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Middle East Models of Jurassic/Cretaceous Carbonate Systems

SEPM Special Publication No. 69, Copyright ©2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-075-1, p.299–314.

Abstract

The Shuaiba Formation of the United Arab Emirates (U.A.E.), forms one of the most important petroleum reservoirs in the Arabian Gulf. Its reservoir quality is controlled by diagenetic processes that were active during early shallow burial to late deep burial. Detailed well log evaluation, petrographic and geochemical studies of carbonate diagenesis, X-ray diffraction, cathodoluminescence microscopy, and oxygen and carbon isotopic determinations from cores and an outcrop section allowed evaluation of the stratigraphic and depositional framework of the Shuaiba Formation, its diagenetic history, and the prediction of its local reservoir potential. The Shuaiba Formation consists of two informal and one formal members: the lower Shuaiba and upper Shuaiba members and the Bab Member, in ascending order. The Shuaiba ranges in thickness from 45 to 145 m, having accumulated in a wide range of depositional settings from shallow to deep shelf. The lithofacies identified within the Shuaiba Formation include peloidal skeletal algal packstone/grainstone; ooidal-peloidal grainstone/packstone; skeletal algal (Lithocodioidea) floatstone; intraclastic and coated packstone/grainstone and skeletal wackestone/ packstone. These facies were deposited during third-order depositional sequences, including two transgression system tracts (TST) separated by a highstand system tracts (HST). Diagenetic alteration of the original carbonate components proceeded through marine, shallow burial, and deeper burial settings related to stabilization of the carbonate matrix, cements, and rudist shells. Oxygen and carbon isotopes of calcific matrix have the least altered components of these rocks (av. δ180 = 5.7%o PDB; δ13C = +2.5 %o PDB), whereas the calcite cements occluding shell porosity and veins have more depleted isotopic values (av. δ180 = 8.8%o PDB;δ13C = +0.5%o PDB). The variations of oxygen and carbon isotopes reflect changes in the water-rock interactions and increasing burial.

The carbonates of the lower and upper Shuaiba members have porosities between 12% and 32% and permeabilities between 1.0 and 160.0 md. The reservoir quality is highly affected by the diagenetic processes which include stabilization of metastable carbonate phases, cementation, dolomitization, stylolitization, and dissolution. The Bab Member, which was deposited in a basinal setting, is organically rich and forms substantial source rock in the eastern and northeastern parts of central UAE and is mature enough in deep troughs to generate and expel hydrocarbons to the reservoirs.

Introduction

The Cretaceous rocks of the U.A.E. represent a time of carbonate platforms and offshore banks and buildups. Rudist bivalves colonized shallow marine settings with strong to moderate currents and turbulence (Alsharhan, 1995, Hughes, 1997) and assisted in creating localized bank complexes. The Shuaiba Formation represents the uppermost section of the Lower Cretaceous Thamama Group and locally forms carbonate buildups. These carbonates contain important reservoirs and produce oil and gas from a number of fields in U.A.E., Qatar, Oman, and Saudi Arabia.

This paper deals with the evaluation of diagenetic modifications using petrology and geochemistry of the Lower Cretaceous Shuaiba carbonates. It also delineates the sequence stratigraphic framework of these carbonates, and documents their depositional settings and hydrocarbon reservoir and source characteristics in the U.A.E.

Materials and Methods

The data and interpretations presented in this study were based on detailed examination of 120 samples from selected cores, sidewall cores, and ditch cuttings of the Shuaiba Formation. These samples came from both onshore and offshore fields in the U.A.E., including producing and nonproducing fields from Bu Hasa, Shah, Asab, Sahil, Jarn Yaphour and Fateh (Fig. 1). In addition, the best exposures of the Lower Cretaceous Shuaiba carbonates in the Wadi Dhayah, in the Northern Emirates (Fig. 1), were sampled, studied, and correlated with the subsurface data. Core samples were studied under a binocular microscope and in 88 thin sections cut from selected intervals. Polished slabs and uncovered thin sections were stained for Fe-calcite using a mixture of potassium ferricyanide and alizarin red-S according to the method of Lindholm and Finkelman (1972). Selective thin sections were examined by cathodoluminescence microscopy using a Technosyn 8200 MKII Model cold cathodoluminescence stage at a voltage of 15 kV and current intensity of about 450 μA. In addition, 47 thin sections were impregnated with an epoxy mixed with a blue fluorescent dye for porosity recognition. Various generations of calcite cements, fossil components, dolomite, and matrix were selected for carbon and oxygen isotopic analysis employing a microscope-mounted drill assembly to extract 0.5 to 4.0 mg of powdered carbonate from polished slabs. Fifty-one samples were treated in vacuo with 100% phosphoric acid at 25°C for calcite and at 50°C for dolomite. All gas extractions were made in the Stable Isotope Laboratory of the University of Windsor, Canada, and the evolved C02 gas was analyzed for isotopic ratios on a SIRA-12 mass spectometer values of O and C isotopes are reported in per mil (%o) relative to the PDB standard. Precision was better than 0.1 %o for both isotopes.

Fig. 1.

Location map of the United Arab Emirates showing the oil and gas fields and the surface samples used in this study.

Fig. 1.

Location map of the United Arab Emirates showing the oil and gas fields and the surface samples used in this study.

Thirty-three surface and subsurface samples were analyzed by X-ray diffraction (Philips, model PW1840), using a Ni filter, Cu radiation (1 = 1.542 Å) at 40 kV and 30 μA and scanning speed of 0.05 %o. The reflection peaks between 2° and 70° 2θ and the corresponding d spacing (Å) and relative intensity (I/I°) were detected and compared with standard data of American Standards for Testing and Materials (A.S.T.M.).

Sonic, gamma ray, neutron, density, and resistivity logs for several wells covering the offshore and onshore parts of the U.A.E. were used, together with the available core and ditch samples, to subdivide the Shuaiba Formation, to create a depositional model and to define sequence boundaries.

Stratigraphy of the Shuaiba Formation

The Shuaiba Formation is composed of thick, porous shelf carbonates (Fig. 2) which show considerable subsurface lateral and vertical lithofacies change over much of the Arabian Gulf. The Shuaiba Formation ranges from 45 to 145 m thick, increasing toward the west and the south. During Early Cretaceous time, the central part of the U.A.E. was a wide trough (intrashelf basin) oriented roughly northeast-southwest and deepening toward the northeast in the onshore area (Murris, 1980; Alsharhan, 1995). This trough was mainly a response to the influence of the deep structural features on the deposition of the Cretaceous sediments. The Shuaiba was deposited during an extensive Tethyan transgression on the northeastern part of the Arabian Shield during the early to mid Aptian. The Shuaiba intrashelf basin was affected by a long-term, second-order eustatic sea-level fall, formed during a third-order transgression in the early Aptian and was filled during the early to mid Aptian.

Fig. 2.

Generalized stratigraphic column of the U.A.E. showing the Shuaiba stratigraphy.

Fig. 2.

Generalized stratigraphic column of the U.A.E. showing the Shuaiba stratigraphy.

This formation rests conformably over the carbonates of the Kharaib Formation upper dense member and is unconformably overlain by the shale and argillaceous, silty limestone of the Albian Nahr Umr Formation. This upper boundary is marked upward by an abrupt increase in the gamma-ray readings and decrease in density readings (Fig. 3).

Fig. 3.

Electric log correlation diagram showing the lateral variations within the Shuaiba Formation. Vertical scale: 1 cm = 30 m (100 ft).

Fig. 3.

Electric log correlation diagram showing the lateral variations within the Shuaiba Formation. Vertical scale: 1 cm = 30 m (100 ft).

The Shuaiba Formation in the U.A.E. has been assigned to the Aptian by most of the researchers in the U.A.E. The upper Bab Member contains upper Aptian ammonites (Hassan et al., 1975), but has been dated as early Albian, based on palynological and paleontological analysis and also by strontium and potassium-argon dating (Azer and Toland, 1993). However, the Aptian age is based on microfossils from the Shuaiba Formation in U.A.E. and Oman (Alsharhan and Nairn, 1986; Scott, 1990; Simmons, 1994; Witt and Gokdag, 1994), rudists (Skelton and Masse, this volume) and by isotopes (Vahrenkamp, 1996; Grötsch et al, 1998).

Although the Shuaiba Formation has been subdivided differently by some workers (e.g., Witt and Gokdag, 1994; Alsharhan, 1985) in the Oman and U.A.E., most divide the Shuaiba Formation into three members, two informal (lower and upper) and one formal (Bab Member). This subdivision is based on the lithostratigraphic and biostratigraphic data and the electric logs and summarized below.

Lower Member

This informal member is known in the oil companies as Thamama I, Thamama A, or Thamama subzone IA, and forms the lowest reservoir facies (Units A and B in ascending order). It is 9 to 24 m thick and consists of blanket-type deposits that are generally present over the whole area. The lower part of this member is composed of wackestone and packstone with local grainstone and colonies of codiacian algae, overlain by microporous lime mudstone and wackestone that are slightly argillaceous, burrowed, and bioturbated. The abundance of algal colonies suggests that deposition occurred in shallow water in the photic zone. The occurrence of calcispheres, miliolids, and other arenaceous foraminifera suggests a lack of open marine circulation and a slight restriction. The upper part of this member is composed of medium-grained wackestone and boundstone with abundant codiacian algae. Occasional fine-grained microporous lime mudstone occurs with abundant stylolites and scattered pyrite. These sediments were deposited in offshore shelf-basin facies with open circulation.

Azer and Toland (1993) suggested a low rate of sedimentation accompanied by a fairly slow rise of the sea level during the deposition of this member, which is reflected as the sheet-like crusts of the Lithocodium algae.

The lower member conformably overlies the dense carbonates of the Kharaib Formation. The boundary is marked up section by a sharp increase in gamma ray, neutron porosity, and density logs (Fig. 3). This member grades conformably upward into the upper member in the west, south, and northwest, and it is overlain unconformably by the Bab Member in the east and northeast.

Upper Member

The informal upper member contains several lithofacies and includes what are known as reservoir units C, D, E, F, G, H, H, and I. It is composed of medium- to coarse-grained packstone and grainstone with abundant rudistid fragments; Orbitolina packstone, dense lime mudstone with scattered fine bioclastic debris, bioturbated argillaceous microporous lime mudstone and wackestone, and fine- to medium-grained wackestone containing abundant and commonly large algal colonies. These different carbonates are recorded in most of the study area but are absent in the eastern and the northeastern parts of the U.A.E. This member represents a wide range of depositional settings from back-biostrome through biostrome to fore-biostrome.

The upper member conformably overlies the lower Shuaiba member; it grades laterally and interfingers with the Bab Member in the east and northeast; and it is overlain unconformably by the Nahr Umr Formation, the contact of which is picked at a sharp up-hole increase in gamma-ray log values. The upper boundary is picked at the sharp transition from the clean carbonates of this member to the shales of the Nahr Umr Formation in the west, north, and south, and the highly argillaceous limestone of the Bab Member in the northeast and east.

Bab Member

The Bab Member, the youngest unit in the Shuaiba Formation, averages 70 m thick and is unconformably overlain by the Nahr Umr Formation. The Bab Member consists of a basinal argillaceous lime mudstone, with abundant planktonic fora-minifera and moderate-high organic content (Hassan et al., 1975; Alsharhan, 1995; Taher, 1997). The Bab Member is restricted in the eastern and northeastern parts of the U.A.E. and was deposited in mid-ramp settings (intrashelf basin). The lower part of the Bab Member is a progradational carbonate cycle of deep-water argillaceous globigerinid limestone. This is overlain by Gymnocodiaceae, peloidal argillaceous packstone-wackestone, and skeletal Everticyclammina and Pseudocyclammina foraminiferal, argillaceous wackestone-packstone. The Shuaiba basinal facies is well developed in central U.A.E. and is known in the area as the Abu Dhabi Intrashelf basin (see Alsharhan and Nairn, 1986).

Shuaiba Surface Section

At Wadi Dhayah (surface sample A, Fig. 1), the upper member of the Shuaiba Formation crops out in the floor of the wadi and is overlain unconformably by the Albian Nahr Umr shales and carbonates (Fig. 4). This section has been discussed on two field trips (Russell, 1996; Alsharhan, 1997). The contact is a scree-covered surface and is picked at the transition from the light gray massive limestone of the Shuaiba Formation to thin-bedded, light to dark brown shales, marls, and limestones of the Nahr Umr Formation (Fig. 4). The Shuaiba Formation displays an average dip of 5° toward the west-southwest. Only the lower and upper members are exposed.

Fig. 4.

Stratigraphic column of the surface section in Wadi Dhayah (compiled with modification from Russell, 1996; Alsharhan, 1997; and our new field data).

Fig. 4.

Stratigraphic column of the surface section in Wadi Dhayah (compiled with modification from Russell, 1996; Alsharhan, 1997; and our new field data).

Only two meters of the lower member crop out in the floor of the wadi, and here it consists of light gray, hard, algal-rich (Lithocodium sp.) packstone to wackestone (Fig. 4). The upper member contains three different lithofacies, representing three reservoir facies units in ascending order based on core and outcrop descriptions.

  • Lithocodium packstone /mudstone, 12 m thick, is known informally in the oil companies as reservoir unit C. This consistsmainly of nodular, stylolitic packstone/mudstone with abundant algae, foraminifera, and shell debris (Fig. 4).

  • Chondrodonta sp. Oyster floatstone/rudstone, 6 m thick isknown as reservoir unit H, and consists predominantly ofnumerous caprinid and caprotinid rudists. It also containsChondrodonta sp. and oysters in floatstone and rudstone (Fig. 4). The x-ray analysis of two samples from this unit showed aremarkable increase in the quartz content in the lower part ofthis unit (Fig. 4).

  • Dasyclad algal packstone/wackstone, 13 m thick, is known asreservoir unit I, and consists of fine-grained dasyclad algae, such as Salpingoporella (?) spp. and Clypeina sp., and foraminiferal packstone/ wackestone and is unconformably overlainby the Nahr Umr Formation.

Sequence Stratigraphy

From our examination of cores and logs we conclude that the entire Shuaiba Formation is a part of a second-order super cycle that we correlate with Zuni B4 (LZB-4) (Haq et al., 1988). It has two third-order sequences: the LZB-4.1 and LZB-4.2 (Fig. 5). The lower part of the older cycle (LZB-4.1) is represented by the upper member of the underlying Kharaib Formation based on facies interpretation, areal extent, and depositional models. Also, correlation of upper dense Kharaib with basal Aptian is supported by Scott (1990), Simmons (1994), and Witt and Gokdag (1994). The Shuaiba Formation is represented by a highstand systems tract (HST) and a transgressive systems tract (TST), with one sequence boundary and two downlap surfaces (Fig. 5). A lower sequence boundary at 112 Ma separates the upper dense unit of the Kharaib Formation from the underlying Kharaib porous zones and we correlate it with LZB4.1 (Haq et al, 1988). The base of the Shuaiba Formation is the lower downlap surface calibrated with the Haq et al. (1988) 111 Ma event. The top of the lower Shuaiba member is a sequence boundary that we correlate with the 109.5 Ma of Haq et al. (1988). This sequence of the uppermost Kharaib and lower Shuaiba records a simultaneous subsidence accompanied by a slow rate of deposition.

Fig. 5.

Correlation of sequence stratigraphic boundaries within the Shuaiba Formation cycles with the Haqet al. (1988) sea-level curve.

Fig. 5.

Correlation of sequence stratigraphic boundaries within the Shuaiba Formation cycles with the Haqet al. (1988) sea-level curve.

The second sequence boundary separates the lower Shuaiba member from the upper Shuaiba member (Fig. 5), and we correlate it with the boundary between LZB-4.1 and LZB-4.2, dated at 109.5 Ma by Haq et al. (1988). We place the second downlap surface separating the HST from the TST at the unconformity between the Shuaiba and Nahr Umr formations and calibrated with the 108 Ma event of Haq et al. (1988). Because of the lack of seismic data the authors could not trace the Bab Member into the eastern Abu Dhabi subsurface. The Bab is absent in the Musandam outcrop because it onlaps and pinches out or because it was eroded (Vahrenkamp, 1996). An alternative sequence stratigraphy shows three sequence cycles in the Shuaiba Formation (Vahrenkamp, 1996) withlowstand facies in the uppermost Kharaib Formation, just below the Bab Member, and in the upper Bab Member. Kendall et al. (this volume) have conducted sedimentary simulations of the Shuaiba utilizing a similar sequence stratigraphy.

Lithofacies

Examination of the available core and ditch samples revealed seven main lithofacies with bed thickness ranging from 10 cm to 10 m. This wide spectrum of lithofacies reflects the complex depositional settings and sub-environments that prevailed during the time of deposition. They were mainly deposited in inner, inner to middle, and middle ramp settings (Fig. 6). Similar depositional models for the Shuaiba were proposed by Hassan et al. (1975) and Hamdan and Alsharhan (1991).

Fig. 6.

Depositional model of the Shuaiba Formation in the U.A.E.

Fig. 6.

Depositional model of the Shuaiba Formation in the U.A.E.

  1. Peloidal skeletal algal packstone/grainstone and algal floatstone/packstone. This facies represents the back shoal setting within the inner ramp (Fig. 6) and suggests a low-energy, semirestricted or restricted lagoonal environment. The skeletal grains include rounded, sand-size shell fragments (Fig. 7A). The average thickness of this lithofacies in the Bu Hasa oil field is 25 m.

    Fig. 7.

    Photomicrographs from the Shuaiba representing different fades; scale: 1 cm = 0.5 mm. A) Algal packstone showing dolomitized matrix. Some micritic fossils molds are occupied by calcite cement, c. B) Floatstone showing well preserved rudist shell. C) Intraclastic packstone showing partially micritized gastropod shell that is leached and filled with the calcite crystals. D) Skeletal packstone showing calcite recrystallization, c, of fossils molds. Note the fracture, f, cutting across the photo. E) Intraclastic packstone showing two sets of fractures intersecting, f and f’. Both fractures are filled by recrystallized calcite. Note the stylolite(s), cutting across the fractures. F) Ooid grainstone showing calcite crystals, c, destroying some of the porosity.

    Fig. 7.

    Photomicrographs from the Shuaiba representing different fades; scale: 1 cm = 0.5 mm. A) Algal packstone showing dolomitized matrix. Some micritic fossils molds are occupied by calcite cement, c. B) Floatstone showing well preserved rudist shell. C) Intraclastic packstone showing partially micritized gastropod shell that is leached and filled with the calcite crystals. D) Skeletal packstone showing calcite recrystallization, c, of fossils molds. Note the fracture, f, cutting across the photo. E) Intraclastic packstone showing two sets of fractures intersecting, f and f’. Both fractures are filled by recrystallized calcite. Note the stylolite(s), cutting across the fractures. F) Ooid grainstone showing calcite crystals, c, destroying some of the porosity.

  2. Ooidal-peloidal grainstone/packstone. This facies accumulated as a prograding shoal (mobile) forming locally extensive sand sheets, and it constitutes limited parts of moderate-quality reservoirs. Common ooids, peloids, and molluscan fragments suggest shoal development within the inner ramp. It is best developed in the upper member, which is the best reservoir in the giant Bu Hasa oil field. The average thickness of this lithofacies in the Bu Hasa oil field is 67 m.

  3. Skeletal algal (Lithocodioidea) floatstone/packstone/grain-stone and skeletal peloidal algal (dasyclads) packstone/grain-stone (Fig. 7B, D). These facies constitute considerable parts of the reservoir sediments. This Jithofacies association suggests deposition in a foreshoal area in an inner-ramp environment (Fig. 6). It ranges in thickness from 3 to 78 m.

  4. Intraclastic packstone/grainstone and cortoid packstone-grainstone (Fig. 7C, E). These facies comprise a minor portion of the reservoirs and occur mostly in dense limestone intervals. They were deposited in a backshoal that may have been developed between relatively high-relief areas of the sea floor in a backshoal lagoon environment. The average thickness of this lithofacies is 35 m.

  5. Skeletal wackestone-packstone, algal (Lithocodioidea) floatstone wackestone, and algal wackestone. This lithofacies and the dense limestone intervals were deposited in a horizontal area extending over the inner fore-shoal to mid-ramp where local accumulations of algal Lithocodium mounds and complex ramp sediment were interbedded. This lithofacies ranges in thickness from 8 to 60 m.

  6. Skeletal foraminiferal (Pseudocyclammina) argillaceous wackestone-packstone, skeletal peloidal (blackened grains) packstone/grainstone, and algal (Gymnocodiaceae) wackestone/ packstone. These facies are most common in dense limestone intervals separating reservoirs. They are thought to have been deposited on a very gentle slope within the mid-ramp (Fig. 6). The range in thickness from 3 to 35 m.

  7. Algal and skeletal packstone/Lithocodium boundstone and argillaceous wackestone/ mudstone. These facies represent the relatively deeper slope of the mid-ramp environment. They are typical of the lower member, informally known as reservoir units A and B or Thamama IA. These lithofacies have more stylolites than any other lithofacies in the Shuaiba Formation, which have an average amplitude of 10 cm. The lithofacies are about 15 m thick and overlie the dense limestone of the Kharaib Formation and contain abundant echinoderms, Orbitolina sp., and colonies of Lithocodium algae.

Paragenesis and Stable Isotopes

Paragenetic Sequences

The diagenetic history of the Shuaiba Formation in the study area includes multiple episodes of calcite cementation, mechanical and chemical compaction, stabilization of shell mineralogy, and dissolution. These diagenetic processes occurred during marine to shallow burial stages and culminated during intermediate to deep burial stages. The paragenetic sequence (Fig. 8) is based on petrographic observations, spatial distribution, cross-cutting relationships between depositional fabrics, cements, compactional features, and geochemical evidence.

Fig. 8.

Paragenetic sequence of the diagenetic processes affecting the Shuaiba Formation.

Fig. 8.

Paragenetic sequence of the diagenetic processes affecting the Shuaiba Formation.

The main skeletal framework in the studied limestones is composed of rudist bivalves, algae and algal aggregates, echinodermi parts, stromatoporoids, and diverse mollusks. These fossil components show variable degrees of preservation in response to their original shell mineralogy, shell structures, and the chemistry of diagenetic fluids (Al-Aasm and Veizer, 1986a, 1986b; Al-Aasm and Azmy, 1996; Hendry et al., 1995). With the accretion of rudist buildups, marine borings in shells commenced contemporaneously accompanied by grain micritization, marine cementation, represented by nonluminescent fibrous calcite cement followed by inclusion-rich equant calcite, syntaxial-rim cement, and internal sedimentation.

Burial of Shuaiba carbonates at shallow depths was accompanied by the formation of minor compactional features such as microstylolites, in-situ broken shells, and close packing of grains. The precipitation of the early generation of dull, prismatic and drusy equant calcite cements (Fig. 7F) and partial replacement of the matrix with first-stage dolomite followed this. As a result of increasing compaction upon burial, solution seams and smooth stylolites developed, and diagenetic stabilization of the shell components was initiated with the neomorphic transformation of aragonite to low-Mg calcite, leaving behind relics of the original microstructure. At this stage also the first generation of microporosity developed (Moshier, 1989), in addition to leaching of some grains.

With increasing burial, a later phase of silicification commenced and microfractures developed in most lithofacies, cutting all cements and enhancing the conduit system where further active solutions and second generation of microporosity developed. At this stage low- and high-amplitude stylolites developed. The formation of the late wide fractures also occurred at this time, and were later infilled with red to dull drusy equant calcite cement (see also Burriss et al., 1983).

Stable Oxygen and Carbon Isotopes

The stable carbon and oxygen isotope compositions of the skeletal and nonskeletal components of the Shuiaba Formation are plotted in Figure 9 (see also Table 1). Shown also on this plot is the distribution of these samples according to their location. Rudist shell layers of presumably aragonitic and calcific original mineralogy (Skelton, 1979; Al-Aasm and Veizer, 1986a) have slightly depleted δ180 values relative to the postulated oxygen isotopic composition of Lower Cretaceous carbonates (Moldovanyi and Lohmann, 1984; Morrison and Veizer, 1990), which may reflect partial reequilibration and mineralogical stabilization by diagenetic fluids (Brand and Veizer, 1980; Al-Aasm and Veizer, 1986b; Al-Aasm and Azmy, 1996). Carbon isotopes seem, however, to have been buffered by the original carbonates. The calcific matrix of the studied carbonates show wider ranges in their δ180 values and slightly depleted δ13C values relative to the Lower Cretaceous carbonates by at least l%o. The isotope values of matrix samples analyzed in this study overlap with values of micro-rhombic calcite reported by Moshier (1989) and Budd (1989) from Sajaa Field and of Shuaiba matrix in Oman (Wagner, 1990). However, our results show more depleted carbon and oxygen isotopes in this matrix (Fig. 9). Early generations of calcite cement have δ180 and δ13C values very comparable to the calcific matrix, which may suggest similar fluids and were responsible for the stabilization of the matrix and precipitation of calcite cement. Later generations of calcite cements that occlude fractures, however, have more negative δ180 and δ13C values (Fig. 9). Dolomite partially replaces the calcite matrix and occurs as scattered euhedral rhombs. Its δ180 values are similar to the calcific matrix but its δ13C signatures show wider and negative values for two samples. These negative δ13C values may reflect some organic contributions during the formation of this dolomite (Durocher and Al-Aasm, 1997). There is a slight co-variation trend between oxygen and carbon isotopes in matrix and cement samples.

Table 1.

Stable-isotope analysis of samples from the Shuaiba Formation in the U.A.E.

Sample no.LithologyMineralogyδ180, %o (SMOW)δ180, %o (PDB)δ13C, %o (PDB)
SurfO matrix calcite 27.19 -3.61 2.66 
Surf 1 algal mud-matrix calcite 24.54 -6.18 1.41 
Surf 2 algal mud-matrix calcite 25.67 -5.08 3.89 
Surf 10 algal matrix calcite 25.45 -5.3 3.72 
Surf 19-1 matrix calcite 25.32 -5.42 3.87 
Surf 19-2 Rudist shell calcite 23.67 -7.02 2.93 
Surf 21-1 matrix calcite 26.06 -4.7 3.89 
Surf 21-2 Rudist shell calcite 25.49 -5.26 3.65 
Surf 24 matrix calcite 25.12 -5.62 4.45 
Surf 27 matrix calcite 26.28 -4.49 2.61 
Surf 26 matrix dolomite 26.00 -4.77 4.07 
FT 9081-1 matrix calcite 25.16 -5.58 2.19 
FT 9081-2 large vein calcite 22.38 -8.27 -2.9 
FT 9081-1 matrix dolomite 20.44 -10.15 -1.34 
FT 9083-1 matrix calcite 24.77 -5.95 2.26 
FT 9083-2 vein calcite 22.87 -7.8 1.5 
FT 9089-1 matrix (fossil) calcite 24.16 -6.55 2.62 
FT 9096 matrix calcite 23.46 -7.23 0.66 
FT 9099-1 matrix calcite 24.92 -5.81 2.17 
FT 9111 matrix calcite 25.35 -5.39 2.82 
FT 9119 matrix calcite 25.07 -5.66 2.66 
FT 9137-1 matrix dolomite 24.56 -6.16 -0.82 
FT 9137-2 fossil calcite 24.67 -6.05 2.19 
FT9151 matrix calcite 22.32 -8.34 1.17 
FT 9161-1 matrix calcite 24.16 -6.55 2.32 
FT 9161-2 cement in fossil calcite 24.41 -6.31 2.43 
FT 9166 matrix-grainstone calcite 23.97 -6.73 1.2 
FT 9172-1 matrix calcite 24.53 -6.19 2.22 
FT 9172-2 cement in fossil calcite 17.05 -13.44 -0.28 
FT 9198-1 matrix calcite 25.18 -5.56 0.67 
FT 9198-2 vein calcite 22.28 -8.38 2.14 
Sa-65 (23) matrix calcite 23.27 -7.31 1.86 
Sa-65 (23) cement ¡n fossil calcite 22.92 -7.75 1.61 
Sb-228(18) matrix calcite 25.56 -5.19 3.89 
Sb-229(17) matrix calcite 25.07 -5.67 2.80 
Sb-229(17) cement dolomite 20.96 -9.65 1.74 
Sb-229(17) cement calcite 25.09 -5.64 2.68 
Sy-16(16) matrix calcite 23.87 -6.83 3.31 
Bb-133(21) matrix calcite 23.13 -7.55 2.95 
Bu-410(19) fossil calcite 24.56 -6.16 4.89 
Bu-410(19) cement in fossil calcite 25.25 -5.49 5.21 
Bu-410(20) fossil calcite 24.03 -6.67 4.99 
Bu-410(20) cement in fossil calcite 24.24 -6.47 5.02 
Sample no.LithologyMineralogyδ180, %o (SMOW)δ180, %o (PDB)δ13C, %o (PDB)
SurfO matrix calcite 27.19 -3.61 2.66 
Surf 1 algal mud-matrix calcite 24.54 -6.18 1.41 
Surf 2 algal mud-matrix calcite 25.67 -5.08 3.89 
Surf 10 algal matrix calcite 25.45 -5.3 3.72 
Surf 19-1 matrix calcite 25.32 -5.42 3.87 
Surf 19-2 Rudist shell calcite 23.67 -7.02 2.93 
Surf 21-1 matrix calcite 26.06 -4.7 3.89 
Surf 21-2 Rudist shell calcite 25.49 -5.26 3.65 
Surf 24 matrix calcite 25.12 -5.62 4.45 
Surf 27 matrix calcite 26.28 -4.49 2.61 
Surf 26 matrix dolomite 26.00 -4.77 4.07 
FT 9081-1 matrix calcite 25.16 -5.58 2.19 
FT 9081-2 large vein calcite 22.38 -8.27 -2.9 
FT 9081-1 matrix dolomite 20.44 -10.15 -1.34 
FT 9083-1 matrix calcite 24.77 -5.95 2.26 
FT 9083-2 vein calcite 22.87 -7.8 1.5 
FT 9089-1 matrix (fossil) calcite 24.16 -6.55 2.62 
FT 9096 matrix calcite 23.46 -7.23 0.66 
FT 9099-1 matrix calcite 24.92 -5.81 2.17 
FT 9111 matrix calcite 25.35 -5.39 2.82 
FT 9119 matrix calcite 25.07 -5.66 2.66 
FT 9137-1 matrix dolomite 24.56 -6.16 -0.82 
FT 9137-2 fossil calcite 24.67 -6.05 2.19 
FT9151 matrix calcite 22.32 -8.34 1.17 
FT 9161-1 matrix calcite 24.16 -6.55 2.32 
FT 9161-2 cement in fossil calcite 24.41 -6.31 2.43 
FT 9166 matrix-grainstone calcite 23.97 -6.73 1.2 
FT 9172-1 matrix calcite 24.53 -6.19 2.22 
FT 9172-2 cement in fossil calcite 17.05 -13.44 -0.28 
FT 9198-1 matrix calcite 25.18 -5.56 0.67 
FT 9198-2 vein calcite 22.28 -8.38 2.14 
Sa-65 (23) matrix calcite 23.27 -7.31 1.86 
Sa-65 (23) cement ¡n fossil calcite 22.92 -7.75 1.61 
Sb-228(18) matrix calcite 25.56 -5.19 3.89 
Sb-229(17) matrix calcite 25.07 -5.67 2.80 
Sb-229(17) cement dolomite 20.96 -9.65 1.74 
Sb-229(17) cement calcite 25.09 -5.64 2.68 
Sy-16(16) matrix calcite 23.87 -6.83 3.31 
Bb-133(21) matrix calcite 23.13 -7.55 2.95 
Bu-410(19) fossil calcite 24.56 -6.16 4.89 
Bu-410(19) cement in fossil calcite 25.25 -5.49 5.21 
Bu-410(20) fossil calcite 24.03 -6.67 4.99 
Bu-410(20) cement in fossil calcite 24.24 -6.47 5.02 

Fig. 9.

Carbon and oxygen isotopic compositions of carbonate components. The box represents the postulated ranges for Lower Cretaceous carbonates (Moldovanyi and Lohmann, 1984; Morrison and Veizer, 1990). Inset is carbon and oxygen isotope compositions of samples according to their locations. See text for details.

Fig. 9.

Carbon and oxygen isotopic compositions of carbonate components. The box represents the postulated ranges for Lower Cretaceous carbonates (Moldovanyi and Lohmann, 1984; Morrison and Veizer, 1990). Inset is carbon and oxygen isotope compositions of samples according to their locations. See text for details.

When isotopic values of the studied carbonates are plotted according to their locations an apparent trend appears. The isotopic composition of surface samples overlaps those from the subsurface. However, the deviation from this general trend arises in two areas. In one, the Bu Hasa samples show very narrow ranges in both oxygen and carbon isotopes (Fig. 9), but samples taken from the Fateh field show wider ranges in both isotopes. This may reflect variations in diagenetic pathways within the studied area, perhaps in response to variations in porosity, permeability, and accessibility of diagenetic fluids, assuming similar lithofacies. Otherwise, it may be reflect variable degrees of recrys-tallization of these carbonates with increasing depth of burial (Moshier, 1989).

Petrographic, burial-history, and isotope data can be used to infer changes in the oxygen isotope composition of diagenetic fluids in the Shuaiba carbonates during progressive diagenesis. The relationships among mineral δ180 values, pore-water δ180 values, and temperature for diagenetic phases from the Shuaiba carbonates are shown in Figure 10. Three possible pathways for pore-water evolution have been suggested and include (a) changes in temperature (increasing burial), (b) changes in fluid chemistry, or (c) a combination of increasing temperature and fluid-rock interaction. In the first scenario, temperature increase alone due to burial is the key factor for stabilization of the carbonate components and the formation of later diagenetic events. The diagenetic fluids of marine parentage did not change significantly during the burial history of these rocks. Moshier (1989) showed that the maximum burial of the Shuiaba Formation was over 3000 m. Accepting a geothermal gradient of 25°C/km and a surface temperature of about 20°C, this translates into ca. 90°C as a maximum burial temperature for the formation of diagenetic phases in the Shuiaba Formation. However, this model cannot explain the covariation of oxygen and carbon isotopes (Fig. 10). Furthermore, it assumes a closed diagenetic system, which may pose difficulty explaining the diagenetic repartitioning of stable isotopes and the formation of early and late calcite cement.

Fig. 10.

Oxygen isotope composition of pore water (δ18O, %o SMOW) vs. crystallization temperature (°C) for Shuaiba carbonates. Isotopic composition of calcite is shown as contours (δ18O, %o PDB). The equation relating temperature, δ180water, αvδδ18Omineral is (103ln αcal-water = 2.78 x 106T 2-2.89) (Friedman and O’Neil 1977). Shown also the isotopic domains for micro-rhombic calcite from other studies (filled oval is from Budd, 1989; and the rectangle is from samples taken from Sajaa Field from Moshier, 1989). See text for details.

Fig. 10.

Oxygen isotope composition of pore water (δ18O, %o SMOW) vs. crystallization temperature (°C) for Shuaiba carbonates. Isotopic composition of calcite is shown as contours (δ18O, %o PDB). The equation relating temperature, δ180water, αvδδ18Omineral is (103ln αcal-water = 2.78 x 106T 2-2.89) (Friedman and O’Neil 1977). Shown also the isotopic domains for micro-rhombic calcite from other studies (filled oval is from Budd, 1989; and the rectangle is from samples taken from Sajaa Field from Moshier, 1989). See text for details.

The other scena rio assumes that most of the diagenetic changes occurred at lower temperatures due to changes in pore-water isotope chemistry, such as incursion of meteoric waters during exposure between deposition of the Shuiaba and the Nahr Umr Formations (Budd, 1989; Scott, 1990; Wagner, 1990; Alsharhan, 1995). This model may explain the stabilization of rudists and matrix mineralogy and development of microporosity, and may also help explain the covariant trend of stable isotopes in some components. Budd (1989) suggested that early diagenesis (formation of micro-rhombic calcite and early cement) of the Shuaiba carbonates in the U.A.E. occurred in an unconfined meteoric aquifer during subaerial exposure immediately after Shuaiba deposition. This suggestion was based on petrographic and isotopic studies of these carbonates. However, this model cannot explain adequately the slight variations in δ13C values, nor does it explain the changes in oxygen isotopes in the later-precipitated diagenetic phases. It may have contributed earlier in the diage-netic history. However, the samples investigated in this study show no clear evidence of paleo-exposure, although their absence does not completely exclude this hypothetical exposure. Vahrenkamp (1996) discounted the possibility of meteoric exposure of the Upper Kharaib and Shuaiba Formations to explain carbon isotope variations. The stratigraphic variation in δ13C of the Shuaiba has been proposed as a correlation tool (Wagner, 1990; Vahrenkamp, 1996; Grötsch et al., 1998).

Increasing temperature accompanied by progressive changes in pore-water isotopic chemistry due to water-rock interaction and/ormixingof fluidsdescribed by Frank and Lohmann (1995) should explain both the covariations in stable isotopes (Fig. 10) and the formation of later diagenetic events during burial. Modification of the original marine pore waters, whether due to incursion of meteoric waters at shallow burial or due to increasing water-rock interactions and temperature, will explain both the negative shift in oxygen isotopes and the buffering of carbon isotopes with the original values.

Hydrocarbon Potential

The Shuaiba Formation carbonates are important reservoirs in the petroleum system of the U.A.E. (see Alsharhan and Nairn, 1997; Alsharhan and Scott, this volume). The future reservoir potential of the Shuaiba is a function of its reservoir characteristics associated with its source rocks and seals.

Reservoir Characteristics

The carbonates of the lower and upper members of the Shuaiba Formation form one of the most important reservoir lithologies in the U. A.E., and produce oil from a number of giant fields, such as the Bu Hasa oil field and the Sajaa gas condensate field (Fig. 1). Their net pay thicknesses range between about 10 and 80 m; porosities range from 13% to 33%, and permeabilities from 1.0 to 160 md. Overall reservoir quality depends on the importance of diagenetic processes such as calcite recrystallization, dolomitiza-tion, fracturing, leaching, and secondary pore filling.

The Shuaiba Formation carbonates are well developed in the central sector of the U.A.E., where they are thick and extensive (e.g., the Bu Hasa field) (Fig. 1). In this giant oil field, the carbonates have a gross pay thickness of 68 m at a depth of 2346 m. Their porosity and permeability averages 23% and 100 md, respectively, and they contain almost 90% of the hydrocarbons present in this giant field (Alsharhan, 1993). The reservoir characteristics of the Shuaiba carbonates varies widely and are generated by the development of the rudist buildup complex over an algal platform.

Lower Member. —

The pore types in this member include vuggy, moldic, and interparticle porosity. The average porosity is 18%. The abundance of stylolites in this member considerably decreases the porosity and permeability. Some of the pore spaces are filled with calcite, baroque dolomite (Alsharhan and Williams, 1987), and pyrite. The permeability is particularly affected by the abundance of stylolites, which decrease the permeability to 2 md. The permeability reaches its maximum 55 md within the algal-rich layers.

Upper Member. —

The porosity and permeability of this member are distributed erratically, corresponding to the heterogeneity of the lithofacies within this unit. The pore types are dominantly vuggy, moldic, and secondary interparticle porosity such as that formed by the dolomitization. The vuggy porosity and moldic porosity are formed by leaching of rudists, oysters, and foraminifers; however, some of these pores have been occluded by calcite cement. Most of the primary interparticle porosity that had been present throughout the sequence was filled by drusy calcite and microc-rystalline cement. Dolomitization has produced a small amount of intercrystalline porosity, which in part has been reduced by later blocky and drusy calcite cementation and microcrystalline silica and pyrite. Some pores are fine to medium, formed by rhombic and mosaic dolomite with relic leached fossils. The porosity ranges from less than 3% in the dense limestones to 27% in the buildup facies of the upper member.

Cementation and sylolitization decrease to a great extent the permeability of the Shuaiba limestones to less than 1 md, forming the dense intervals of the member. This can be seen in an increase in the responses of the density and gamma-ray logs and decreases in the neutron-porosity log. The best permeability coincides with greater quantities of rudists, calcareous algae, and corals, and consequently reaches up to 150 md in the buildup facies.

Source Potential

The major source rocks defined in the U.A.E. are the Diyab-Dukhan formations (Oxfordian-Lower Kimmeridgian) and the Shilaif-Khatiyah formations (Upper Albian to Cenomanian) (Alsharhan, 1989).

The source potential of the Aptian Shuaiba Formation has been studied in detail by Azzam and Taher (1995) and Taher (1997). They described the organic-rich carbonate intervals within the basinal facies Bab Member of the Shuaiba Formation with excellent to poor source potential, with total organic carbon (TOC) and thicknesses ranging from 4.3% and 30 m in the central part of the Shuaiba basin to 0.7% and 100 m toward the shelf margins, respectively. Some of the offshore wells such as in Umm Shaif, the Bab Member shows a pyrolysis yield up to 10 kg/ton (Alsharhan, 1989). In other areas, the Bab carbonates were not analyzed but are characterized by light brown to black, fine-grained, laminated limestones, and lower formation density and higher gamma ray in comparison with the underlying carbonates. These are the typical characters for carbonate source rocks described Meyer and Nederlof (1984) and Ohler (1984). The Bab Member, deposited in an intrashelf setting, was exposed to restricted water circulation, anoxic conditions, and deposition below wave base. The Bab carbonate extracts have a sulfur content of 4.5%, a pristane/phytane ratio of 1.4%, and an average δ13C value of -25%o (Mohamed and Ayoub, 1992), indicating shallow to deep marine environments of deposition.

Generally, the average geothermal gradient in the U.A.E. increases southward (Alsharhan, 1989). In the northern offshore part of the U.A.E., it is 1.5°F/100 ft and reaches up to 2.3°F/100 ft in the southern part. However, in the southeastern corner of the U.A.E., all source rocks are immature. Based on the Time-Temperature Index (TTI) method, Taher (1997) concluded that the basinal facies of the Shuaiba is immature only in the most northwestern and southeastern part of the U.A.E. and is mature everywhere else.

The geochemical characteristics of the Bab Member extracts were compared by Taher (1997) with those of the Jurassic Diyab source rock (the main source rock for the oils in the U.A.E.), using analytical gas chromatography, and showed significant differences. He concluded that extracts from the Bab basinalcarbonates could be geochemically correlated with oil samples from the Shuaiba oils in some oil fields in the onshore part of the U.A.E. (Fig. 11).

Fig. 11.

Gas chromatograph plots of oil/source correlation showing the genetic relationship between the Bab Member extracts and the Thamama oils.

Fig. 11.

Gas chromatograph plots of oil/source correlation showing the genetic relationship between the Bab Member extracts and the Thamama oils.

Sealing mechanisms

Two potential sealing rocks for hydrocarbon accumulations are associated with the Shuaiba Formation carbonates in the U.A.E., including (1) the fine clastics of the Albian Nahr Umr Formation and (2) the dense lime mudstone within the lower and upper members. The shales of the Nahr Umr Formation are considered to be the ultimate seal for hydrocarbon accumulations in the Shuaiba reservoirs. They form potential vertical seals above the underlying lower and upper members, and lateral seals where adjacent to the Shuaiba reservoirs across folds and/or faults. The dense lime mudstone, wackestones, and packstones in the lower and upper members act as seals for the reservoirs in the Shuaiba (see also Alsharhan and Scott, this volume).

Conclusions

  1. The Aptian Shuaiba Formation in the U.A.E. can be divided into the informal lower and upper members, and the Bab Member. The formation represents a wide range of deposi-tional settings from shallow shelf to intrashelf starved basinal facies.

  2. The carbonates within the formation comprise seven different lithofacies: peloidal skeletal algal packstone-grainstone; ooidal-peloidal grainstone/packstone; skeletal algal (Lithocodioideae) floatstone; intraclastic packstone-grainstone and coated packstone/grainstone; skeletal wacke-stone/packstone, algal (Lithocodioidea) floatstone; skeletal foraminiferal (Pseudocyclammina) argillaceous wackestone/ packstone; and skeletal foraminifera/lithocodium argillaceous wackestone/packstone.

  3. The Shuaiba carbonates underwent various diagenetic modifications during shallow to deep burial stages. These modified their original altered textural and geochemical attributes and affected reservoir porosity. Diagenetic modification of the Shuaiba carbonates occurred at variable burial depths and water-rock interactions.

  4. Oxygen and carbon isotopic data demonstrate that the calcific matrix has the least altered values in both isotopes relative to the postulated Cretaceous marine carbonates (average δ180 = -5.7% PDB; δ17C = +2.5%o PDB), whereas the calcite cements occluding shell porosity and infilling veins have more depleted isotopic values (average δ180 = -8.8%o PDB; δ13C = +0.5%o PDB). The isotopic data also show a co-variant trend between these isotopes. The variations of oxygen and carbon isotopes reflect increasing water-rock interaction and /or fluid mixing during progressive burial.

  5. The Shuaiba carbonates are one of the most prolific reservoir lithologies in the U.A.E. oil province, and produce oil and/or gas from several giant fields in the basin. The average porosity is 23%, and permeabilities range from 1.0 to 150 md. Their net pay thicknesses reach up to 78 m, as in the Bu Hasa field. Basinal facies within the formation (Bab Member) have poor to good source potential.

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Wagner
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Witt
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(ed.), Micropalaeontology and Hydrocarbon Exploration in the Middle East:
Chichester, U.K.
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242
.

Acknowledgements

The authors are especially grateful to Abu Dhabi Company for Onshore Operation (ADCO) and Dubai Petroleum Company (DPC) for supplying some core data. They express their appreciation to Drs. Christopher Kendall, David Budd, G.W. Hughes, and Robert W. Scott for reviewing this manuscript; and to S.D. Russell (ADCO) who introduced us to the Shuaiba outcrop area. I.S. Al-Aasm is thankful to the Canadian Natural Sciences and Engineering Research Council (NSERC) for its continuous support.

Figures & Tables

Contents

GeoRef

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