Great exploration targets in the Persian Gulf: the North Dome/South Pars Fields

Wednesday, February 13, 2013

This article is by Behrooz Esrafili-Dizaji (ESRAFILI@Khayam.ut.ac.ir), Hossain Rahimpour-Bonab, Farkhondeh Kiani Harchegani, Vahid Tavakoli and Mehrangiz Naderi of the University of Tehran.

The world’s largest gas accumulation is found in the the Permo-Triassic carbonates of Qatar Dome structure, Persian Gulf. This single supergiant gas-bearing structure is shared between Qatar and Iran, and is known as the North Dome and South Pars fields, respectively. These fields contain approximately one fifth (19%) of the world’s total discovered gas reserves. In addition, this structure has giant oil reservoirs in the Late Jurassic and Early to Middle Cretaceous carbonates. Here the general characteristics of these supergiant gas and giant oil reservoirs in this exceptional hydrocarbon structure are reviewed.

A Platform of South Pars Gas Field (SPQ-1) in the Persian Gulf (Photo from POGC).

Introduction

In 1971, Qatar discovered an enormous gas reservoir (North Dome Field) in the Qatar Dome structure of Persian Gulf waters. Two decades later, the northern extension of this reservoir (South Pars Field) was explored by Iran, based on seismic surveys, in 1990. Complementary studies showed large amounts of gas and condensate accumulated in a broad dome or anticline structure (with 9600 km² area). This single gas bearing structure is separated by the political borders between Iran and Qatar. According to the latest estimates approximately 19% of the world's total gas is hosted by this huge structure.

After these surprising discoveries, Iran’s and Qatar’s gas reserves increased about 36% and 99%, respectively, and easily made them the second and third largest natural gas reserves holders in the world, after Russia. These jointed fields exceed the entire reserves of the Americas, Western Europe and sub-Saharan Africa combined. Now, Qatar is the world's largest producer of (LNG), accounting for about 15% of world liquefaction capacity.

During 1971 to 1985, several appraisal wells were drilled to quantify the reservoir gas accumulation. Development of the North Dome field began in the late 1980s. In 1991, Qatar began producing natural gas and condensate from this reservoir during the Alpha project. If current production of Qatari counterpart continues, the North field has a reserve-production ratio of more than several hundred years, although that expected lifetime will fall given Qatar's aggressive plan to boost production from the field. In the other part, the Iranian government is hoping to develop the South Pars field in 25 to 30 phases. Gas production from the Iranian portion started in 2002.



Comparison between South Pars and North Dome gas reserves and other countries’ gas resources (Data from Oil and Gas Journal, 2011, and Edwards, 2009).

Evaluations indicate abnormally-pressured and sour (high H2S content and also CO2) natural gas at 2750-3200 m depth in this shared reservoir. In addition, some researchers believe in several hydraulic connections in this gas/condensate bearing structure (between the Qatari and Iranian parts); accepting this theory, reduction of reservoir pressure with over-production in one part may lead to gas migration from the other part.

Besides the supergiant gas reserves, during the 1990s, Qatar and Iran discovered large oil reservoirs in both parts of the Qatar Dome. There are several main oil layers in the Jurassic and Early to Middle Cretaceous carbonates of this structure. Seemingly, similar to the gas reservoir, Iran's South Pars Oil Layers (SPOL) is shared with Qatar's oilfield (Al-Shaheen Field). The Qatari counterpart discovered the Al-Rayyan oil field in 1976, in the Dome. The Jurassic Arab A and C members are two thin, oil-bearing reservoirs in this field that both contain relatively heavy, low gas-oil ratio (GOR) oil.




Location of the South Pars and North fields in the Persian Gulf. The political border separates this geologically single supergiant reservoir structure (between Iran and Qatar). The location of some wells (exploration/appraisal wells) is displayed in these fields; because of confidentiality well names are not labelled.

Overall, there are a main gas and several major oil layers in the subsurface Qatar Dome structure that shared between Iran and Qatar. Latest reports show that Qatar overtakes Iran in gas production from the Qatar Dome field. Currently Qatar's production of liquefied natural gas (LNG) from the North Dome is twice Iran's production. Moreover, Qatar currently produces from oil reservoirs, while Iran has not yet developed the SPOL Field. Hence a great competition between the two countries has prompted them to boost their gas and oil production from these co-joined reservoirs. Because there are no detailed publications regarding the geology of the North Dome Gas Field and its Oil layers, here only characteristics of gas and oil reservoirs of South Pars fields are reviewed.



A generalized cross section illustrating main reservoir zones and stratigraphy of the South Pars Field in the Persian Gulf. The Upper Dalan–Kangan carbonates (Upper Khuff) host the world’s largest gas reserve in these fields. In addition to the supergiant gas reservoirs, this structure holds three oil layers in the Late Jurassic to the Early and Middle Cretaceous carbonates.

Name	Ownership (area)	Gas in- place (Tcfg)	Recoverable gas (Tcfg)	Condensate in-place (B bbl)	Recoverable condensate (B bbl)	Percent of world's proven gas reserves
North Dome	Qatari (6000 km2)	1300	900	30	10	5%
South Pars	Iran (3700 km2)	436	360	18	9	14%

Comparison between the general characteristics of South Pars and North Dome gas/condensate reserves. After Edwards, G. 2009. An overview of Qatar Petroleum’s multidisciplinary database project. IPTC 13411 in Doha, Dec (7-9).

Description of Gas reservoirs

The major gas-bearing layers in the Qatar Dome Fields (North Dome/South Pars) are Permo-Triassic carbonates, known as Upper Khuff carbonates in Qatar and Dehram group carbonates in Iran. Lower Permian sandstones, the Faraghun Formation, are most probable reservoirs rocks in the Persian Gulf but it is not proven yet. It is time equivalent to the Unayzah or Wajid Formations in the Arabian countries; the Unayzah Formation hosts enormous oil and gas reserves in the several fields of Saudi Arabia. In future deep drillings, the Faraghun Formation is a great target as a new reservoir in the Qatar Dome structure.

Faraghun Formation; Probable reservoir

The Early Permian Faraghun Formation is a thick siliciclastic sequence (sandstone, siltstone, shale) with minor limestone interbeds. In fact, this formation represents Post-Hercinian continental deposition in fluvial to river dominated deltaic systems. In deep drilled wells, the Faraghun Formation is not penetrated completely so, unfortunately, there is no accurate data about hydrocarbon potential and total thickness of this formation. Several meters of Faraghun Formation are drilled in South Pars (in 4 km depth subsea) but no hydrocarbon has been discovered in this interval. Data available from the Faraghun interval including wire line logs, cutting, seismic data and tests show that this formation could be one of the largest potential reservoirs in Iran. Petrophysical evaluation of wire line logs shows that the upper part of the formation consists of limestone interbeded with sand layers. With increasing depth, the dominant lithology is sand and siltstone.

In the cored intervals of this formation in the nearby fields, the Faraghun consists of gray silt and sandstones, white sandstones, red and greenish gray claystone. The average porosity of the formation is 10.8 percent and average permeability is 8.9 mD. The dominant facies in drilled wells in this field are calcite cemented quartz arenite, calcite cemented quartz wacke and silt and claystone, respectively. The porosity nature is uniformly distributed interparticle in sand intervals, and by dissolution in lime dominated parts. Reservoir quality decreases in clay rich intervals, whether sand or lime dominated. The reservoir quality (both porosity and permeability) improvess with decreasing cement content and dissolution. Future deep drilling and subsequent analysis will clarify the hydrocarbon potential of this unit in this large Dome structure.



A close-up and two microscopic photographs of Faraghun facies from nearby field of South Pars Field. Faraghun formation has good reservoir properties but is not proved as hydrocarbon reservoir yet.

Supergiant gas reservoirs; Permo-Triassic carbonates

The Permo-Triassic carbonates (Upper Dalan member and Kangan Formation in Iran and its equivalent Upper Khuff carbonates) host enormous gas reserves in the Qatar Dome structure. The reservoir intervals were deposited during a marine transgression (of the large Arabian plate) starting in the Lower Permian times which covered the Qatar Arch throughout the Permian and during the Lower Triassic.

In the South Pars Field, the Upper Dalan member and Kangan Formation are fully cored and studied. Reservoir rocks extend from depths of roughly 2750 to 3200 m and gently dip toward the NE. This unique reservoir primarily consists of dolomite and limestone with some interbedded anhydrite and shale. The mean thickness of the reservoir units increases from the Qatari side (385 m) to South Pars (some 450 m). This thick succession consists of carbonate and evaporates layers and is divided into five subdivisions, K1 through K5. The productive zones are K2 through K4 due to high reservoir quality, near initial reservoir pressure and high net pay thickness. On the whole, reservoir quality of the K4 and K2 units is better than the K3 and K1 units. The average gross and net pay zones in the field are some 450 m and 328 m, respectively. Gross reservoir quality is around 9.7% porosity and 26.8 mD permeability. Many geologists believe the field is a complex reservoir.



Microscopic and slabbed cores photomicrographs of important reservoir facies (Oolitic shoal grainstones) in the South Pars Gas field.

Most studies indicate that reservoir rock in the South Pars is highly stratified in nature and displays layer cake geometry. More than 60% of the reservoir rocks at South Pars have been dolomitized during diagenesis. The most common pore types (mouldic and solution enlarged, up to 60%) in the reservoir rocks were produced by dissolution of the unstable components. Also, interparticle and intercrystalline pores are abundant in the grainy and dolomite facies, correspondingly.

The best reservoir quality can be found commonly in dolomitized and non-dolomitized grain-dominated rock types. Other good reservoir rocks are sucrosic and recrystalized dolomite (with high intercrystalline poroperm) and fractured rock types, but the latter are not common in the reservoir.

In regard to the origin of gas, the Lower Silurian Sarchahan Formation (or Qusaiba Hot shales) is believed to be the main source rock of this petroleum system. The reservoir rocks are capped by a thick, impermeable anhydrite and shale succession of the Triassic Dashtak Formation (and its lateral equivalent Sudair Formation in Arabian countries).

Description of Oil Layers

Three giant oil reservoirs have been discovered on along with the supergiant gas accumulation, in the Late Jurassic (Arab units in Al-Rayyan) and Early to Middle Cretaceous carbonates (Al-Shaheen and SPOL Fields). The Cretaceous formations (Khatiyah, Mauddud , Nahr Umr ,Shuaiba and Kharaib B intervals) in the Al-Shaheen Field and Jurassic Arab reservoirs in the Al-Rayyan are reservoir horizons. Three main reservoirs of South Pars oil layers (SPOL) are Cretaceous carbonates Upper Dariyan, Lower Dariyan and Maddud members. Reservoir rocks are drilled and incompletely cored over about 1 km. The Upper Dariyan is the main oil container that has not yet been developed.

On the whole, in contrast to gas layers, oil reservoir rocks in the Qatar Dome are mud-dominated, low-permeability and highly fractured in nature with heavy oil content. The main source rock for the oil layers is believed to be the Jurassic Hanifa (Tuwaiq group or Surmeh equivalents) Formation to the Southeast of the Qatar peninsula.

Dariyan reservoirs

The Lower Cretaceous Dariyan Formation in the SPOL is divided two units (Lower Dariyan Formation, and Upper Dariyan Formation). The Lower and Upper Dariyan have reservoir properties and are separated by the Hawar shale.

The Lower Dariyan is composed of white limestone (wackestone–packstone, partially grainstone) with moderate to good porosity (average porosity is 20% and average permeability is 5 mD). This formation deposited in a high energy shallow water carbonate ramp and consists of white light brown to gray, shallow, open marine chalky limestone. This reservoir interval approximately shows constant thickness in all of the SPOL.

The Upper Dariyan is the equivalent of the Shuaiba in the Arabian countries. It consists of mud-dominated limestone (skeletal wackestone and packstone, partially boundstone). Secondary porosity has developed in the uppermost interval of this unit. Average porosity is 25% and average permeability is approximately 10 mD in the Upper Dariyan Formation. Compared with the Mauddud and Lower Dariyan formations, this reservoir has better reservoir quality. Its depositional environment includes lagoon, small bioclastic shoals and narrow proximal open marine environment located in a carbonate ramp. Orbitolina is the dominant foraminifera in this reservoir so it is called the Orbitolina-bearing Formation in some literatures.

Mauddud reservoir
The Upper Cretaceous Mauddud member of the Sarvak Formation consists of limestone, wackestone to packstone, bioturbated skeletal deposited in a shallow lagoon environment. In some intervals, there are skeletal grainstones/packstones with vuggy, channel and fracture porosity and stylolite features. Secondary porosity is common in the whole reservoir section, mostly near its top, originating from leaching. The thickness of the Mauddud is variable throughout the field; the average porosity of this unit is about 20% and average permeability is approximately 5 mD.



Three microphotographs from South Pars oil layers (Lower Dariyan, Upper Dariyan and Maudud units) in the Persian Gulf. The giant and heavy oil reserves are accumulated in the mud-dominated and low-permeable rocks of these rocks.

Carbonate Reservoir and Geochemistry Geoteam During the last 8 years, the South Pars petroleum system (both gas and oil layers) was the subject of many sedimentological, diagenetic, geochemical and reservoir characterization studies (MSc and PhD thesis) in the University of Tehran. This research are mainly supervised by Professor Rahimpour-Bonab. The University of Tehran and POGC (Pars Oil and Gas Company of Iran) supported these investigations. The results of these studies have been published in several domestic and international scientific journals and conferences.
Behrooz Esrafili-Dizaji undertook his undergraduate studies at the University of Zanjan (2001–2005) and received his MSc from the University of Tehran in 2008. During the last 5 years, Behrooz studied the sedimentology, diagenesis and reservoir characteristics of South Pars Gas layers. At the present he is a Ph.D student in sedimentology and sedimentary petrology at the University of Tehran. His research projects focus on the characterization of Zagros/Persian Gulf carbonate reservoirs.	Hossain Rahimpour-Bonab studied geology at the University of Tehran, Iran and received his BSc and MSc, in Geology and Sedimentology, in 1988 and 1991. Then, he received his Ph.D. from the University of Adelaide, Australia, in Sedimentary Geochemistry and Sedimentology, in 1997. Since 1997 he has been Professor of Sedimentology and Geochemistry in Department of Geology, at the University of Tehran. His research interests include carbonate reservoirs geochemistry, depositional model, diagenesis and characteristics in the sequence stratigraphic framework.
Farkhondeh Kiani Harchegani received her B.Sc in geology from the University of Isfahan in 2000, and her Master’s degree in sedimentology and sedimentary petrology from Azad University (Khorasgan Branch), Iran in 2010. Her area of interest is carbonate reservoirs, sedimentology and diagenesis.	Vahid Tavakoli has a Ph.D from the University of Tehran. He works on carbonate and sandstone reservoirs studies for exploration and production projects. He is assistant professor in University of Tehran teaching petroleum geology courses. Vahid spent 2 years in studying newly cored Faraghun intervals in the Persian Gulf.
Mehrangiz Naderi has a M.S from the University of Tehran. She works in National Iranian Oil Company (NIOC) as a geologist. She has studied South Pars oil layers during last 2 years. Her interests are carbonate reservoir geological studies, geological modeling and well logs interpretation.