TURKEY - Geology.

With an area of 780,580 sq km, Turkey has many tectonic features. It has a narrow continental shelf along the Black Sea to the north, prospects for gas in the Marmara Sea and for oil and gas in shallow waters of the disputed Aegean Sea to the west, and oil basins in the south and south-east.

Tectonic features, together with hydrocarbon prospectivity, relate to pre-Mesozoic eras and a later phase of continental movements which affected the entire Middle East as well as big parts of Asia and Africa.

In the Levantine region, near today's Sea of Marmara and Thrace for example, an ancient seaway used to link the south-eastern shores of the European Plate to the Indian Ocean at a point not far from the Ionian Sea. There developed sedimentation at later phases.

In the Marmara Sea, there is the Thrace-Gallipoli Basin with natural gas reservoired in an Upper Eocene limestone called Sogucak, at a depth of 1,120 metres.

Developments during the Paleozoic and great earth movements in the Early Mesozoic brought about many types of sedimentation, such as the now oil-rich Zagros Fold Belt running all the way from Turkey to south-western Iran. They caused the Taurus/Zagros mountain ranges, and the spreading of the Tethys.

Subsequent events featured the Owen Fracture Zone in the Indian Ocean to the south, the separation of Arabia from Africa, and the Levant Fracture System from the Gulf of Aqaba up to the foothills of the Taurus range.

Sediments of various ages were then emplaced from Cyprus and south-western Turkey, through the Taurus and Zagros mountain ranges, down to Oman. The closure of the Tethys marked a phase of subduction, collision and compression, as well as the migration of ancient sediments to newer layers.

Most prominent among the results is the Hakkari Basin, which has sourced nearly all of Turkey's current oil production. A thrust belt crossing Turkey from Iran to the Mediterranean has a great deal of geological resemblance to the Overthrust Belt of the Rocky Mountains in the US. Amoco (now part of BP), a main operator in the Rockies, in the late 1980s chose the part of the Hakkari Basin in Turkey which was near the borders with Iran and Iraq as a most prospective area - but it was a politically dangerous zone in view of Kurdish guerrilla attacks there. Amoco also chose to explore areas at the western end of the belt, where the geology resembles the Prinos field off Greece in the disputed Aegean Sea.

Turkey consists of many tectonic faults and zones of severe earthquakes. Some of the most destructive earthquakes occur at the north-western end of the thrust belt near Istanbul and the south-west near Bay of Iskenderun, as well as in areas to the east of the Gulf of Izmir where hydrocarbon prospectivity has been indicated.

It is in the seven southern provinces (Hakkari, Si'irt, Mardin, Diyarbakir, Gaziantep, Hatay and Adana) that most prospects are probably located. The equivalent of Iran's Oligocene-Miocene Asmari limestones in Hakkari, which sourced many oilfields in Iran, occur at a depth of about 9,850 feet.

It is said the formation has not been drilled properly in Turkey, although it is partly equated with the older Midyat (or Hoya) limestones. Such limestones are found as source rocks in northern Iraq.

It was speculated in the late 1980s that Amoco's interest in Hakkari limestones was related to its experience in Iran during the 1960s, when it developed the Kharg group of oilfields. The related Asmari generally has poor matrix porosity. The rock is fractured within the folds of the Zagros Belt, has high fracture porosity, and an incredibly high permeability.

(In Iran the ultimate seal to the Asmari is the Gachsaran evaporite. It is a measure of the seal's efficiency that it has remained intact in the same Iranian structures, where the underlying limestone has become a highly fractured, high pressure reservoir).

Along the rest of the thrust belt, from Si'irt to the Adana Basin, several formations of the Triassic, Permian and Silurian/Ordovician systems are similar to, but much smaller than, the reservoirs in Syria and Iraq. However, the details of sedimentation in south-eastern Turkey are not the same.

Many of the fields along this belt, known as the Hakkari Basin, are producing from Upper and Middle Cretaceous formations. Most of the oil there is heavy and of high sulphur content.

Some medium oil, 30.5 deg. API, is produced from the Derdere formation of the Mardin group. Other formations of the Mardin group producing medium oil include the Turonian Karababa carbonates.

The Cretaceous Mardin formation is a fractured carbonate. It is the primary production zone for 32 deg. API oil being extracted in the Karakilise area close to the city of Diyarbakir. JKX Oil & Gas of the UK in 2005 framed into three onshore exploration licences in that area operated by Aladdin (see Part 2).

The Raman West field in Si'irt has in place about 2 billion barrels of 13.3 deg. API oil with 5.7% sulphur content, reservoired in Maestrichtian Garzan carbonates. But it can only be partly recovered by special enhanced oil recovery (EOR) methods which are very expensive.

There are Triassic carbonates of the Butmah formation locally called Camurlu. Such formations, together with the Campanian Karabogas, Boloka and others, can be found in Diyarbakir, Si'irt and Gaziantep. Some Turonian Karababa carbonates produce 35.3-36 deg. API oil in Gaziantep (District 12), where several discoveries have been made in the past 14 years.

There are carbonate formations of the Eocene, with the sediments developed before the Levant Fracture System. A Miocene reservoir produces oil in one field.

Triassic light oil and condensate, as well as Paleozoic light oil and gas, have been found but in small quantities. Some discoveries have been made in formations beneath ophiolites in the Amanus area, on both sides of the Levant Fracture called Kara Su Valley. Geologists also class this area as being within the sphere of the sinistral East Anatolian Fault and outside that of the Levant Fracture System.

Analysis of space imagery has indicated a splintering of the Levant Fracture from the Dead Sea towards the East Anatolian Fault. The splintering has deranged modern drainage systems, indicating that the region is still active.

In Diyarbakir, a discovery of light oil and condensate at Barbes made by Shell in 1972 has produced from a Devonian Katin sandstone reservoir. It is said to have been sourced by Ordovician/Silurian organic shales of the Bedinan formation.

These shales have sourced other minor occurrences in that region, including the surface seepages of the Hazro area, where the formation is a Lower Permian sandstone. But Bedinan shales are only partially preserved in south-eastern Turkey, stripped off extensively as a result of an uplift over the Mardin swell. Similar deposits extend widely in Syria, west of the Ga'ara-Khleisia uplifts.

The area from Hakkari to Gaziantep lies in the northern margin of the Arabian Plate. The margin differs in detail from those in the adjacent parts of Syria and Iraq. Its pattern of sedimentation began in the Late Cretaceous and continued.

In places, a hiatus occurs between the Cretaceous and Tertiary. Flysch-type deposits are of continental clastics, greywackes, shales of the Germav formation, and reef complexes of the Sinan formation, all deposited during the Paleocene.

In the Early Eocene, a regression occurred giving rise to the Beciraman formation. Over-lain by a red bed sequence of the Gercus formation, the Beciraman is believed to represent a marine off-lap corresponding to continental beds related to extensive uplifts to the north and east.

A regional transgression followed, giving rise to shallow carbonates of the Midyat (Hoya) formation, of the Middle-Upper Eocene. This now represents a stable-shelf condition over the whole region.

Lower Miocene sediments of the Firat, of a Euphrates graben's formation where Shell operates on the Syrian side, overlie the Midyat and Germik, indicating structure building movements in the region. They later controlled Miocene sedimentation.

In the northern part, the major structures were capped by carbonates of the Miocene Silvan formation, separating an intra-deep basin and a subcontinental platform to the south, where the Mio-Pliocene clastic Selmo formation was laid down.

The intra-deep basin, between the foothill belt and the Taurus ridge, was filled with a flysch sequence called Lice formation.