Greater Persian Gulf Permian-Triassic stratigraphic nomenclature requires study

Carbonates of the Dalan and Khuff formations, which are essentially of Permian age, form extensive gas reservoirs in the Greater Persian Gulf area (Iran, Iraq, Saudi Arabia, and the eastern portion of the Arabian Peninsula, Fig. 1). These sediments were deposited unconformably on Lower Paleozoic rocks in western Iran and on basement rocks in Oman, Yemen, and Arabia. The term "Khuff formation" has been widely used by oil companies in this region, and its equivalent (the Dalan formation) is a rather new name which is currently used in Iran.

However, as with many other litho- stratigraphic units in the Middle East, poor correlations and confusing nomenclature are the rule rather than the exception. This problem is even worse for the Permian-Triassic rocks as the data base (particularly the subsurface data) is scattered and largely inaccessible. Consequently, some information regarding the Permian-Triassic reservoir rocks has been presented incorrectly in various parts of the Middle East.

Introduction

Some of the names that have been given to Permian-Triassic stratigraphic units in the Greater Persian Gulf area and which appear in the literature are inconsistent (e.g. Khuff, Unayzah, Jamal, and Dalan formations).

Previous discussions of the Permian-Triassic units and their hydrocarbon production history by authors from various parts of the Middle East have regrettably included the misuse of terminology and the misinterpretations of a number of formations and of their areal extent. Oil companies have generally established their own terminologies and unit names, whereas academic groups have in some cases tried to introduce new terms unrelated to those used by the oil industry.

It is the intention of this study to give a brief description of the major Permian and Triassic rock units in the Middle East and to discuss the validity of the terminology used to describe them. Attempts have been made to eliminate some of the inconsistencies and to establish a common ground between the oil industry and academia. These units are prolific reservoirs for gaseous hydrocarbons.

Regional stratigraphy

The Permian carbonate unit known as the "Khuff formation" in the Persian Gulf region occurs in Bahrain, Qatar, Abu Dhabi, and Saudi Arabia. In south-southwest Iran and the coastal provinces, the thick carbonates were formerly known as the Khuff formation and are now known as the Dalan formation¹ (Fig. 2). The Permian Dalan and Khuff formation constitute very extensive gas reservoirs in the Greater Persian Gulf area.²

The Khuff formation was deposited during a regional transgression over a stable shelf of very low relief which had a minor clastic supply on the margin of the Arabian Peninsula³ (Fig. 3A). However, in Oman the Khuff formation changes into a continental red-beds facies⁴ and in the Oman Mountains, the lateral equivalent of the Permian part of the Khuff (the Saiq formation) lies with angular unconformity on Ordovician, Precambrian, and basement rocks.⁴

Lovelock et al.⁵ also noted that the Akhdar Group equivalent of the Khuff in the Oman Mountains has overstepped to lie with a strong angular contact on basement rocks. However, major oil reserves have been discovered recently in Central Arabia within the Permian-Carboniferous clastics which lie directly beneath the Khuff formation.⁶

In Yemen, fluvial clastic rocks which lie unconformably on basement are now considered to be Early Triassic in age as a result of both macrofossil studies⁷ and also microfossil dating.^{8 9}

Because only a single well has penetrated into the Paleozoic in Kuwait, the stratigraphy of the Permian succession is poorly understood.¹⁰ However, Murris11 stated that the Permian Khuff formation in Kuwait consists of 406-610 m of limestones with some clastics comprising mainly shallow-water carbonates (Fig. 3B).

Alsharhan and Kendall¹² used the term "Jamal formation" for the Khuff formation in Iran. This is misleading and incorrect. The term "Jamal" is better applied to the entire Permian carbonate succession in the northeast corner of Iran, near the border with Turkmenistan, and is not relevant to the Persian Gulf region (Fig. 1). Furthermore, these authors stated that the Khuff formation was deposited in the form of sabkha carbonates and shelfal limestones in "central and northern" Iran.

However, the Khuff (Dalan formation) is present only in the Zagros fold belt and does not extend to central or northern Iran. It is also fully-marine and is not a sabkha sequence. The Dalan carbonates marked the beginning of marine sedimentation in the Zagros basin, which continued almost uninterruptedly in to the Early Miocene. These carbonates contain fusulinids and corals and correspond to the Khuff formation of the Persian Gulf area.^{2 13}

In 1976, the Stratigraphic Committee of Iran adopted a new term for the Permian succession in southern Iran (the Deh Ram Group) which was named after a village on the southwestern flank of Kuh-e-Surmeh (Fig. l) in south Iran.

Subsequently, the Deh Ram Group in southern Iran was divided into three formations.

They are, in ascending order:

1. The Lower Permian Furghun formation, which is composed of terrigenous clastics and is 200 to 420 m thick;13
2. The Upper Permian Dalan formation, comprising a lower limestone succession (160 m thick), a middle section of anhydrites and dolomites (about 300 m thick), and an upper carbonate succession consisting of limestones and dolomites (about 180 m in thickness); and
3. The Lower Triassic Kangan formation (160 m thick), which forms a massive carbonate section at the top (Fig. 2).

The equivalent of the Furghun formation, which previously was included in the basal part of the Khuff formation in Saudi Arabia, is now known as the Unayzah formation in that area.¹⁴ Further- more, McGillivray et al.⁶ identified the lower (clastic) portion of the Khuff (below the carbonates) as the Unayzah formation, a new Lower Permian unit in Saudi Arabia. They defined the Unayzah formation as a shallow-marine, fine-grained sandstone which was deposited above a major regional unconformity. The Unayzah formation recently became an important hydrocarbon reservoir in central Saudi Arabia.

The term "Khuff" was first introduced by Bramkamp et al.¹⁵ for a 172-m thick carbonate section of Late Permian age in Saudi Arabia. Since then, this term has been widely used by oil companies in the Greater Persian Gulf area. However, the upper part of an approximately 1,100-m thick succession of sedimentary rocks used to be referred to as the Khuff formation in southern Iran but is now recognized from paleontological data as being the Lower Triassic.¹⁶

A prominent feature of the Permian succession in Iran is the common occurrence of disconformable sandstone beds (Furghun) at the base (Fig. 2). The Furghun formation is a rather thick terrigenous clastic unit with a minor carbonate content.

The unit thins southwestward in Iran and becomes dominantly shaly.² These relatively thin, basal terrigenous sediments have been dated as Late Carboniferous (Pennsylvanian).^{1 13} However, other studies have shown that the Upper Permian carbonate section overlies these sandstones conformably, which strongly suggests that they should be assigned to the Lower Permian. The Upper Permian Khuff formation was deposited on a shallow shelf covered by an epeiric sea that covered a vast, virtually flat area.¹⁷

The thickest Permian section in the Greater Persian Gulf area occurs in the eastern Oman Mountains,¹⁸ where nearly 1,830 m of sediments, whose base is not exposed, are present. However, as a result of the obduction of Upper Cretaceous basic igneous rocks including ophiolites, Permian rocks in this location are partially metamorphosed.

A regional unconformity separates the Permian from the Triassic in southern Iran. A shale bed in the lower part of Kangan formation contains the pelecypod Claraia, which indicates an Early Triassic age for this part of the formation; it overlies Permian carbonates which contain abundant Permian fusulinids.¹³ This unconformity is also present above the Permian sections in Central Saudi Arabia and the eastern Oman Mountains.

Production history, petroleum geology

In Saudi Arabia, high pressure gas from the Upper Permian section was encountered in the Damman No. 43 well in 1957. The gas zone is in the Khuff formation at a depth of 2,588-2,619 m.¹⁹ The Permian gas pay zone lies about 914 m below the oil-producing Arab zone (Upper Jurassic) in Dammam field. Beydoun et al.²⁰ and Bushara²¹ discussed deeper Permian gas accumulations which have been discovered in the southeast portion of the Zagros-Persian Gulf basin.

Gas from the Permian Khuff formation has been produced from a depth of 2,750 m in Bahrain since 1970.¹² Shehabi²² and Stephens et al.²³ noted that the Khuff formation in Awali field in Bahrain consists of dolomitized carbonates rocks with many anhydrite beds and has a thickness of 600 m.

Farther to the southeast, dolomites of the Permian-Triassic Khuff formation and clastic equivalents of the Sudair formation are also reservoir objectives in Yemen.⁹

A huge, nonassociated gas discovery in 1972 in the Permian Khuff (now Dalan) formation in the Kangan No. 1 well in southwest Iran increased Iranian gas reserves, which, with subsequent discoveries within the same sequence, have reached over 450 tcf.²⁴

Mainly carbonate units within the Permian section constitute potential reservoir rocks elsewhere in the Greater Persian Gulf area. These are:

1. The Dalan formation in southern Iran;²
2. The Khuff formation in Saudi Arabia and Bahrain;¹⁴
3. The Chia Zairi formation in Iraq;²⁵
4. The Qamar limestone, Bih dolomite, Hagil and Asfar formations in southeast Arabia;^{2 6 12 14} and Haushi and Akhdar in Oman.³

However, the equivalent of the Furghun formation, the Unayzah sandstones contain light oils in parts of Central Arabia within the Greater Persian Gulf.⁶

The Permian sequence is generally fractured and bituminous in outcrop. Permian limestones and dolomites exposed in the Zagros Mountains (Kuh-e-Dinar, Kuh-e-Surmeh, and Kuh-e-Furghun) have a strong gasoline-like smell and sulfurous odor, especially where freshly exposed. Similar hydrocarbon shows have been reported from exposed Permian section in southeast Saudi Arabia and the Oman Mountains.^{18 26 27 28 29}

Porosity and permeability are generally low in these carbonates, but secondary fracture porosity and dolomitization are common. Joint and fracture porosities are well developed where the thick carbonate sections have been folded; this is generally the case for any carbonate in the Zagros basin. Two sets of perpendicular joints and random fractures are often conspicuous in outcrops of these massive carbonates. However, in the less-folded area of the foreland shelf of Arabia and Iraq (Fig. 1), oolitic limestones with good primary porosities replace the fractured and jointed carbonates which prevail the northern parts of the Persian Gulf.

Permian reservoir rocks are generally sealed by impervious strata. However, in the Zagros basin to the southeast and across the Persian Gulf to northern Oman, thick Permian carbonates pass upwards into Triassic carbonates with no interbedded impermeable sections. Therefore, the continuous Upper Permian-Lower Triassic carbonate succession may act as a single reservoir, especially where overlying Triassic evaporites serve as a cap rock.

The overlying predominately evaporitic sequence is known as the Dashtak formation in these areas (Fig. 2). In Kuh-e-Surmeh and Kuh-e-Dinar, the lower part of the Dashtak formation (cap rock) consists of anhydrite and minor shale beds with intercalated dolomites in the upper part. However, the Lower Dashtak formation becomes a massive dolomite unit towards interior Fars in Iran.

The calcareous shales of the Mirga Mir formation in Iraq (Lower Triassic) conformably cap the carbonates of the Upper Permian Chia Zairi formation.³⁰

In Saudi Arabia, a 50-m-thick sequence of massive, impervious, overpressured Sudair shale, equivalent to the lower part of the Dashtak formation (Fig. 2) known as the Sudair formation, and thought to be of latest Permian age conformably overlies the Permian Khuff formation. This can support a gas column of several thousand meters. In the Rub al Kha* (Fig. 1), the lower part of the Sudair formation forms a cap rock for the Khuff in the southeast part of the Arabian Peninsula.^{28 31 32}

In southern Iran, the same Lower Triassic calcareous shales overlay the Permian carbonate reservoirs.

Locally, thin beds of impermeable marls, shales, and evaporites within the Permian carbonates act as permeability barriers. Zones of dolomitization and secondary porosity within the Khuff formation in Bahrain are separated by thin impermeable layers of anhydrite and anhydritic limestones. In Saudi Arabia, thin layers of gypsiferous clays in the upper part of the Khuff limestone also serve as barriers to contain gas within the formation.

Conclusions

The Persian Gulf Permian basin was an elongate trough with the greatest subsidence occurring in the southeast; therefore, the thickest sediments accumulated near the Oman Mountains.

The Permian section in this area is composed mainly of shallow-water carbonates, with some interbedded evaporites, and locally changes into a continental red-beds facies. Previously, it was believed that only carbonate units with secondary fracture porosity constitute potential reservoir rocks; it is now apparent that Upper Permian strata may have the potential to produce hydrocarbons from their primary porosity, essentially in the south-southwest portion of the basin. The evaporitic part of the section may serve as an effective cap rock. But in recent discoveries in the south-southeast of the basin, shales may form an effective cap rock.

Permian-Triassic stratigraphic names are generally only valid locally, and it is difficult to make a regional correlation. More work is required to address the problem of Permian-Triassic stratigraphy in the Persian Gulf. Due to a lack of subsurface data, the Permian of Kuwait is the least studied of any country in the region.

References

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The author

Mansour S. Kashfi is a consultant with Kashex International and Associated Resource Consultants Inc. He also lectures at Richland College and Collin County Community College near Dallas. He was senior geologist with National Iranian Oil Co. during 1971-78, exploration manager with Petroleum Corp. of Jamaica in 1981-82, and chief petroleum geologist with the Navajo Tribe (Arizona) in 1982-84. He holds a BS degree in geology from the University of Tehran, an MS degree in geology/subsurface stratigraphy from Michigan State University, and a PhD in geology/tectonics and sedimentology from the University of Tennessee. E-mail: [email protected]