|Petroleum systems of the Tethyan region|
|Author||Lisa Marlow, Christopher C. G. Kendall, and Lyndon A. Yose|
The geology of the Arabian plate, North Africa and parts of Southern Eurasia are often discussed independent of one another. Our approach has been to step back and look at the region from a plate-scale geologic perspective. The present day Arabian and North Africa plates evolved together as a single unit from the Precambrian. They were physically connected as they moved around the globe from the southern polar regions to their present day position in the northern hemisphere, sub-tropical belt. Throughout the Phanerozoic, these plates have been on the margin of a series of Tethys seaways, including the Proto-Tethys, Paleo-Tethys, and Neo-Tethys. The common evolution of these plates along the Tethys Margin led to many similarities in the regional geology, including similar paleogeographic, tectonic, stratigraphic and depositional settings which, in turn, led to the development of similar petroleum systems. Though there are numerous similarities across the margin, variations occur too, and these can be compared in the chapters that follow. One primary difference across the region was the transition from uniform platform sedimentation which occurred across North Africa and Arabia, to intracontinental sub-basins and rift-basins (Arabian plate and North Africa, respectively) in the Permian, which led to “pots” of source rock across the Tethyan Margin.
|Area||Giant Fields: 500 MMboe< Recoverable Reserves (Proven+Probable) <5000 MMBOE||Supergiant Fields: Recoverable Reserves (Proven+Probable) >5000 MMBOE|
|Eastern Tethyan Margin (Countries approximately east of Dead Sea fault zone; including Syria, Jordan, Iraq, Iran, Turkey)||194||47|
|Western Tethyan Margin (Countries approximately west of Dead Sea fault zone)||45||2|
|Total Giant & Supergiant fields along Tethyan Margin||239||49|
Estimated recoverable reserves of countries along the Eastern and Western Tethyan Margins are shown in Figure 1. Many of the world’s giant and super-giant fields have been discovered in these various countries (Table 1). As shown in Table 1, these Tethyan countries account for 239 and 49 of the world’s giant and supergiant fields, respectively. There is a predominance of proven hydrocarbons on the former Eastern Tethyan Margin (Table 1). The Middle East region alone accounts for a significant amount of the global petroleum produced. The lack of giant and super-giant fields on the Western Tethyan Margin could in part be attributed to under exploration rather than less resource potential. Thus, identifying similarities across the Tethyan Margin and extrapolating known hydrocarbon systems to underexplored regions (onshore and offshore) will likely lead to more discoveries.
Paleozoic petroleum systems
During the Paleozoic, the Tethyan margin faced the Proto- and Paleo-Tethys Oceans and the paleogeography was primarily nonequitorial. Paleozoic sedimentation across the margin was dominantly siliciclastic with interbedded continental, fluvial-deltaic, transitional marine deposits and intermittent carbonates. Numerous proven and potential source-reservoir-seal rocks of Paleozoic age exist across the margin. 
The most prolific and regionally extensive Paleozoic source rock, is of Silurian age. At a sub-regional scale, source rocks are reported in nearly all of the other Paleozoic intervals (in places even Precambrian).  These regional and sub-regional proven source rocks are likely present in underexplored or undrilled areas across the entire Tethyan Margin. For example, there are proven Devonian-Carboniferous source rocks in Iraq and Turkey. The equivalent of this source rock may exist in other countries along the margin, but it is yet undocumented. Similarly, Precambrian source rocks that are responsible for most of the hydrocarbon accumulations in Oman may provide new play concepts in other areas. Cambrian reservoirs are documented in Jordan and Libya indicating additional potential for Cambrian or older source rocks. The presence and distribution of Paleozoic reservoirs may be used to help identify other working Paleozoic petroleum systems. The Cambrian reservoir Salib in Jordon can be attributed to what possible deeper source? Likewise the Permian reservoirs in the UAE were charged from what source? Were the Permian Triassic carbonates in Southern Fars and its contiguous offshore charged by much older source rocks? Tracking reservoirs and source rocks across the margin may lead to discovery of hydrocarbons in the underexplored rocks of the Alamein Basin and the Paleozoic section in northern Iraq-Kurdistan.
Mesozoic/Cenozoic petroleum systems
During the Mesozoic and Cenozoic, the Tethyan Margin faced the Neo-Tethys Sea whose paleogeography was primarily equatorial. At this time, sedimentation across the Tethyan Margin included dominantly shallow-marine carbonate-evaporite settings, some fluvial-deltaic systems and the advent of shallow (generally <200 meters) intra-shelf basins. Intra-shelf basins were commonly bordered by shallow-water carbonates and filled by carbonates and siliciclastics, including deposition of widespread organic rich sediments (source rocks). Collectively these systems resulted in the development of world class petroleum systems that are widespread across the Tethyan Margin. 
Jurassic and Cretaceous source rocks are important across the Tethyan Margin and account for the majority of the petroleum generated in Saudi Arabia, Iraq, Kuwait, Iran, Qatar, the United Arab Emirates, Libya and Turkey. Nearly all of the more than 137 Bbo and 106 Tcfg that occur in over 100 fields in Iraq have been generated from extremely organic-rich Jurassic and Cretaceous source rocks. The organic-rich Jurassic and Cretaceous source rocks extend into Syria. Jurassic source and reservoir are equally important in Saudi Arabia, where they have resulted in the world’s largest conventional oil field, Ghawar. Yemen, which remains an overall frontier area, has established basins (Sab’atayn and Say’un-Masilah) with proven Jurassic and Cretaceous source rocks where recent oil discoveries were made in the Upper Jurassic-Lower Cretaceous carbonates and clastics. Aptian/Albian source rocks in Iran have charged the oil in dominantly carbonate Cenomanian and Lower Miocene reservoirs in the Zagros Mountain anticlines; with these reservoirs accounting for an estimated 8% of global oil reserves. In Turkey, Middle Cretaceous source rocks are probably responsible for charging the majority of the reservoirs in this country. In Libya, the Campanian sedimentary section forms world class source rocks with great generative potential. Upper Cretaceous source rocks have sourced 85% of Libya’s 45 Bboe reserves. Egypt has multiple proven source rocks in the Mesozoic and Cenozoic sections. In Lebanon, numerous shows/seeps are likely generated from Mesozoic sources, while in Israel 16 oil and gas fields exist that have been sourced by dominantly Mesozoic source rocks. Triassic-Jurassic-Cretaceous source rocks that exists onshore of the Eastern Mediterranean      likely extend into the East Mediterranean-Levantine Basin.  In the southeastern part of the Levantine Basin, the Mango well tested Lower Cretaceous sands (10,000 bo/d); these sands are likely charged from Jurassic source rocks. 
Although Jurassic and Cretaceous source rocks are very important for the Tethyan region, there are other regionally correlative proven or potential source rocks of Triassic age.        Cenozoic source rock potential also exists in some deeper basins where the source rocks may be mature. Though there are many proven plays in the Mesozoic and Cenozoic across the Tethyan Margin, there are significant areas that remain undrilled. Correlating stratigraphy and hydrocarbon systems across the margin will undoubtedly facilitate the growth of new exploration ideas and new production.
Future potential and new approaches
Though the Tethyan Margin is a prolific hydrocarbon province, discovering additional hydrocarbons in mature basins and in frontier areas will need an integrated approach to exploration and development, including:
- Selective acquisition of new seismic and well data,
- Application of exploration concepts and the challenging of old dogmas,
- Application of new technology,
- Pursuit of unconventional resources,
- Extrapolation of onshore hydrocarbon systems to offshore areas, and
- Evaluation of new play types, i.e. subsalt and stratigraphic plays.
Notable examples that support these points include:
- Acquisition of 2-D and 3-D seismic surveys in the Levantine Basin that has been a key factor in exploration success and recent discoveries by Noble Energy.   The recent discoveries in Miocene plays in the Levantine Basin have led to changes in the USGS’s estimate of undiscovered reserves in the basin from zero to 122 Tcfg gas and 1.7 Bbo oil.   
- New exploration concepts and challenging dogmas will lead to discoveries in immature and mature basins. In the Levantine Basin there are numerous play types that have yet to be tested.    In the past, the Levantine basin was overlooked partly because it was thought to be underlain by oceanic crust. Challenging dogmas like this and other early assumptions can open up new areas for exploration. Lebanon is under-explored and has undiscovered potential, including the Triassic which is the top producer in Syria. In Upper Egypt, the Kom Ombo Basin has had recent discoveries of commercial oil in a basin roughly the size of the Gulf of Suez. Yemen has several frontier onshore basins including the San’a Basin that may have working petroleum systems in the Paleozoic and Mesozoic sections. The Rub’ Al-Khali Basin, part of which extends partially into Yemen, is an established petroleum basin in Saudi Arabia, however, it remains a frontier in Yemen.
- Incorporating technological advances can lead to exploration success and also improved recovery i.e. Qatar’s latest increase in oil production from the mid Cretaceous offshore Al Shaheen giant oil field utilized long horizontal wells to unlock previously recognized potential. Low- angle and multi-lateral wells are being successfully applied to increase productivity and recovery from low permeability carbonate reservoirs (e.g. microporous rocks) that are widespread across the Tethyan region and contain vast hydrocarbon accumulations.
- Unconventional resources are gaining increased importance in the Tethyan region. Oman is one of the first countries in the Tethyan region where unconventional resources are being actively pursued. Pilot wells into liquid-rich source rocks in the Precambrian and Cretaceous intervals have been drilled and are currently being tested. The Permian-Carboniferous Unayzah in Saudi Arabia is being tested for shale-gas. Tight-oil plays in fractured Jurassic and Upper Cretaceous source rocks have been tested in Kuwait and the plays could extend into Iraq and Iran. In Iraq, heavy oil accumulated in shallow reservoirs and at the surface. Upper Cretaceous also holds potential elsewhere across the margin, for example the Campanian Sirt Shale in Libya and the Campanian-Maastrichtian Shale in Jordon. Oil-shale exploration in Jordan is underway and according to a World Bank consultant on oil and energy, “utilised quantities of oil shale in Jordan can generate around 35 Bbo barrels of oil, more than the US oil reserves.” In Syria, the main Paleozoic, Triassic, Jurassic and upper Cretaceous/Eocene source-rocks which have fed the Palmyrides, Euphrates and NW corner hydrocarbon fields are potential shale oil and gas opportunities. In Egypt, unconventional plays are being pursued in the Western Desert in the Abu Gharadig Basin. Silurian source rock is a potential target for shale gas in many regions. The Silurian Tanezzuft Shale Formation in Libya and its equivalents in adjacent countries (including Tunisia and Algeria) is prospective and the Silurian Dadas Formation in Turkey is being drilled for unconventional resource. Candidate source rocks for unconventional resources are ubiquitous across the Tethyan Margin.
- Extension of onshore petroleum systems to the adjacent offshore can lead to exploration opportunities in the Eastern Mediterranean-Levantine Basin and Arabian Peninsula.      
- Historically, structural plays have been the focus of exploration in the Tethyan Region. Expanding exploration to include stratigraphic plays, pre-salt and drilling deeper targets may open up new exploration opportunities in discovered fields.
- Phanerozoic Tethys region
- Israel petroleum systems
- Libya hydrocarbon provinces
- Jordan petroleum geology
- Iraq petroleum geology
- Iran petroleum systems
- Petroleum basins of Turkey
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