9

The Khuff Petroleum SystemVast hydrocarbon resources (>5000 MM BOE) are suspected in the Arabian epicontinental basin (BP 2012). The Permo-Triassic Khuff layer is an important part of the basin fill covering some 3.7MM km2, one of the largest carbonate ramps in the Earth’s history. Its reserves are estimated at 2680 TcF of gas, of which the North Dome Field alone contains some 1826 TcF (IHS 2012).

Some 70 Khuff discoveries have been made up to December 2011 in the epicontinental basin. Of those, 23 are in UAE, 20 in Iran, 16 in Saudi, 7 in Oman, 5 in Qatar and 1 in Bahrain. These are largely gas-condensate accumulations with only 6 oil or oil-gas discoveries. Of those discoveries however only a small percentage, i.e., 21 accumulations, are producing at the time of publication (IHS 2012). Despite the vast extent of the Khuff play, hydrocarbon accumulations are largely concentrated around the wider Qatar Arch (Konert et al. 2001) and hosted predominantly in grainstone. Conceptual geological models are paramount for Khuff exploration because conventional seismic-based techniques are of limited value. Khuff grainstone is largely of sub-seismic thickness and exhibits only subtle seismic geometries (Figure 2). The fact that seismic surveys are commonly plagued by multiples further reduces the value of seismic play characterisation.

Figure 1: Map of the Arabian Peninsula and surrounding areas, indicated are study areas covered by the individual chapters of this volume.

Chapter Author

1 Pöppelreiter

2 Oosterhout

3 Singh

4 Richoz

5 Forke

6 Angiolini

7 Kavoosi

8 Faqira

9 Aqrawi

10 Esrafili

11 Knaust

12 Janson

13 Husain

14 Perez-Gomez

15 Pöppelreiter

1 The Khuff Formation: Play Elements and Development History of an Epicontinental Carbonate Platform

10

Chapter 1

The elements of the Khuff petroleum system and its evolution through the Earth’s history are reflect-ed in this EAGE publication (Figure 1). Its 15 chapters cover 6 countries across the Arabian basin (Figure 1). The Khuff petroleum system and its exploration and development history on a plate-wide scale are outlined in chapter 1 by Pöppelreiter and Marshall.

StructurePalaeomorphology is one important control of facies distribution of the Khuff play. It influences source rock occurrence and richness, grainstone occurrence and thickness and evaporite and shale distribution that constitute the top seal. Genetically palaeomorphology is typically linked to structural evolution. Examples are basement composition, deep-seated faults, location of Cambrian salt basins and the dynamic interaction of these components with paleotectonics through the Earth’s history. Virtually all known Khuff accumulations are structural traps, some with stratigraphic components. Trap style is controlled by the structural setting of the region and the evolution of the Arabian Plate through time as outlined in chapter 2 by Oosterhout and Pöppelreiter. The Proterozoic consolidation of the Arabian plate through amalgamation of some 20 individual terrains (Teasdale, oral communication), the following Infra-Cambrian rifting and associate salt deposition as well as inverted basement fault tectonics influ-enced structure and facies distribution through the stratigraphic record (Ziegler 2001, Johnson 2008). The interplay and imprint of the early structural-stratigraphic framework on Khuff facies distribution, burial history and structuration are discussed by Singh et al. in chapter 3. Specific examples of technol-ogy, i.e., gravity and seismic inversion used to visualise basement position are shown in this chapter. The case study from Kuwait on basement morphology is used as a proxy for the prediction of source rocks. The investigations showed that two broad trap classes can be distinguished on a basin-wide scale: anticline structures that include N-S structures above Cambrian basement highs (e.g., North Field, Qatar/Iran) and Zagros trending anticlines (e.g., Kangan Field, Iran). The second type of structures are salt dome structures: Hormuz (Cambrian) salt domes (e.g., Zakum Field, UAE, offshore Fars, Iran) (Szabo and Kheradpir 1978).

In purely stratigraphic traps however, no economic hydrocarbon accumulations have been encountered so far. Potential stratigraphic geometries in the Khuff include unconformity traps (e.g., Central Oman and W Kuwait); traps below intraformational seals (e.g., Iran, Iraq), toe of slope grain-stone at the platform edge (e.g., Iraq, NW UAE); off platform isolated platforms (e.g., Oman) and grainstone in bypass channels at the platform edge (e.g., Oman). Evolving conceptual models and improved seismic resolution might reveal potential stratigraphic trap configurations, particularly at the platform edge, as exploration targets in the near future.

Tectonic elements and their relationship to Khuff facies across the platform margin are outlined by Richoz et al., in chapter 4. The sections described are palaeogeographically located at distal

Figure 2: Layer-cake type reservoir as seen in outcrops of NE Oman and the subsurface of Oman (from Aigner and Pöppelreiter 2011).

11

The Khuff Formation: Play Elements and Development History of an Epicontinental Carbonate Platform

platform, slope, tilted block, basement and deepwater settings. The observations derived from out-crop descriptions in Oman/UAE might be applicable to similar settings in Iran and Iraq (Aqrawi et al. 2009). The longevity of structural movements, including volcanic activity is documented from well data near the Iranian platform margin in chapter 7 by Karvoosi. Contrasting to commonly held opinion that volcanism and related stratigraphic expressions are restricted to initial Khuff deposition there is evidence of top Middle Permian volcanism and block faulting. Syn-sedimentary movements of basement blocks, erosion, seismic activity and volcanism have been documented previously from the lowermost Khuff (Weidlich and Bernecker 2012, Bendias et al. 2013. Walz et al. 2013). Minor seismic activity was recognised in the Lower Triassic Upper Khuff and Sudair at the platform edge in Oman and the UAE (Pöppelreiter et al. 2011, Toland 2006).

SourceThe source of Khuff gas is Silurian organic-rich graptolite shale in vast parts of the Arabian basin, as discussed by Faqira in chapter 8. The shale is part of the Qusaiba Formation and its time equivalents. It is a typical type II source rock with a distinct geochemical signature (Cole et al. 1994). Elements of the Khuff play, specifically the source rock distribution and maturity, are characterised across the vast Arabian platform in chapter 8 and together with migration, put into a context of structural evolution and salt movements. Source rock distribution and richness are controlled by syndepositional palaeotopography and post-depositional Hercynian tectonics. Mini-basins for example developed above glacial erosive relief (Spaak and Ross 2011), which tend to accumulate shale with higher TOC concentrations (Konert 2001). Similarly areas of stronger differential subsidence preferentially accumulated hot shale. Lower organic content is found above morphological highs, areas of slower differential subsidence and the landward platform margin (Bell et al. 2008). More importantly Hercynian uplift caused erosion of Silurian organic-rich shale particularly atop inverted basement faults. Examples of such missing Silurian hot shale intervals are the Burgan High and the Ghawar High (Strohmenger et al. 2002, Faqira et al. 2009). Another proven source rock is the Pre-Cambrian Q-oil in Oman (Terken et al. 2001) where escaped oil from older, leaky Palaeozoic reservoirs is re-trapped in Khuff carbonates (Perez-Gomez chapter 14).

Speculative sources are Intra-Khuff shale that has been observed in the KS-6 and the KS-2 sequences in Iraq, Iran and Oman (Aqrawi et al. chapter 9, Leda et al. 2013, Bendias et al. 2013) or intraformation-al microbial laminites, which are suspected to generate hydrocarbons (Dessort et al. 2006). Potentially Upper Devonian shales and the Mississippian Ora Formation in northern Iraq and Iran (Aqrawi et al. chapter 9) may act as source rocks in places such as the Proto Palmyra Graben (Konert 2001).

These alternative Palaeozoic source rocks might extend the prospectivity of the Khuff play.

ChargeCharge is controlled by regional basin evolution (Faqira et al. 2009, Perez-Gomez et al. chapter 14). Migration into Khuff structures is likely related to faults, as basal Khuff shales, tight carbonates in the lower Khuff and the overlying Median anhydrite make migration through matrix unlikely. Hence the established charge model assumes reactivated basement faults acting as pathways for vertical migration (Wender et al. 1998). Charge risk is present in areas with absent or overmature Qusaiba source rock such as in parts of Kuwait, Iraq and Oman. Deeply buried structures exhibiting high levels of CO2 and N2 are known from north-western Lorestan, Iran and deeply buried areas in the Gulf, along the Bandar Abbas fault (Iran-UAE-Saudi) and from the Zagros fore-deep in Iraq, Iran and Kuwait. In similar areas, the risk of encountering largely non-hydrocarbon gasses (H2S, CO2, N2, He) is significant (Nederloff et al. 2011). Basin, organic geochemistry and diagenesis modelling are vital tools for assessing this risk (Faquira et al. chapter 8).

12

Chapter 1

ReservoirEconomic success in the Khuff play is primarily controlled by the presence of permeable grainstone. Khuff rocks with mud-dominated textures may have some porosity, but low permeablity, due to cemen-tation by anhydrite or carbonate. Khuff grainstone occurs either as limestone or dolomite and consists largely of ooids and peloids with skeletal components being less common constituents (Haase et al. 2011). The Khuff ranges in thickness from 100 m (500 ft) in Saudi Arabian outcrops to 1500 m (4000 ft) in Iran platform margin settings (Al-Jallal, 1995). The Khuff and Permian Pre-Khuff clastics are separated from the underlying Palaeozoic clastics by the Hercynian unconformity.

This unconformity is covered by Permian clastics as shown from outcrops in Saudi Arabia (Evans et al. 1997). These Permian clastics in turn are truncated by a break-up unconformity associated with the opening of the Neotethys Ocean (Bell and Spaak 2011).

The base of the Khuff is a time-transgressive lithostratigraphic boundary above this break-up uncon-formity and covered by patchily developed Pre-Khuff clastics (Le Nindre et al. 2012).

The stratigraphic framework of the Khuff presented by Forke et al. in chapter 5 is based on extensive biostratigraphic analysis tied to sequence stratigraphy.

This framework is extended to the Lower Khuff type sections in the interior of Oman by Angilioni et al. in chapter 6.

High-resolution biostratigraphy and detailed documentation of macrofossils (Partoazar 2002) content of this skeletal-carbonate-dominated succession highlight the reservoir characteristics of the less well-known lower Khuff. Skeletal grains, commonly occurring in the foreshoal section at the seaward edge of the Khuff platform, are portrayed by Angiolini, chapter 6. The fauna consists of moderately diverse, high-abundance fossil association i.e. corals, brachiopods, foraminifer, algae, gastropods, crinoids. Typical carbonate producers such as corals and green algae are interdispersed. A few decimetre thick microbial biostroms (Heydari et al. 2003) are widespread above the Permo-Triassic boundary (Svensen et al. 2009). However build-ups or reefs are largely absent in the Khuff Formation. Environmental con-ditions such as water stratification and hypersalinity might have reduced fossil abundance and diversity. A narrow fringe of skeletal components exists during the Permian in the most open marine parts of the platform. Deepwater deposits are uncommon in the Khuff and restricted to the outer platform margin extending through Oman, Iran and Iraq. Importantly some of these open marine layers (KS-6, KS-2) may contain organic-rich shales (Bendias et al. 2013, Aqrawi et al. chapter 9, Leda et al. (2013).

Reservoir characteristics of oolitic grainstone deposited in shoal environments, is exemplified by the Iranian South Pars Field presented in chapter 10 by Esrafili et al. Reservoir quality is attributed to the interplay of relative sea level and basin morphology (Harchegani et al. 2011).

Several post-depositional factors are important in controlling Khuff reservoir properties. Early dolo-mitisation may significantly improve reservoir quality whereas precipitation of pores occluding anhydrite cements can transform originally porous rocks to non-reservoir rock. Carbonates exhibiting muddy textures and poorly cemented grain-dominated rocks tend to show physical and chemical compaction-related poros-ity loss.

Compaction, particularly associated with structures buried deeper than 3000 m, can be significant, especially in limestone. Dolomite tends to withstand depth-related porosity loss by physical and chemi-cal compaction to greater depth than limestone. Consequently porosity in dolostone is often preserved at a greater depth compared with limestone (Ehrenberg et al. 2005). Early charge, especially gas, tends to reduce physical and chemical compaction (Zampetti et al. 2010). Breaching of structures and hydrocarbon loss can negate the porosity preserving process. Burial diagenesis, in particular charge related processes such as fault-related thermochemical sulphate reduction (TSR), can further modify reservoir quality on a 100s to 1000s m scale.

The authors characterise shoal and back shoal Khuff grainstone from the subsurface of Oman that

13

The Khuff Formation: Play Elements and Development History of an Epicontinental Carbonate Platform

experienced a complex evolution in a structurally complex setting at the edge of an Infra-Cambrian salt basin. They authors relate structural position and overall basin evolution to the succession of shallow and burial diagenetic processes that modified the permeability architecture of this grainstone (Gomez-Perez et al. in chapter 14).

The anatomy of oolitic grainstone bodies in a near shore inter-tidal environment is illustrated by Janson in chapter 12. The outcrop study from Saudi Arabia places particular emphasis on grainstone architecture and its link to petrophysics. This occurrence of thin but clean and porous grainstone, near the platform margin, exposed to shallow burial only, highlights the potential of similar palaeogeographic settings, such as the Palmyra Graben in Syria and comparable areas in NE Iraq.

Predominantly low-energy lagoonal deposits are described in chapter 13 by Husain et al. The authors emphasise the genetic link between primary facies and reservoir quality. Chapter 13 illustrates the expres-sion of a palaeo-high, the Burgan Arch, on reservoir quality. The area above the high is covered preferen-tially by muddy texture that formed in low-energy probably lagoonal settings. This example illustrates the interplay between basement tectonics, palaeomorphology and Khuff sedimentation.

The diversity of sedimentary structures in the Khuff Formation is generally low. Common are normal grading, low-angle lamination and trough cross-bedding. Widespread in lower energy settings is microbial lamination and bioturbation. The nature and importance of bioturbation on petrophysical properties are outlined by Knaust in chapter 11. A new tool for reservoir characterisation is petrophysically-biased biotur-bation analysis. Khuff grainstone typically contains porosity of 3–30% and with a permeability of 0.1 to 1000 mD. Porosity in limestone is largely mouldic and thus the permeability is commonly below 1 mD. However bioturbation may enhance permeability significantly on a bed-scale. The influence of bioturbation on permeability architecture in the Khuff is shown from the South Pars Field in Iran.

Figure 3: Methodology for a Khuff outcrop study in Oman (from Aigner and Pöppelreiter 2011).

14

Chapter 1

SealThe Khuff reservoir is regionally sealed by the overlying Sudair Formation and its stratigraphic equivalents, as outlined by Pöppelreiter and Obermaier in chapter 15. The widespread Sudair Formation, often misleadingly referred to as ‘Sudair shales’, is 300 ft to 1200 ft thick (Alsharhan and Nairn 1997). Its evaporites and shales, probably deposited in lagoonal and mudflat environments, act as top seals to the Khuff reservoir below.

Distinct facies belts are mapped in the Sudair (Ziegler 2001, Poppelreiter et al. 2011, Obermaier 2013). Shale is dominating a landward belt, extending along the southern platform margin. It passes into a shale-anhydrite belt covering the platform interior. Largely tight carbonates (Ziegler 2011) constitute a seaward platform-margin belt.

Sealing lithology above the Khuff reservoir seems present across the Arabian platform. Hence seal risk is generally low.

Only in areas where the Khuff Formation is truncated by erosive unconformities it may be sealed by shale of Jurassic or Cretaceous age. Examples are found in Oman and Kuwait (Konert 2001). Top seal presence and integrity are minor exploration risks.

The bottom seal can be tight carbonate. Established intraformational seals are tight carbonates (e.g., KS-4 in Oman) and anhydrites such as the Median anhydrite in Iran.

Overview of exploration and development of the Khuff FormationAlthough exploitation of the Khuff reservoir was initially slow, the emergence of gas as a preferred energy source, along with significant technical advances (LNG and GtL) in the development of sour gas resources have resulted in an upturn in the number of Khuff developments coming on-stream in recent years. The steady discovery of significant hydrocarbon volumes in Khuff strata, as shown in Figure 5, highlights the future potential of the play.

Reserves Cumulative Khuff / Dalan Reservoirs Creaming Curve showing Cumulative Gas MMboe and Cumulative Liquid MMbbl

Figure 4: Creaming curve of the Khuff.

15

The Khuff Formation: Play Elements and Development History of an Epicontinental Carbonate Platform

BahrainThe first Khuff gas discovery was made in Bahrain in 1948 when a shallow crestal well, Awali-52, was deepened to a depth of 10,078 ft (3,072 m) and encountered gas in Khuff carbonates. The reservoir remained undeveloped for some 20 years until 1969 when Bahrain embarked on an industrialisation scheme and gas was required to provide feedstock for a newly constructed aluminium smelter.

Two commercial development wells were drilled in 1969 and Khuff gas was brought on-stream at an initial production of 50 MMcf/d, gradually increasing over time to meet the expansion of power plants and gas-dependant industries.

A second phase of development took place during the 1970s. Khuff gas from an additional nine wells replaced previously-used, condensate-rich Arab Formation gas used for injection and artificial lift in oil reservoirs, power generation and as feed stock and fuel in refinery operations. With the continued indus-trial expansion in Bahrain further development campaigns took place throughout the 1980s to 2000s. BAPCO most recently undertook a Khuff gas development program between 2008–2011, including the drilling of the first directional gas well in Bahrain to the pre-Khuff.

Saudi ArabiaAramco first encountered non-associated Khuff gas in commercial quantities in 1957 at Dammam-43, a wildcat in Dammam field that produced 25,000 Mcf/d.

Although Khuff gas was discovered in south Ghawar Field in 1971, it was not until 1975, with an ever increasing need for non-associated gas that an exploration campaign commenced in earnest. Khuff discoveries followed at Qatif and Berri fields in the mid to late 1970s; north Ghawar Field in 1975, east Ghawar Field, south Ghawar Field and Abu Sa’fah in the early 1980s; Khursaniyah Field in 1987; and Harmaliyah Field in 1990.

Aramco brought the first Ghawar Field Khuff well on-stream in December 1983, with subsequent development centred on the Shedgum/Ain Dar and Uthmaniyah gas plants. In 1995, Saudi Aramco embarked on an aggressive non-associated gas development program, aimed at fully exploiting Ghawar Fields’s non-associated gas resources. The program focused on the Haradh and Hawiyah areas. Saudi Aramco’s first gas plant dedicated to processing non-associated gas was brought on-stream at Hawiyah in September 2001. It was initially designed to process 1.6 Bcf/d feed gas, 1,200 MMcf/d from the Khuff and 400 MMcf/d from the Pre-Khuff. By September 2001, 44 Khuff wells were on-stream, pro-ducing 1.4 Bcfg/d. Trial production from the Haradh gas plant started on 23 April 2003, including 770 MMcf/d from the sour Khuff reservoir. Between 2000 and 2001, the Khuff gas processing facilities were expanded to 2.4 Bcf/d at Shedgum, through a 400 MMcf/d de-bottlenecking project and to 2.3 Bcf/d at Uthmaniyah.

Between 2003 and 2009, a further eight Khuff discoveries were made. The most significant of these, Karan Field, was the first non-associated gas discovery in Saudi Arabian territorial waters in the Gulf. The field was discovered in 2006, when Karan-6 found 770 ft (235 m) net pay over four Khuff intervals, believed to be the thickest Khuff reservoir section in the country. Karan will eventually contribute 1.8 Bcfg/d to Saudi Aramco’s Master Gas System. Early production of some 400 MMcf/d was commis-sioned during 2011, with three other platforms scheduled for completion and tie-in by June 2012, with a final platform due on-stream in April 2013. Gas will then be transported onshore to the Khursaniyah Gas Plant (KGP). The success at Karan Field was followed by Khuff discoveries at Arabiyah and Hasbah fields in 2008 and 2009. Both discoveries were fast-tracked for development. Saudi Aramco intends to produce 1.2 Bcf/d from the Arabiya and 1.3 Bcf/d from the Hasbah offshore gas fields by 4Q 2013 as part of the Wasit Development.

16

Chapter 1

OmanWhilst Khuff exploration had been underway for some time in the other Gulf States, it was not until oil was discovered in the formation at Yibal in 1986 that the Khuff Formation became a significant target for oil exploration in Oman. The deep exploration well, Yibal-85, drilled on the Yibal structure in North Oman 1977, confirmed sour gas and condensate from the K2 unit at a depth of 2,910 m. However, the presence of oil was not identified and the gas was classified as non-associated. The well was suspended as sweet gas was abundant in shallower and economically more attractive reservoirs.

Almost a decade later, a deep gas exploration well, Yibal-192, drilled in 1985 under the Government Gas Exploration Programme, tested oil from the K2, K3 and K4 Khuff units, gas/condensate from K1 and K5 and volatile oil from the deeper Gharif sands. As a result, the K1, K2, K3 reservoir units were re-classified as saturated oil reservoirs with a gas cap. Later drilling confirmed high levels of H2S and CO2. A long-term production pilot was conducted at Yibal in 1987, though terminated in 1997 follow-ing corrosion problems and operating difficulties.

A 2006, re-evaluation of the Khuff reservoir at Yibal concluded there was scope to develop one oil rim reservoir and two sour gas reservoirs through a single facility at the field. The FDP for the Yibal Khuff sour development project was scheduled for completion in 2011, with full Khuff reservoir development anticipated by 2019.

Following the early success at Yibal, a Khuff appraisal well drilled in 1987 at Al Huwaisah-46 tested 2,360 bo/d from Khuff K1 and 5.5 MMcf/d sour gas from Khuff K5.

UAEIn 1975, the first Khuff Formation exploration well, Mender-1, was drilled in Abu Dhabi. The well, located in the south-east of the state, proved unsuccessful. It was not until 1979, with the drilling of the Umm Shaif-88 offshore deep exploration well that gas was discovered in commercial quantities within the Khuff, at rates of 220 MMcf/d. Subsequent Khuff discoveries were made throughout the 1980s at Hair Dalmah, Fateh, Zakum, Nasr, Abu Al Bukhoosh, Satah, Bu Haseer, Sath Al Raaz Boot and Arzanah fields.

Significant Khuff development projects have included the launching of an US$ 350 million major gas development project by Adma-Opco in the 1980s for the Umm Shaif field. Khuff gas is used for pressure maintenance of the Middle Jurassic Uweinat reservoir and as a feedstock for the Abu Dhabi Gas Liquefaction Company (ADGAS) LNG plant located on Das Island. The Umm Shaif Gas Development project was commissioned in 1994. In 2002, Adma-Opco completed a pilot project using Khuff Formation gas from Abu Al Bukhoosh Field for gas lift at Upper Zakum Field.

The exploration and development of the Khuff reservoir throughout the region are illustrated in Figure 5. Discoveries are shown by decade from the 1940s to 2010s, from the first Khuff gas discov-ery in Bahrain in 1948 until the surge in Khuff-equivalent discoveries in Iran during the 2000s. Resulting developments by decade are presented from the 1960s to 2010s, highlighting the develop-ment activity that has taken place in the central Gulf region, notably through the 1990s and 2000s.

QatarQatar’s giant North Field was discovered by Shell in 1971 with the drilling of the North West Dome-1 exploration well on a broad regional high. The well reached a TD of 11,350 ft (3,459 m). It penetrated the entire Khuff succession and tested 46 MMcf/d plus condensate and water from a 83 ft (25 m) interval of Khuff K2 between 8,687 ft (2,648 m) and 8,770 ft (2,673 m). The field is the world’s largest non-associated gas field, extending over 6,000 sq km and contains approximately 900 Tcf recoverable natural gas within the Khuff, Khuff K4 being the most prolific.

Figure 5: Khuff reservoir discoveries by decade (1940s to 2010s) and developments by decade (1960s to 2010s).

17

The Khuff Formation: Play Elements and Development History of an Epicontinental Carbonate Platform

The magnitude of North Field’s non-associated gas reserves enables it to support world class gas uti-lisation projects. The field includes three main contract areas; RasGas, Qatargas and North Field Alpha. The North Field Alpha complex, operated by Qatar Petroleum, was the first to come on-stream and was officially inaugurated on 3 September 1991. The first phase of the Al Khaleej Gas Project started up in 2005 and the Dolphin Gas Project became operational in 2007. The Barzan Gas Project is scheduled for completion in 2015.

Since 1997, Qatar has been exporting LNG from the North Field. Some 14 LNG trains are now operational; seven operated by RasGas and seven by Qatargas. In 2006, Qatar became the world’s largest LNG exporter. The advent of Gas-to-Liquids technology has enabled the construction of the Oryx GTL Plant, the first of its kind in Qatar. The Oryx GTL plant was commissioned in 2007 and the giant Pearl GTL plant is currently under construction.

It is not only Qatar’s North Field that holds significant Khuff non-associated gas reserves. Gas was discovered in the Khuff at Dukhan Field in 1940, although the reservoir was not developed until 1976. The Khuff reservoir was shut-in when production began from the North Field and from 1992 surplus North Field gas was re-injected into Dukhan’s Khuff reservoir. The Khuff reservoir at Dukhan Field has since become a strategic gas reserve, where excess North Field gas is stored. The Khuff wellhead treatment plants at Dukhan Field are run on a rotational basis to work as a ‘hot’ standby, to ensure rapid start-up if the North Field platforms are shut-down.

The increasing interest in gas is also reflected in the time from discovery to first production (Figure 6).

OmanWhilst Khuff exploration had been underway for some time in the other Gulf States, it was not until oil was discovered in the formation at Yibal in 1986 that the Khuff Formation became a significant target for oil exploration in Oman. The deep exploration well, Yibal-85, drilled on the Yibal structure in North Oman 1977, confirmed sour gas and condensate from the K2 unit at a depth of 2,910 m. However, the presence of oil was not identified and the gas was classified as non-associated. The well was suspended as sweet gas was abundant in shallower and economically more attractive reservoirs.

Almost a decade later, a deep gas exploration well, Yibal-192, drilled in 1985 under the Government Gas Exploration Programme, tested oil from the K2, K3 and K4 Khuff units, gas/condensate from K1 and K5 and volatile oil from the deeper Gharif sands. As a result, the K1, K2, K3 reservoir units were re-classified as saturated oil reservoirs with a gas cap. Later drilling confirmed high levels of H2S and CO2. A long-term production pilot was conducted at Yibal in 1987, though terminated in 1997 follow-ing corrosion problems and operating difficulties.

A 2006, re-evaluation of the Khuff reservoir at Yibal concluded there was scope to develop one oil rim reservoir and two sour gas reservoirs through a single facility at the field. The FDP for the Yibal Khuff sour development project was scheduled for completion in 2011, with full Khuff reservoir development anticipated by 2019.

Following the early success at Yibal, a Khuff appraisal well drilled in 1987 at Al Huwaisah-46 tested 2,360 bo/d from Khuff K1 and 5.5 MMcf/d sour gas from Khuff K5.

UAEIn 1975, the first Khuff Formation exploration well, Mender-1, was drilled in Abu Dhabi. The well, located in the south-east of the state, proved unsuccessful. It was not until 1979, with the drilling of the Umm Shaif-88 offshore deep exploration well that gas was discovered in commercial quantities within the Khuff, at rates of 220 MMcf/d. Subsequent Khuff discoveries were made throughout the 1980s at Hair Dalmah, Fateh, Zakum, Nasr, Abu Al Bukhoosh, Satah, Bu Haseer, Sath Al Raaz Boot and Arzanah fields.

Significant Khuff development projects have included the launching of an US$ 350 million major gas development project by Adma-Opco in the 1980s for the Umm Shaif field. Khuff gas is used for pressure maintenance of the Middle Jurassic Uweinat reservoir and as a feedstock for the Abu Dhabi Gas Liquefaction Company (ADGAS) LNG plant located on Das Island. The Umm Shaif Gas Development project was commissioned in 1994. In 2002, Adma-Opco completed a pilot project using Khuff Formation gas from Abu Al Bukhoosh Field for gas lift at Upper Zakum Field.

The exploration and development of the Khuff reservoir throughout the region are illustrated in Figure 5. Discoveries are shown by decade from the 1940s to 2010s, from the first Khuff gas discov-ery in Bahrain in 1948 until the surge in Khuff-equivalent discoveries in Iran during the 2000s. Resulting developments by decade are presented from the 1960s to 2010s, highlighting the develop-ment activity that has taken place in the central Gulf region, notably through the 1990s and 2000s.

QatarQatar’s giant North Field was discovered by Shell in 1971 with the drilling of the North West Dome-1 exploration well on a broad regional high. The well reached a TD of 11,350 ft (3,459 m). It penetrated the entire Khuff succession and tested 46 MMcf/d plus condensate and water from a 83 ft (25 m) interval of Khuff K2 between 8,687 ft (2,648 m) and 8,770 ft (2,673 m). The field is the world’s largest non-associated gas field, extending over 6,000 sq km and contains approximately 900 Tcf recoverable natural gas within the Khuff, Khuff K4 being the most prolific.

Figure 5: Khuff reservoir discoveries by decade (1940s to 2010s) and developments by decade (1960s to 2010s).

18

Chapter 1

IranThe first discovery in Iran’s Khuff-equivalent Dalan and Kangan formations was Ferdows Field in 1966. However, Iran’s giant South Pars field, which represents the northward continuation of the North Field of Qatar, holds the greatest reserves, some 500 Tcf recoverable. The National Iranian Oil Company (NIOC) is planning to develop the field in 24 phases, capable of producing between 25 and 30 Bcf of natural gas per day.

At least 20 large fields and discoveries have since been made in Khuff-equivalent reservoirs in Iran. Development activities have taken place at Salman Field (an extension of Abu Al Bukhoosh Field), Kangan, Aghar, Nar, Homa, Day and Sefid Zakhur fields.

IraqIn Iraq, limestone and shale of the Upper Permian Chia Zairi Formation are equivalent to the Khuff Formation of the Gulf region and the Dalan Formation of southwest Iran. The formation, which reaches up to 810 m thickness in the area, has been drilled in only a handful of wells. Atshan-1, drilled in 1955 by the Mosul Petroleum Company, intersected 295 m of oolitic and argillaceous limestone, silty dolo-mite, shale and sandstone. The formation was found to be tight at this location. The Chia Zairi Formation was also penetrated at Jabal Kand-1 and Kifl West-1. The formation may be a prospect for gas, with the intraformational Satina Evaporite Member providing the seal.

KuwaitThe Khuff Formation has rarely been drilled in Kuwait. In 1984, the deeper pool well Umm Gudair-33, drilled on the Umm Gudair High, intersected 546 m of the Khuff Formation. The Khuff C reservoir is 107 m (350 ft) thick over the West Umm Gudair Field. Seismic inversion and lithology modelling studies of the Khuff Formation have been carried out by Rahaman et al. (2011) and Shereef et al. (2011). Results

Figure 6: Khuff Discovery and time to production.

Time to production from discovery: Khuff reservoirs

19

The Khuff Formation: Play Elements and Development History of an Epicontinental Carbonate Platform

indicate that the porosity of the Khuff C reservoir varies from 2–3% in the Minagish/Umm Gudair area. Best reservoir potential is observed over the East Umm Gudair anticline, suggesting that this may be an area for future exploration.

SynthesisThe presentations, summarised in this volume, reflect a paradigm shift in Khuff interpretation; the importance of tectonics in controlling all aspects of the Khuff play, a stratigraphic unit previously considered ‘atectonic’ by some workers. Integrated investigations of epicontinental platforms might improve predictability of rock properties in plays such as the Khuff where layer-cake type architecture on a seismic-scale is in sharp contrast to permeability variations over several orders of magnitude. The papers in this volume aim at shedding new light on the elements of the Khuff petroleum system. The Khuff reservoir is largely associated with high-energy (oolitic, peloidal) lime or dolo-grainstone as they constitute a permeable reservoir. Reefs as potential reservoirs are largely absent. Khuff grainstone is typically of sub-seismic thickness.

Gentle platform geometries yield subtle seismic geometries and attributes that typically are only interpretable in high-energy grainstone zones that mimic palaeomorphology. Grainstone occurs in irregular sheets deposited as interplay between climate, eustasy and palaeomorphology resulting in for example differential subsidence of basement structures and salt basins. Diagenesis may have either a positive or negative impact on the storage capacity of grainstone. More importantly it modifies grain-stone permeability through pore type alterations. Non-grainstone facies generally have a poorer storage capacity but thick intervals can hold significant volumes.

The very subtle expression of palaeotectonics across the Arabian platform has emerged as a key controlling factor on the Khuff play.

The authors, editor and reviewers hope to provide the reader with a comprehensive overview of the latest understanding of stratigraphy, reservoir occurrence and technology as set out at the beginning of this conference and enthusiasm in unravelling controls on epicontinental platforms.

AcknowledgementsThe meticulous reviews of this chapter by R. Koepnick, M. Vroon and P. Wagner are gratefully

acknowledged.

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