The Middle Permian to Early Triassic Khuff Formation occurs in the subsurface of the United Arab Emirates at depths that range from 3,688–6,188 m (12,097–20,297 ft) in Abu Dhabi and Dubai, and as outcrops in mountainous areas of the northern United Arab Emirates. The formation consists of a shallow-water carbonates that include limestones, dolomitic limestones with subordinate anhydrite and dolomites. It reaches a thickness of as much as 625–970 m (2,050–3,182 ft) in the subsurface and 125–960 m (410–3,149 ft) in outcrops. The Khuff Formation is interpreted as a second-order composite sequence represented by the KS1 through KS7 third-order sequences. The Khuff transgressive systems set starts with the KS7 event and ends at the maximum flooding surface of KS4. The highstand systems set starts in the upper portion of the Khuff with a second-order maximum flooding surface (MFS-4) and ends with a sequence boundary at the top of KS1 that characterizes the top of the Khuff Formation. The formation is subdivided into ten facies units distinguished on the basis of their depositional textures that represent an overall regressive carbonate-evaporite sequence. Based on the paleoecology, sedimentary structures and lithology, four distinct depositional settings can be recognized: (1) supratidal (sabkha), (2) lagoon, (3) shoal and (4) shallow shelf. The formation can be broadly subdivided into two major carbonate units deposited in two different hydraulic regimes, which are separated by an anhydrite bed (the Middle Anhydrite marker).
A detailed petrographic study of the Khuff carbonates reveals a complicated diagenetic history. Four diagenetic settings have been identified: (1) marine phreatic, (2) mixed phreatic, (3) meteoric phreatic, and (4) burial. The Khuff Formation has both primary and secondary porosity. Most open pores are a result of interparticle, intercrystalline, dissolution vug or enlarged mouldic porosity. The diagenetic features in these sediments are mainly partial cementation, dolomitization and the development of secondary anhydrite. Porosity ranges from 6–20% and permeability from less than 1.0 to more than 500 md. Horizontal permeability is greatly enhanced by subvertical partings of the open pores, common in microcrystalline dolomites. Stylolites are common, but unimportant as vertical barriers. Extensive fracturing of the reservoir has produced a dense network of intersecting vertical and subvertical fractures. These fractures have a significant impact on the enhancement of the effective porosity and permeability.
The Khuff Formation has large volumes of proven gas reserves in Bahrain, Iran, Qatar, Saudi Arabia and the United Arab Emirates and minor oil in Oman. While the Khuff Formation forms prolific gas and condensate reservoirs in the offshore United Arab Emirates, no hydrocarbons have been found in the onshore area. Locally here the reservoir is capped by the shales and dolomites of the overlying Sudair Formation (Early Triassic). The formation is sourced from Silurian Qusaiba shales.
The Middle Permian to Early Triassic Khuff Formation is a widespread rock unit in the Arabian Basin. It is composed of shallow-water carbonates and evaporites, representing a second-order transgressive-regressive sequence, which is composed of seven third-order composite sequences (Figure 1; Sharland et al., 2001; Strohmenger et al., 2002). In the subsurface of the United Arab Emirates (UAE). the formation consists of a sequence of microcrystalline dolomites with subordinate limestones and anhydrites that collectively range from 625 to 970 m (2,050 to 3,182 ft) in thickness. Dolomite is the dominant lithology and constitutes some 75% of the formation in the western Abu Dhabi fields, increasing to about 85% eastwards towards the Dubai fields. Original textures are relatively well-preserved in the dolomite. Anhydrite constitutes about 5–10% of the Khuff Formation to the west. Eastwards there is a conspicuous diminution of the anhydrite content. The Khuff Formation accumulated as a series of cyclic units. Each cycle commences with a dolomitic subtidal grainstone/packstone that passes upwards into lagoonal/intertidal dolomites capped by subtidal/supratidal anhydrite or dolomitic anhydrite.
The Khuff Formation is divided into two major carbonate units separated by an anhydrite bed known as the Middle Anhydrite marker (Figure 2). The Lower Khuff lies between the sandstones of the Unayzah Formation and the base of the Middle Anhydrite. The Upper Khuff begins with a sharp boundary at the base of the Middle Anhydrite and ends with a minor erosional unconformity at the base of the Lower Triassic shaley dolomitic limestone of the Sudair Formation.
Exploration drilling for deep hydrocarbon reservoirs started in 1975 with the onshore well Mender-1, which was dry. In 1979, the first Khuff gas discovery was made in the offshore area (Umm-Shaif-88 well). The formation has since proved to be a significant gas reservoir and is now a major producer and exploration target. A series of deep wells were drilled, both in the offshore and onshore areas, which presently number a total of 25 wells covering 18 structures (Figure 3). Eleven structures proved to be commercially gas-bearing in the Khuff Formation; two have sour gas and nine sweet gas. The gas shows variations in composition with the methane being the dominant component. Diagenesis plays a significant role in altering the original petrophysical characteristics of the rocks; also fractures have a significant impact on the porosity and permeability of the reservoirs (Alsharhan, 1993).
This paper describes several of the Khuff Formation characteristics in the UAE including: (1) the sequence stratigraphy of the Khuff Formation in the subsurface and the analog in outcrop formations in the northern part of the United Arab Emirates; (2) the diagenetic history and its effect on reservoir quality; (3) the porosity distribution, origin and relationship to lithology and reservoir characteristics; and (4) the hydrocarbon potential.
STRUCTURE AND VARIATION IN THICKNESS
In the United Arab Emirates (UAE), the Khuff Formation is present at depths that range from 3,688–6,188 m (12,097–20,297 ft) below sea level. It exhibits regional variations, deepening in eastern Abu Dhabi towards Dubai, and a regional shallowing trend towards the west and southeast (Figure 4). The regional isopach map of the formation in the UAE shows two pronounced anomalies (Figure 5). The first is local thickening within the Khuff basin in eastern Abu Dhabi, with the axis of maximum subsidence running in a NNW-SSE direction through the Nasr, Jarn Yaphour and Mender areas (Elbishlawy, 1985). This suggests that the eastern part of Abu Dhabi subsided at a faster rate relative to western Abu Dhabi, and received more sediment throughout the Late Permian to Early Triassic times. This depocenter received deeper-water sediments that matched the increase in accommodation that developed here. The second anomaly is an elongated NS-trending thin zone located east of – and parallel to – the Qatar Arch. This zone passes through the Hair Dalma, Satah, and Idd Al Shargi fields, and indicates the existence of a pre-Khuff paleohigh that formed an actively emerging block during deposition.
Sharland et al. (2001) and Strohmenger et al. (2002) interpreted the Khuff Formation as a second-order transgressive-regressive sequence that is composed of seven third-order composite sequences (termed Khuff sequence KS1 through KS7) (Figure 2 and Table 1). The maximum flooding surfaces at the Khuff sequence KS4 (dated as 254 Ma) (Sharland et al., 2001; Strohmenger et al., 2002) is a second-order maximum flooding event. The transgressive systems set of the Khuff starts with KS7 and ends at the maximum flooding surface of KS4. The highstand systems set starts above the second-order maximum flood surface and ends at the top of the Khuff sequence KS1 (Strohmenger et al., 2002).
The Khuff Formation in the subsurface of the UAE can be broadly divided into two major carbonate units (the Upper and Lower Khuff units) separated by the Middle Anhydrite marker (Figure 2).
The Lower Khuff is about 425–500 m (1,394–1,640 ft) thick and composed, in the basal part, of argillaceous dolomites with minor terrigenous mudstones and thin-bedded shales. This is followed upward by lime mudstones and wackestones, minor interbeds of packstones and grainstones. The limestone is partially dolomitic with some of the dolomite exhibiting rhombic textures and relict wackestones to packstones textures. This style of sedimentation continued through the lower Khuff with the thickest sequence of the cycle dominated by microcrystalline and rhombic dolomite, with relict peloidal, oolitic and bioclastic grains and scattered patches and nodules of anhydrite, with minor (occasional) intercalations of lime mudstones and wackestones. This limestone is abundantly fossiliferous, containing fusulinids, molluscan and echinoid fragments, algae and bryozoa. Deposition of the Lower Khuff ended with deposition of thin lagoonal microcrystalline dolomites before reaching the massive (marker bed) of anhydrite (Middle Anhydrite).
The Middle Anhydrite is a clear and obvious marker horizon of regional significance, and occurs approximately at the midpoint of the formation. It acts as a barrier to vertical fluid flow, and is used to subdivide the Khuff into two thick informal members: the Upper and Lower Khuff, deposited in two different hydraulic regimes. The Middle Anhydrite can be traced from eastern Saudi Arabia to offshore UAE, and reaches the onshore Fars Province of Iran. This anhydrite is about 12–17 m (20–39 ft) thick and consists of massive anhydrite with minor thin interbeds of dolomite. The anhydrite is interpreted to have formed largely in a coastal sabkha. Characteristically for this setting it exhibits chicken-wire and nodular fabrics, and is interbedded with tidal flat sediments. Local laminated and massive fabrics suggest hypersaline lagoons co-existed with the coastal sabkha.
The Upper Khuff is about 360–470 m (1,180–1,542 ft) thick, and formed by a rather complex series of depositional cycles. It commenced with accumulation of dolomitized peloidal, oolitic and molluscan packstones and grainstones with abundant anhydrite cement. This is followed upward by dolomicrites and rhombic dolomites with some relict peloids and oolites, and also undolomitized or slightly dolomitized grainstones and bioclasts packstones. The upper part of the Khuff Formation commenced with the accumulation of mainly high-energy dolomitized peloidal bioclastic, foraminiferal and oolitic packstones with some minor thin lime mudstones interbeds and thin interbeds of anhydrites. The uppermost part of the Khuff sequence consists of lime mudstones, dolomicrites and some peloidal packstones/grainstones. This Upper Khuff is abundantly fossiliferous containing gastropods, pelecypods, ostracods, algae and fusulinids.
Outcrops in northeastern UAE, close to the Oman Mountains, provide additional information for the late Paleozoic rocks of the region. Throughout the Oman Mountains, large detached blocks of poorly bedded, white, recrystallized limestone of Permian-Triassic age are found in the sedimentary melange between the Hawasina Thrust sheets and the Semail Ophiolite. These olistoliths in the Dibba Zone form the Al Qamar (North and South) mountains of the Idhan area. Both these blocks of carbonate and clastics are several kilometers across and contain several different recognizable units (Figure 6; Hudson et al., 1954; Robertson et al., 1990; Alsharhan and Nairn, 1997).
Asfar and Qamar formations, Al Qamar Mountains
The Early Permian Asfar Formation comprises up to 25 m (82 ft) of sandstone and limestone, passing-up from fine- to medium-grained siltstone and sandstone to lenticular, cross- and wavy-bedded, fossiliferous limestone, which in turn give way upwards to recrystallized, bioclastic limestone. The top of the formation consists of alternating bioclastic sandstone, quartzose sandstone and stylolitic, micritic limestone. The sandstone appears as the fill of channels cut into the shallow-water carbonates. The Asfar sandstone and limestone contain fossils that include productids, spiriferids and fenestellid bryozoans of Early Permian age (Hudson et al., 1954).
The Qamar Formation of Late Permian age ranges in thickness from 60 to 100 m (197–328 ft), and consists of brown, weathered, mainly fine-grained limestone, which is generally recrystallized. Massive to weakly or rubbly-bedded boundstone is interpreted as patch reefs developed on fault blocks. The limestone rests disconformably over the beds of the Asfar Formation. The Qamar limestone was laid down in a shallow-marine setting and contains fusulinids, corals and brachiopods.
Russ Al Jibal Group, Ras Al Khaimah
In the Ras Al Khaimah Emirate, United Arab Emirates, a thick sequence of Permian-Triassic rocks crop out in Jabal Hagab and are found in upthrusted blocks at Wadi Ghail and Wadi Hagil. They are formed by mainly gray, dolomitic, karstic, limestone, collapse breccias with stromatolites, fenestral structures and edge-wise conglomerates. These rocks were originally deposited in an intertidal to supratidal setting. Three formations have been recognized and are collectively known as the Russ Al Jibal Group: Permian Bih and overlying Hagil formations, and the lower part of the Triassic Ghail Formation (Figure 7; Hudson et al., 1954; Hudson, 1960; Alsharhan and Kendall, 1986; Alsharhan and Nairn, 1997; Strohmenger et al., 2002). The Russ Al Jibal Group is interpreted to be coeval to the Khuff Formation in the subsurface of Abu Dhabi.
The Middle Permian Bih Formation consists of about 200 m (650 ft) of gray to dark-brown, medium-grained, burrowed, saccharoidal dolomites, with occasionally interbedded fossiliferous, dolomitized grainstone and some minor porcellaneous, dolomitic limestone. The sediments display the characteristics of shallow-water, moderate to high-energy deposits laid down in a subtidal to supratidal setting and whose content of fusulinids and algae are the basis of age assignment (Alsharhan and Kendall, 1986). The Bih Formation corresponds to the Khuff sequences KS4 to KS7. The top of the Bih Formation consists of cryptalgal laminates that are mudcracked and pock-marked with possible raindrop imprints.
Above the Bih Formation, the Upper Permian Hagil Formation consists of about 260 m (853 ft) of alternating fine, gray, argillaceous limestone (which are often cross-bedded with shaly partings or partings of shale) and fine, dark-gray, dolomitized, slightly oolitic limestone containing algae and foraminifera. The formation is interpreted to have accumulated in a shallow-water, supratidal to subtidal setting.
The contact between Hagil Formation and massive dolomites of the overlying Ghail Formation is unconformable and shows an erosive surface with karst breccias. The contact is placed above a dark-gray, limestone breccia containing prominent fragments of light-colored, porcellaneous limestone. The Early Triassic Ghail Formation consists of 500 m (1,640 ft) of massive, sucrosic dolomite interbedded with dolomitized limestone. Relict peloidal and bioclastic structures can be distinguished, particularly in the upper half of the succession. In the lower part, large-scale current bedding is apparent. Solution breccias, cavernous weathering, fracturing and buckling of strata also are features of the beds of this formation, which was laid down in an intertidal to supratidal setting. Occasional marl can be found in the lower and middle parts of the formation. The formation has a sparse ostracod, algal and foraminiferal assemblage (Hudson et al., 1960).
LITHOSTRATIGRAPHY AND LOG CORRELATIONS
Based on gamma-ray/porosity log correlations, together with lithological descriptions from cores and cuttings, the Khuff section has been subdivided from the base upwards into four major lithostratigraphic units that can be mapped across the Abu Dhabi area with a fair level of confidence (Figures 2 and 8) (Alsharhan, 1993; Elbishlawy, 1985; Lutfi and Elbishlawy, 1989).
The Khuff-A unit is about 220 m (722 ft) thick and consists of lime mudstones and wackestones to packstones that are interpreted to have been deposited within a shallow water, relatively quiet open-marine setting. At the base, the sediments are argillaceous dolomites with minor terrigenous mudstones and thin-bedded shales. The percentage of dolomite increases toward the east (from Umm Shaif to Fateh fields). These dolomitic limestones are mainly rhombic dolomite that represents relict wackestones and minor interbedded packstones and grainstones. The upper part of Khuff-A is a thick sequence of microcrystalline and rhombic dolomites with scattered relict peloidal/oolitic/bioclastic wackestone to packstones, with some grainstones and patches and nodules of anhydrites. The Khuff-A unit is characterized by rather poor porosity development (less than 1%). This unit is defined as the interval between gamma-ray log marker-1 (base of Khuff Formation) and log marker-2 and is characterized by a spiky gamma-ray log response. The unit contains abundant marine fossils including fusulinids and smaller foraminifers, algae, bryozoans, molluscan and echinoid fragments (Table 2) dated as middle Murghabian (= Wordian = Kazanian) in age.
The Khuff-B unit has an average thickness of 188 m (617 ft) and is dominated by dolomicrite, microcrystalline dolomites, nodular anhydrite and subordinate grainstones, lime mudstone, and wackestone. The sediments consist mostly of oolites, pellets, foraminifers and algae. Anhydrite is common and sedimentary structures are mainly planar and ripple cross-laminations. The unit has low to fair porosity averaging 8%, but exceeding 20% in some parts. This high porosity is related to rhombic dolomite development with good intercrystalline and vuggy porosity. This unit is defined at the base and top by log markers-2 and 3 respectively, and is characterized by a very high gamma-ray response that indicates a high clay and organic matter content.
The Khuff-C unit reaches a maximum thickness of 470 m (1,542 ft) and is composed of foraminiferal oolitic and dolomitic grainstones and packstones, dolomicrite, stromatolitic mudstones and microcrystalline dolomite and anhydrites. This unit has low to moderate porosity with a maximum of 18% and a mean of 6%. The lower part of this unit consists mostly of microcrystalline dolomite with varying degrees of anhydritization, which reduced the porosity and permeability. The upper part of the unit consists of dolomite and dolomitic grainstones with good permeability (75 md) and moderate porosity (10%). The rhombic dolomite has good intercrystalline and vuggy porosity. Some grainstone intervals have low porosity and permeability interpreted to be the result of partial dolomitization and cementation by anhydrite. The unit is defined at base and top by gamma-ray log markers-3 to 6. The gamma-ray log response varies from low to spiky.
Based on common faunas including fusulinids, smaller foraminifers and algae (Table 2) found in units B and C, a broad late Murghabian (= Wordian) to indeterminate Djulfian (= Wuchiapingian) age is assigned for these units.
The Khuff-D unit has a maximum thickness of about 147 m (482 ft) and is dominated by oolitic dolomitized grainstones, dolomicrites, mudstones, peloidal packstones and subordinate interbeds of anhydrite. The unit is characterized by low- and high-energy oolitic grainstone (a shoal setting). The Khuff-D unit is dominated by moderate to low porosity with a mean of 10%. The permeability is very low (less than 1 md) with a maximum of 30 md. The unit is made up of rhombic dolomite with intercrystalline and vuggy porosity and with grainstones, which are strongly recrystallized and characterized by interparticle and vuggy pores. This unit is defined as the interval between gamma-ray log marker-6 and log marker-7 at the top of the Khuff Formation. The gamma-ray signature is distinct and reflects the two sedimentary cycles that show a decrease followed by an increase in the gamma-ray log response. The unit contains small pelecypods (Claria sp.) and abundant marine fossils (Table 2) that indicate a possible age of Late Djulfian-Scythian (= Late Wuchiapingian to Induan).
BIOSTRATIGRAPHY AND AGE
Based on foraminiferal evidence, the lowermost part of the Khuff Formation is interpreted to be Kazanian (Wordian) or younger in age. The section is characterized by abundant foraminifers dominated by the Hemigordioisidae (Hemigordius) and Biseriamminidae (Globivulina) associated with some Nodosariidae and Fusulinacea. Also present are dasycladacean and gymnocodiacean algae. The uppermost part of the Khuff is Early Triassic in age. The occurrence of Claraia spp. (a small pelecypod) dates this section as Early Triassic.
Three micropaleontological zones (A to C) in the Khuff Formation are reported by Elbishlawy (1985). Assemblage Zone-A (in the lower part of the Khuff is comprised of nodosariid foraminifers, coral fragments, bryozoans and occasional fusulinids) is of Late Permian age (late Kazanian to early Tatarian = Wordian to Wuchiapingian). Assemblage Zone-B (lower and upper Khuff sections), is characterized by miliolid species and calcareous algae, which indicate an early to late Tatarian (= Wuchiapingian) age. Assemblage Zone-C (the uppermost Khuff to lowermost overlying Sudair), is characterized by abundant microgastropods, ostracods and thin-shelled bivalve filaments and is assigned an Early Triassic (Scythian) age. The main faunas listed in Table 2 are found in sedimentary sequence of the Khuff Formation, which gave an age of Ufimian-Tatarian to Early Scythian (Middle Permian-Early Triassic) in Abu Dhabi.
NATURE OF CONTACT BOUNDARIES
In the subsurface, the lower boundary of the Khuff is placed at the contact between the carbonate of the Khuff and the underlying terrigenous clastics of the late Carboniferous-Early Permian Unayzah Formation (Alsharhan, 1989). In the Umm Shaif field, the contact is marked by the change from micritic limestone, argillaceous in parts, of the Khuff Formation to silty claystone of the underlying Unayzah Formation. Eastwards from the Zakum to Fateh fields, the basal Khuff consists of microcrystalline dolomite, partly argillaceous and sandy with thin interbeds of shale and terrigenous mudstone underlain by the argillaceous siltstone of the Unayzah Formation.
Evidence of an unconformity or erosional surface at the boundary between the Paleozoic and Mesozoic Khuff is lacking.
A minor erosional surface marks the upper boundary of the Khuff Formation in the United Arab Emirates. The upper boundary in the Umm Shaif field is taken at the change from clean limestone of the Khuff to argillaceous dolomites, dolomitic limestones with thin beds of shales, which mark the basal part of the Lower Triassic Sudair Formation; this is possibly an intra-Khuff lateral facies change before the “Sudair Shales”. At Zakum field, the boundary is positioned at the contact between pyritic shales of the basal Sudair and slightly dolomitic limestone of the upper Khuff. At Fateh field the basal Sudair consists of microcrystalline dolomite with thin interbeds of shale, which overlie calcareous dolomite with anhydrite inclusions of the Upper Khuff.
FACIES ANALYSIS AND DEPOSITIONAL MODELS
The sediments of the Khuff Formation in the United Arab Emirates have been grouped for this study into ten facies units (A-J) (Table 2), each of which represents a distinct environment or subenvironment of deposition. The facies-type numbering (J-A) is from open-marine to restricted inner-shelf, shoal, lagoon to sabkha (Figure 9a). The original carbonate sediments of the Khuff Formation have been replaced extensively by dolomite. The original depositional textures, sedimentary structures and faunas are often recognizable although poorly preserved. The carbonates of the Khuff Formation show the environmental characteristics of a typical cyclic carbonate-evaporite sequence, matching the sequence described by Wilson (1975) in other parts of the world. Recognition of these settings is based on ecology, sedimentary structures and sedimentary facies. Four major settings are recognized (Table 3). These are supratidal (sabkha), lagoon, shoal and shallow-shelf. The generalized sedimentary cycle is illustrated in Figure 9b. The cycle begins with sediments from high-energy settings represented by the seaward side of the shoal, grading upward to moderate high energies of the leeward side of the shoal and followed by the protected area of lagoon and locally ending in a supratidal (sabkha) setting.
The primary processes that affected the Khuff are dolomitization and leaching. Dolomitization occurred in two stages, resulting in several textures: (1) a first generation of dolomite in the form of both fine- and medium-grained rhombs and a microcrystalline (sucrosic) fabric; and (2) a second generation of mosaic and baroque (saddle) dolomite.
Cementation and recrystallization were significant in the study area. Three primary calcite cement textures are recognized: (1) a medium-grained neomorphic calcite spar, which developed where original fibrous cements and skeletal grains recrystallized to low Mg-calcite; (2) a fine, equant, sparry calcite, which replaced the aragonitic carbonate mud matrix in mud-supported limestones and fills some fractures and pressure solution seams; and (3) a coarse, mosaic calcite cement, which formed during burial, filling fractures and pressure solution seams. In addition, syntaxial overgrowth cements developed on echinoid fragments and locally engulf the earlier druse cement of adjacent grains.
The anhydrite found in the study area is believed to have formed in sabkha and hypersaline lagoonal settings. The anhydrite textures are either blocky or in the form of laths, and exhibit a chicken-wire texture. This latter is believed to be a product of compaction of earlier-formed anhydrite textures during burial.
Marine Phreatic Stage
Sediments at – or just below – the sediment/water interface were affected at this stage of diagenesis. Mechanical and biological reworking of the sediment occurred in conjunction with the micritization of grains by boring algae (Plates 1a and 1b). Accumulation of grains by biological reworking, burrowing and excretion of faecal pellets is prevalent in the sediment. Cracks and fenestrae are observed locally and the original voids are occluded by anhydrite or dolomite. Most peloids were probably micritized skeletal grains whose fabric is a product of microboring cyanobacteria and fungi which formed micrite envelopes (Plate 1c). The process goes on centripetally around the grains and may completely convert peloids to micrite (see also Bathurst, 1966).
Former marine cement fabrics (Plate 1d) are selectively preserved on some skeletal grains as 10–50 μm square-tipped fibers/stubby blades often in association with a micrite envelope, which surrounds the grain. Folk and Assereto (1976) and Assereto and Folk (1976) have suggested that square-tipped fibers are indicative of aragonite. The micrite envelopes are thin around skeletal grains (5–10 μm) and, like the square-tipped fibers/blades which may encrust them, are probably aragonitic. The micrite envelopes, also of marine origin, are commonly indistinguishable from the micritized outer layers of non-skeletal allochemical grains, ranging in thickness from 5–30 μm.
Patches of ferroan calcite are common in mudstones and wackestones of the phreatic environment. In those limestones that are dolomitized, the replacive dolomite rhombs forming within the ferroan patches contain Fe inclusions. This suggests early incorporation of Fe into the calcite crystal lattice in the marine phreatic environment. The phreatic waters are reducing, so Fe is in the ferroan (Fe2+) state and is easily incorporated into the calcite crystal lattice (James and Choquette, 1990).
Mixed Phreatic Stage
Sediments on the order of tens to hundreds of meters are affected by mixed meteoric/marine diagenesis (Loucks and Budd, 1981). The dominant diagenetic process in the Khuff Formation during this stage is dolomitization, which occurred in several forms. Aphanocrystalline dolomite rhombs (15–45 μm) occur in the mud-supported textures and in some oolitic grainstones (Plates 2a and 2b). Some of the original micrite and bioclasts are still associated with both dolomite and felted anhydrite nodules, suggesting that the early dolomitization is linked to early evaporitic diagenetic environments. Anhedral to subhedral sucrosic dolomite occurs as fine-grained (5–25 μm) to medium-grained (25–80 μm) crystals (Plate 2c). Early burial dolomitization also affected some grainstones and packstones in which grains have been altered to dolomicrite and/or microcrystalline dolomite (Plate 2d).
Selective dolomitization in some grainstones/packstones occurred where the cores of ooids, peloids and bioclasts were replaced by clusters of medium- to coarse-grained (45–60 μm) rhombic dolomite. Penecontemporaneously, neomorphism of ooids, peloids and bioclasts was taking place, the grains recrystallizing as a mosaic of fine-grained (5–15 μm) to medium-grained (25–35 μm) sparry calcite (Plate 2e). The interparticle areas are cemented by calcite crystals, coarsening toward the center of interparticle spaces.
Hanshaw et al. (1971) and Land (1973) described a model for mixing-zone dolomitization to explain Quaternary dolomite in limestones of Florida and Jamaica. This model suggests that the mixing of marine and fresh water creates brackish water undersaturated with respect to Mg2+ (Hanshaw et al., 1971; Land, 1973; Longman, 1980). Theoretically, the CO2-saturated water mixes with sea water, dissolving CaCO3 and precipitating dolomite. The Mg2+ is supplied by the sea water and dissolved CO3 (Longman, 1980). Uncemented fractures, stylolites and the widespread intercrystalline porosity of the dolomites of the Khuff Formation create enhanced permeability through which the mixed water could then circulate, modifying the depositional proper of the uncompacted grainy sediments.
The optimal fresh water to sea water ratio is 5:1 for mixing zone dolomitization (Longman, 1980). This implies that the water was undersaturated with respect to evaporite minerals. Thus, the anhydrite, commonly present in association with dolomite, would have to have formed after the dolomite because any evaporites would have been dissolved. However, when hypersaline water is diluted by fresh water, a large drop in salinity occurs in association with only a slight drop in the already high Mg to Ca ratio (Longman, 1982). The result is a schizohaline environment characterized by rhombic dolomite formation when the rate of crystallization is low to moderate (Folk and Land, 1975; Longman, 1982). The Mg to Ca ratio of water flooding the sabkha increases landward due to gypsum precipitation. The Mg-rich water sinks downward into the sediment and moves seaward through seepage refluxion (Morrow, 1990). Underlying intertidal and subtidal sediments are dolomitized to a depth of 2–3 m (7-10 ft) beneath the sabkha landward of the algal mat zone (Bush, 1973; DeGroot, 1973). This model provides a source for the Mg2+ ions and a means for the formation of the aphanocrystalline dolomite rhombs in association with anhydrite.
The arid conditions during the Late Permian and Early Triassic times, and the association of dolomite and anhydrite suggest hypersalinity and sabkha diagenesis (Alsharhan and Kendall, 2003).
Micrite accretion is common in laminated mudstones but is often recrystallized and visible only under SEM. These calcite overgrowths appear as crystalline facets, perched on micrite particles.
The Middle Anhydrite layer separating the Upper and Lower Khuff is characterized by “chicken-wire” anhydrite (Plate 2f). Penecontemporaneous in origin are anhydrite nodules observed in mud-supported sediments (Plate 2g). They are composed of felted masses of rod-shaped crystals (Plate 2h). These lath-shaped rods can reach over 200 μm in length depending on available pore space, and are composed of elongate crystals whose c-axis ranges from 10–160 μm in length (more commonly between 35 and 70 μm). The anhydrite nodules are thought to have formed: (1) from gypsum rosettes by burial dehydration; (2) by displacement of the sediments that contained anhydrite and directly influenced the morphology of the depositional surface; and (3) in grainstones which were subject to hypersaline conditions.
Meteoric Phreatic Stage
Associated with this environment is dissolution (leaching) and incipient solution-compaction (Loucks and Budd, 1981). Leaching is a major pore-producer in the grain-supported textures of the Khuff Formation. Early leaching produced moldic porosity and dissolution vugs prior to dolomitization (Plates 3a and 3b). This is evidenced by the fact that, in most cases, the dolomite rhombs are developed on the walls of the dissolution vugs and infilled oomolds. Late leaching includes dissolution of calcitic remnants from a dolomitized matrix. Aragonitic bioclasts have suffered total dissolution and many ooids and peloids have been partially to completely removed. Original grain outlines are commonly preserved through the existence of micrite envelopes or submarine rim cements (Plates 3a and 3c).
In the rhombic and moderately crystalline dolomites, intercrystalline pores are commonly cemented by a later blocky equant calcite spar (Plates 3b, 3c and 3d). This equant spar is void-fill cement and differs from the mosaic sparry calcite produced in the mixed phreatic stage, which represents the neomorphism of allochemical grains. Patches of lath-shaped anhydrite and poikilotopic anhydrite in fractures are locally replaced by this equant blocky spar. However, many fractures remain uncemented and occur along mudstone-wackestone boundaries.
Syntaxial overgrowth cements that developed on echinoid fragments locally engulf the earlier druse cement of adjacent grains. This syntaxial calcite cement has grown in optical continuity with these fragments, elongate along the c-axis and generally surrounding the grains (Evamy and Shearman, 1965).
Compaction and pressure solution during the burial stage cause extensive fracturing and stylolitization, as well as microcrystalline dolomitization. Porosity reduction is common during this stage due to compaction and cementation (Loucks and Budd, 1981; Longman, 1982; Prezbindowski, 1985; Morrow, 1990). Late burial cements and replacement fabrics in the Khuff include anhydrite, coarse crystalline calcite, silica, sulfur and baroque dolomite.
Microcrystalline dolomite is preserved in oolitic/peloidal grainstone/packstone and localized finegrained subhedral to rhombic dolomite also precipitated along fractures and stylolites.
Compacted and deformed ooids are present in a few intervals. Compaction effects are generally minor and localized although distinctive (Plate 4a). Grain- compaction effects post-date rim cementation and leaching, and include distortion of micrite envelopes, spalling of dolomite rim cements and flattening of cement-rimmed ooids and peloids. Within the intraclastic-peloidal packstones-grainstones compaction has resulted in overpacking of grains.
Nearly all recorded stylolites in the Khuff are subhorizontal at angles ranging from 5–45°. Rare subvertical stylolites occur. The dominant types of stylolites are rectangular, sharp-peaked and wave-like. Many stylolites are located at the boundary of two rock types (mainly mudstones and wackestones). Stylolites which belong to dissolution seams are very low amplitude and tend to represent organic shaley seams (Plate 4b). Sharp-peaked stylolites with small amplitudes (less than 1 mm) are usually found in microcrystalline dolomites. The rectangular stylolites that formed in limestones and rhombic dolomites are commonly open, but are occasionally filled by fine equant sparry calcite (like that described in the meteoric phreatic stage) or poikilotopic anhydrite. The stylolites cut across all grains and cements, confirming their late diagenetic timing. Locally, where fissures are associated with the stylolites, they act as blocking planes across the fissures.
Most of the fracture and fissure planes appear subvertical. Some fractures are calcite-cemented and less than 1 mm wide; others are wider (up to 5 mm) but very commonly anhydrite-cemented (Plate 4c). Uncemented fractures may reach 30 cm in length and a few fractures exceed one meter. In the Khuff Formation in Abu Dhabi, 25–45% of the section is affected by fractures with an average of 6–10 fractures per vertical meter of section. Nearly all the fractures are either completely free of cement or nearly completely cemented by dolomite, calcite or anhydrite. Most of the fractures are of high angle (more than 70°) and are oriented parallel to the structural axis of the fields (Lufti and Elbishlawy, 1989).
Cementation by anhydrite is of late deep burial origin and is composed of coarsely crystalline, poikilotopic void-fill cement (Plate 4d). In grainstones and packstones, it is present in depositional void spaces, occluding intercrystalline, interparticle and moldic pores as well as dissolution vugs and many fractures. Poikilotopic anhydrite tends to occur in very large single crystals that are both displacive and replacive. Porosity has been markedly reduced by void-filling anhydrite (lath-shaped and elongate crystals) and the precipitation of multiple sulphate generations in the grain-supported textures. Gypsum, which occurs as scattered crystals from phreatic zone precipitation, is converted during later burial to anhydrite. Primary anhydrite developed as displacive and replacive nodules in the vadose zone (Plate 4e).
According to Worden et al. (2000), four types of anhydrite forms occur in the Khuff Formation.
(1) Anhydrite nodules of up to 1–2 cm in diameter either formed as gypsum nodules that underwent dehydration during burial or directly as anhydrite during burial.
(2) Fine to coarse nodules with crystal sizes ranging from 250–600 μm in length.
(3) Anhydrite comprise crystal sizes ranging from 500–1,000 μm in length. Anhydrite occurs in fractures representing post-depositional, very early diagenetic shrinkage (dehydration) cracks that subsequently filled with anhydrite.
(4) Anhydrite poikilotopic crystal sizes ranging from 500 μm to 3 mm in length, enclose detrital bioclastic grains and ooids, and fill primary pore spaces between the detrital components.
Isotopic analysis of the sulfur show that anhydrite in the Khuff Formation have δ34S values of +10 to +20‰ CDT.
Coarse crystalline calcite occurs in molds of dissolved bioclasts, suggesting late cement origin. This calcite cement is formed in the deep subsurface and is characterized by coarse crystal size (45–90 μm), straight euhedral crystal edges (observed in thin section) and abrupt size changes compared with earlier calcite spar cements (see also Loucks, 1977; Badiozamani et al., 1977).
The Khuff anhydrite nodules exhibit various degrees of calcite replacement of the anhydrite with greater depths of burial. Calcite crystals (between 200 μm and several millimeters) can be found within replacive rinds enveloping the surface of the nodules and extending progressively towards the core of the anhydrite.
Mosaic dolomites are rare and occur as tightly interlocking 45–135 μm crystals replacing blocky sparite (Plate 4f) similar to that described by Mattes and Mountjoy (1980). They contain good intercrystalline porosity unless the pores in between the crystals are plugged by calcite, anhydrite or organic matter. Relict structures are usually altered and are difficult to recognize.
Silicification occurs most commonly in mudstones and wackestones and appears as euhedral crystals of quartz with abundant inclusions of the original host sediment. The presence of authigenic quartz is probably the product of the interaction of saturated brines rich in silica with dolomites, limestones and sulphate. Also, locally, barites and anhydrite are replaced by chalcedony or megaquartz.
Traces of sulfur have been observed in dolomitized sections of the Khuff in local vugs, molds, interparticle pores and microfractures. It is the result of the reduction of sulfate by hydrogen sulfide. δ34S for anhydrite from the Khuff Formation in Fateh field has values ranging from 16.5 to 20.2‰ CDT, suggests a sulfate source for the evaporite other than Late Permian seawater, probably from older Paleozoic rocks through which diagenetic fluids moved also supports non-marine burial origin for anhydrite (Videtich, 1994).
Fluoride commonly associated with saddle dolomite, calcite spar and anhydrite cements is interpreted to have precipitated in a mesogenetic environment. Oil entrapped in fluorite cement is the most mature, which supports that this fluorite was among the latest cement to precipitate (Videtich, 1994).
H2S may have been generated by thermal interaction of hydrocarbons and anhydrite. Baker and Kastner (1981) reported that the presence of sulfate is believed to adversely affect the dolomitization process by slowing the precipitation of dolomite and/or slowing the dissolution of calcite (Morrow and Rickets, 1988). However, dolomite forms in coastal Abu Dhabi, where brines have sulfate concentrations two to five times that of normal sea water (Patterson and Kinsman, 1981; Hardie, 1987). In spite of this, the late diagenetic timing of the sulfate in the Khuff appears to have rendered it relatively insignificant in terms of dolomite formation and calcite dissolution.
H2S probably formed by a thermochemical reaction involving the reduction of some of the abundant anhydrite by methane to various products including H2S and calcite. δ13C values for the methane in shallow Khuff Formation reservoirs is about -43‰ PDB. As discussed in Hunt (1995) this value is typical of thermogenic gas sources. δ13C values for methane in deep sour Khuff reservoirs are about -25‰ PDB. These data reported by Worden et al. (2000) suggest that residual methane became relatively enriched in C13. δ13C values for CO2 in shallow Khuff reservoirs are about -6‰ PDB, whereas in deep sour Khuff reservoir about -14‰ PDB. This suggests that CO2 became depleted in C13 as thermochemical sulfate reduction advanced (Worden et al., 2000). Khuff gas is composed of methane dominated light hydrocarbons, CO2, N2 and H2S with minor quantities of noble gases (Worden et al., 1995). With increasing H2S content hydrocarbon gases decreases, indicate that hydrocarbons are being replaced by H2S rather than CO2 or N2 by thermochemical sulfate reduction. Worden et al. (1995) used a “gas souring index” H2S/(H2S + CH4) to track the changing amounts of H2S and methane in the Khuff gases of Abu Dhabi (Figure 10). Petrographic and fluid inclusion data indicate that reservoir gas becomes sour only at depths greater than 4,300 m (14,104 ft) and temperature greater than 140°C. In reservoir shallower than 4,300 m (14,104 ft) the H2S concentration is less than 5%, but when greater than this depth, H2S contents reach 30–40% and are the result of thermochemical sulfate reduction within the reservoir.
Saddle (baroque) dolomite occurs with curved crystal faces, undulose extinction and a generally clouded appearance, locally replacing calcite or filling pores. Alsharhan and Williams (1987) reported that this kind of dolomite was formed at temperatures ranging from 65° to 112°C. It has been reported that baroque dolomite is a late diagenetic subsurface cement (Loucks, 1977; Loucks et al., 1977; Radke and Mathis, 1980).
The saddle dolomite crystals are very fine to coarse crystalline (0.06–0.7 mm), crystals that are generally moderately luminescent and mottled to zoned, but do not have the same zonation pattern expected if these formed at the same time from the same fluids (Videtich, 1994). Aqueous fluid inclusions were observed in saddle dolomite and saddle dolomite matrix, calcite, anhydrite and fluoride cements and suggested formation at temperatures of 80–123°C (Videtich, 1994), most apparently formed from brines with ≥16 equivalent weight % NaCl at deep burial origin.
In the Fateh field dolomitization and cementation occurred after hydrocarbon maturation, which is assumed to have begun at about 0.6% Ro, and the Khuff Formation is now overmature (as overmaturation begins at about 1.5% Ro) (Videtich, 1994). The paragenetic sequence for cement and matrix components in which fluid inclusions were observed in the Fateh field matches the burial and maturation history (Figure 11). Note in Table 5, the carbon and oxygen and strontium stable isotope values for dolomite matrix and calcite cement plotted in Figure 12.
Videtich (1994) made detailed isotope analysis for dolomite and calcite, and concluded that the stable isotope values of dolomite matrix had δ18O ranges -3.7 to -6.4‰ and δ13C ranges -0.3 to +1.0‰. The relatively heavy δ18O corresponds to an interval with high anhydrite content. The relatively light δ18O and δ13C corresponds to a thick layer of saddle dolomite matrix, which probably formed at a higher temperature than the non-saddle dolomite matrix, resulting in a temperature effect on δ18O (higher temperatures, lighter δ18O). The relatively light δ13C in the saddle dolomite matrix may have resulted from incorporation of carbonate derived from the oxidation of CH4 which is enriched in C12. The δ13C of calcite cement ranges from +1.5 to -28.5‰. The very light δ13C is probably a product of carbon derived from oxidation of CH4, while the heavier δ13C indicating an absence of a carbon contribution from CH4 in these cements.
The various rock components (matrix dolomite, anhydrite nodules and calcite and fluorite cements) have widely varying 87Sr/Sr86 ratios (0.70780 to 0.71084) than expected for Late Permian seawater, which supports a non-marine burial origin, and indicating that they formed at different times from waters containing variable amounts of strontium-87 (Videtich, 1994).
Factors which have controlled the creation and destruction of porosity are:
(1) grain size and depositional texture;
(2) the development of early rim cements creating rigid grainstone frameworks;
(3) the degree of leaching; and
(4) the availability of suitable pore fluids for replacement and cementation.
The effect of the early diagenetic processes resulted in some diminution of porosity, but much of the porosity was still retained by the Khuff sediments. The various late burial diagenetic processes led to partial or complete obliteration of the remaining porosity. Much of the remaining porosity is secondary in origin. A favorable aspect of late diagenetic process is the generation of intercrystalline porosity and the development of the systems of fractures, which significantly enhanced permeability.
The Khuff reservoir constitutes only about 35% of the total thickness of the formation, and is generally characterized by moderate to low porosity and low matrix permeability. Well-developed and uncemented fractures are widespread in this formation and result in significant enhanced vertical permeability. The dolomites with intercrystalline porosity have the best-interconnected pore systems and are highly permeable. The secondary porosity results from leaching associated with dolomitization, fractures and the early development of oomoldic pores.
Leaching of grains and bioclasts results in creation of production vugs and moldic porosity. The development of secondary anhydrites, replacing dolomite in dolomicrites as current infilling of fractures. The development of coarsely crystalline dolomite replacing various lithologies results in the creation of intercrystalline porosity, which represents one of the important types of porosity in the Khuff.
As described earlier, regionally the Khuff Formation has been divided into two major carbonate units (Lower and Upper Khuff). The Upper Khuff section has a relatively higher porosity than the Lower Khuff. The maximum porosity value of the Upper Khuff is around 20%, while the maximum value in the Lower Khuff is no more than 14%. Moderate to good porosity values are related to grainstone and rhombic dolomite development. The matrix permeability is very low, ranging from 1–10 md in the Lower Khuff with a maximum of 80 md; permeability in the Upper Khuff, which is greatly enhanced by a system of open fractures and reaches a maximum of 500 md in some wells.
The mud-supported sediments of the Lower Khuff have poor overall reservoir quality, while the grain-supported sediments of the Upper Khuff have better developed reservoir quality. Dolomitization and cementation are the major causes of porosity reduction and variations throughout the Khuff Formation. The porosity distribution is also a function of the original volume of the grainy/sandy depositional facies. The sabkha anhydritic facies commonly have their interparticle pores cemented by anhydrite. The shoal facies commonly have both moldic and interparticle pores partially or completely cemented by anhydrite. The dolomites of the lagoonal facies commonly contain both intercrystalline and vuggy porosity. The porosity also decreases with an increase in burial depth.
Stylolites are abundant and include microstylolites (amplitude less than 10 mm) and macrostylolites (rectangular) with amplitude of more than 10 mm. Microstylolites are common in the dolomicrites, while the rectangular forms occur in grainstones and in rhombic dolomite. Small fractures are associated with the large stylolites.
Fractures are important features in the Khuff Formation, because of the relatively high density of their occurrence, and their significant contribution to the total porosity. Cores show that fractures are either vertical or subvertical and vary in length from a few centimeters to 30 cm, but a few larger fractures also occur in the Khuff. Fractures in the dolomites section exhibit higher density than in the limestones and tend to be shorter. Fractures are either: (1) open (free of cement); (2) few infilled with calcite, dolomite or anhydrite; (3) cemented with dolomite, calcite, anhydrite; and (4) in a few cases with sulfur. The high porosity intervals have a lower frequency of fractures, and low-porosity dolomitic intervals have the greatest number of open fractures.
The Khuff Formation was subdivided into seven porous reservoir units from top to base (K1–K7) separated by dense units (Lufti and Elbishlawy, 1989; Alsharhan, 1993). The lithological and petrophysical characteristics of each reservoir unit of some wells are shown in Tables 3, 6, and 7. The K4 reservoir unit is the thickest and most significant reservoir zone. The K5 reservoir unit underlies the Middle Anhydrite and marks the top of the Lower Khuff section. The K6 reservoir is the thickest reservoir unit of the Khuff Formation. The most significant of these reservoirs are found within the Upper Khuff section, and individual porous intervals may reach 15 m (49.2 ft) in thickness with up to 30% porosity, while permeability varies and reaches several tens of millidarcies. The porous reservoir zones of the Lower Khuff section have lesser magnitudes and are thinner than those found in the Upper Khuff with porosities not exceeding 17%.
The recorded temperature of the Khuff Formation is relatively high and ranges between 130° and 260° C (Figure 13). Pressure in the gas zone in the Upper Khuff reservoir is about 6,990 psi @ 12,150 ft (3,645 m), while in the Lower Khuff reservoir it is about 8,010 psi @ 13,635 ft (4,091 m). In the water zone the pressure measurements are 7,600 psi at 14,150 ft (4,245 m) and 7,990 psi @ 14,925 ft (4,478 m). The pressure measurements show that the Upper and Lower Khuff are separate reservoirs, with the middle anhydrite acting as the barrier between the two reservoirs. The pressure measurements of the Khuff Formation indicate that it is moderately over-pressured. This could be due to late burial extensive stylolitization and fracturing; late digenetic precipitation of poikilotopic calcite, baroque dolomite, sulfur; or due to the prevailing high temperatures.
Porosity of the Khuff Formation is generally low, and may be reduced further by the cementation of interparticle pores, although dolomitization may enhance the porosity. Intercrystalline and interparticle porosity and permeability values are quite low in the lime mudstone and wackestone (less than 2.5% and 1 md, respectively), but are at least partly compensated by their greater degree of fracturing. Higher porosities are found in the lagoonal-intertidal deposits, which contain sand-sized carbonate particles (oolitic limestones) in a high-energy storm washover in lagoon-shoal setting (Alsharhan, 1993).
Fracturing may enhance the intergranular porosities. Fractures, however, may be partly or wholly sealed by anhydrite or other minerals. The development of rhombic dolomite with good intercrystalline porosity has been interpreted as the driving force in the creation of secondary porosity in the reservoirs.
If the reservoirs can be related to a particular depositional facies, detailed understanding of the Khuff stratigraphy and sedimentology may permit: (1) prediction of reservoir locations in undrilled areas, and (2) regional correlation. If the existence of the reservoirs is controlled, in part, by sedimentation (lateral continuity or discontinuity of depositional facies), and in part by secondary diagenetic effects, sedimentological studies must be combined with tectonic studies to determine possible locations susceptible to fluid movement within the Khuff.
Since the discovery of a large accumulation of gas in the Khuff Formation in 1979 at Umm Shaif field, several gas discoveries have been located in salt-cored domal and anticlinal structures including the Abu Al Bukhoosh, Nasr, Satah, Hail, Hair Dalma, Bu Haseer, Arzanah and Sath Al Raaz Boot fields (Figure 3). The gas composition in these fields consists of hydrocarbon gases (C1 and C2) associated with varying proportions of H2S, CO2 and N2 (Table 8). The Zakum and Fateh fields produced appreciable amounts of H2S, CO2 and N2 in association with hydrocarbon gas. The gas in these fields is probably formed in response to the high-temperature reaction between methane and anhydrite. Sulfur is present in vugs, partially replacing of anhydrite nodules and in moldic and intercrystalline porosity. Sulfur is derived from H2S, which is formed at high temperatures as a result of the reaction between the anhydrite and organic compounds. The only offshore structure with no Khuff production is the Ghasha field.
In onshore Abu Dhabi, the Khuff Formation at the Bab, Bu Hasa, Jarn Yaphour, Mender, Shah and Shuwaihat fields has been tested as water-bearing. This may reflect the fact that these onshore structures are younger and generally more deeply buried than offshore. The timing of the formation of structures and hydrocarbon generation and migration apparently controls hydrocarbon occurrence in the Khuff Formation. Structural traps in the offshore area have existed since Jurassic times, while the onshore area structures are believed to have gone through maximum development during the Late Cretaceous times.
The Khuff Formation represents approximately 19% of the estimated total gas in place in the UAE. The fluid distribution model shows four zones of wet and dry gas (Figure 13) in this formation (Marzouk, 1989). In the northwest corner of offshore Abu Dhabi, a wet gas zone occurred where the condensate/gas ratio increases in this direction. Southeast onshore and central/western offshore Abu Dhabi represent a dry gas zone. There are several dry gas zones in eastern offshore Abu Dhabi which extend to offshore Dubai, as well as central and western onshore Abu Dhabi. Dry gas also occurs in eastern onshore Abu Dhabi and in the Falaha Syncline in the onshore area.
Geochemical analysis of the Khuff carbonates indicates that: (1) the vitrinite reflectance values range from 1.5 to 2.0 Roe; and (2) a poor source rock potential with maximum total organic carbon (TOC) of 0.6 wt% and a low pyrolysis yield of less than 0.5 kg/ton (Elbishlawy, 1985; Alsharhan, 1989). The source for the gas in the Khuff Formation in offshore Abu Dhabi is probably from the Silurian Qusaiba shales, which are known to have good source rock potential in Saudi Arabia. Peak hydrocarbon generation from the Qusaiba shale was in the Jurassic time and postdates significant trap growth in offshore areas, but predates trap formation in onshore areas.
SOURCE OF KHUFF GAS
In the Arabian Gulf region geochemical analysis shows fair to good source rock potential in the graptolitic Silurian (basal Qusaibah) shale, which are sufficiently mature and rich in organic matter (0.3 to 6.1 wt%) (Mahmoud et al., 1992; Elbishlawy, 1985) to have acted as hydrocarbon sources. There does not appear to be any evidence for a potential Devonian source rock bed, although such source rocks of Devonian age are not uncommon in North Africa.
The potential of an infra-Cambrian source rock, such as those generating the oil found in the Lower Paleozoic Huqf Supergroup rocks of Oman would require a thermal conversion of oil to gas as proposed by Grunau (1977). The rocks of the latter group are sufficiently deeply buried so that any oil would be thermally converted into gas, which could then migrate upwards through the porous clastics to traps in the Permian carbonates. The carbon isotope ratio data, however, indicate that the CO2 present is more likely to be derived from highly mature source rocks rather than from the breakdown of carbonates. I believe that if the CO2 is a result of a carbonate breakdown, gas would be expected in the Khuff of the central part of the basin, and oil would be expected in the less deeply buried Khuff of southern and westcentral Oman and Saudi Arabia.
Nevertheless, the infra-Cambrian remains a potential source for the oil found in the Khuff reservoir of the Yibal field in Oman, as this oil is similar to oil found in the Paleozoic clastic reservoirs of the South Oman fields. The Yibal oils are related to the Q type of Grantham et al. (1988) classification, which has some characteristics in common with the Early Paleozoic Huqf oils, but differs in the predominance of C-27 steranes and high concentrations of tricycline terpanes. The Yibal oil also has X-branched hydrocarbons but relatively low concentrations of n-alkanes. The oils cannot be correlated with any sampled source rock in Oman, but because of some of the characteristics which they have in common with the Huqf oils, it has been suggested that they are derived from the deeply buried infra-Cambrian Huqf Supergroup in the salt basins in Central and South Oman.
Hydrocarbons generated in the Khuff Formation may be derived from the Khuff carbonate itself, and from the Silurian source where rich organic matter occur in shales in the Qusaiba Formation. An alternative source may be the thermal conversion of oil to gas as proposed by Grunau (1977). However, if Grunau’s (1977) mechanism is correct, the composition of the gas might be expected to change due to an increase in temperature if thermal cracking occurred much below 5,300 m with hydrocarbons beginning to be replaced by H2S, CO2 and N2. This does not appear to be the case for the Khuff gas in the Arabian Gulf region, where the principal methane gas amounts to as much as 89% of the total gas content, with small amounts of carbon dioxide, nitrogen, and heavier hydrocarbon compounds. The hydrogen sulfide content ranges from less than 1% to as much as 5%. The source of the sulfur may be thermal cracking of dispersed organic matter, or the reaction of organic matter with anhydrite at fairly high temperatures as described by Hunt (1995).
Geochemical analyses of the Khuff carbonates in the United Arab Emirates yield vitrinite reflectance values of 1.5 to 2.0 Ro, maximum TOC values of 0.6% by weight, and a low pyrolysis yield of less than 0.5 kg/ton (Elbishlawy, 1985; Alsharhan, 1989), suggesting poor source-rock potential. This fact increases the likelihood that the underlying Silurian shales are the source.
SEDIMENTOLOGICAL AND DEPOSITIONAL ASPECTS IN THE ARABIAN BASIN
In common with many other areas of the globe, the early Late Permian was a time of major transgression and marks the first extensive Phanerozoic spread of marine carbonates over the Arabian Platform. This transgression ended with regression in earliest Triassic time when the terrigenous clastics (shaly dolomitic limestones) of the Sudair Formation replaced the carbonates over much of the platform (Figure 14).
The lithologies recognized in the Khuff limestones indicate a range of depositional settings with a shallow-marine tidal-flat influence (Figure 15). There are also several shoaling-upward sequences, although their actual number does not yet appear to have been documented unequivocally. One of the more obvious signs of this cyclicity in the Khuff is the presence of widespread sheets of anhydrite in the Arabian Gulf region. However, angular unconformities are unknown within the Khuff Formation and local thickness changes may be attributed, at least in part, to active salt doming of the infra-Cambrian and Early Cambrian salts. The major changes in the Khuff distribution are the results of subsequent tectonic events and minor climatic and sea-level changes.
At a regional stratigraphic scale (parasequence third-order cycle), the general succession of the Khuff carbonates shows a characteristic shallowing-upward cycle. The first member of the cycle consists of lime mudstone deposits of a normal open-marine environment. As water depth decreases, dolomitic, lagoonal-intertidal deposits form, including sand-sized carbonate particles moved by storm waves to form shoals in conditions similar to those existing in the present-day Arabian Gulf. As shallowing continues and still more restricted environmental conditions develop, the lagoon waters become more saline and ultimately hypersaline. Sabkhas formed parallel to the lagoon margins where gypsum and anhydrite were deposited under supratidal conditions. The high-energy deposits tend to concentrate over local bathymetric highs, which may have been caused by the rise of salt domes or folds. These oolite and carbonate sands were abundant and widespread during the later stages of Khuff deposition, and the packstones and grainstones now have the best reservoir characteristics. Although interparticle pore volume may be reduced by cementation, it could also be enhanced by dolomitization and the porosity and permeability of many Khuff reservoirs has been increased by fracturing.
Water depths were sufficiently shallow to allow periodically extensive supratidal conditions to prevail. Under such conditions, small changes in sea-level caused significant changes in the depositional environment, including the development of sabkhas (Alsharhan and Kendall, 1986). These basic conditions extended all the way from Oman to southeast Turkey (Murris, 1981; Sharief, 1982); certainly the cyclic developments described above did not require major changes in sea-level. As illustrated in paleogeographic maps by several authors (e.g Murris, 1981; Al-Jallal, 1995; Alsharhan and Nairn, 1997), a shallow-water belt extended southwestward from east of the Fars Province of Iran to Oman and probably occurred over bathymetric highs where carbonate buildups and even reefs formed. The significance of this feature is that it acted as a partial or complete barrier restricting the free inflow of fresh marine waters into the Arabian Shelf sea area. East of the present Zagros High lay the open waters of the Tethys Ocean. The deposition of the Khuff Formation ended during Early Triassic (Scythian) time with a major regression as indicated by the prevalence of the continental deposits of the Sudair Formation.
CHARACTERISTICS OF KHUFF HYDROCARBONS IN THE ARABIAN BASIN
Numerous hydrocarbon shows have been reported from Permian surface outcrops in the Arabian Plate. The Permian limestones and dolomites cropping out in Southern Turkey contain light oil and oil stains (Tasman and Egeran, 1951; Edgell, 1976). According to Morton (1959) traces of oil were recognized in some outcrops of Permian carbonate rocks in Oman. Fetid limestones with a sulfurous odor have been reported from the Upper Permian carbonates in Jabal Al Qamar and Jabal Hijab of the United Arab Emirates. Until the discovery of a large non-associated gas reserve in the Awali field (Bahrain) during 1948, the Permian Khuff attracted relatively little attention, because: (1) the abundance of oil in the overlying stratigraphic units, and (2) the low general interest in gas. This situation was changed dramatically with the discovery of the other fields in Saudi Arabia and Iran. All of these discoveries are of non-associated gas from a variety of depths ranging from 5,300 m (17,384 ft) in offshore Abu Dhabi to 2,450 m (8,036 ft) in eastern Saudi Arabia.
Awali Field, Bahrain
In Bahrain, the Khuff gas reservoir in the Awali field was discovered in 1948 when well no. 52 was deepened to 3,150 m (10,332 ft). Two other deep wells, nos. 244 and 245, were drilled in 1969, followed by others in the 1970s. During this time, the drier Khuff gas replaced the richer Upper Jurassic Arab zone gas for injection and artificial lift operations in the field. By 1986, 21 wells had been drilled and completed in the Khuff zones. The non-associated gas production from the Khuff reservoir started in 1969 at an initial rate of 50 million standard cubic feet per day (MMSCFD) rising to 753 MMSCFD in 1986. The Khuff Formation, which is here about 660 m (2,165 ft) thick, consists of dolomitized carbonate/anhydrite interbeds.
The Khuff in the Awali field contains four reservoirs (K0 to K3 in descending order) of which the principal production is from the reservoirs, K1 and K2 (Janahi and Dakessian, 1985; Al-Khayat and Mohammed, 1987). The K0 reservoir is about 90 m (295 ft) thick with porosity up to 36% and permeability ranging from 0.01 to 64 md. Reservoir K1 has a thickness of 156 m (512 ft), an average porosity of 17% and permeability in the range of 55–300 md. In reservoir K2, which is about 100 m (328 ft) thick, the uppermost and most porous part (porosity is about 16%) has an average permeability of about 83 md. The presence of fractures in the lower 47–63 m (154–207 ft) section is the major factor determining its high productivity. The K3 reservoir has a total thickness of 290 m (951 ft), but its effective thickness is only 5–15% of this total (Janahi and Dakessian, 1985; Janahi and Mirza, 1991). The porosity is in the interparticle/intercrystalline pores and hairline fractures of microsucrosic oolites and peloidal grainstones. The Khuff gas is fairly sweet with a H2S content of 370–2,000 ppm, CO2 about 6.2% and N2 about 11.5%. Janahi and Mirza (1991) reported that no free water has been produced from the reservoir which continues to behave as a depletion type reservoir.
In Saudi Arabia, significant gas production was reported from the Khuff Formation in the Dammam field in 1957, and since then gas reserves of great significance have been discovered in other major fields including Ghawar, Abu Sa’fah, Berri, Harmaliya, Khurais and Qatif. The formation, about 510 m (1,673 ft) thick, has been divided into four reservoir units, Khuff-D to A in a shoaling-upward sequence (Al-Jallal, 1987, 1995), each formed during a different depositional cycle. The cycle commences with mainly subtidal carbonates and shallows upward into a regressive phase of mainly intertidal and sabkha sediments.
Reservoir quality is controlled by lateral continuity or discontinuity of the facies and also by diagenesis. High porosity and permeability is usually associated with primary interparticle pore spaces. Khuff gas from the Saudi Arabian fields is sour and contains hydrogen sulfide and carbon dioxide. Production is accompanied by water (1.5–2.0 barrels per million standard cubic feet of gas) and moderate amounts of heavy condensate (API gravity 47.5°, 30–50 barrels per million standard cubic feet of gas) (Kasnick and Engen, 1989). Analysis of the gas shows that it contains approximately of 20% non-hydrocarbons of which H2S forms about 4.1 mole %, CO2 3.7 mole % and N2 12.3 mole % (Kasnick and Engen, 1989). The condensate has 0.81% sulfur and significant quantities of heptanes and heavier components.
North Field, Qatar
In Qatar, the North field (formerly called the Northwest Dome), discovered by Shell in well NWD-1 (1971), is one of the world’s largest reservoirs of non-associated gas with reserve of more than 900 TCF. The total gas-bearing area is estimated to be more than 5,800 sq km. The effective reservoir thickness is about 280 m (918 ft) of the total Khuff succession of about 845 m (2,772 ft), and consists mainly of a shallow-marine carbonate sequence. There are five potential reservoir beds (K1 to K5) (Alsharhan and Nairn, 1997) that exhibit good porosity (15–20%) and very high permeability. The carbonate sediments comprising the Khuff Formation in Qatar accumulated in laterally contiguous islands peritidal, lagoonal, high-energy shoal, shallow-subtidal and deep subtidal deposited environments.
Yibal Field, Oman
In Oman, sour gas was tested in the Permian-Triassic rocks of well Y-85 in the Yibal field, western Oman Mountains. These Khuff carbonates contain five reservoir zones (Kl–K5) (Bos, 1989; Abu Risheh and Al Hinai, 1989). The Yibal field is a domal structure covering an area of about 19 x 5 km. The field is divided by a NE-trending fault into two areas with different fluid distributions. On the western downthrown side there is a 100 m (328 ft) oil column with a relatively small gas cap, whereas on the eastern upthrown side, a 60 m (197 ft) oil column has a large gas cap (Bos, 1989). Gas was first discovered in 1977 in well Y-85 from the K2 bed. Gas was next found in 1985 in well Y-192, which was drilled as a gas exploration well. The same well tested oil in the K2, K3 and K4 beds, and gas and condensate in the K1 and K5. An oil extraction rate of 7,550 barrels per day was tested in the K2 reservoir.
Towards the end of 1986, two step-out appraisal wells (Y-212 and Y-214) were drilled on the western flank of the structure. The Y-212 well found oil shows in KI, but produced only water on test. The exploratory appraisal well Y-230, drilled in 1987, established the fluid contacts in the eastern part of the field and tested oil at the rate of 1,250–1,890 barrels per day in the low permeability K2 bed. Well Y-236, drilled near the end of 1987, found gas in Kl and the upper part of K2. The lower part of the K2 bed and K3 were found to be in the oil column (Bos, 1989).
The carbonate reservoirs K1–K5 generally have porosity values in excess of 20%, but show significant lateral variations in both thickness and reservoir properties. Porosity is primarily oomoldic, partially enhanced by intercrystalline porosity, and permeability ranges from less than 1.0 to more than 100 md (Abu Risheh and Al Hinai, 1989; Bos, 1989). The basal reddish and grayish-green laminated anhydritic shales of the Early Triassic Sudair Formation form a good seal over the Khuff (Hughes-Clarke, 1988). Within the Khuff Formation itself, there are also good seals including those at the base of K4 (grayish-green claystone, anhydritic dolomite, and anhydrite) and also at the intra-K5 bed where low porosity and tight anhydritic dolomites occur.
The Khuff Formation in the United Arab Emirates consists predominantly of dolomite with subordinate limestones and anhydrite. It is a second-order sequence consisting of seven transgressive-regressive cycles (KS1 to KS7) that were deposited in settings that ranged from shallow unrestricted marine to restricted shoal-lagoonal and supratidal. It ranges in thickness from 625–970 m (2,050–3,182ft), and in depth between 3,688–6,188 m (12,097–20,297 ft).
Reservoir porosity ranges from 2–8%, but locally exceeds 20%. Permeability is generally low but greatly enhanced by open vertical fractures (0.1 to 80 md but locally exceeds 500 md). Both primary and secondary porosity exist. The latter is affected by different diagenetic processes during early to deep-burial stages. Fractures are common and have significant impact on enhancing the effective porosity and permeability. The Khuff reservoir contains sweet gas, consisting predominantly of methane but also considerable amounts of H2S, CO2 and N2. The source of this is probably the Silurian Qusaiba shales. The cap rock is the basal Triassic Sudair Formation.
A series of four distinct diagenetic (chronological) stages have been recognized: (1) marine phreatic stage (aggregate grains, micritization, isopachous fibrous/bladed cement); (2) mixed phreatic stage (micrite accretion, anhydrite nodules, dolomitization, neomorphism); (3) meteoric phreatic stage (syntaxial overgrowth, leaching, equant calcite cement, mosaic dolomite); and (4) burial stage (anhydrite cementation, coarse crystalline calcite, silicification, sulfur, compaction, saddle (baroque) dolomite, stylolites and fractures).
The author would like to thank Christopher Kendall and GeoArabia’s anonymous reviewers for their constructive suggestions and criticisms which have greatly improved the manuscript. He also thanks his colleagues in the Geology Departments of ADMA and ADCO for their fruitful discussions on the different aspects of the Permian carbonates in the United Arab Emirates; and to their companies for providing thin sections and cuttings. The design and drafting of the final graphics was done by GeoArabia.
ABOUT THE AUTHOR
Abdulrahman S. Alsharhan is Professor of Geology and was Dean of Faculty of Science at United Arab Emirates University, Al Ain. He received his MSc in 1983 and PhD in 1985 in Geology from the University of South Carolina. His research interests include Holocene coastal sabkhas of the United Arab Emirates, and the geology and hydrocarbon habitats of the Middle East. He has authored and published over 80 scientific papers in refereed international journals. In 1997, Abdulrahman co-authored a book with A. Nairn, “Sedimentary Basins and Petroleum Geology of the Middle East”, and in 2001 co-authored with Z. Rizk, A. Nairn, D. Bakhit and S. Al-Hajari, “Hydrogeology of an Arid Region: Arabian Gulf and Adjacent Areas”. He has also edited four books, (1) in 1998 with K. Glennie, G. Whittle and C. Kendall (Quaternary Deserts and Climatic Change); (2) in 2000 with R. Scott (Jurassic/Cretaceous Carbonate Systems); (3) in 2003 with W.W. Wood, A.S. Goudie, A. Fowler and E. Abdellatif (Desertification in the Third Millennium); and (4) in 2003 with W.W. Wood (Water Resources Perspectives: Evaluation Management and Policy). Also he is preparing of a book with C.G.St.C. Kendall, and A.S. Alsuwaidi titled “Evaporite Stratigraphy, Structure and Geochemistry, and their Role in Hydrocarbon Exploration and Exploitation”. Abdulrahman is a member of the Editorial Advisory Board of GeoArabia and is also a member of the AAPG, IAS and Geological Society of London.