ABSTRACT

The Aptian Shu’aiba Formation forms a major carbonate reservoir in the Shaybah field of eastern Saudi Arabia. Lack of exposures and poor seismic data have forced the cored intervals to be fully exploited to provide evidence of the depositional environment and layering of the reservoir rocks and associated lithofacies. Rudist, foraminiferal and coccolith evidence indicates an Aptian age for the entire Formation, most of it being early Aptian. A major unconformity at the top of the Shu’aiba separates it from the overlying Nahr Umr Formation. Rapid biofacies variations suggest possible sequence boundaries within the Shu’aiba Formation. Semi-quantitative macropaleontological and micropaleontological analyses indicate significant paleoenvironmentally influenced lateral and vertical bioassemblage variations. Lagoon, rudist-associated back-bank, bank-crest and fore-bank, and upper-ramp depositional environments have been interpreted, of which the bank represents the gradual amalgamation of earlier isolated rudist shoals. Integrating the micropaleontological analyses with rudist assemblages has facilitated the prediction of rudist-associated reservoir facies. Variations in the micro- and macrofacies permit the Formation to be divided into three layers. (1) The “lower Shu’aiba” (without rudists) is dominated by a regionally extensive, moderately deep marine planktonic foraminiferal/algal association of Palorbitolina lenticularis-Hedbergella delrioensis-Lithocodium aggregatum and the benthonic foraminifera Debarina hahounerensis, Praechrysalidina infracretacea, Vercorsella arenata and rotalids. (2) The “middle Shu’aiba” shows the significant lateral and vertical differentiation of a rudist-rimmed shallow carbonate platform typically associated with a marine highstand. A predominance of rudist species Glossomyophorus costatus and Offneria murgensis occurs together with Lithocodium aggregatum, Palorbitolina lenticularis, Trocholina spp. and miliolid foraminifera. (3) The “upper Shu’aiba” represents an expansion of the lagoon (associated with a marine transgression), and a predominance of Agriopleura cf. blumenbachi and A. cf. marticensis rudists, together with Debarina hahounerensis, Praechrysalidina infracretacea and Vercorsella arenata. The localized distribution of the rudist Horiopleura cf. distefanoi in association with corals, is a feature of the eastern flank of the field. A coarse assemblage-based biozonation for the Shu’aiba has been proposed, but a detailed scheme is precluded by rapid diachronous biofacies variations across the Shaybah field. In addition to the major biocomponent assemblages, minor variations reveal high-frequency depositional cycles that may assist in the interpretation of the distribution and correlation of reservoir facies. The identification of bioassemblages, and the paleoenvironmental interpretation of formation micro-imager logs from vertical cores in exploration wells, has assisted the calibration of images from uncored horizontal development wells.

INTRODUCTION

The purpose of this paper is to provide a comprehensive account of the spatial relationships of the various micropaleontological and macropaleontological components of the Shu’aiba Formation in the Shaybah field of eastern Saudi Arabia (Figure 1). It is a pioneer approach to determining the internal architecture of the Shu’aiba carbonate reservoir. The integration of the biocomponents has yielded a coherent paleobiological interpretation. Coincident with the biofacies distribution are the various reservoir aspects of the carbonates, the distribution of which may now be better understood and predicted. Sequence stratigraphic implications of the various biofacies configurations are outlined, but the reader is referred to Aktas et al. (1999, 2000) for a detailed account of the sedimentology and sequence stratigraphy and its application to the reservoir model.

The Shu’aiba Formation has been studied in great detail in the United Arab Emirates (UAE) and Oman, where it is an important hydrocarbon reservoir. Some key references are Litsey et al. (1983), Frost et al. (1983), Harris et al. (1984), Alsharhan (1985, 1987), Scott (1990), Alsharhan and Kendall (1991), Hamdan and Alsharhan (1991), Abou-Choucha and Ennadi (1993), and Azer and Toland (1993).

The Shaybah field of approximately 700 sq km produces oil and gas from the Shu’aiba Formation at a depth of 4,900 ft (Saudi Aramco, 1999). The Formation is of Aptian age (almost entirely early Aptian). It consists wholly of carbonates that mostly accumulated as rudist-rimmed, shallow-marine platforms on the southern flank of an intra-shelf basin (Figure 2). It has an average thickness of approximately 400 ft (Saudi Aramco, 1999; Hughes, 1998a, Figure 3).

In Saudi Arabia, rudists have been recorded only from the Aptian Shu’aiba and Maastrichtian Aruma formations, although they are also known from other stratigraphic levels in other parts of Arabia. Of the two formations, only the Aruma is exposed, and the study of the Shu’aiba rudists has been based entirely on subsurface data obtained from core samples. The Shu’aiba Formation has been reported as outcropping in Wadi Nisah (Figure 2) although seismic and well-based data suggest that erosion associated with the pre-Aruma unconformity probably removed the Shu’aiba east of the published Shu’aiba outcrop limit. When integrated with other macro- and micropaleontological information, rudists have contributed significantly to the paleoenvironmental interpretation of the Shu’aiba (Hughes, 1999). In addition, the contribution of rudists to reservoir quality has encouraged a detailed investigation of their distribution as a means of predictive modeling away from cored wells. A lack of good-quality seismic data for the Shaybah field has placed additional importance on core-based information to provide guidance for reservoir facies distribution. In the UAE, where seismic resolution is better owing to the thinner cover of sand dunes (Calavan et al., 1992; Fischer et al., 1994, 1997), the seismic data has assisted structural interpretation of the Shu’aiba-rimmed carbonate platform. The study of the macropaleontological and micropaleontological components, when integrated with petrographical, sedimentological, petrophysical and wireline log data, has provided important information for mapping depositional facies and establishing a reservoir model (Aktas et al., 1999, 2000).

The Shu’aiba Formation was deposited over an extensive stable platform during a period of relative tectonic quiescence during the early Aptian. The Arabian Peninsula at that time was located approximately 8°S of the Equator and the Aptian paleogeography was similar to that of the Albian shown in Figure 3 (Smith et al., 1981; Hulver, 2000). An intra-shelf marine basin occurred to the north and east of the Shaybah area (Figure 2) and deep marine, planktonic foraminiferal-bearing sediments are present at certain levels within the Shu’aiba of Shaybah field, and persistently in wells on the eastern flank of the field. The basin margin was extremely gentle, with a slope angle of only a few minutes to less than 2° (Aldabal and Alsharhan, 1989). Rudists preferentially colonized the rim of the basin (Grabowski and Norton, 1995, Figure 15). The generalized paleoenvironmental features of the Shaybah field are shown in Figure 4. The field developed on a peninsular-like projection of a near-shore rudist-bank complex into the open marine waters of the basin.

The primary applications of the study have been the interpretation of the paleoenvironmental evolution of the possible peninsula-like carbonate platform into the intra-shelf basin (Figure 4); the definition of the paleoenvironmentally controlled depositional layers; and the prediction of the location of the rudist banks. Lateral variations of biofacies within the individually correlated layers have enabled construction of a “pseudo layer-cake” stratigraphy (in the manner of Aigner, 2000). A three-dimensional model of the evolution of the Formation at Shaybah is based on the vertical stacking of the micro- and macrofaunal variations.

STRATIGRAPHY OF THE SHU’AIBA FORMATION

The holostratotype section for the Shu’aiba Formation is the 9,870 to 10,132-ft interval in the Zubair-3 well in southern Iraq (Owen and Nasr, 1958). In the absence of confirmed exposures of the Formation in Saudi Arabia, the hypostratotype is in Saudi Aramco Abu Hadriya-4 well, between 2,136.7 and 2,197.6 m depth.

Shaybah field

There is no formal subdivision of the Shu’aiba Formation in the Shaybah field but a three-tiered, sequence-bounded, compound scheme has been established using biofacies constraints (Figure 5). Three layers are recognized: “lower” (Shu’aiba I); “middle” (Shu’aiba II); and “upper” (Shu’aiba III).

The Formation consists of non-dolomitic carbonates (planktonic foraminiferal mudstones, rudist packstones and coralgal boundstones). The basal unit (Shu’aiba I, or “lower Shu’aiba”) includes basal dense, dark-gray orbitolinid and planktonic foraminifera-bearing carbonates considered in Saudi Arabia to be a carbonate facies of the Biyadh Formation. In this study, this lower dense unit is considered to be genetically related to the Shu’aiba where it represents the lowermost transgressive unit. The base of the Biyadh Formation is considered to unconformably overlie the rudist-bearing carbonates of the Buwaib Formation. The Biyadh, or lowermost Shu’aiba, is equivalent to the “upper dense unit” of the Kharaib-B Formation in the UAE and to the Hawar shale in Qatar and Oman (Alsharhan and Nairn, 1997). The upper part of the “lower Shu’aiba” is cream-colored. Characteristics of the “middle Shu’aiba” will be discussed later in detail.

In Shaybah, the Shu’aiba is unconformably overlain by marine shale equivalent to the Khafji Member of the Wasia Formation (Alsharhan and Nairn, 1997), that is regionally referred to as the Nahr Umr Formation. The unconformable contact at the top of the Shu’aiba is placed at the marked transition from limestone to shale.

The Shu’aiba III (or “upper Shu’aiba”) contains dissolution features that are infilled with siliciclastic sediments which resemble material from the Nahr Umr. The phase of emergence and karstification that produced the karstic features probably occurred during the later part of the early Aptian, prior to the deposition of the Nahr Umr Formation (Immenhauser et al., 2000).

UAE and Oman

In the UAE, the shallow-water Shu’aiba Formation is considered to be overlain by the onlap of the basinal late Aptian Bab mudstones. Other interpretations, however, have the Formation passing vertically into the intracratonic basinal Bab Member mudstones of “middle” Aptian age (Alsharhan and Nairn, 1986; Scott, 1990), or consider the Bab mudstones to be a lateral deep-water facies equivalent of the Shu’aiba Formation’s platform carbonates (Alsharhan and Kendall, 1991). Witt and Gokdag (1994) describe, for the Shu’aiba in Oman, a mid-Aptian regression that exposed the shallow-marine Shu’aiba sediments in the south while late Aptian deeper marine sediments were deposited to the north.

AGE OF THE SHU’AIBA FORMATION

Shaybah field

In the Shaybah field, larger benthonic foraminifera, rudists and calcareous nannofossils have been used for age determination, as the predominantly shallow-marine deposits preclude the application of ammonite stratigraphy. Throughout this paper, the use of the qualifiers “early” and “late” in relation to age follow the abridged version of the International Stratigraphic Guide (Murphy and Salvador, 2000) when applied to the Aptian. Figure 6 shows the fossil evidence for the age of the Shu’aiba.

The Shu’aiba carbonates of the Shaybah field are dated as early Aptian, based on the presence of rudists Offneria murgensis, Agriopleura cf. blumenbachi and Glossomyophorus costatus (Masse, 1985, 1992; 1999; Skelton and Masse, 2000; Skelton, 2000) and the absence of Eoradiolites spp. (Masse and Gallo Maresca, 1997). Further rudist evidence for an early Aptian age is the apparent absence in the platform carbonates of the Shu’aiba in the Shaybah field of the drastic reduction in species that conventionally marks the early-late Aptian boundary (Masse, 1999).

Schemes for using the larger benthonic foraminifera Orbitolina for stratigraphic purposes were established by Schroeder (1975) and Moullade et al. (1985). The presence of Palorbitolina lenticularis and Choffatella decipiens in the Shu’aiba Formation of Shaybah confirms an early Aptian age (M. Moullade, written commun., 1999; Moullade et al., 1985, 1998; Hussinec et al., 2000) (Figure 6). The presence of Salpingoporella dinarica throughout the section confirms an age no younger than Aptian (Simmons et al., 1991). The apparent absences of any faunal and lithological manifestation of the Selli global anoxic event of latest early Aptian age (Cobianchi et al., 1999), may further restrict the age. Scott (1990) has concluded an early Aptian age for the Shu’aiba Formation, and a late Aptian age for the Bab Member in Abu Dhabi, based on ammonite faunas. However, no ammonites have been found in the Shu’aiba (Kennedy and Simmons, 1991). In certain deeper marine carbonates of the Zumul and Ramlah regions to the east of Shaybah, calcareous nannofossils also indicate early and late Aptian ages (O. Varol, written commun., 1999); Aktas et al., 1999; Hughes et al., 1999).

An attempt to quantify the stratigraphy of the Shu’aiba Formation by Vahrenkamp (1996) and Follmi et al. (1994) by means of carbon isotope dating has not proved useful at Shaybah so far (Hughes et al., 1999). Strontium isotope analysis of selected cores produced results of limited value because of the apparent inaccuracy of the standard Sr seawater curve. Compared with the carbon isotope record at the Aptian type section (Moullade et al, 1998), the marked positive excursion during the later part of the early Aptian is not readily identifiable in the Shaybah field. This lends further support to the earliest early Aptian age based on rudist and foraminiferal evidence.

UAE and Oman

The Shu’aiba Formation in the UAE has been variously dated as early Aptian (Hughes Clarke, 1988; Simmons and Hart, 1987; Scott, 1990) or early to early middle Aptian (Harris et al., 1984; Alsharhan and Nairn, 1986; Alsharhan and Kendall, 1991). Fischer et al. (1994) considered the Shu’aiba of Abu Dhabi to have been deposited during the early Aptian eustatic sea-level rise, and to have been terminated by subaerial exposure in the late Aptian.

Rudist and foraminiferal assemblages in the Shu’aiba of Oman are identical to those in the Shaybah field and provide an early Aptian age (Masse, 1997). Benthonic foraminiferal (Palorbitolina lenticularis, P. cormyi and Mesorbitolina lotzei) and calcareous nannofossil evidence from the Al Huwaisah field of Oman suggest an early Aptian age. However, the presence of foraminifera Mesorbitolina texana, and M. parva in the Lekhwair and Yibal fields to the north of Al Huwaisah indicates that the formation extends into the late Aptian (Witt and Gokdag, 1994).

METHODOLOGY

The Shu’aiba Formation has been cored in most of the vertical wells drilled in the Shaybah field (Figure 4). Paleontological analysis was based on approximately 17,000 ft of core from 51 wells that penetrated the Formation in the Shaybah field. Micropaleontological records, based on the analysis of thin-sections, identified planktonic and benthonic foraminifera, calcareous algae, sponge spicules, calcispheres and ostracods. Macrofossils present are rudists, chondrodontid oysters, corals, gastropods, and the encrusting calcareous alga Lithocodium aggregatum. Bivalve and echinoid debris is present throughout most of the Formation.

The coexistence of certain species in each well section provides biostratigraphic assemblages, or local biozones, of paleoenvironmental significance. For example, Vilas et al. (1995) described paleoenvironmental controls on Orbitolina morphological variations, and Banner and Simmons (1994) used calcareous algae and foraminifera as water-depth indicators in the early Cretaceous carbonates of northeastern Arabia. When integrated with lithological, petrographic and log data, these biozones have enabled the recognition and correlation of depositional layers (Aktas et al., 1999, 2000). In addition, the lateral variations of the biofacies provide evidence for lateral facies migration. A further application of these depositional layers is the development of a three-dimensional lithofacies distribution, with certain important implications for possible reservoir continuity and flow behavior. This novel use of biofacies for the detection of depositional layers within shallow-marine carbonates, and their eventual integration with other disciplines for application to reservoir characterization, has been used for late Permian and late Jurassic successions by Hughes (1996, 2000, in press a, b) and Geel (2000).

BIOCOMPONENTS AND THE PALEOENVIRONMENT

Hughes (1998a) provides a detailed review of fossil assemblages from the Shaybah field. Microfossils of the Shu’aiba exposures in Oman have been described by Simmons and Hart (1987) and Simmons (1994). In addition, a cursory review of Shu’aiba macrofossils and microfossils from Al Huwaisah in Oman is given by Vahrenkamp and Grotsch (1995) and from Bu Hasa, Abu Dhabi, by Russell et al. (1996). Representative fossils of the Shu’aiba Formation are illustrated in Plates 1 to 6. Note that reference to Plates is in the form (1.3); i.e. Plate 1, illustration 3.

Rudists

The majority of Shu’aiba rudists are of the elevator type. They include Agriopleura cf. marticensis (1.1; 1.5; 6.1), Agriopleura cf. blumenbachi (1.2), Glossomyophorus costatus (1.3; 1.4; 1.5), less common Horiopleura cf. distefanoi (2.1; 2.2), a new large genus and species aff. Retha (2.3) (Skelton et al., 1999; Skelton, in press) and rare Offneria nicolinae (3.1; 3.2) (Skelton, 1997; Skelton et al., 1999). This species assemblage closely resembles that from lower Aptian carbonates in Greece (Skelton and Steuber, 1999). Horiopleura cf. distefanoi has been found locally on the northeastern flank of the Shaybah field. Glossomyophorus costatus is particularly well represented within the Formation and is interpreted as having occupied moderately shallow-marine conditions. Such conditions could have occurred either on localized highs within the shelf or lagoon, or in the moderately sheltered leeward, back-bank location and possibly windward positions of the higher energy rudist bank dominated by Offneria murgensis (P.W. Skelton, oral commun., 1998). Offneria murgensis (3.3–3.5) is the only recumbent form identified and no clinger morphotypes have been recognized.

Elevator rudist mud-sticker communities usually grew in restricted carbonate-platform conditions to form tabular biostromes (Skelton et al., 1995; Gotz, 1999). Estimates of their paleobathymetric preferences are difficult to obtain. However, an association of Glossomyophorus costatus with Palorbitolina lenticularis (4.2, 5.5 and 6.2) and the calcareous alga Lithocodium aggregatum (4.1; 5.2) and Salpingoporella dinarica (5.3; 6.12) suggests a depth of from 5 to 20 m (Banner and Simmons, 1994). Glossomyophorus costatus is typically found within a fragmented Offneria matrix, overlying Agriopleura or Lithocodium-Orbitolina facies, and mostly overlain by Offneria facies. Agriopleura species are believed to have occupied a range of environments, although net-positive sedimentation and low-energy conditions would seem to favor their establishment (P.W. Skelton, oral commun., 1998). The tightly fitting, lid-like free (left) valve of Agriopleura cf. marticensis (1.5) would possibly permit survival through periods of temporary exposure under shallow to very shallow conditions. Agriopleura species are typically present above the Lithocodium-Orbitolina facies and below the Glossomyophorus facies (Figure 8a) where they are considered to represent moderately deep, low-energy conditions at the beginning of a new depositional cycle. The Agriopleura species are especially well developed in the upper part of the Formation.

The sediments of the shallower environments consist almost entirely of pallial canal fragments derived from Offneria murgensis (3.3-3.5). Also present is the robust new genus, new species elevator rudist aff. Retha (2.3) (Skelton, 1999; 2000b). It is a new arrival and probably occupied a shallower, higher energy zone than did Glossomyophorus costatus. Grainstones consisting of monospecific O. murgensis debris suggest high-energy sites on the shallow crests of rudist biostromes within the fair-weather wave-base. The empty shells were easily broken to provide very porous sand that was swept predominantly into the lagoon, but also onto the open marine flanks of the bank, possibly through inter-bank channels. Stossel and Bernouli (in press) have described a similar process from the Cenomanian of the Maella platform of central Italy.

Rudist mounds and shoals that are as much as 180 m in diameter have been described from the Aptian-Albian carbonates of Arizona (Hartshorne, 1989) and Texas (Kerans et al., 1997). Flank beds of Offneria grainstones could theoretically attain dips of 30° (Schlager and Camber, 1986; Kenter, 1990). Agriopleura cf. marticensis and Agriopleura cf. blumenbachi are considered to have preferred quiet, shallow platform interiors with net sediment accumulation where extensive tabular bodies could develop. They are often associated with chondrodontid oysters, and thereby form a typical lagoonal assemblage (Cucchi and Pugliesi, 1999; Caffau et al., 1999). Horiopleura cf. distefanoi forms a monospecific assemblage associated with corals on the eastern flank of the platform, but is also rarely found above the Glossomyophorus assemblage on the carbonate platform. It is difficult to estimate the width of the rudist belt in the Shaybah field, but isolated rudist mounds in Mexico form a belt 7 km wide (Alencaster et al., 1999).

Calcareous algae

The dominant calcareous alga in the Shu’aiba Formation is the encrusting codiacean Lithocodium aggregatum (4.1; 5.2) (Banner et al., 1990). It forms several concentrated, localized occurrences, especially in the lower half of the section where it is concentrated in beds that overlie those containing planktonic foraminifera such as Hedbergella. This association suggests cycles that shoal upward from the moderately deep-water Hedbergella biofacies into the slightly shallower Lithocodium biofacies. Another important Lithocodium biofacies is associated with corals (see p. 557).

Other common calcareous algae include Salpingoporella dinarica and Coptocampylodon lineolatus. S. dinarica (5.3; 6.12) is a calcareous dasyclad alga that is concentrated in the uppermost and lowermost parts of the Formation, but is also sporadically represented within the deeper back-bank Glossomyophorus costatus facies within the lagoon. C. lineolatus (6.11) is an aragonitic dasyclad alga that is well represented within the deeper marine sediments although it is not clear if this species prefers deeper conditions, or if it has been transported from the shallower margin of the platform. A rather robust, rare dasyclad form (6.13) is also present in the “middle Shu’aiba” and “upper Shu’aiba”.

The Lithocodium-coral biofacies is overlain by various biofacies in different parts of the field. At localities that later developed into the rudist-bank regime, it is relatively thin and overlain by the elevator rudists Agriopleura cf. marticensis and Glossomyophorus costatus (“foramol” facies of Carannante et al., 1994). Thick Lithocodium-dominated successions are typical within the “algal platform” facies, where they are progressively overlain by G. costatus-bearing sediments at controlled distances from the rudist-bank complex. Where rudists are absent, the Lithocodium-coral biofacies represents a lateral, deeper marine isochronous lagoonal equivalent of the aggrading and prograding rudist-bank facies. The Lithocodium-dominated deep lagoonal sediments are considered to have aggraded to fill the accommodation space that the adjacent, more-rapidly aggrading rudist banks had already saturated. At localities in the east of the study area, the Lithocodium-coral biofacies is overlain directly by a deep-marine biofacies characterized by a Hedbergella (planktonic foraminifera)-Palorbitolina lenticularis (low trochoid morphotype) biofacies.

Corals

Solitary, branching, platy and massive corals occur but are generally rare within the Shu’aiba Formation. However, in the lower part of the Formation, platy microsolenid corals are relatively common and are typically found in association with Lithocodium aggregatum. Corals are an important bioconstituent of the Formation only along the northeastern flanks of the Shaybah field. As microsolenids are related to increased heterotrophic feeding, their occurrence suggests reduced light levels associated with relatively deep marine conditions (P.W. Skelton, written commun., 1999). Microsolenid corals associated with the laterally extensive Lithocodium aggregatum biofacies may represent localized deeper-water conditions in a new depositional cycle, but could also result from the influx of episodic storm-induced debris.

Benthonic foraminifera

Benthonic foraminifera, especially agglutinated genera, are present throughout most of the Formation. The most significant examples are Palorbitolina lenticularis (4.2; 5.5; 6.2), Praechrysalidina subcretacea (4.4; 6.6), Debarina hahounerensis (6.3), Vercorsella arenata (4.7; 6.10), Textularia species (4.3; 5.10), Reophax spp. (5.6; 5.7), Bigenerina sp. (5.8), and Ammobaculites sp. (5.11). Cribellopsis neoelongata (6.4; 6.5), Nautiloculina (6.7) and cf. Pfenderina spp. (6.8) are confined to the lagoonal sediments in the “upper Shu’aiba”. Miliolid foraminifera (typically Quinqueloculina spp.), are preferentially concentrated within the shallow inner neritic regime, and are well represented in the “upper Shu’aiba”. They are not normally found in association with Lithocodium aggregatum, but are consistently present in the lagoonal sediments overlying L. aggregatum of the “middle Shu’aiba”. Palorbitolina lenticularis is well represented. Moullade et al. (1985) suggested that high conical forms of Orbitolina are typically found within the carbonate facies of the “shallow infralittoral zone”, or inner shelf, and rarely occur in the outer shelf environment. The low conical and larger forms are not, however, so restricted and have been found within carbonate shelf facies and also in the outer basin. Other authors, for example, Sartorio and Venturini (1988), place the orbitolinids within the inner shallow platform and also on the outer platform, but absent from the rudist-dominated build-up on the edge of the inner platform. By using the depth ranges of associated algae, Banner and Simmons (1994) have provided depth ranges for Palorbitolina lenticularis of 5 to 60 m, with a preference for 10 to 60 m.

Debarina hahounerensis is well represented within the uppermost and lowermost parts of the Shu’aiba Formation and Praechrysalidina infracretacea is also consistently present. Textularia spp. is present throughout most of the Formation, but is less well represented within the deeper environments. Trocholina alpina (5.4) is only found within the lower part of the open algal platform, where it is associated with Lithocodium aggregatum and rare planktonic foraminifera (4.8; 4.9). Vercorsella arenata is typically restricted to the uppermost and lowermost parts of the Formation.

Rotalid foraminifera are consistently well represented within the deeper marine facies as well as the lagoonal facies, but are absent from the bank and associated environments. A variety of rotalids have been recorded, and include evolute, involute, costate and Lenticulina species (4.6). Nodosarids (5.12) are rare and restricted to the deeper, planktonic-associated sediments of the “lower Shu’aiba” and basal “middle Shu’aiba”.

Planktonic foraminifera

Planktonic foraminifera are found only in certain parts of the Shu’aiba Formation, and range from rare to abundant, but have very low species diversity. Small planktonic foraminifera are well represented at the base of the Formation, where they are often abundant (Figure 7). They are also consistently present on the deeper-marine flanks of the rimmed platform and locally present within the lagoonal sediments where they are considered to be associated with short-term rises in sea level. Calcispheres are calcareous algal cysts, and are present with the planktonic foraminifera in what is considered to be moderately deep marine facies (Leckie, 1986; Banner and Simmons, 1994). Identification of the forams is not always possible owing to the random orientation of the thin sections, but Hedbergella delrioensis (4.8; 4.9) and H. planispira have been positively identified.

Various microfossils and macrofossils

In addition to the fossils discussed above, the Formation also contains bivalves and echinoid debris, together with less common bryozoans, platy corals (5.1), solitary corals, microsolenid corals, gastropods, ostracods, sponge spicules and worm tubes. Bivalve debris is present throughout much of the Formation, and in many localities may represent Glossomyophorus costatus debris. Echinoid debris is also well represented throughout much of the Shu’aiba; for example, echinoid spines (4.10; 4.11 from the “lower Shu’aiba”). Highly spoked echinoid spines are present within the muddier sediments from the deeper environments. Chondrodontid and undifferentiated oysters and nerineid gastropods are generally rare and mostly restricted to the upper part of the formation.

BIOFACIES AND DEPOSITIONAL CYCLICITY

Semi-quantitative micropaleontological and macropaleontological analysis of closely spaced samples reveals localized changes in relative species abundance in addition to localized absolute variations within each well. These changes are related to high-frequency depositional cycles, the characteristics of which vary in response to the environmental differences that occur from well to well. Vertical transitions between elevator and recumbent rudists are probably controlled by changes in energy levels related to sea-level changes, whereas lateral trends are related to lateral energy variations associated with the microenvironments of rudist banks. Integration of the three-dimensional variations in the macro and microbiofacies has enabled a broad subdivision of the Shu’aiba Formation in the Shaybah field into three stacked major depositional layers (Figure 8). However, the high spatial variability of the micro- and macrofauna evident in Figure 8 precludes the establishment of a unified pan-Shu’aiba biozonation scheme.

The lowest paleoenvironmental layer (“lower Shu’aiba”) represents a widespread, low relief, moderately deep carbonate shelf with a laterally extensive, rather uniform biocomponent that is characterized by the absence of rudists. It contains a distinctive basal succession informally equated with the Biyadh Formation, and characterized by dense, dark-gray, orbitolinid-dominated beds that alternate with paler beds containing the alga Lithocodium aggregatum. The planktonic foraminifera Hedbergella delrioensis and H. planispira are present within the orbitolinid beds. This distinctive basal part is overlain by a thicker unit that is consistently of a pale color, and consists of two layers characterized by the common presence of Lithocodium aggregatum with platy corals, separated by a denser layer in which planktonic foraminifera are well represented. This association has been informally interpreted to represent two episodes of “open algal platform” development, separated by “deep, open platform” facies. The absence of rudists in the “lower Shu’aiba” may have resulted from the unrestricted open marine conditions.

The “lower Shu’aiba” is overlain by the thicker “middle Shu’aiba”. Rudist colonies became established in the “middle Shu’aiba”, possibly on the rims of fault-controlled blocks that caused the differentiation of the platform into lagoonal, rudist-bank and offshore marine provinces (Figure 8). Within the lower of the lagoonal successions, evidence for open marine access suggests that the rudist-bank complexes were rather ineffective. Faults that terminate at the boundary between the “lower Shu’aiba” and the “middle Shu’aiba”, interpreted from formation micro scanner (FMI) logs (K. Sadler, oral commun., 1999), may have caused small submarine topographic highs to form that favoured rudist colonization. A similar structural episode commenced at the Biyadh-Shu’aiba contact in the Al Huwaisah field of Oman according to Boichard et al. (1995). Variations in the positions of specific rudist and foraminiferal facies, supported by log correlation, suggest that the rudist-bank complex aggraded, retrograded and prograded during the middle Shu’aiba, presumably in response to changes in sea level (Figure 8) on the gently subsiding platform.

The upper paleoenvironmental layer (“upper Shu’aiba”) contains a characteristic micro- and macrofaunal lagoon assemblage. The lagoon has a minor rudist-bank rim that is narrower than that of the “middle Shu’aiba”. A facies dislocation is recognized at the base of the lagoonal complex that resulted from a minor basinward shift of the underlying rudist-rimmed lagoon of the “middle Shu’aiba”. Many of the species of foraminifera in the “upper Shu’aiba” are the same as those that characterize the “lower Shu’aiba”, and may suggest moderately deep lagoonal conditions. In this study, it was decided that assignment to informal biofacies-characterized environmental regimes is probably more accurate than estimates of specific water depths.

“LOWER SHU’AIBA” (SUB-RUDIST SUCCESSION)

The foraminifera and calcareous algae of this layer (Figure 8) are remarkably similar across the entire study area, and indicate a regionally similar paleoenvironment of a stable and regionally extensive carbonate platform. The “lower Shu’aiba” consists essentially of a duplicated succession of two major biofacies that represent two shallowing-upward cycles. The deeper Palorbitolina lenticularis-dominated biofacies with planktonic foraminifera, underlies a shallower biofacies dominated by Lithocodium aggregatum.

Deep open platform “Biyadh”

Of the four main depositional units of the “lower Shu’aiba”, the lowermost, dense part consists of meter-thick beds of dark-gray, orbitolinid packstone with planktonic foraminifera (4.6, 4.7; and Figures 7 and 8) and equates with the uppermost part of the Biyadh Formation. These pass upward into cream-colored wackestone/boundstone characterized by an abundance of encrusting Lithocodium aggregatum. Large, low trochoid Palorbitolina lenticularis are dominant and well packed. Vilas et al. (1995) identified this composition and fabric as outer shelf. Well-packed, large, low trochoid Palorbitolina lenticularis, Textularia spp. and rotalid foraminifera are dominant, with subordinate miliolids and rare planktonic foraminifera. Bivalve, echinoid debris and the dasyclad alga Salpingoporella dinarica are well represented, and there are rare coral fragments.

Open algal platform

This unit forms the basal depositional unit of the Shu’aiba Formation in the strict sense and maintains a remarkable consistency of fossils across the entire Shaybah field. Praechrysalidina infracretacea and Textularia spp. are well represented together with rare Hedbergella spp. and rare Choffatella decipiens, rotalids, Palorbitolina lenticularis and Vercorsella arenata. The calcareous alga Lithocodium aggregatum is present throughout, with rare, localized Salpingoporella dinarica.

Deep open-marine platform

Planktonic foraminifera, calcispheres and large, low-trochoid Palorbitolina lenticularis are commonly abundant within this section. Foraminiferal diversity is typically high, and the relative uniformity of the biocomponents suggests a relatively deep carbonate shelf depositional environment. The combination of a rich planktonic assemblage and rare Lithocodium aggregatum suggests deposition within the shallow middle neritic zone (30–60 m). Calcispheres, Hedbergella spp., Bolivina spp., Lenticulina spp., rotalids, Palorbitolina lenticularis and Textularia spp. are well represented, together with rare Reophax spp. and Vercorsella arenata. Some small, highly ornamented echinoid spines occur within this assemblage.

Moderately deep open-algal platform

This unit also displays an extensive and uniform biocomponent composition across the Shaybah field. It is characterized by the almost constant presence of the calcareous alga Lithocodium aggregatum whose presence suggests deep inner neritic, low energy conditions and a low sedimentation rate. The foraminifera Palorbitolina lenticularis and Textularia spp. are well represented, and there are subsidiary rotalids, pteropods and spicules. Planktonic foraminifera are rare or absent.

Coralgal platform/ramp

The calcareous alga Lithocodium aggregatum and scattered simple corals characterize this depositional environment. The planktonic foraminifera Hedbergella spp. are locally present, and a deep inner neritic (15–30 m), low energy depositional environment with low sedimentation rate is indicated. The benthonic foraminifera Trocholina spp., Reophax spp., Palorbitolina lenticularis and rotalids are typically present. Lithocodium aggregatum is commonly present and is accompanied by subordinate Salpingoporella dinarica and simple corals.

“MIDDLE SHU’AIBA” (MAIN RUDIST COMPLEX)

Deep lagoon/partly barred ramp or platform

This environment (Figure 8) contains similar fossil assemblages to those described for the coralgal platform. However, there is a notable decrease in the abundance of planktonic foraminifera and an increase in Palorbitolina lenticularis, Trocholina sp. and Textularia spp. Lithocodium aggregatum is well developed in association with subordinate Salpingoporella dinarica and coral fragments, and is commonly interbedded with Palorbitolina lenticularis. Deep inner neritic water depths are indicted, and the presence of rudist biostromes suggests that a partly barred shelf or incipient lagoon may have existed but with limited open marine access, as indicated by the presence of calcispheres and planktonic foraminifera. A belt of rudist-infested mobile shoals and channels, as described by Gili et al. (1995), rather than a barrier reef, probably rimmed the platform.

Shallow lagoon

The shallow lagoon environment is characterized by a moderately high foraminiferal diversity. Miliolids are present, together with Palorbitolina lenticularis and Textularia spp. rotalids, Trocholina sp., Debarina hahounerensis, Everticyclammina hedbergi, Vercorsella arenata and Praechrysalidina infracretacea. Orbitolinids are typically small and highly trochoid. Lithocodium aggregatum displays a rather inconsistent regional distribution, and is commonly replaced by the dasyclad alga Salpingoporella dinarica and rarely Coptocampylodon lineolatus. Water depths of less than 15 m are thought to have occurred, corresponding to the inner neritic zone. The elevator rudists Glossomyophorus costatus, and Agriopleura cf. marticensis are well represented.

Deep back-bank

The deep back-bank regime is characterized by the co-occurrence of abundant bivalve and echinoid debris with well represented Glossomyophorus costatus and Lithocodium aggregatum. Higher benthonic foraminiferal species diversity occurs than in the shallow back-bank. Agriopleura spp. is poorly represented. The abundance of Offneria debris is significantly very low to absent. Palorbitolina lenticularis (high trochoid form) and Textularia spp. are common, together with subordinate Debarina hahounerensis, rotalids, miliolids, Everticyclammina hedbergi and Praechrysalidina infracretacea.

Shallow back-bank

The elevator rudist Glossomyophorus costatus and abundant bivalve debris dominate this moderately shallow depositional regime. Offneria debris, in the form of fragments of pallial canals, is a common component of the sediment, and was derived from the physical destruction of O. murgensis shells on the biostromal crest and its immediate flanks. The co-occurrence of debris of elevator and recumbent rudists is common (Gili et al., 1995). The elevator rudists lived partially buried in bioclastic sand and calcareous sandy mud that accumulated around them, and this implies that they occupied low to moderate energy environments (Gili et al., 1995). Elevator accumulations have been defined elsewhere (Skelton and Gili, 1991) as being of meter thicknesses and essentially lenticular or tabular in shape, showing no topographical relief. They are characterized by having low species diversity and are commonly monospecific to paucispecific. Miliolid foraminifera are well represented with subordinate Palorbitolina lenticularis (high trochoid). Trocholina spp. Lithocodium aggregatum is locally present.

Bank crest

The very shallow, high-energy conditions associated with this kind of environment are responsible for the absence of fine-grained sedimentary material such as foraminiferal debris. The predominance of the recumbent caprinid rudist Offneria murgensis is indicative of unstable, current-swept conditions where the by-passing of sediment predominated over its accumulation (Gili et al., 1995; Skelton et al., 1995). The dasyclad alga Coptocampylodon lineolatus (6.11) is typically rare.

Shallow fore-bank

Sediments that accumulated within the shallow fore-bank were derived mostly from the bank crest, and therefore consist predominantly of coarse and fine-grained debris of the recumbent rudist Offneria murgensis. Fragments of Lithocodium aggregatum and scattered specimens of the elevator rudist Glossomyophorus costatus are possibly allochthonous, having been transported through inter-bank channels from the deep back-bank or lagoon. The moderately high sedimentation rates of the shallow fore-bank would have been unsuitable for the establishment of L. aggregatum (Banner et al., 1990) that required low energy, low sedimentation rates. G. costatus, however, is considered to have preferentially occupied protected, low-energy environments in which there were only occasional episodes of physical disturbance (Gili et al., 1995; Skelton, oral commun., 2000). It is conceivable that such an environment was available locally and that it also permitted the development of the dasyclad alga Coptocampylodon lineolatus. An allochthonous source for many of the species, however, is suggested by the coarse grain size that would typically exclude in-situ benthonic foraminifera such as Palorbitolina lenticularis, miliolids and Textularia spp.

Deep fore-bank

This depositional environment is characterized by sparse Offneria debris together with planktonic foraminifera that imply long-distance transport from a rudist bank. The sediments contain predominantly fine-grained fragments of Lithocodium aggregatum, with miliolids, Lenticulina spp., rotalids, large, low-trochoid Palorbitolina lenticularis, textularids, Trocholina spp., Debarina hahounerensis, Everticyclammina hedbergi, Vercorsella arenata and Reophax spp. These, together with common bivalve and echinoid debris, Coptocampylodon lineolatus, Cylindroporella arabica, coral fragments, and gastropods are all probably derived from a shallower source. Pteropods and high-spoked echinoid spines, however, are probably in-situ components.

Open marine-upper slope

The planktonic foraminifera Hedbergella spp. and benthonic forms such as rotalids, low trochoid Palorbitolina lenticularis and Textularia spp. characterize this environmental regime in the absence of rudist debris. “Shallow” and “deep” variations are recognizable. Lenticulina spp. and rotalids are common, with subordinate thin-walled miliolids, Textularia spp. and Debarina hahounerensis. Debris of echinoids, especially multi-ribbed and high-spoked spines, and thin-walled bivalves is common together with rare sponge spicules, pteropods and Coptocampylodon lineolatus.

It is possible that some of these species (especially in the “shallow” open marine-upper slope) may have been transported from shallower environments during debris flows, such as those described from the late Aptian carbonates of the southern Alps (Sartorio, 1992).

“UPPER SHU’AIBA” (UPPER RUDIST COMPLEX)

Lagoon

The “upper Shu’aiba” often displays a distinctive rudist (Agriopleura)-dominated lower layer, and an upper forminiferal-dominated layer. High benthonic species diversity and the consistent presence of the elevator rudist Agriopleura spp. and the dasyclad alga Salpingoporella dinarica are characteristic of this environment (Figure 8). Agriopleura displays two distinctive morphotypes: (1) the common “V-type” (widely conical or squat Agriopleura cf. marticensis; 1.1; 1.5; 6.1); and (2) the “U-type” (thin elongate Agriopleura cf. blumenbachi; 1.2). The elongate Agriopleura cf. blumenbachi was probably adapted to higher rates of sedimentation, with the tendency to increasing fixed valve length being a survival technique to maintain the commissure above the sediment-water interface. Agriopleura cf. marticensis may have responded to low rates of sedimentation that allowed the fixed valve to develop laterally and thus increase its stability in possibly muddy thixotropic sediments.

The alternation of both morphotypes within each depositional cycle suggests an association on the one hand between long narrow forms and the deeper conditions of a marine transgression, and on the other between low conical forms and lower rates of rising sea level typical of highstand conditions. Initial investigations of morphometric analysis of Agriopleura specimens in Shaybah cores by the author support the presence of two Agriopleura species, and not intraspecific polymorphism as described by Floquet (1998) for Praeradiolites ciryi.

In a few wells, a planktonic foraminiferal-bearing layer (interpreted as being associated with a maximum flooding event), separates the transgressive-associated A. cf. blumenbachi assemblage from the overlying highstand-associated A. cf. marticensis assemblage. Chondrodontid oysters commonly accompany Agriopleura cf. marticensis. Miliolids (Quinqueloculina spp.), Debarina hahounerensis, Praechrysalidina infracretacea and Textularia spp. are consistently present, with subordinate Everticyclammina hedbergi, Orbitolina spp. and Vercorsella arenata. Ovalveolina reicheli and Nautiloculina sp. are rare but confined to this environment. Dark-green to gray beds of mudstone within the lagoonal sediments in the northern part of the Shaybah field may represent soils. Many cores from the “upper Shu’aiba” show evidence of karst features (with pyritic-mud infill in the northern part of the field), and provide support for an episode of emergence and dissolution prior to the deposition of the overlying Nahr Umr Formation.

Rudist bank

The lagoon is fringed by a poorly developed rudist-bank complex in which the recumbent rudist Offneria murgensis and the elevator rudist Glossomyophorus costatus are present.

Fore-bank

Corals and the localized presence of the elevator rudist Horiopleura cf. distefanoi define the fore-bank environment on the northeastern flank of the Shaybah platform.

BIOCOMPONENT VARIATIONS

The vertical distribution of the macro- and micro-biocomponents in the various wells has enabled “generic predominance facies” to be recognized that characterize the major and subordinate foraminiferal and associated microfaunal/floral biocomponents of the entire Shu’aiba Formation. As a single, detailed assemblage-based biozone scheme for the whole study area is difficult to establish owing to the diachronous nature of the assemblages, a multiple biozonation scheme is proposed (Figure 9; see also Figure 8). The scheme is based on the most typical species associations representative of the lagoon, rudist-bank and basin-flank regimes, based on the information provided in the preceding section. The biozones assigned to the “lower Shu’aiba” and “upper Shu’aiba” should be recognizable across the Formation of the whole region and, if so, could be used to support isochronous correlation. Biozones of the “middle Shu’aiba” should also be recognizable throughout the region but their utility, because of their diachronous nature, lies in paleoenvironmental tracking rather than isochronous correlation.

Various biocomponent associations have been recognized that indicate the paleobathymetry of species for which there was no previously published information. Figure 10 summarizes the paleoenvironmental and paleobathymetric interpretation for benthonic and planktonic foraminifera, pteropods (opisthobranch gastropods), rudists and calcareous algae of the Shu’aiba Formation. Aspects of rudist paleoecology and morphological variations are summarized in Figure 11. Repeated complete or incomplete vertical stacking cycles of Agriopleura, Glossomyophorus and Offneria, respectively, are considered to have been influenced by small-scale sea level changes typical of the Cretaceous “greenhouse” conditions (Skelton et al., 1997). This biotic feedback in the rudist facies provides important guidelines for depositional layer recognition, and may represent a response to complex paleobiological and paleophysical conditions, as described by Gili and Skelton (1999) and Gotz (1999).

The lagoon, rudist-bank and fore-bank/open marine regimes illustrate the stacked assemblages of the “middle Shu’aiba”, as related to high-frequency shoaling-upward cycles within a general shoaling upward trend (Figure 12). Integration of these smaller cycles with logs, especially large-scale gamma-ray logs, has shown a simultaneous upward decrease in gamma value in relation to the upward-shoaling trend of the cycle. This observation was instrumental in assisting recognition and correlation of high-frequency events determined by sedimentological studies of cores and wireline-log interpretations (Aktas et al., 1999). A gradual upward replacement of certain species is noted from cycle to cycle, as each high-frequency cycle was deposited in increasingly shallow conditions. A similar pattern was observed within the foraminiferal variations of the Upper Jurassic Arab-D carbonates (Hughes, 1996).

A series of seven paleoenvironmental maps (Figure 13) is based on the current interpretation of depositional layers. The seven time-slices assist in the determination of the lateral and vertical reservoir continuity, as described by Payne et al. (1999). Each layer has been assigned informally to zones 7 to 1 (in ascending order) and applied in developing a reservoir zonation scheme. Zone 7 (base) equates with the laterally extensive open-marine paleoenvironment assigned to the planktonic foraminiferal-Orbitolina-Lithocodium biofacies of the Biyadh, or “lower Shu’aiba”. Zones 6 to 2 display the initiation, development and expansion of the differentiated rudist-rimmed platform. Zone 1 (top) shows a marked restriction of the width of the rudist-bank complex and equates with the lagoon-dominated, Agriopleura-associated “upper Shu’aiba”.

SEQUENCE STRATIGRAPHIC ASPECTS

Depositional cycles of the Shu’aiba Formation from Abu Dhabi have been described by Boichard et al. (1995) and from Oman by Vahrenkamp and Grotsch (1995). A sequence-stratigraphic interpretation of the Abu Dhabi succession concludes that the “Kharaib upper dense” unit (equivalent to the Biyadh or basal “lower Shu’aiba” in the Shaybah field), represents a transgressive systems tract. The maximum flooding event of this sequence coincides with the Biyadh-Shu’aiba boundary in Abu Dhabi, with the Shu’aiba (in the strict sense) containing the subsequent highstand, lowstand and transgressive systems tracts.

Little attention has been paid to the application of micropaleontology to the sequence stratigraphic analysis of shallow-marine carbonates, although Simmons and Williams (1992) and Emery and Myers (1996) described aspects of this technique. The Shu’aiba Formation in Oman has been examined in terms of sequence stratigraphy by M.D. Simmons, M.B. Preobrazhensky and I.J. Bugrova (written commun., 1994) and the base of the Shu’aiba may coincide with the eustatic event of Haq et al. (1987) at 112 Ma. This basal transgressive systems tract is associated with orbitolinids and planktonic foraminifera, and is followed by an early highstand systems tract and the development of the algal blanket facies. A late highstand systems tract is related to the establishment and aggradation of rudist buildups, and with orbitolinid and miliolid wackstones and packstones in the lagoon.

Gradual, autocyclic biovariations are relatively easy to identify in the shallow-marine carbonates of Saudi Arabia (Hughes, 1998b; in press 2000a). Examples occur in the Khuff Formation (Hughes and Kamal, 2000), Arab-D member of the Arab Formation (Hughes, 1996) and the Shu’aiba Formation. They are especially evident where the sample interval is small enough to display the detailed vertical distribution of bathymetrically sensitive species. In this way, prograding, retrograding and aggrading biofacies can be recognized in the style of Everts and Reijmer (1995). Abrupt vertical changes, however, are conventionally attributed to rapid alteration of the depositional environment, such as at a depositional cycle boundary, where explanations due to storm-derived allochthonous transport can be excluded.

Paleontological variations cannot be independently used to establish a sequence stratigraphic framework, and ideally need to be integrated with sedimentology, wireline logs and seismic data. This approach, but without seismic assistance, has been attempted for the Shu’aiba Formation in Shaybah by Aktas et al. (1999; 2000). The entire succession is characterized by a series of shallowing-upward cycles, some of which occur at the meter scale. Variations in the micropaleontology and macropaleontology of the Formation have led to the recognition of discrete biofacies. As each of these biofacies represents the reaction of the microorganisms to syndepositional environmental conditions, they may be used as indicators of repeated marine transgressions and regressions. In this way, each depositional cycle of the “middle Shu’aiba” would tend to become increasingly influenced by wave energy and lead to coarsening-upward cycles that could, theoretically, develop good reservoir properties in the upper part. As the Shu’aiba was deposited over a relatively short time interval, the lack of biostratigraphic time constraints has led to difficulty in providing confident lateral correlation of the palaeoenvironmentally controlled biocycles. Aktas et al. (1999, 2000) have selected the coarsening-upward layers as potential reservoir layers, each of which is composed of a several smaller coarsening-upward cyclic sub-units.

Maximum regressions, as represented by cycle boundaries (Armentrout and Clement, 1991; Simmons and Williams, 1992), would tend to be displayed as events of minimum faunal abundance, reduced diversity, reworking and rapid changes in biofacies character. Such changes are indicated in Figure 12 for the lagoon, rudist-bank and open-marine regimes, respectively. Reworking at the base (transgressive lag) of each new cycle would be expected to cause a localized increase in species abundance and diversity.

Marine transgressions would tend to have moderately high species diversity and abundance, and would commonly contain planktonic foraminifera and deep marine species. The tendency for carbonate sediments deposited during a marine transgression to be relatively easy to correlate laterally would tentatively associate the “lower” and ‘‘upper” Shu’aiba (as described in this paper), with transgressive systems tracts.

Maximum flooding events tend to be recognized by peaks of microfaunal abundance, (although not necessarily of diversity), especially of planktonic foraminifera, deep-marine benthonics and oligosteginids. The concentration of orbitolinids with planktonic foraminifera in the basal parts of the Shu’aiba is best interpreted as being maximum flooding events. These concentrations are too thin to be of use for confidently separating the transgressive biofacies from that of maximum flooding, although an abundance of planktonic foraminifera and elevated gamma values could be used for the purpose. Where open-marine influences were minimal, as in the lagoons, such an event may be represented by localized concentration of rotalids and miliolids.

During highstand events, gradual shallowing is recorded by lower microfaunal diversity and, in the rudist complex, by the replacement of the Glossomyophorus costatus assemblage by the recumbent form Offneria murgensis. In contrast to what happens during marine transgressions, carbonates deposited at the time of highstands are typically shingled and laterally discontinuous and it is commonly difficult to trace such beds over long distances. Such is the case of the “middle Shu’aiba”.

The lagoon and rudist-bank environments display major facies shifts that can be considered as possible sequence boundaries. In the lagoon, the upward succession from planktonics to Orbitolina to Lithocodium to Glossomyophorus is interpreted as a gradual shoaling trend. It could represent migration of the Glossomyophorus back-bank facies into a shallowing lagoon as a possible retrogradational response to a marine transgression. The contact between this facies and the overlying biosuccession of Orbitolina–Lithocodium–Glossomyophorus indicates a major facies shift and may represent a sequence boundary. Within the rudist bank complex, alternations between back-bank Glossomyophorus and bank-crest Offneria facies could also be similarly interpreted in terms of individual shoaling-upward cycles, each of which was “reset” by the succeeding marine transgression.

The junction between the “middle” and lagoonal “upper” layers, dominated by Glossomyophorus costatus-Offneria murgensis and Agriopleura cf. blumenbachi-Agriopleura cf. marticensis, respectively, represents a major basinward-shift of facies, with similar sequence boundary implications. The deeper foraminiferal assemblages within the “upper Shu’aiba” indicate a response to a marine transgression at the base of this layer.

RESERVOIR IMPLICATIONS OF BIOFACIES

Paleoenvironmental controls on biofacies had a sympathetic effect on petrofabrics. Consequently, they have influenced reservoir-rock quality and the distribution of reservoir facies. The lagoonal and basinal muds have relatively high porosities (based on their microporosity), but low permeabilities, whereas high levels of porosity and permeability occur in the rudist-bank, grain-dominated facies, despite local occlusion by blocky spar cement. Transitional bank-lagoon and bank-basin biofacies have intermediate porosity-permeability values.

Calibration of biofacies with FMI logs and reservoir facies in the vertical, cored wells is being used to interpret the facies in uncored, horizontal development wells.

SUMMARY

The Shu’aiba Formation of the Shaybah field can be considered in terms of three stacked layers, each with a distinctive micro- and macro-biofacies. The Formation consists almost entirely of carbonate rocks that were deposited on the southern flanks of an intrashelf basin during the early Aptian, although there is evidence for late Aptian sedimentation off the eastern flank of the field. Paleontological evidence shows that the Formation evolved from a laterally extensive, moderately deep platform (“lower Shu’aiba”) into a rudist-rimmed platform with a well-developed lagoon (“middle Shu’aiba”) and finally into an extensive, deep lagoon with a narrow rudist-bank rim (“upper Shu’aiba”).

The “lower Shu’aiba” consists essentially of two main shoaling-upward units, each of which is characterized by two biofacies. The layer is polycyclic and consists of a vertical succession commencing with deep-water marine deposits containing the planktonic foraminifera Hedbergella delrioensis and the low trochoid morphotype Palorbitolina lenticularis conformably overlain by beds with calcareous algae (Lithocodium aggregatum) and platy coral. The orbitolinid-dominated basal biofacies equates with the unit known regionally as the Biyadh Formation (Saudi Arabia), “Kharaib upper dense” (UAE) and Hawar shale (Oman), and is extensively developed with only slight lateral variations. The basal biofacies also contains the benthonic foraminifera Debarina hahounerensis, Lenticulina spp., Nodosaria spp., Praechrysalidina infracretacea and Choffatella decipiens. No rudists are present in the “lower Shu’aiba”.

The “middle Shu’aiba” is characterized by considerable lateral biofacies variations in which rudists are well represented. It is possible that a fault-controlled change of submarine relief caused the sudden termination of the laterally extensive “lower Shu’aiba”. Colonization by rudists, possibly on local submarine highs, resulted in differentiation of the basal “middle Shu’aiba” into lagoon, rudist-bank and slope environments, each of which has a distinct biofacies. The interpreted lagoon environment typically contains a lower biofacies that includes encrusting Lithocodium aggregatum and platy corals together with the benthonic foraminifera Trocholina alpina, Palorbitolina lenticularis and various unornamented miliolids. In the upper part of this regime, increasingly adverse environmental conditions caused a decrease in biotic diversity and a concentration of miliolid foraminifera.

The rudist assemblages are spatially clustered to form a network of banks, and display an orderly species range. The deep lagoonal flank of the bank complex is characterized by the presence of small elevator morphotypes such as Agriopleura spp. and Glossomyophorus costatus. Two distinct Agriopleura morphotypes are present. One morphotype is a rapidly enlarging form assigned to Agriopleura cf. marticensis and the other is a cylindrical, elongate form assigned to Agriopleura cf. blumenbachi. Elevated energy conditions toward the bank crest enabled colonization by robust elevator rudists (aff. Retha sp.), together with rare specimens of the pencil-like caprinid elevator Offneria aff. nicolinae. The highest energy conditions of the bank crest allowed colonization by the recumbent caprinid rudist Offneria murgensis. The fore-bank regime is dominated by rudist debris. It passes laterally into deeper-water conditions that are dominated by low trochoid Palorbitolina lenticularis and the planktonic species Hedbergella delrioensis and rare H. planispira. Evidence exists for tempestite-triggered allochthonous beds of shallow-marine bioclasts that fine upward into autochthonous, planktonic foraminiferal-bearing mud.

The marked contrast in biofacies between the “middle” and “upper” Shu’aiba is related to a basinward-shift in the lagoonal biofacies, suggestive of a sequence boundary. The “upper Shu’aiba” is typically characterized by the widespread occurrence of Agriopleura cf. marticensis and A. cf. blumenbachi, although Glossomyophorus costatus and Offneria murgensis are also present, but with much reduced extent. The elevator rudist Horiopleura cf. distefanoi is particularly well developed along the eastern flank of the Shaybah field. The “upper Shu’aiba” is dominated by deep lagoonal conditions in which foraminiferal diversity is typically high. Species include Praechrysalidina infracretacea, Debarina hahounerensis, Vercorsella arenata, textularids and miliolids. Within the lagoon, the dasyclad alga Hensonella/Salpingoporella dinarica is well represented but on the outer flanks of the banks it is replaced by Coptocampylodon lineolatus. The “upper Shu’aiba” contains localized layers or pods of gray, non-calcareous, pyritic clay that may be related to karst-fill that took place during the phase of emergence corresponding to the top of the Shu’aiba Formation.

Further investigations of the lateral distribution of the various biocomponents may reveal preferential concentration with respect to leeward and windward sides of the platform.

ACKNOWLEDGMENTS

The author wishes to thank Saudi Aramco and the Saudi Arabian Ministry of Petroleum and Minerals for permission to publish this paper. Dr. P. W. Skelton of the Open University, UK provided the author with valuable instruction in the identification of rudists and their paleoenvironmental significance, for which he is acknowledged with much gratitude. The illustrations of fossils 1.1, 2.1 to 2.3, 3.2 and 3.4 are copied from photographs taken by Dr. Skelton (Skelton, 1997). The author thanks GeoArabia’s staff for designing and drafting the final figures and plates.

This paper was originally presented at the Fifth International Rudist Conference, Erlangen, Germany in September 1999 and later at GEO 2000, Bahrain in March 2000. Constructive comments to the oral presentation of the paper at Erlangen by Dr. P.W. Skelton, Dr. E. Gili of Departament de Geologia, Universitat Autonoma de Barcelona and Dr. J.-P. Masse of Université de Provence, Marseille are also gratefully acknowledged. The author acknowledges the valuable discussions with G. Aktas and D.R. Cantrell of Saudi Aramco regarding the depositional layering of the Shu’aiba Formation together with those of a general nature by Dr. I.A. Al-Jallal, Chief Geologist of Geological Research and Development, Saudi Aramco. A poster describing the depositional architecture of the Shaybah field, incorporating the biocomponent distribution, was presented by G. Aktas et al. at the Annual Convention of the American Association of Petroleum Geologists at San Antonio in 1999, and at GEO 2000 in Bahrain in March 2000.

ABOUT THE AUTHOR

Geraint Wyn ap Gwilym Hughes is a Consultant Geologist with the Research and Development Division of Saudi Aramco and has been a micropaleontologist in Saudi Aramco since 1991. He received his BSc, MSc and PhD from Prifysgol Cymru (University of Wales) Aberystwyth, U.K. His nearly 30 years of biostratigraphic experience include 10 years as a field geologist/micropaleontologist with the Solomon Islands Geological Survey and 10 as a Biostratigraphic Consultant and head of the North Africa-Middle East-India region with Robertson Research in Singapore and North Wales. His professional activities are focussed on the integration of micropaleontology with sedimentology to enhance sequence stratigraphic interpretations for characterization of Permian to Miocene Saudi Arabian carbonate reservoirs. Wyn is an Adjunct Professor (invertebrate paleontology) at the King Fahd University of Petroleum and Minerals, Dhahran and a reviewer for GeoArabia. He is a member of the British Micropalaeontological Society and the Dhahran Geoscience Society and a Fellow of the Cushman Foundation for Foraminiferal Research.