Understanding the ZagrosAuthor Andrew Horbury

Tags: Middle East Exploration Issue 6, Volume 9, 2012

The Zagros fold-thrustbelt is one of the oldest studied and exploited petroleum provinces of the world, yet for mostly political reasons it has not been fully developed and its exploration has generally taken place in a very piecemeal fashion. Attempts are now being made to develop play concepts that may encompass the whole orogenic belt.

Asmari carbonates above Pabdeh marls, Tang-e Gurguda, adjacent to Gachsaran oilfield, showing progradation of slope systems from top left down to bottom right. These Oligocene-age carbonates form the main shallow reservoir system of the Masjid-i-Suleiman, Kirkuk and Gachsaran fields, amongst others. Source: Neil Pickard

Aside from historical occurrences of extraction of petroleum from the basin, the initial modern commercial discoveries of petroleum in Iran (then Persia) at Masjid-i-Suleiman in May 1908 and Iraq (Kirkuk, October 1927) marked the beginning of a period of prolific development. Production was initially from Cenozoic reservoirs and during this early phase of exploration there was considerable interchange of pertinent information between the Iranian (mostly Anglo-Persian, later BP) and Iraqi operations (IPC, a consortium mostly comprising elements of the ‘Seven Sisters’).

Some remarkably insightful pieces of work, which can be considered industry ‘firsts’, emerged from the region during this early phase of work. Probably the most significant of these were Dunnington’s (1958) review of the petroleum geology of northern Iraq, Henson’s studies of larger benthic foraminifera (1948) and his 1950 paper on reef distribution as influenced by changes in relative sea-level, which preceded the Vail model by 24 years. Similarly, the pioneering use of microfacies and

micropaleontology in calibrating reservoir-scale models of Oligo-Miocene carbonates of the region, pre-dating the carbonate microfacies ‘classics’ such as Wilson (1975) and Flugel (1982), was developed in the works of Thomas (1950) and van Bellen (1956). On a production scale, the history of the development of the Masjid-i-Suleiman field by Gibson (1948) is a remarkable account of the early trial-and-error development of the first-discovered field in the region; whilst later work of Daniel (1954) and McQuillan (1970s) on mesostructures has proven critical in the understanding of production-scale fracture networks.

Mid-Oligocene palaeogeography ‘Highstand above the Pg40 MFS of Sharland et al., 2001’ showing extent of basin and adjacent shelf systems, produced by integrating data from the whole Zagros orogenic belt. Source: Cambridge Carbonates Ltd

Complex Lithostratigraphy

During this phase, however, there was a tendency for over-complex lithostratigraphic schemes to develop, that reflected differences between subsurface and outcrop, and the individual license areas. For example, in Iraq the Campanian-Maastrichtian shelf formations were named the Hartha and Tayarat in the Basra area and to the west, Pilsener in the Mosul area, but Bekhme and Aqra in outcrop areas of the Iraqi Zagros; all are essentially the same unit/facies. Formations of the same age in Iran and Turkey are named differently again. Any along-strike integration of models and nomenclature was cut short by the nationalization of IPC in 1961, and was then further hindered by the Iranian Revolution of 1979, followed by the various major conflicts in the region.

By this time, exploration had extended stratigraphically into the mid-Cretaceous reservoirs, of which the understanding was relatively good, but deeper Cretaceous, Jurassic and Triassic/Permian reservoirs were relatively poorly understood. Consequently, and particularly for these deeper formations, national

industries tended to develop their own ‘in house’ models. There was also much re-naming of formations for political reasons (such as the Lower Fars Formation being re-named Gachsaran in Iran, and Fat’ha in Iraq) that often veered away from the possibly more integrated earlier stratigraphic models. Several events in the recent past have caused a reverse in this ‘local’ thinking. Firstly, the re-opening of the Iranian petroleum industry to western companies in the mid-1990s until the late-2000s, and the later opening of the Iraqi petroleum industry in the mid-2000s (ironically at a time when it was becoming more difficult to work in Iran). Added to this, a more plate-wide approach in terms of both fold-thrust belt tectonics and stratigraphy (e.g. Alsharhan and Nairn, 1997; Sharland et al., 2001) emerged at about the same time. Whole-country syntheses, such as Jassim and Goff (2006) and Aqrawi et al. (2010) have also assisted in regional-scale integration of data. In addition, the ability to build large map-based datasets in ArcGIS and Petrel has allowed a properly integrated view of the margin, possibly for the first time.

Mish anticline, a typically steep-sided Zagros whaleback fold, adjacent to the Gachsaran oilfield. Source: Neil Pickard

Inversion Structures

In terms of structural geology, a big factor that has proven to be of importance along the whole Zagros fold-thrustbelt is the absence of any significant long-distance overthrusting, or development of complex structures, like imbricated thrusted stacks with internal seals, which are successful in other fold-thrustbelts such as the gas plays of the Canadian Rockies. Most folds are very simple, steep-sided whalebacks. The consequence of this is that play types are relatively simple and rely upon the preservation of only one or two regional seals.

Another feature that has emerged relatively recently is that there are wide areas of the folded zone where anticlines have a largely inversion origin, after reverse movement of early normal faults of Triassic and/or Late Cretaceous age. Fields such as Ain Zalah (norh-west Iraq) and structures like Jebels Sinjar and Abd-el Aziz (north-west Iraq and north-east Syria respectively) fall into this category. Development of inversion structures has important consequences for play development because wells placed on the anticline crest are, by definition, testing former basin-center facies (e.g. the fractured

basinal limestones that produce at Ain Zalah). Meanwhile, traditionally better reservoir facies may instead be developed on untested footwall blocks, where reserves could be present within stratigraphic or combination traps. Thus, a full investigation of the origins of individual structures may reveal many details that impact prospectivity; it should not be assumed that every structure is a purely late Neogene feature with no history of earlier deformation.

Pir-i-Mugrun Mountain in Iraqi Kurdistan, the significance of which to the understanding the Middle Cretaceous basin margin was first noted by Henson (1950). These carbonates and their Iranian equivalents of the Sarvak form a deeper reservoir system in the Jambur, Kirkuk and Gachsaran fields, amongst others. Source: Saad Jassim

Significant Unconformities

The structural complexity of the Zagros belt and identification of major phases of generally extensional basin development and Cenozoic foreland basin evolution have important consequences for stratigraphic development and thus reservoir distribution. Basin extension occurred in the mid-Permian, intra-Triassic, mid-Jurassic and latest Jurassic/earliest Cretaceous, with strike-slip passing into more compressional basins during the late Cretaceous. By way of contrast to the ‘stable’ Arabian Plate interior stratigraphy of countries such as Saudi Arabia and U.A.E., the Zagros margin shows the development of many very significant unconformities. These both destroy reservoir potential by removing regionally-widespread reservoir and source rocks, such as much of the Middle and Late Jurassic, as well as provide new reservoir potential where karstification of carbonates has enhanced reservoir properties, as in the Oligocene reservoirs of the Kirkuk Field.

Indeed, it can perhaps be said that the major influence on much of the stratigraphy and petroleum potential of complex areas, such as northern Iraq, are the unconformities and the way they interact with structural development; and in the often thick and localized formations that were then deposited in the intervening basins. Thus, a lack of appreciation of the significance of these unconformities and consequent careless application of formation names has led to extreme confusion in stratigraphies that have been applied to some of the more recent wells, particularly in Iraqi Kurdistan. For example, on some Late Cretaceous fault footwalls, up to 2 km of stratigraphy can be eroded (note the blocks of Carboniferous or possibly older olistoliths sitting in the Maastrichtian basin fill of Jebel Abd-El Aziz in Syria); whilst lower-relief regional erosion has removed much of the Jurassic and later Triassic from regional paleohighs such as in the Khleisia-Mosul area.

Promising Iraqi Kurdistan

Probably of most interest to the oil industry at the present-day is Iraqi Kurdistan. Licensing here is undertaken in an environment that, in general, can be considered more favorable than has been offered

in neighboring areas over the past fifty years, confirmed by the very high uptake in license areas and the general high level of activity. In terms of reservoirs and stratigraphy, Iraqi Kurdistan offers an interesting insight into the use of along-strike analogs because both the exploration history of south-east Turkey and south-west Iran are relevant. Parts of the Turkish petroleum province extend into Iraq and indeed the 2006 Tawke discovery in northern Kurdistan can be seen as having more in common with some of the Turkish fields than with fields in adjacent parts of Iraq.

By contrast, the main part of Iraqi Kurdistan is proving to have much in common with many of the deeper Iranian plays, in that fields such as Kabir Kuh and Veyzenhar, which produce gas and condensate from Liassic-Triassic strata, clearly function in the Iraqi Kurdistan region. Recent discoveries have been reported from these reservoir units in the Miran West and Shaikan structures. Other plays that have proven to be worthwhile are shallower reservoirs at Pila Spi (Eocene) and Shiranish (Campanian-Maastrichtian) level, originally suggested by old discoveries considered sub-commercial, such as Chemchemal and Taq Taq. However, these only function properly in the outer parts of the foldbelt because they ideally require the presence of Lower Fars caprocks. In the inner parts of the foldbelt these are absent, so viable plays require lower regional seals – usually Middle–Upper Jurassic and Lower Cretaceous bituminous marls and shaly limestones – to be present and not breached.

Mountains near Erbil in Iraqi Kurdistan. Source: Genel Energy

Developing Play Concepts

Other reservoir types that have emerged in recent years include the regionally important hydrothermal dolomite bodies. The importance of these was first recognized in Iran, with the discovery and appraisal of the Azar field, based in large part on analogs of the dolomitized Sarvak Formation in the Anaran Mountains (Sharp et al. 2005, 2006, 2010). This has led to a revision of examples of unusual dolomitization which can be listed, beginning with the dolomitization of the Shiranish Formation in Taq Taq, and massive late diagenetic dolomitization of the Qamchuqa Formation along much of the Iraqi fold-thrustbelt. Identification of these dolomite reservoirs pre-drill requires some degree of good

fortune because commonly seismic data quality is not good over anticlinal areas and may not be sufficient to determine rock properties at depth. It would appear that the dolomite bodies developed during the main Neogene compression of the basin. This is similar to high temperature dolomites elsewhere (e.g. Jurassic and Cretaceous dolomite reservoirs of southern Mexico) and it is likely that their formation was in response to orogenic breaching of regional seals and mobilization of deep formation waters rich in magnesium that had been trapped for millions of years in a passive margin setting.

Finally, our understanding of the petroleum systems has developed significantly since the 1958 publication of Dunnington’s masterly review. Dunnington recognized the overriding importance of the Middle Jurassic-Lower Cretaceous source rocks, but subsequent exploration and advances in organic geochemical analysis have highlighted the importance of other sources, not least of which is the Silurian, critical for gas generation and responsible for the success of Khuff reservoirs in the foreland basin. More recent work on the Triassic in northern Iraq, Syria and south-east Turkey has demonstrated an active petroleum system in that part of the stratigraphy, which is ripe to be tested in other parts of the orogenic belt. In fact, the main issue with understanding hydrocarbon migration in the basin is that it may prove to be very difficult to unravel the relative importance and significance of each system because there is likely to be a high degree of superimposition and mixing of active petroleum systems of different ages and types within any given area.

Thus, the present time is the ideal opportunity for the development of play concepts that may encompass the whole orogenic belt. In this regard a forthcoming conference at the Geological Society of London (January 23–25, 2013) aims to address issues where concepts developed in one part of the Zagros fold-thrustbelt can be applied to exploration elsewhere, as well as presenting the first glimpses of new data that are emerging from the first thorough exploration of the Kurdistan region of Iraq.

It is to be hoped that lessons learnt from present Kurdistan exploration can in the future be applied to analogous areas of Turkey and Iran which at present see little activity.

For further information on the structural geology of the Zagros Mountains, see GEO ExPro Vol. 9, No. 1 Share on email Share on facebook Share on twitter More Sharing Services 0

Understanding the ZagrosAuthor Andrew Horbury

Tags: Middle East Exploration Issue 6, Volume 9, 2012

The Zagros fold-thrustbelt is one of the oldest studied and exploited petroleum provinces of the world, yet for mostly political reasons it has not been fully developed and its exploration has generally taken place in a very piecemeal fashion. Attempts are now being made to develop play concepts that may encompass the whole orogenic belt.

Asmari carbonates above Pabdeh marls, Tang-e Gurguda, adjacent to Gachsaran oilfield, showing progradation of slope systems from top left down to bottom right. These Oligocene-age carbonates form the main shallow reservoir system of the Masjid-i-Suleiman, Kirkuk and Gachsaran fields, amongst others. Source: Neil Pickard

Aside from historical occurrences of extraction of petroleum from the basin, the initial modern commercial discoveries of petroleum in Iran (then Persia) at Masjid-i-Suleiman in May 1908 and Iraq (Kirkuk, October 1927) marked the beginning of a period of prolific development. Production was initially from Cenozoic reservoirs and during this early phase of exploration there was considerable interchange of pertinent information between the Iranian (mostly Anglo-Persian, later BP) and Iraqi operations (IPC, a consortium mostly comprising elements of the ‘Seven Sisters’).

Some remarkably insightful pieces of work, which can be considered industry ‘firsts’, emerged from the region during this early phase of work. Probably the most significant of these were Dunnington’s (1958) review of the petroleum geology of northern Iraq, Henson’s studies of larger benthic foraminifera (1948) and his 1950 paper on reef distribution as influenced by changes in relative sea-level, which preceded the Vail model by 24 years. Similarly, the pioneering use of microfacies and micropaleontology in calibrating reservoir-scale models of Oligo-Miocene carbonates of the region, pre-dating the carbonate microfacies ‘classics’ such as Wilson (1975) and Flugel (1982), was developed in the works of Thomas (1950) and van Bellen (1956). On a production scale, the history of the development of the Masjid-i-Suleiman field by Gibson (1948) is a remarkable account of the early trial-and-error development of the first-discovered field in the region; whilst later work of Daniel (1954) and McQuillan (1970s) on

mesostructures has proven critical in the understanding of production-scale fracture networks.

Mid-Oligocene palaeogeography ‘Highstand above the Pg40 MFS of Sharland et al., 2001’ showing extent of basin and adjacent shelf systems, produced by integrating data from the whole Zagros orogenic belt. Source: Cambridge Carbonates Ltd

Complex Lithostratigraphy

During this phase, however, there was a tendency for over-complex lithostratigraphic schemes to develop, that reflected differences between subsurface and outcrop, and the individual license areas. For example, in Iraq the Campanian-Maastrichtian shelf formations were named the Hartha and Tayarat in the Basra area and to the west, Pilsener in the Mosul area, but Bekhme and Aqra in outcrop areas of the Iraqi Zagros; all are essentially the same unit/facies. Formations of the same age in Iran and Turkey are named differently again. Any along-strike integration of models and nomenclature was cut short by the nationalization of IPC in 1961, and was then further hindered by the Iranian Revolution of 1979, followed by the various major conflicts in the region.

By this time, exploration had extended stratigraphically into the mid-Cretaceous reservoirs, of which the understanding was relatively good, but deeper

Cretaceous, Jurassic and Triassic/Permian reservoirs were relatively poorly understood. Consequently, and particularly for these deeper formations, national industries tended to develop their own ‘in house’ models. There was also much re-naming of formations for political reasons (such as the Lower Fars Formation being re-named Gachsaran in Iran, and Fat’ha in Iraq) that often veered away from the possibly more integrated earlier stratigraphic models. Several events in the recent past have caused a reverse in this ‘local’ thinking. Firstly, the re-opening of the Iranian petroleum industry to western companies in the mid-1990s until the late-2000s, and the later opening of the Iraqi petroleum industry in the mid-2000s (ironically at a time when it was becoming more difficult to work in Iran). Added to this, a more plate-wide approach in terms of both fold-thrust belt tectonics and stratigraphy (e.g. Alsharhan and Nairn, 1997; Sharland et al., 2001) emerged at about the same time. Whole-country syntheses, such as Jassim and Goff (2006) and Aqrawi et al. (2010) have also assisted in regional-scale integration of data. In addition, the ability to build large map-based datasets in ArcGIS and Petrel has allowed a properly integrated view of the margin, possibly for the first time.

Mish anticline, a typically steep-sided Zagros whaleback fold, adjacent to the Gachsaran oilfield. Source: Neil Pickard

Inversion Structures

In terms of structural geology, a big factor that has proven to be of importance along the whole Zagros fold-thrustbelt is the absence of any significant long-distance overthrusting, or development of complex structures, like imbricated

thrusted stacks with internal seals, which are successful in other fold-thrustbelts such as the gas plays of the Canadian Rockies. Most folds are very simple, steep-sided whalebacks. The consequence of this is that play types are relatively simple and rely upon the preservation of only one or two regional seals.

Another feature that has emerged relatively recently is that there are wide areas of the folded zone where anticlines have a largely inversion origin, after reverse movement of early normal faults of Triassic and/or Late Cretaceous age. Fields such as Ain Zalah (norh-west Iraq) and structures like Jebels Sinjar and Abd-el Aziz (north-west Iraq and north-east Syria respectively) fall into this category. Development of inversion structures has important consequences for play development because wells placed on the anticline crest are, by definition, testing former basin-center facies (e.g. the fractured basinal limestones that produce at Ain Zalah). Meanwhile, traditionally better reservoir facies may instead be developed on untested footwall blocks, where reserves could be present within stratigraphic or combination traps. Thus, a full investigation of the origins of individual structures may reveal many details that impact prospectivity; it should not be assumed that every structure is a purely late Neogene feature with no history of earlier deformation.

Pir-i-Mugrun Mountain in Iraqi Kurdistan, the significance of which to the understanding the Middle Cretaceous basin margin was first noted by Henson (1950). These carbonates and their Iranian equivalents of the Sarvak form a deeper reservoir system in the Jambur, Kirkuk and Gachsaran fields, amongst others. Source: Saad Jassim

Significant Unconformities

The structural complexity of the Zagros belt and identification of major phases of generally extensional basin development and Cenozoic foreland basin evolution have important consequences for stratigraphic development and thus reservoir distribution. Basin extension occurred in the mid-Permian, intra-Triassic, mid-Jurassic and latest Jurassic/earliest Cretaceous, with strike-slip passing into more compressional basins during the late Cretaceous. By way of contrast to the ‘stable’ Arabian Plate interior stratigraphy of countries such as Saudi Arabia and

U.A.E., the Zagros margin shows the development of many very significant unconformities. These both destroy reservoir potential by removing regionally-widespread reservoir and source rocks, such as much of the Middle and Late Jurassic, as well as provide new reservoir potential where karstification of carbonates has enhanced reservoir properties, as in the Oligocene reservoirs of the Kirkuk Field.

Indeed, it can perhaps be said that the major influence on much of the stratigraphy and petroleum potential of complex areas, such as northern Iraq, are the unconformities and the way they interact with structural development; and in the often thick and localized formations that were then deposited in the intervening basins. Thus, a lack of appreciation of the significance of these unconformities and consequent careless application of formation names has led to extreme confusion in stratigraphies that have been applied to some of the more recent wells, particularly in Iraqi Kurdistan. For example, on some Late Cretaceous fault footwalls, up to 2 km of stratigraphy can be eroded (note the blocks of Carboniferous or possibly older olistoliths sitting in the Maastrichtian basin fill of Jebel Abd-El Aziz in Syria); whilst lower-relief regional erosion has removed much of the Jurassic and later Triassic from regional paleohighs such as in the Khleisia-Mosul area.

Promising Iraqi Kurdistan

Probably of most interest to the oil industry at the present-day is Iraqi Kurdistan. Licensing here is undertaken in an environment that, in general, can be considered more favorable than has been offered in neighboring areas over the past fifty years, confirmed by the very high uptake in license areas and the general high level of activity. In terms of reservoirs and stratigraphy, Iraqi Kurdistan offers an interesting insight into the use of along-strike analogs because both the exploration history of south-east Turkey and south-west Iran are relevant. Parts of the Turkish petroleum province extend into Iraq and indeed the 2006 Tawke discovery in northern Kurdistan can be seen as having more in common with some of the Turkish fields than with fields in adjacent parts of Iraq.

By contrast, the main part of Iraqi Kurdistan is proving to have much in common with many of the deeper Iranian plays, in that fields such as Kabir Kuh and Veyzenhar, which produce gas and condensate from Liassic-Triassic strata, clearly function in the Iraqi Kurdistan region. Recent discoveries have been reported from these reservoir units in the Miran West and Shaikan structures. Other plays that have proven to be worthwhile are shallower reservoirs at Pila Spi (Eocene) and Shiranish (Campanian-Maastrichtian) level, originally suggested by old discoveries considered sub-commercial, such as Chemchemal and Taq Taq. However, these only function properly in the outer parts of the foldbelt because they ideally require the presence of Lower Fars caprocks. In the inner parts of the foldbelt these are absent, so viable plays require lower regional seals – usually Middle–Upper Jurassic and Lower Cretaceous bituminous marls and shaly limestones – to be present and not breached.

Mountains near Erbil in Iraqi Kurdistan. Source: Genel Energy

Developing Play Concepts

Other reservoir types that have emerged in recent years include the regionally important hydrothermal dolomite bodies. The importance of these was first recognized in Iran, with the discovery and appraisal of the Azar field, based in large part on analogs of the dolomitized Sarvak Formation in the Anaran Mountains (Sharp et al. 2005, 2006, 2010). This has led to a revision of examples of unusual dolomitization which can be listed, beginning with the dolomitization of the Shiranish Formation in Taq Taq, and massive late diagenetic dolomitization of the Qamchuqa Formation along much of the Iraqi fold-thrustbelt. Identification of these dolomite reservoirs pre-drill requires some degree of good fortune because commonly seismic data quality is not good over anticlinal areas and may not be sufficient to determine rock properties at depth. It would appear that the dolomite bodies developed during the main Neogene compression of the basin. This is similar to high temperature dolomites elsewhere (e.g. Jurassic and Cretaceous dolomite reservoirs of southern Mexico) and it is likely that their formation was in response to orogenic breaching of regional seals and mobilization of deep formation waters rich in magnesium that had been trapped for millions of years in a passive margin setting.

Finally, our understanding of the petroleum systems has developed significantly since the 1958 publication of Dunnington’s masterly review. Dunnington recognized the overriding importance of the Middle Jurassic-Lower Cretaceous

source rocks, but subsequent exploration and advances in organic geochemical analysis have highlighted the importance of other sources, not least of which is the Silurian, critical for gas generation and responsible for the success of Khuff reservoirs in the foreland basin. More recent work on the Triassic in northern Iraq, Syria and south-east Turkey has demonstrated an active petroleum system in that part of the stratigraphy, which is ripe to be tested in other parts of the orogenic belt. In fact, the main issue with understanding hydrocarbon migration in the basin is that it may prove to be very difficult to unravel the relative importance and significance of each system because there is likely to be a high degree of superimposition and mixing of active petroleum systems of different ages and types within any given area.

Thus, the present time is the ideal opportunity for the development of play concepts that may encompass the whole orogenic belt. In this regard a forthcoming conference at the Geological Society of London (January 23–25, 2013) aims to address issues where concepts developed in one part of the Zagros fold-thrustbelt can be applied to exploration elsewhere, as well as presenting the first glimpses of new data that are emerging from the first thorough exploration of the Kurdistan region of Iraq.

It is to be hoped that lessons learnt from present Kurdistan exploration can in the future be applied to analogous areas of Turkey and Iran which at present see little activity.

For further information on the structural geology of the Zagros Mountains, see GEO ExPro Vol. 9, No. 1 Share on email Share on facebook Share on twitter More Sharing Services 0

The Zagros UpliftAuthor Rasoul Sorkhabi, Ph.D.

Tags: Middle East Giant Fields Issue 1, Volume 9, 2012

“We climbed for most of that day, conquering step by step the Morvarid Pass, only to drop down again, having reached the top; and as evening fell we came down on the lovely valley of Deh Diz, with its single sentinel poplar and a ruined castle in the distance, and the long ridge of the snowy Kuh-e Mangasht beyond. Our camping-place this time was in an orchard of pomegranates, beside a clear mountain stream, on a grassy terrace strewn with rocks and boulders.”

Vita Sackville-West, Twelve Days in Persia: Across the Mountains with Bakhtiari Tribe (1928)

A summit of Zard Kuh (‘Yellow Mountain’ in Persian), 3,972m, in the High Zagros, exposing Jurassic and Cretaceous carbonate sediments dipping north-east. Source: Dr. M. Fakhari

The quote above from a famed British writer’s travelogue is an expression of mankind’s natural attraction to the majesty and serenity of high mountain landscapes. The Zagros Mountains form an important segment of the Alpine-Himalayan orogenic (‘mountainbuilding’) system that originated from the closure of the Neo-Tethys Ocean and the collision of Africa-Arabia-India continental plates with Eurasia. Studies of Zagros are important for a variety of reasons. Firstly, this active mountain range of colliding continents provides a natural laboratory to investigate how mountains of similar type form. Secondly, the foreland basins of Zagros are home to some of the world’s largest oil and gas reserves, and Zagros outcrops exhibit these subsurface petroleum source-reservoir-cap rocks for direct observations at reservoir scale. Thirdly, the Zagros uplift exerts enormous impacts on the climate, landscape development, ecology, earthquake hazards, and cultural history of south-west Asia.

The Anatomy of Zagros

Anatomy helps us to understand the pathways of evolution, in geology as in biology. Mapping of the Zagros Range over the past century has led geologists to divide these mountains into several tectonic belts which run, for most part, parallel to one another along the north-north-west to south-south-east strike of Zagros. These belts are separated by major, generally north-east dipping, thrusts

or reverse faults. Internal structural complexity in each tectonic zone definitely exists and detailed examination of many areas remains to be conducted. Nevertheless, the following description drawn on from the works of Jovan Stocklin, Norman L. Falcon, Manuel Berberian, and Mehdi Alavi offers a simplified framework of Zagros, useful for understanding its geologic history and architecture.

The Urumieh-Dokhtar magmatic belt (50-80 km wide) lies to the north-east of Zagros and is composed of plutonic and volcanic rocks which largely formed from the subduction of Neo-Tethys oceanic floor beneath the Central Iran block. Magmatic activity began in the Middle Cretaceous and climaxed in the Eocene (50-35 Ma) but it has continued sporadically to the present day. In places, these igneous bodies are thrust north-eastward over the Central Iranian Plateau. The south-western boundary of the Urumieh-Dokhtar belt, mapped as a thrust zone, is considered by some geologists to be the ‘suture zone’ (continental plate boundary) between the Arabian and Asian plates.

Location of the Zagros Mountains in the plate tectonic framework of the Middle East. The plateau of Iran is sandwiched by the Zagros on south-west and the Alborz and other ranges to the north and north-east. The northern belt initially formed by the closure of the Paleo-Tethys Ocean during the Triassic-Jurassic (but was reactivated in the Cenozoic). The Zagros belt was produced by the closure of Neo-Tethys. The south-east boundary of Zagros is often considered to be a fault zone in the Strait of Hormuz (the ‘Oman Line’); its north-west boundary, however, is not clearly defined as it merges with several other mountain structures. Geographically, the Urumieh Lake may be considered as the north-west end of Zagros, which then makes this mountain range about 1,500 km in length, but some geologists, on tectonic grounds, extend it some 500 km further north-west to the East Anatolian strike-slip fault - Arabian plate velocities with respect to a fixed Eurasia from Sella et al. (2002, Journal of Geophysical Research, 107,B4: 2081) and Vernant et al. (2004, Geophysical Journal International, 157: 381-398) Source: Satellite image: NASA; Structural map: Rasoul Sorkhabi

The Sanandaj-Sirjan belt, named after the two towns to the northwest and south-east respectively, is a highly-deformed and relatively wide (about 150 km) zone which consists of overthrust Paleozoic and Mesozoic sediments (mostly

deformed and metamorphosed), and volcanic and granitic rocks. The tectonic origin of this belt is related to the closing of Neo-Tethys.

The High Zagros or Zagros Imbricated Zone is a topographic high and a relatively narrow zone (up to 80 km) of imbricated thrust faults. This zone is bounded on the north-east by the Main Zagros Thrust (or Main Zagros Reverse Fault, initially called “Zagros Crush Zone”), which many geologists have regarded as the Arabia-Asia suture zone because its hangingwall are the Cretaceous-age ophiolites (ocean-floor volcanic rocks) and deep ocean sediments of Neo-Tethys. There is apparently little seismic activity assignable to this thrust zone today. However, the north-western strands of the structure, mapped as the Main Recent Fault, show right-lateral strike movements of up to 200 km. The High Zagros is thrust over the Simply Folded Belt along the High Zagros Thrust, which is exposed to the surface.

The Simply Folded Belt (also called the Zagros Folded Belt), at up to 300 km the widest zone of Zagros, is dominated by numerous large ‘whale-back’ and small anticlines as well as localised thrust faults which trend NNW-SSE parallel to the general strike of Zagros. These anticlines are renowned for their petroleum accumulations. The folding mainly occurred during Late Miocene and Early Pliocene times. This belt has also been affected by highly active, north-south trending wrench faults (strike-slip faults across the zone), the largest of which is the Kazerun Fault. The Simply Folded zone is divided into two arc-shaped areas or salients, the Lurestan (or Posht Kuh) arc to the north-east and the much larger Fars arc to the south-west. The Mountain Frontal Fault (Flexure) marks the southwest boundary of the Simply Folded Belt.

Associated with the Simply Folded Belt, there are two particular blocks, the Kirkuk embayment to the north-west and the Dezful embayment to the southeast, which deserve special attention because they contain a large number of oil fields. Although these embayments or promontories have anticlines and thrust structures like the Lurestan and Fars salients, they have experienced much less uplift and erosion. Bounded by active strike-slip faults, they appear as re-entrants in front of the Simply Folded Zagros, and their origin is a matter of debate among geologists studying Zagros.

The Zagros Deformation Front (also called the Zagros Foredeep Fault) separates the Zagros orogenic belt from the foredeep basins of the Mesopotamia-Persian Gulf which currently receive enormous amounts of sediments from Zagros as well as from the Taurus Mountains in southern Turkey.

As in many other fold-and-thrust belts, the faults that have sculptured the Zagros Mountains are joined at depth to a basal docollement (detachment) fault. In the case of Zagros, this basal decollement is often considered to be the ductile (low-friction) layer of Hormuz Salt at the base of the Phanerozoic sediments. The uniform features and parallelism of anticlines in the Simply Folded Belt indicates such a common detachment horizon at depth. This notion suggests a ‘thin-skinned’ tectonic style in which the deformed sedimentary cover is detached from the underlying Precambrian basement. While this appears to be the case

especially in the Simply Folded Belt, it is conceivable that basement faulting in the High Zagros and beyond has also taken place at deeper levels but that these structures have not emerged to the surface. This is supported by the distribution of earthquake hypocentres. Moreover, the areal extent of the Hormuz salt in the entire Zagros region is not well known; it is possibly absent or insignificant in the north-western parts of Zagros.

Counter-clockwise plate motion of the Arabian Plate from its position in Gondwana at 200 Ma (Early Jurassic) across the Neo-Tethys Ocean to the Cenozoic collision with the Iranian block (Eurasian Plate) to produce the Zagros Mountains. Le Pichon et al. (1988, Geological Society of America Special Paper 218, p. 111-131), who analysed this plate motion history, estimate about 5,700 km of Neo-Tethys ocean floor subduction (since 190 Ma) and 1,020 km of continental collision between Africa-Arabian and Eurasian plates (since 110 Ma). Source: Rasoul Sorkhabi based on Le Pichon et al., 1988)

Acts of a Zagros Drama

Precambrian basement rocks have not been exposed in Zagros and previous suggestions of Proterozoic metamorphic rocks or granite bodies have been refuted by new data. The deepest wells drilled in the Zagros basin have not reached the Proterozoic basement. The oldest rocks exposed in the High Zagros are of Lower Cambrian sediments at the base of major thrust zones indicating that these structures have cut through the entire Paleozoic-Mesozoic succession. Besides these, the emergent plugs of the Hormuz Salt, abundantly in the Simply Folded Belt, have brought debris of Infracambrian sediments and metamorphic rocks to the surface.

The story of Zagros may be described in five acts of a geologic drama spanning the past 500 million years or so.

Act 1, the longest, is the deposition of vast amounts of sediments on the passive-margin, continental shelf and slope of the Tethys Ocean when Arabia was part of the supercontinent of Gondwana in the southern hemisphere. The thickness of these Paleozoic-Mesozoic sediments is 4,000-7,000m.

Act II begins in the Middle Jurassic when Africa-Arabia started to split from Gondwana and the Tethys oceanic floor, being denser, began to subduct beneath the Central Iranian continental block. The partial melting of the subducting oceanic slab produced an Andean-type igneous arc on the Iranian block. In the Late Cretaceous, compressional stresses produced the first deformation in Zagros, notably in the obduction (overthrusting) of ocean-floor slivers onto the marine sediments.

Act III is the head-on continental collision between Arabia and Central Iran, pushing the ophiolites further south-west. Geologists have no consensus on the age of the Arabia-Asia collision partly because of the scarcity of data (especially paleomagneticgeochronlogic data from key areas) and partly because of different concepts of ‘continental collision’ for a geologically complex region. Suggestions for the timing of the collision range from latest Cretaceous to the Miocene. This event likely took place (or was completed) at the end of the Eocene, after which both the subduction-related igneous activity and the plate motion of Arabia substantially decreased and there is a widespread Oligocene unconformity in Zagros. It is notable that shallow marine sedimentation (notably the Asmari limestone) continued even after the continental collision, just as it is still continuing in the Persian Gulf.

Act IV is the rise of the Zagros Mountains by compressional stresses and crustal shortening and the south-westward sequential development of major thrust structures. Quantitative studies of the spatial and temporal development of the Zagros structures in various transects and by integrating geophysical and well data and detailed structural-stratigraphic mapping are critical for better evaluation of the subsurface petroleum systems in the Zagros basin. Since the Miocene the rifting of the Red Sea and the Gulf of Aden and the subsequent separation of Arabia from Africa have supplied renewed stress for tectonic

deformation in Zagros. Recent studies have also postulated the ‘slab break-off’ of the subducted Neo-Tethys ocean floor beneath the Central Iran block after the continental collision. This break-off, is considered to have served as a force for an orogen-scale ‘thermal uplift’ of Zagros in the Neogene, and also as a heat source for the continued volcanic activity in the Urumieh-Dokhtar belt.

Act V (coeval with Act IV) involves the development of a long, wide foreland basin in front of the rising Zagros and the in-filling of this basin with sediments carried by rivers from these mountains. The ‘antecedent’ pattern of river drainage in Zagros is a remarkable feature. Many (but not all) rivers were already established before new mountains (mainly anticlines) popped up; therefore, the streams continued their flow and valley incision simultaneously with tectonic uplift of mountains. In his classic work The Zagros Streams (1965), Theodore Oberlander has documented this cooperation of erosion and uplift to produce spectacular sceneries in Zagros.

Structural map of Zagros (between latitudes 38° N and 26° N) showing the major tectono-stratigraphic zones and bounding faults. Zone A: The Urmieh-Dokhtar magatic arc; Zone B: Sanandaj-Sirjan belt; MZT: Main Zagros Thrust (Reverse Fault); MRF: Main Recent Fault. Zone C: High Zagros ‘Imbricated Zone’; HZT: High Zagros Thrust. Zone D: Simply Folded Belt. MFF: Mountain Front Fault (Flexure). Kh.F: Khanaqin Fault. Br.F.: Bala Rud fault. H.F.: Hendijan Fault. K.F.: Kazerun Fault. ZDF: Zagros Deformation Front (also called Zagros Foredeep Fault). Zone E: Mesopotamian-Persian Gulf foreland basin (Quaternary plains on the Arabian platform). (Modified after Mehdi Alavi, 1994, Tectonophysics, 229, p. 211-238 Manuel Berberian, 1995, Tectonophysics, 241, p. 193-224). Source: Rasoul Sorkhabi

Why So Much Oil?

The reasons why the Zagros basin and the Arabian platform are so prolific for oil and gas have been discussed in a previous article in Geo Expro (Vol. 7, No. 1).

Suffice it to mention a few points in relation to the Zagros tectonics. It is important to note that the Zagros foreland basin did not develop on a Precambrian basement (as it happened in the central parts of the Himalayas) but was superimposed on the shelf and slope sediments of Tethys. The thickness of Cenozoic sediments in Zagros is 3,000-5,000m. The overburden of these sediments as well as the thickening of the basin by fold-thrusting deepened the organic-rich Mesozoic source rocks to the oil and gas generation window. Moreover, the upper parts of the Zagros foreland sediments are extensive evaporites (Gachsaran Formation) which provide an excellent regional cap rock for petroleum accumulation. Finally, the Zagros tectonic stresses have not only produced large anticlinal and thrust-fault traps for oil and gas pools but also contributed to the fracturing of carbonate reservoirs which dominate this region.

A fabulous Landsat image (taken on 2 February 2002) of large ‘whale-back’ shaped anticlines in the Simplyfolded Zagros. Such anticlines, home to enormous oil and gas reserves, are large folds produced by Hormuz Salt movement at the base and compressional stresses of the Arabia-Asian collision. The town of Khormuj (with a population of about 35,000) and the Mand River (both in the Province of Bushehr, Iran are visible in the figure. A salt dome has intruded

through the crest of the anticline. Source: NASA

Environmental Impacts

The average altitude of Zagros in its northern and central parts is 2 to 2.5 km while in the south-east it is 1.5 km. Some of the highest summits in Zagros are Zard Kuh (4,548m), Mt. Dena (4,359m), and Oshtran Kuh (4,140m). The Zagros uplift has produced a lofty barrier in south-west Iran against the rainbearing air currents from the Mediterranean and the Atlantis, thus producing rain shadow and a dry desert climate on the Iranian plateau, but a zone of high precipitation in the foreland. Precipitation is usually higher in the northern and central parts of Zagros, between 300 and 1,200 mm a year, but is reduced to 100-500 mm a year in the southern parts close to the Persian Gulf. Large rivers originating in the Zagros Mountains supply freshwater in that region. These rivers and streams have also created alluvial plains on which some of the earliest agricultural communities appeared some 10,000 years ago (shortly after the last glacial age). The majority of the Zagros rivers flow into the Persian Gulf, such as the Karun River (the longest) which passes through the city of Abadan (where the first refinery in the Middle East was built in 1909-1913). But some rivers drain the interior areas behind the High Zagros; for example, the Zayandeh River which flows through the famed city of Isfahan. Seasonal climate and pastures of Zagros have also motivated nomadic life, most notably of the Bakhtiari tribes about which Vita Sackville-West wrote in her 1928 travelogue.

In 1996, Patrick McGovern of the University of Pennsylvania Museum published chemical analysis of wine stains from 7,000-year old jars found from a mud-brick building in Haji Firuz Tappe (near Lake Urumie) in northern Zagros. This is the oldest inery on record, and it comes from a region where grape grew in the wild.

The story of Zagros has not come to an end. Numerous earthquakes are a (sometime tragic) testimony to the tectonic activity of the Zagros area. GPS measurements of a large network of geodetic benchmarks published in the recent decade indicate that structural shortening and strike-slip motions are taking place at rates up to 10mm/yr in Zagros; these data indicate that almost one-third of the convergence of the Arabian plate (about 2-3 cm a year) is deforming Zagros, the rest of convergence being taken up by the other parts of Iran.

An outcrop of the fractured Asmari limestone (Early Miocene) in south-west Zagros; this is a major carbonate reservoir in the Zagros foreland basin. Source: S. Tabrizi

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Kurdistan: Safe and Secure, with Billion Barrel ProspectsAuthor Jane Whaley

Tags: Middle East Country Profiles Issue 2, Volume 7, 2010

Where in the world can you still find undrilled billion barrel prospects? Iraqi Kurdistan is one answer. But what is it like to work in an area thought by many to be a war zone?

The Miran West Field is 55 km east of the giant Kirkuk field and is thought to have more than 1 Bbo recoverable. Photo: Heritage Oil

“Kurdistan is an exciting and very safe place to explore,” exclaims Steve Curd, Chief Geophysicist with Heritage Oil, one of the first foreign companies to move into this region of Iraq after the second Gulf war, back in 2004. “It is a beautiful country, and the scenery is spectacular - from our Miran Block the Zagros Mountains can be seen rising in the distance, giving a wonderful backdrop. And of course, it was the collision of the Arabian and Asian plates in the Late Cretaceous and Early Tertiary which formed not only these mountains, but also the structural traps we are exploring, which have the same north-west to south-east trend as all the major accumulations in the area.”

“The structures can be seen at the surface"

“Many of these structures can be seen at the surface, which helps in the design and cost-effectiveness of our seismic work,” he continues. “It also means we can look at our reservoirs at outcrop, and come across oil seeps which prove what promising acreage we hold.”

Billion barrel prospects

According to CFO Paul Atherton, Heritage Oil has been interested in Iraq for over a decade . “We decided to concentrate on Kurdistan for political and security reasons, but also for prospectivity, as the undiscovered potential of the region is estimated by the USGS to be about 40 Bbo and 60 Tcfg (11.4Bboe),” he explains. Having built up an extensive database on the area, and with the help of expert advisers, the geologists at Heritage knew exactly the area they wanted, and the company was one of the first to be awarded a Production Sharing Contract by the Kurdistan Regional Government. “First mover advantage meant that we were able to cherry-pick a prized block,” Paul continues. “Our 1,015 km2 Miran block is just 55 km east of the giant Kirkuk field, which has remaining reserves in excess of 10 Bbo. We were awarded it in October 2007 and just 15 months later, in December 2008, we spudded our first well.” 

“Our initial seismic survey, conducted in the second quarter of 2008 – the first ever on the block – enabled us to identify two large and very promising anticlinal structures, Miran West and Miran East, together covering 330 km2,” explains Steve. “Initial estimates, reinforced through an independent study by RPS Energy, suggest that the block contains in-place reserves of 3.4 billion barrels of oil. Where else in the world nowadays have you a chance of discovering giants like this?”

The Kurdistan Region of Iraq is in the north-east corner of the country and is bordered by Syria, Iran and Turkey, all of which have indigenous Kurdish populations Image: Heritage Oil

Kurdistan – the other Iraq!

Nestling between with the Tigris and Euphrates Rivers, Kurdistan is known as the ‘Cradle of Civilisation’, as it was here that agriculture, metal work, pottery and weaving all first appeared. In fact, the Citadel in Erbil dates from 6,000 BC and is said to be the longest continually inhabited city in the world.

Covering about 30,000 km2 of the north-east corner of Iraq, with a population of nearly 4 million, the Kurdistan Region of Iraq was initially established in 1970 to stop continual fighting between Iraq and Kurdistan separatists. During the regime of Saddam Hussein, however, much of Kurdish self-rule was lost, culminating in the chemical gas genocide of the 1980’s and oppression by Saddam’s forces after the Kurdish uprising of 1991, when hundreds of thousands of Iraqi Kurds were killed or displaced.

The new post-Saddam constitution, which was ratified in 2005, established the Kurdistan Region of Iraq as a federal entity recognized by Iraq and the United Nations – effectively an autonomous region within a federal state. It is ruled by the democratically elected Kurdistan National Assembly, which has the right to issue production sharing contracts and exploration rights over new acreage, although not existing fields. Proceeds from the sale of oil goes to the Iraqi government and then is redistributed, with 17% of the total Iraqi budget, the vast majority of which stems from oil sales, going to Kurdistan.

As a result of its relative stability, the Kurdistan Region of Iraq has the fastest growing economy with the highest standard of living in Iraq, with many modern hotels and offices in the capital, Erbil. As a more secular society, Kurdistan has managed to avoid much of the religious and sectarian violence that has affected other areas in the country.

Fractured limestone reservoirs

As reported last year (GEO ExPro, vol. 6, no. 4, p.74), the first well, Miran West-1, drilled to a depth of nearly 3,000m, found a gross oil-bearing column of 710m with three Cretaceous reservoir zones. Estimates for the field suggest that it has in excess of one billion barrels recoverable, and although testing was incomplete, Miran West-1 flowed at a maximum rate of 3,640 bopd from a single upper reservoir interval. “As the seismic revealed structures with substantial vertical relief, possibly trapping a significant column of hydrocarbons, and we had no other knowledge of the prospect, standard industry practice meant that we drilled with high mud weights, in case we encountered a high pressure gas column,” Steve explains. “However, we found oil, under much less pressure, so our high mud weights tended to invade and contaminate the reservoirs in the vicinity of the well. This is not a major issue, but it meant that we were not able to properly test the lower horizons. The second well, Miran West-2, is being drilled with the knowledge gained from the first well and therefore with appropriate drilling parameters and equipment to gain maximum information.” The main targets in this part of Iraq are all Cretaceous in age , although in northern Kurdistan the Triassic is also hydrocarbon bearing, and in other parts the Jurassic may be prospective. The principal reservoir horizons in this structure are the Late Cretaceous Shiranish and Kometan Formations, and the Early Cretaceous Qamchuqua. These are all carbonates, principally limestones, extensively cracked as a result of folding and therefore exhibiting excellent fracture porosities. “The highly fractured nature of the rock means that we

expect recovery rates of between 50 and 70%,” says Steve. “Data gathered from the initial test indicates that a production rate of 10,000 bopd per well should be achievable.”

A seismic line across the Miran block clearly shows the two structures of Miran West and Miran East Image: Heritage Oil

Secure and safe

Miran West-2 was spudded in November 2009, about four kilometres north-west of the first well, and is expected to take four months to drill, so the field is already well on the road to development, with first oil expected as early as this summer. “Initially, we expect to produce about 5,000 bopd for local consumption,” explains Paul. “We will eventually export much larger quantities via the Iraq-Turkey pipeline, which has a capacity of 1.6 MMbopd. At the moment the export payment mechanism via Iraq has not been clarified, but we are confident that this will be resolved, as the money from exports from this area will benefit the whole country, not just Kurdistan. As soon as that happens, we will look to begin exports from Miran.”

“We can go about our business without fear” Which brings us to the inevitable question: bearing in mind everything we read about Iraq, what is it like to work in Iraqi Kurdistan? “It’s great,” says Steve. “It’s very stable and very safe. Since March 2003, there has only been one bomb in the areas administered by the Kurdistan Regional Government, no foreigners have been kidnapped and not a single coalition soldier has died. The local Kurdistan security force is excellent and we have very few fears for our safety. The main towns have comfortable western-style hotels and the locals throughout the State are welcoming and hospitable. It is also very interesting historically, so there are plenty of places to explore in my time off.”

Heritage has a Chinese crew working on the drill site, but uses and trains local staff wherever possible. “Our drivers, security and labourers are all local, and we

also have Kurdistan geoscientists, administrators and logistics people,” says Paul. “This is a very important policy for us. We are also involved at the community level, working with schools and mosques in the Miran area. Heritage’s objective is to support development in local communities. We recognise the importance of engaging with local stakeholders and believe that by working closely with host communities we are better enabled to meet the challenges facing us.”

“There are now about 25 international oil companies working in the Kurdistan Region of Iraq, and flights come to Erbil from all over the world,” adds Steve. “There are no issues – basically, it’s not the Iraq you hear about in the news!”

Photo: Heritage Oil

Heritage Oil

Tony Buckingham, CEO and founder of Heritage Oil, has more than 30 years experience in the oil industry, but comes from a very different background to that of most senior managers in the business. He started out as a deep sea diver, working on rigs in the North Sea, before moving into the security business, acting, among other things, as an adviser to large independent companies like Premier Oil and Ranger. Having worked for many years in Africa, in 1992 he set up Heritage Oil, specifically to concentrate on exploration in Africa and the Middle East.

Heritage acquired pioneering assets in the Albert Basin in Uganda in 1997 – the first company to do so. After the initial seismic in the area showed promise, it farmed out 50% to Africa Energy, (later purchased by Tullow) and together they went on to discover over 700 MMbo in the Albert Graben. Since then, Heritage has been awarded or acquired exploration acreage in Mali, Tanzania, Pakistan, the Democratic Republic of Congo and Malta, and producing fields in Russia and Oman.

Since acquiring its first assets six exploration wells have been drilled on Heritage acreage, and all of them have found hydrocarbons, a remarkable success rate. Late in 2009 the company decided to sell its Ugandan assets, in order to concentrate resources on the development of Kurdistan, while diversifying further elsewhere. In total Heritage spent about US$150 million in Uganda and is selling the assets for about US$ 1.5 billion – not a bad return on investment, as Paul points out.

Interested in Iraq, the company strategically chose to float on the Toronto Stock Exchange in 1999, taking advantage of Canada’s politically neutral status. At the time it had a market capitalisation of less than US$20 million. In 2008 it moved to the London Stock Exchange, where Heritage Oil is listed on the Main Board, is a member of the FTSE 250 group of companies and is now valued at US$ 2.2 billion.

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