SYRIA - Tectonic Background.

Syria lies in the western continental margin of the Arabian Plate. In pre-Mesozoic times, this Levantine margin developed along a subsidiary arm of the spreading Tethys in the north (not far from the Ionian Sea of our time) where a seaway was linked to the Indian Ocean. Several tectonic developments during the Paleozoic and great earth movements in the Early Mesozoic era brought about elevations in Syria called paleohighs. Fragments of the Early Paleozoic caused the Rutbah and Aleppo Highs. The former was an extension of the Hail-Jawf swell of Saudi Arabia and the latter extended from the Khleissia High of Iraq.

In later times the two paleohighs were separated by the Euphrates graben in a cross-plate shear zone across Syria and Iraq. A number of subsequently mobile zones developed between these highs. A thick Palmyrid aulacogene separated the Rutbah and Aleppo highs, as well as the Sinjar trough which today extends to Iraq. A shear zone along the Euphrates, together with a trough, developed at later times.

Several major movements during the Mesozoic and Cenozoic eras introduced new tectonic features in various parts of Syria, including the Levant Fracture trending from the Gulf of Aqaba into Turkey. Among these was basaltic volcanism spread on both sides of the fracture zone during the Late Neogene and Pleistocene. A plateau of basalt developed from north-western Jordan to the Houran region in southern Syria in an area of 45,000 sq km. This is where Total of France encountered a 500-metre thick layer of black basalt before it found oil from further drilling. It is in view of such tectonic developments that geologists now are concentrating on pre-Mesozoic horizons.

The Paleozoic: There are several interesting Paleozoic sequences in Syria, as well as in Turkey, Iraq and Jordan. Important among these are the older sequences in Jordan, from Ordovician to Silurian/Devonian layers which have sourced gas and condensate. But in Syria and Iraq, there are major unconformities between the Lower and Upper Paleozoic. Some of the Upper Silurian and all of the Devonian are missing in Syria.

Only a partial section of the Carboniferous developed in the Euphrates graben and the Mesopotamian Foredeep. In Iraq, the outcrop section consisting of the Zagros mountain range exposes a good development of Ordovician sediments. But all the Silurian, most of the Devonian and a major part of the Carboniferous and Lower Permian sequences are missing. An Upper Devonian shale sequence and a Lower Carboniferous carbonate sequence have been encountered in western Iraq.

Lower Silurian shales, widely believed to have sourced the huge gas reserves of the Late Permian Khuff in the Persian Gulf, are being examined in Syria. Apart from finds in Jordan, a Devonian light oil discovery made in the 1980s in southern Turkey is said to be sourced by these shales. Most of the Fertile Crescent (Levant) region has a favourable distribution of Silurian source rock and the potential for extensive hydrocarbon generation. The region to the west of the Jawf-Ga'ara-Khleissia axis, i.e., a major part of Syria, has good oil prospectivity because of shallow depth and relatively easy access for the drill.

The Mesozoic: An era of major tectonic movements, the Mesozoic in Syria began with a rich Triassic sequence of carbonates, mainly limestone and dolomite. Shales and evaporites developed in a succession called "Mulussa" or "Dolaa". There were also migrations from sands of the Late Permian age (close to the terminal Paleozoic). Good anhydrite seals developed, holding fairly large hydrocarbon accumulations.

Major hydrocarbon traps are found within the Triassic sequence. They consist of interbedded Mulussa carbonates, shales, and evaporites. They include the gas and condensate discoveries made by Marathon of the US in a block and adjacent areas of the Palmyrid Belt (the gas fields of Al Shaer, Cheriffe, Soukhne, etc.), as well as SPC's fields of heavy oil at the edge of the Palmyrid Basin. It is thought that the gas and condensate accumulations in the deeper part of the Palmyrid Basin are due to a greater maturity in the same source rock. The Triassic hydrocarbon occurrences of north-east Syria, north-western Iraq and south-eastern Turkey are sourced from interbedded shales in the Mesopotamian Foredeep.

Charge into many of the Triassic hydrocarbon occurrences is said to be a late event. Many of the traps in those Mesopotamian Foredeep areas were developed in the Late Neogene.

There are good evaporite seals under Syria's Triassic carbonates (reservoirs), both in the Palmyrid region and in the north-east. Oil discoveries have been made from Triassic sandstones in the east, said to be sourced from Late Permian sands.

As a result of slab-pull forces moving northwards at the beginning of the Upper Cretaceous, rifting took place in the Palmyra region and adjacent areas in Syria as well as in western Iraq. At the end of the Cretaceous, this led to collision between the Arabian Plate margin and the Bitlis-Poturge Massif in the west, and an Iranian arc in the east. A great deal of sedimentation occurred and a number of fairly large oil reservoirs developed during that age. Most of the Syrian oil discoveries made in 1956-1979 were from these reservoirs.