rocks of Late Cretaceous and in the north the basin bordered by the Kirklareli metamorphics and granites Cretaceous or older age. The rocks in center of the basin may reaches 9,000m of thickness; a few wells drilled more than 3,500m, the deepest well, Corlu-3A reached 5,043m depth. There are about 350 wells drilled and 13 gas (one offshore in Marmara Sea) and 3 oil fields have been discovered, mainly by TPAO. The most important reservoir rocks are; in Eocene: Hamitabat Sandstone, Ceylan Tuffs (related picture), Sogucak Reefal Limestone (related picture) and in Oligo-Miocene: Osmancik and Danisment Sandstones. Mezardere (TOC runs between 1-7%, related picture) and Hamitabat Shales have the best sources potentials. Hamitabat Gas Field is the first gas field discovered in the basin in 1970 and the largest in the Late Eocene turbiditic sandstone where daily production is around 15MM SCFD.

Litho and chrono-stratigraphic units of Thrace Basin (click for larger image)
(After M. Siyako of TPAO, Ankara, Turkey, 2005)

Adana – Iskenderun Basin

The Tertiary Adana Basin exhibits varying sedimentary facies continental to marine environments in the southeastern corner of Mediterranean Sea, north of Iskenderun Bay area. It is a rift basin. It is bounded to the north and northwest by the Taurus Mountain and to the east and southeast by the Missis highs. In the south the basin extends as far as to Cyprus under the Mediterranean Sea. There are several live oil seepages; one in the south of Iskenderun, oil seepage at Kepirce and Kurtbag gas seepage, south of Arsuz (related picture).

The Adana Basin was formed during the Late Tertiary on irregular paleo topography of deformed Paleozoic-Mesozoic rock units. The basin was filled with 9,000m of Oligocene -Miocene and younger sediments. Most of the sediments display of rapid subsidence. There are no Paleocene-Eocene rocks in the basin. In general, the northern part of the basin includes mainly sandy and calcareous sediments of Late Tertiary and they rest diconformably overlie the Pre-Miocene basement rocks. The older rocks are composed of four different lithologic units. Late Paleozoic shallow marine carbonates/dolomites and shale, Mesozoic carbonates, an ophiolitic complex and Oligocene flysh. Adana Basin was formed during the Late Tertiary on the irregular paleo topography of deformed Paleozoic and Mesozoic rock units in an extension basin. The total Tertiary rocks in the basin are between 6000-9000 meters. A total of 60 exploration wells have been drilled but only 16 of them had reached more than 3,000 m depth, including 12 deep off-shore wells. 11 wells had H-C shows, however, only one oil field was discovered, Bulgurdag, by Mobil in 1960 and the oil is still being produced from the Miocene Reefal Carbonates and the source is from the Devonian carbonates. The field is run by a domestic AME company.

Tuz Gölü (Salt Lake) Basin

The Tuz Gölü basin located 150km south of Ankara around Tuz Gölü (Salt Lake). There are also two small sub basins called Haymana and Cankiri Basins nearby. The Basin is poorly known and tectonically very complex. The Cretaceous through Eocene rocks are filled the basin and the thickness of sediment in the basin may reach to 10,000m. Sediments in the basin consist of turbite clastics, sand & shale and reefal limestone. Half a dozen of live oil seepages are present north of the basin, in the Eocene sandstone and reefal limestone, near Ankara.

Tuz Gölü from space (click for larger image)
(Courtesy of General Directorate of Mineral Research and Exploration (MTA), Ankara, Turkey)

In the basin, there are several large and thick salt domes (more 1500m thick) located in the south. About 16 deep and 12 shallow exploration wells were drilled mainly by TPAO prior to 1980 and several of the wells were abandoned due to high pressed gas during drilling. Most of the drilling times were more than a year and most of the wells had several drilling problems; when the gas was encountered, the wells were killed and drilling continued deeper to reach their targets to test the oil, but they never reached their targets and most of the wells were suspended and/or abandoned. Plus some wells cut thick salt (on the salt domes) and proper casings and well designs should have been used. Six deep wells were tested gas and one well blew up during the testing. When the drilling activates were going on in the basin there was no gas market and no gas infrastructures in the country, the companies were exploring only oil not gas. According to geochemical studies, there should be some gas potential deeper in the basin. Also there is a new study going on the area to store the exported Russian Gas under those salt domes.

The Black Sea Offshore Basin

The Turkish Black Sea offshore section covers more than 20 million acres of deep sea and the coast line is 1600km long, Bulgaria is on the western Black Sea and Georgia is on the eastern site. The Turkish section of the Black Sea is still largely unexplored and very virgin, presenting a high risk, but high-gain perspectives as well. The International marine borders (the median line) for the Black Sea are very well defined by the international agreements by all the neighboring countries and there are no marine border disputes. The Turkish Black Sea is the most important one for hydrocarbon exploration, due to being the largest unexplored offshore basin and where only few wells were drilled. The offshore Turkish Black Sea has had only sporadic attention till the year 1997, since after ARCO’s interests for the Western Black Sea which resulted signing a joint venture agreement with TPAO and drilling of two exploration wells back to back by the ARCO-TPAO joint venture. The deeper offshore areas get more attention and there have been several joint ventures that have been signed with TPAO to explore.

The basin can be divided into three sub basins, Western-Central and Eastern. The Western offshore section is mostly covered by thick Miocene and underlined by the Cretaceous (related picture). The central part started with thick Eocene and underlined by Cretaceous and Paleozoic. The East mostly covered by thick Miocene and underlying Cetaceous. The geochemical data indicates that the western part is mostly gas-prone and the central and the eastern sections may have gas/oil prone. There are several live oil seeps onshore and offshore several licenses and prospects are being farmed out by TPAO at depths of between 1500-2000 meters and in recent new drilling technology can manage to work at those water depths. The region could be potentially another North Sea if the big structures can be drilled, such as in the central that is called Androsov Ridge.

So far only five exploration wells were drilled at the water depths of 100-1500m, not beyond. Fours wells in the West, 25km offshore, Igneada-1 and Karadeniz-1 by TPAO (1978) at water depth of 150 meter range and 60km offshore, Limanköy-1 and Limanköy-2 by TPAO-ARCO joint venture (1999) in the western Black Sea. Limanköy-1 had tested some gas in Miocene at 2000m, in 850 m water depth. The tested gas was not economical at that water depth, and the tested gas was in the Miocene diatomites which had very poor permeability. Also, two shallower wells were drilled at water depth less than 100m at Akcakoca-1 (some gas has been reported to be tested in Eocene sand) and Akcakoca-2 by TPAO in 1976. And also 5 more shallow wells recently were drilled by TPAO-Toreador joint venture nearby Akcakoca offshore (10 km) and some gas was reported to be tested in the sands. The deepest well, Hopa-1, was drilled by TPAO-BP joint venture, in 2005, in the Eastern Black Sea (in District-V) at 1500m water depth and 50km offshore. The result of this well has not been disclosed yet. Also, two shallower wells were drilled at water depth less than 100m at Akcakoca-1 (some gas has been reported to be tested in Eocene sand) and Akcakoca-2 by TPAO in 1976. And also 5 more shallow wells recently were drilled by TPAO-Toreador joint venture nearby Akcakoca offshore (10 km) and some gas was reported to be tested in the sands.

There are ample evidences of hydrocarbons in the region: