Late Oxfordian micropalaeontology, nannopalaeontology and palaeoenvironments of Saudi Arabia

A study was initiated by Saudi Aramco to provide a core-based map of the various depositional settings that existed during the deposition of the uppermost Hanifa Formation over Saudi Arabia. Its purpose was to define the regional distribution of potential reservoir facies and also areas of possible source-rock accumulation (Figure 1). Acquisition of such data provides valuable guidance to locate exploration targets, and for focusing seismic surveys in remote frontier areas. The Hanifa Formation extends from the outcrop belt, west of Riyadh, into the subsurface beneath the Arabian Gulf. The study has attempted to sample as many locations within the outcrop and subcrop, within the time constraint of the project (Figure 1a). To achieve this goal, well locations were carefully selected for core availability and geographic distribution, together with published and otherwise documented data from within and peripheral to the study area. Analytical work included sedimentological study of nearly 1,000 m of core from 41 wells and semi-quantitative micropalaeontology of 3,021 thin-sections of core plugs. Petrography of the thin-sections recorded the textural fabric following Dunham (1962). Selected samples from various wells were submitted for calcareous nannopalaeontological analysis to provide age control and to assist correlation.

Early stratigraphic studies on the Hanifa Formation in Saudi Arabia include Bramkamp and Steineke (inArkell, 1952), Powers et al. (1966) and Powers (1968), in which the age, thickness and contact relationships with the underlying Tuwaiq Mountain Limestone Formation and the overlying Jubaila Formation were described. The reference section is well-exposed at Jabal Abaqqayn (Figure 2). Vaslet et al. (1983) recognised two members, the Hawtah and Ulayyah, within the Hanifa at outcrop and used this lithostratigraphic nomenclature in all successive publications resulting from the Saudi Arabian geological mapping project (Figure 3). Sequence stratigraphic interpretations of the Jurassic succession include Le Nindre et al. (1990a, b), Sharland et al. (2001) and Hughes (2004a, b, c; 2006; 2007). Mattner and Al-Husseini (2002) interpreted the Hanifa Formation to consist of two third-order sequences, equivalent to the Hawtah and Ulayyah Members, respectively (Figure 3). Using an orbital-forcing approach, Al-Husseini et al. (2006) maintain this interpretation and have further subdivided the Hawtah and Ulayyah Members into five and seven fourth-order cycles, respectively.

Previous regional palaeoenvironmental interpretations for the Hanifa Formation include Murris (1980) (Figure 4) with Callovian maps proposed by Moshrif (1984), Al-Husseini (1997) and Ziegler (2001) (Figure 5) for which very limited palaeoenvironmental detail is presented other than an approximate shoreline and an undifferentiated shallow-carbonate platform. A palaeogeographic map for the Oxfordian in the northern Arabian Gulf was provided by McGuire et al. (1993, Figure 6, inMattner and Al-Husseini, 2002) in which the influence of regional eustatic changes was considered, following Hallam (2001). Droste (1990) described the Hanifa Formation in Qatar, where intra-platform, basinal laminated, dark, organic-rich lime-mud wackestones and local anhydrites are present. Aspects of the Hanifa stratigraphy, lithology and palaeoenvironment are described by Alsharhan and Kendall (1986), McGuire et al. (1993), Luthy and Grover (1995), de Matos and Hulstrand (1995), Alsharhan and Nairn (1997), Al-Naji (2002) and McGuire (2003).

The Hanifa Formation accumulated in and around a relatively shallow depression that represents a typical example of an intra-shelf basin, which formed within the interior of an extensive broad epeiric, shallow-water carbonate platform termed the “Arabian Hanifa Intra-shelf Basin” (Aigner et al., 1989). The bathymetry of this basin is considered to have been responsible for the deposition of prolific source-rocks in the Upper Jurassic of eastern Arabia. Intra-shelf basins develop as a result of a rapid eustatic sea-level rise in which carbonate margins build-up around an isostatically sagged deeper basin floor while the sedimentary fill continues at a slower rate (Read, 1985). The rate of subsidence of the Hanifa, based on the thickness at outcrop and latest absolute ages for the Jurassic (Gradstein et al., 2004) is approximately 42m/Ma based on a Middle to Upper Oxfordian age range. This is slightly higher than that calculated by Le Nindre et al. (2003). A basin-margin complex is described for the Hanifa Formation, as part of the Diyab Formation, in Abu Dhabi by Al-Suwaidi et al. (2000) and Ayoub and En Nadi (2000). Strohmenger et al. (2004) calibrated a sequence interpretation of this succession to the J60 and J70 sequence boundaries of Sharland et al. (2001). The lowstand and transgressive systems tracts were found to contain five correlative high-frequency sequences and to favour source-rock accumulation, and the highstand systems tract to contain six correlative high-frequency sequences in which reservoir-quality barrier shoal facies were developed. Subsidence of the Hanifa Formation has been examined by Le Nindre (2003) and Hughes (2006, 2007). A palaeoenvironmental interpretation of outcrop and well-based data has been provided by Hughes (2004a, b, c)

The Hanifa Formation forms one of seven formations that constitute the Shaqra Group of Saudi Arabia (Manivit, 1990). It overlies the Tuwaiq Mountain Limestone with apparent paraconformity in the outcrop and is disconformably overlain by the Jubaila Formation, as evidenced by a pebble conglomerate in exposures in Wadi Birk. It has been defined from exposures in Saudi Arabia (Powers et al., 1966; Powers, 1968), where the reference section (24°57′48″N, 46°11′29″E) (Figures 1 and 2) consists of a lower wackestone to packstone succession, 94.4-m-thick, and contains colonial corals in the lower part. The upper unit consists of an 18.9-m-thick succession of packstones and grainstones. The top of the Hanifa is described by Powers as “marked by a massive bed of oolite-pellet calcarenite”. Recent examination of this contact has revealed, in Wadi Birk, an oyster-encrusted, iron-stained surface overlain by a bed containing rounded carbonate pebbles of the basal Jubaila (Franz Meyer, oral communication, 2006). In the roadside exposures west of Riyadh, the top of the Hanifa Formation is characterised by thin beds of coated pelloidal grains with abundant nerineid gastropods (Figure 7).

Regional mapping by Vaslet et al. (1983, 1984, 1991) and Manivit et al. (1985a, b) has enabled subdivision of the Hanifa Formation into two members as shown in Figures 2 and 3. The Hawtah Member, also termed H1, was measured as being 57 m thick and abruptly overlies the Tuwaiq Mountain Limestone. It begins with a yellow clayey limestone containing a brachiopod fauna, and continues with more indurated beds of gray bioclastic limestone and nodular limestone with corals and ammonites. It is mud-dominated, and forms the characteristically recessive lower slopes of the escarpment. The top of the Hawtah Member is formed by an oyster-rich bed informally termed the “Nanogyra lumachelle” (Vaslet et al., 1983, 1984) (Figure 7).

The Ulayyah Member, also termed H2, is 71 m thick, is grainier than the Hawtah, and the contact is placed at the base of a series of brown, cross-bedded, pelletoid and bioclastic packstones/grainstones. Vaslet et al. (1983) describe the Ulayyah Member “with a carbonate-pebble conglomerate base filling channels in the Hawtah Member”, an observation that is inconsistent with the interpretation that the member boundary coincides with the formational maximum flooding zone. The Ulayyah Member forms resistant cliffs of the upper escarpment and contains a greater proportion of calcareous algae, stromatoporoids and corals and is considered to represent the Hanifa highstand. Le Nindre et al. (1990a) and Manivit et al. (1990) have described red-stained surfaces at the top of the Hanifa Formation, in addition to reworking at the base of the Jubaila Formation. The topmost beds of the Ulayyah in outcrop display abundant coated gastropods of Nerinea sp. (Figure 7b).

The maximum duration of the Hanifa Formation is 5.5 million years (My), based on the recently revised geological time scale of Gradstein et al. (2004). This estimated duration of the Hanifa Formation compares reasonably well with the 4.86 My duration estimated by Al-Husseini et al. (2006) based on the Arabian Orbital Stratigraphy (AROS) framework of Al-Husseini and Matthews (2005). In this framework the Hawtah and Ulayyah Members were each correlated to a third-order sequence, using an orbital-forcing, glacio-eustatic model (Matthews and Frohlich, 2002).

The Oxfordian age of the Hanifa Formation has been determined from a variety of macrofossils, microfossils and nannofossils recovered from exposures. Brachiopods from the Hawtah Member include Somalirhynchia africana, Somalithyris bihendulensis and Terebratula bicanaliculata (Boullier, in Manivit et al., 1985, 1990), and indicate an Early to Middle Oxfordian age. Fischer et al. (2001) documented the gastropod fauna of the Hanifa Formation, but these do not provide additional chronostratigraphic information. Almeras (1987) while providing a comprehensive review of the Early and Middle Jurassic brachiopod faunas of Saudi Arabia, did not include the Hanifa. The middle and upper parts of the Hawtah Member are Middle Oxfordian based on the presence of the ammonite Euaspidoceras cf. catena perarmatum (Vaslet et al., 1983; Enay et al., 1987) and the nautiloids Paracenoceras aff. arduennense, P. macrum and P. aff. hexagonum (Tintant, 1987). No Early Oxfordian ammonites have been found and Enay (oral communication, 2006) is convinced that the Early Oxfordian is not present, and dismissed the brachiopod evidence as being insignificant.

Micropalaeontological evidence for age determination is sparse. The absence of the benthonic foraminiferal species Trocholina elongata is used, if not environmentally excluded, to possibly suggest an Oxfordian age despite the presence of other Trocholina species, as this species is common in the underlying Callovian Tuwaiq Mountain Limestone (Manivit et al. 1985; Hughes, 2004c). The calcareous nannofossil coccolith evidence for age determination from the outcrop samples is not conclusive (Manivit, 1987), but recovery from cores from the WLBN-1 well in Wadi Laban, west of Riyadh, and the road cut on the Mecca-Riyadh highway, west of Riyadh indicates an Oxfordian age, based on the presence of Cyclagelosphaera deflandrei, Crepidolithus crassus and Watznaueria manivitae. Of these species, Cyclagelosphaera deflandrei and Watznaueria manivitae have been found to be of greatest value in the studied wells because they are not present above the Hanifa Formation. This study confirms the previous stratigraphic restriction of Watznaueria manivitae to the Hawtah Member (Manivit, 1987), and would support an Early Oxfordian age for this member (Bown, 1998), although the age assignment of this locally-defined species has been calibrated with evidence that still remains questionable, as discussed above.

The Ulayyah Member is Late Oxfordian in its basal part, based on the presence of Paracenoceras aff. sulcatum (Tintant, 1987). The coccolith Stephanolithion bigotii, of Oxfordian to Early Kimmeridgan age, is present within the maximum flooding zone, together with an influx of Ellipsagelosphaera britannica and Lotharingius crucicentralis. Lotharingius crucicentralis is not younger than intra-Late Oxfordian (Bown, 1998). The common and consistent presence of the benthic foraminifera Alveosepta jaccardi throughout the Ulayyah Member provides a Late Oxfordian age (Manivit et al. 1984; Andreieff, inManivit, 1990; Hughes 2004c), although rare specimens have been documented by the first author and also reported by Manivit et al. (1985) in the Hawtah. A Late Oxfordian age has been assigned to part of the Ulayyah Member based on brachiopods (Boullier, inManivit, 1990), while the upper part of the member yielded echinoid faunas including Pygurus smelthei and Polycyphus parvituberculatus (Clavel, in Manivit et al. 1985; 1990) that suggest a possible Early Kimmeridgian age (?Hypselocyclum zone).

Inspection of the micropalaeontological and macropalaeontological assemblages has revealed a preferred coincidence of certain species and other biocomponents within the studied part of the Hanifa Formation. In addition to the uppermost parts of various wells distributed across Saudi Arabia, complete sections through the Hanifa were analysed for micropalaeontology and nannopapaleontology. These include the roadcut on the Riyadh-Mecca highway (24°31.454′N, 46°24.805′E) (Figure 8, see Enclosure; Figures 9 and 10), shallow borehole WLBN-1 (24°29′01.1″N, 46°35′59.8799″E) (Figure 11, see Enclosure; Figure 12) and an extensively cored section from a field in eastern Saudi Arabia (Figure 13, see Enclosure). These groups of biocomponents are interpreted to represent a response to variations in environmental conditions, mostly related to hydraulic energy levels and possibly light penetration. Five biofacies have been determined for the upper part of the Ulayyah Member in this regional study. They are based on the coexistence of selected, environmentally-sensitive species and their relative abundance, and are related to the simple depositional model by Moore (2001). The recognition of such biofacies has been instrumental in constructing the palaeoenvironmental map for the uppermost Hanifa across the entire region.

The Hanifa Formation has yielded a variety of biocomponents in thin-section, including microfossils and numerous macrofossil fragments, the most representative being illustrated in Plates 1 to 7; age-significant calcareous nannofossils are illustrated in Plates 8 and 9. Variations in their relative abundance and composition provide significant information regarding the variations in the depositional setting of the Hanifa sediments. Semi-quantitative micropalaeontological analysis of thin-sections has revealed a number of discrete biofacies and these have been used to interpret regional variations in the depositional setting. A depositional model, modified from Tucker and Wright (1990) and Moore (2001) of a rimmed shelf (Figure 1b), has been used to interpret the data obtained in this study, because of the presence of encrusting stromatoporoids of basin-rimming affinity. The information gained from the sedimentological analysis has been integrated with the micropalaeontological data to determine the depositional environments for the studied sections. Grainstones are typically found in association with the stromatoporoid-bearing sediments, but are also found within the shallow-lagoon sediments. Mudstones and wackestones are typical of the intra-shelf basin depositional facies, and packstones are found associated with the Clypeina-bearing sediments typical of the deeper lagoon (Hughes, 2005). In addition, the relative abundance and diversity of coccolith nannofossils has been used as an indicator for the degree of open-marine influence. The significance of ascidian spicule calcareous nannofossils has yet to be fully understood in terms of palaeoenvironmental significance. The distribution of various biofacies within the uppermost 40 ft of the Ulayyah Member and interpreted palaeoenvironmental and hydrocarbon-significant regimes are displayed in Figure 1a.

Two sections have been sampled and analysed for micropalaeontology and nannopalaeontology, and provide representative locations at the extreme west and east of the study area. The complete 210-ft-thick Hanifa succession is exposed in the roadcut on the Riyadh-Mecca highway, west of Riyadh (24°31.454 N, E 46°24.805′E) and has been sampled (samples HL1–HL20 and HU1–HU22) and analysed for semi-quantitative micropalaeontology and quantitative nannopalaeontology (Figure 8, see Enclosure). It is here stressed that closely-spaced samples from each bed remain to be collected and that the existing samples were collected routinely at 5 ft spacing, regardless of the position of the samples in relation to the depositional cycle. This type of sampling was imposed by time constraints and is not to be followed as a standard procedure, because it is believed to obscure detailed biofacies trends related to transgressive and regressive tracts components of each depositional cycle through the exposure.

The contact between the Tuwaiq Mountain Limestone and the Hanifa Formation is clearly exposed and from which six samples (TM 1–6) were collected from the uppermost Tuwaiq Mountain Limestone. The Hawtah-Ulayyah contact is clearly visible, as it forms a topographic low, and separates beds that are generally muddier in the Hawtah (c. 95 ft thick) than in the Ulayyah Member (c. 110 ft thick). Micropalaeontological and nannopalaeontological differentiation between the Tuwaiq Mountain Limestone and the Hawtah Member is well defined but the Hawtah biofacies display only subtle differences when compared with the Ulayyah. The micropalaeontological and nannopalaeontological distribution indicates the following three features that suggest a moderately deep-lagoon environment for the Hawtah and a shallow-lagoon environment for the Ulayyah.

• (1) The upper part of the Tuwaiq Mountain Limestone consists of wackestones and packstones with biofacies consisting of abundant sponge spicules, Kurnubia palastiniensis, Nautiloculina oolithica, echinoid fragments, the dasyclad Salpingoporella spp., Quinqueloculina spp. This biofacies differs from the overlying Hawtah Member by the presence of fragments of a branched stromatoporoid that resembles Shuqria or Cladocoropsis mirabilis, the foraminifera Redmondoides lugeoni, Trocholina alpina and the absence of coccoliths, although ascidian spicules are well-represented. Trocholina elongata is, unusually, not present in the studied section.

• (2) The Hawtah Member consists predominantly of wackestones with Kurnubia palastiniensis, Nautiloculina oolithica, echinoid fragments, Quinqueloculina spp., Siphovalvulina spp., Bositra buchi, sporadic Salpingoporella spp., quartz grains and triaxon/tetraxon spicules with an abundance of coccoliths including species of Cyclagelosphaera, Ellipsagelosphaera and Watnaueria barnesae. The presence of Siphovalvulina spp. and peaks in abundance of Cyclagelosphaera argoensis, C. margarelli and W. barnesae suggests that a maximum flooding zone for the Hawtah sequence could be located between samples HL4 and HL7, with a possible maximum at HL7. The lower, transgressive part of the Hawtah is characterised by the presence of Nautiloculina oolithica, Bositra buchi. The upper, regressive part of the Hawtah sequence is characterised by the presence of Kurnubia palastiniensis, Quinqueloculina spp. and undifferentiated ostracods.

• (3) The Ulayyah Member consists of wackestones, packstones and grainstones, in which angular quartz grains, Quinqueloculina spp. and Kurnubia palastiniensis are more consistently present than in the Hawtah and the agglutinated Late Oxfordian species Alveosepta jacardi displays a scattered presence. It is noteworthy that the abundance of coccoliths decreases markedly in the Ulayyah Member, with Ellipsagelosphaera britannica and Cyclagelosphaera deflandrei becoming almost absent. Watznaueria barnesae is consistently present, but in reduced abundance. The maximum flooding zone is less distinct for the Ulayyah sequence and may be located very near the base between samples HU2 and HU3, based on the tentative evidence suggested by the concentration of probable windblown angular quartz grains, due to decreased carbonate productivity, and absence of Kurnubia palastiniensis.

The biostratigraphic evidence based on semi-quantitative analysis of core samples from Wadi Laban-1 well (WLBN-1) (24°29′01.0″N, 46°35′59.879″E) drilled west of Riyadh (Figure 11, see Enclosure) displays a micropalaeontological succession that can be calibrated with the gamma-ray log. The top of the core is located at ground level in Wadi Laban (Figure 12), at the Hanifa-Jubaila formational contact. This well continuously cored the entire Hanifa and upper part of the Tuwaiq Mountain Limestone. The contact between the two formations is placed at c. 300 ft based on a marked microfaunal biofacies shift, and the top occurrence of the benthonic foraminifera Trocholina elongata, known to characterise penetration of the Tuwaiq Mountain Limestone and to regionally mark the top of the Callovian. This event coincides with a marked uphole increase in the gamma-ray values, interpreted to represent the mud-rich sediments associated with the transgressive and maximum flooding zones of the lower part of the Hanifa sequence. The gamma-ray values gradually decrease until 145 ft, where a rapid increase is interpreted as the contact between the Hawtah and overlying Ulayyah members. The Ulayyah Member also displays a gradual decrease in gamma-ray values, related to an increase in sediment granularity. The micropalaeontological and nannopalaeontological distribution indicates the following two features.

• (1) The Hawtah Member consists of a microfaunal assemblage dominated by Kurnubia palastiniensis, sponge spicules, Lenticulina sublenticularis, with scattered specimens of Everticyclammina sp., dwarf Valvulina spp., scattered small echinoid spines with an excess of ten spokes, and scattered specimens of the pelagic bivalve Bositra buchi. Angular quartz grains are common throughout. It is noteworthy that Alveosepta jacardi is present throughout the Hawtah Member, as this species has been considered to display a common presence only within the Ulayyah, where the increase in abundance has been stated as indicative of the Upper Oxfordian.

• (2) The Ulayyah Member consists of a microfaunal assemblage that displays lower species diversity than the Hawtah, and may suggest palaeoenvironmental stress, possibly related to the generally shallower conditions. Forms that extend from the Hawtah into the Ulayyah include Nautiloculina oolithica, brachiopod fragments, echinoid fragments, Alveosepta jacardi, and rare Lenticulina spp. with sporadic angular quartz grains. Kurnubia palastiniensis and sponge spicules are conspicuously absent or rare. The presence of a localised abundance of Bositra buchi and ostracods with the only miliolid specimens between 142 ft and 114 ft, together with their coincidence with the elevated gamma-ray value may suggests that a maximum flooding interval for the Ulayyah sequence could be located at this position. It is also noteworthy that although situated approximately 20 km east of the outcrop section described above, the Hanifa at Wadi Laban-1 contains almost no specimens of Kurnubia palastiniensis, although this species is well-represented in the exposure.

The biostratigraphic evidence based on semi-quantitative analysis of cores samples from a well in Berri field, near the eastern coast of Saudi Arabia (Figure 13, see Enclosure) indicates the presence of two clearly differentiated biocomponent assemblages. The micropalaeontological and nannopalaeontological distribution indicates the following two features.

1. The Hawtah Member consists of the consistent presence of Quinqueloculina spp., echinoid and brachiopod debris. The presence of localised Lenticulina sublenticularis with large echinoid spines and Valvulina spp. approximately coincident with the elevated gamma-ray value between 8,725–8,735 ft, suggests that a maximum flooding interval for the Hawtah sequence could be located at this event. The lower, transgressive part of the Hawtah is characterised by the presence of dasyclad algae, ostracods and scattered Kurnubia palastiniensis and Nautiloculina oolithica. The upper, regressive part of the Hawtah sequence is characterised by the presence of consistent Quinqueloculina spp. and echinoid debris.

2. The Ulayyah Member consists of microfauna and microflora that include consistent Quinqueloculina spp., Pseudoclypeina distomensis, Clypeina sulcata, echinoid debris, Cladocoropsis mirabilis, encrusting stromatoporoid fragments and Thaumatoporella parvovesiculifera. The base of the Ulayyah is placed at the marked uphole appearance and abundance of dasyclad algae, considered to represent a basinal shift of biofacies, and consistent with a sequence boundary. The presence of localised abundant sponge spicules, coincident with the elevated gamma-ray values around 8,545–8,565 ft, suggests a possible maximum flooding zone. The lower, transgressive part of the Ulayyah is characterised by the presence of Kurnubia palastiniensis, Nautiloculina oolithica, Pfenderina salernitana and Alveosepta jacardi. The upper, regressive part of the Ulayyah sequence is characterised by the greater abundance of Cladocoropsis mirabilis and Thaumatoporella parvovesiculifera.

As Alveosepta jacardii (Schrodt) is located at the junction of both assemblages, it is here suggested that the lower unit is comparable with the Hawtah Member, and the upper one with the Ulayyah Member. The consideration that these members represent sequences is supported by the presence of deeper-marine forms within each member, and these may represent the maximum flooding zones, including the deeper-marine forms such as Lenticulina cf. sublenticularis within the middle part of the Hawtah Member, and a concentration of sponge spicules in the lower to middle part of the Ulayyah Member. This interpretation is further supported by the gamma-ray log, as it displays an increased value adjacent to the interpreted lower, or transgressive portions of the interpreted Hawtah and Ulayyah members, and a reducing intensity towards the upper part of each member equivalent to the regressive tract.

Sedimentological evidence supports this interpretation, as there is a vertical trend from wackstone and packstones to grainstones within both members. The distinctive polarisation of the dasyclad-bearing sediments to the upper member is highly suggestive of a basinward shift of biofacies characteristic of a sequence boundary, wherin the shallower dasyclad lagoon facies has rapidly prograded over the underlying deeper, dasyclad-free assemblage of the Hawtah Member. The replacement of Lenticulina cf. sublenticularis by sponge spicules in the respective maximum flooding intervals is also consistent with the upper sequence being shallower than the lower.

The Hawtah and Ulayyah members of the Hanifa Formation may each represent third-order sequences (Mattner and Al-Husseini, 2002, Al-Husseini et al., 2006) of which the upper part of the Ulayyah Member, here studied, represents late highstand conditions. Palaeoenvironmental subdivisions based on the various biofacies range in order of water-depth control related to increasing distance offshore, from foraminiferal lagoon, Clypeina/Pseudoclypeina lagoon, Cladocoropsis (branched stromatoporoid) back-bank/lagoon, stromatoporoid bank complex to foraminiferal-spicule intra-shelf basin. Within basinal areas, highstand conditions would be expected to be poorly represented, except for the possible increased incidence of storm-triggered, carbonate-debris flows originating from an adjacent flank. On the basin margin, however, the variety of biofacies would be expected to form a tiered succession in Waltherian style, progressing from basinal, through domed stromatoporoid, branched stromatoporoid, dasyclad algal and foraminiferal biofacies. Such a vertical succession is also recognised in the Jubaila and Arab-D associations. Rapid progradation of a biofacies, as seen in the study well, would indicate a sequence boundary for which the responsible fall in sea level would have caused the rapid basinward migration of the biofacies. In Figure 13 (see Enclosure), this is well-defined by the sudden uphole appearance of dasyclad algae that rapidly prograded over the underlying deeper Hawtah sequence.

As the purpose of this study was to define the areal distribution of the uppermost Hanifa biofacies and interpreted palaeoenvironments of potential Hanifa reservoir facies, a model based on biofacies associations and their tiered relationships (both within this study and numerous intra-reservoir micropalaeontological studies carried out by Saudi Aramco) has led to the association summarised in Figure 1b. Because of the absence of extensively cored sections across the area, the regional palaeoenvironmental interpretation has been based on the youngest Hanifa Formation, essentially equivalent to the uppermost 40 ft of the Ulayyah Member. Future work could utilise cuttings-sample micropalaeontology to extend the biofacies characterisation to lower levels within the Hanifa Formation. Environmentally-controlled biofacies boundaries have been tentatively drawn between wells that have different biofacies, with a degree of confidence that decreases rapidly within the areas of low well density. Neverthless, even the wells in the southern part of the study region provide point sources of data that can be interpreted within the depositional model established for the Hanifa. An orderly biofacies progression is evident, grading from proximal lagoon, to deeper dasyclad lagoon, to back-bank Cladocoropsis lagoon and the higher-energy domed stromatoporoid-ooid grainstone shoals. The stromatoporoid-bank complex is distally flanked by the distal aspect by deep-marine biofacies representative of an intra-shelf basin.

Biofacies A (Plates 1 and 2) is characterised by high foraminiferal species diversity and recovery, in which the main component species include Kurnubia palastiniensis (Henson), Redmondoides lugeoni (Redmond), Alveosepta jaccardi (Redmond), Levantinella egyptiaca Fourcade, Mouty and Teherani, previously mis-assigned to the Cretaceous species Mangashtia viennoti (Kaminski, 2004), Quinqueloculina spp., Palaeofenderina salernitana (Sartori and Crescenti) and Nautiloculina oolithica (Mohler) with scattered coarsely agglutinated forms that resemble Reophax horridus following the taxonomy of Kuznetsova et al. (1996). It should be noted that rare, robust specimens of Lenticulina spp. are found within this biofacies, and testify to the moderately wide palaeobathymetric tolerance of this species during the Late Jurassic. This genus is also present within the low-diversity, deep-marine biofacies described below, where its association with triaxon and tetraxon spicules, Bositra buchi (Roemer) and calcareous dinocysts confirm a deep-marine setting.

Foraminifera are well-represented in most samples, with high-recovery and high-diversity, except for those deposited in the basinal setting where diversity is very low. Agglutinated foraminifera predominate over miliolid and rotalid calcareous species. The foraminifera have been the most useful, in combination with the stromatoporoids and calcareous algae, for discriminating subtle differences in the interpreted palaeoenvironment. Foraminiferal aspects of the Hanifa Formation in well samples and from outcrop have been published by Hughes (2004a, b, c, 2005) and presented by Dhubeeb and Hughes (2005b). Brachiopod and echinoid fragments are common components of the Hanifa carbonates, of which the latter are present both as plate fragments and discrete spines. The smaller, high-spoked spines seem to be preferentially concentrated within the mudstones and wackestones. Ostracods are rare.

Monaxon sponge spicules are present within certain wackestones and packstones of the lagoonal biofacies, but are also well-represented in the mudstones and wackestones that are interpreted to represent the basinal sediments. In the basinal setting, however monaxon spicules are accompanied by triaxon and tetraxon forms probably derived from hexactinellid sponges, and are typically associated with sparse microfaunal assemblages. Brachiopod and echinoid debris are common throughout. Stromatoporoids and dasyclad algae are absent, but the above species are also found together with such forms in the deeper-lagoon and back-bank facies. At certain localities, such as in the northwest, quartz grains and fragments of wood are present that, together with charophyte oogonia, provide evidence for proximity to a source of terrestrially-derived sediment (Plate 3).

Biofacies B (Plates 3 and 4) consists of a combination of most of the biofacies described for Biofacies A, but accompanied by dasyclad algae. The well-preserved forms indicate that there has been little sediment transport and disturbance, as these fragile forms are easily disarticulated. Moderately deeper or protected conditions within the lagoon are interpreted from the lower energy conditions, possibly close to or below fairweather wave base. The calcitic form Pseudoclypeina distomensis Barattolo and Carras is well-represented, and very well-preserved when compared with the moulds of the primarily aragonitic Salpingoporella spp. Dasyclad algae are well-represented in many of the studied sections, and are mostly assignable to Pseudoclypeina distomensis Barattolo and Carras (Hughes, 2005).

These delicate forms are interpreted to occupy moderately deep, normal salinity parts of the lagoon, where low-energy conditions predominate (Banner and Simmons, 1994). These forms are typically preserved as disaggregated fronds, except in the northwestern part of the study area, where entire stems and branches are preserved. Microbialites are present within this biofacies and include Rivularia piae. The encrusting alga Thaumatoporella parvovesiculifera (Raineri) is also well-represented.

Biofacies C (Plate 5) is characterised by the presence of the branched stromatoporoid assigned to Cladocoropsis mirabilis Felix together with the encrusting algal form Thaumatoporella parvovesiculifera (Raineri), and rare branched corals. Branched stromatoporoids are considered to have required moderately low-energy conditions in order to avoid breakage, and are considered to have best-developed in the distal part of the lagoon, in the lee of a bank, where the higher wave energy would be dampened. Branched corals occupy a similar niche today, and their distribution has provided support to this palaeoenvironmental interpretation of these extinct stromatoporoids. The branching form probably represents a response to the need for accelerated vertical growth within areas where the sedimentation rate was relatively high and this was, therefore, a survival strategy. It is noted here that the branching tendency of many corals appears to be a strategy for rapid regrowth in an areas where fragments may be prone to breakage due to periodically elevated energy conditions (Leinfelder, oral communication 2005; Lirman, 2000; Wallace, 2004). Biocomponents of Biofacies A and B are also present within this facies.

Stromatoporoids present within the Hanifa Formation also include unspeciated encrusting, dome-shaped forms that resemble Burgundia ramosa Pfender, based on the criteria of Wood (1987). Both stromatoporoid morphotypes are present within the Saudi Arabian Middle and Upper Jurassic carbonates (Hughes, 2004c), and together with the limited published data on Tethyan stromatoporoid carbonates (Leinfelder, 2001; Leinfelder et al., 2005) suggest a response to variations in environmental energy levels. The existing model proposes that the domed stromatoporoids adapted to higher-energy conditions such as would be expected on the oceanward flanks of a bank margin. As with extant branched corals such as Acropora cervicornis, A. palmata and species of Porites and Goniolithum, the branched stromatoporoids would be expected to have occupied the relatively lower energy, not necessarily deep, relatively sheltered region within the lagoon, on the leeward side of the stromatoporoid bank. A “back-reef” environment is also suggested for branched stromatoporoids by Turnsek et al. (1981).

Biofacies D (Plate 5) is characterised by the presence of the massive, domed and encrusting stromatoporoid forms, with comparatively rare corals. These forms are considered to have best-developed in association with high-energy grainstone banks with ooids, where the elevated wave energy would be effective in inhibiting most biological activity. Domed encrusting corals occupy a similar niche today, and their distribution has provided support to the palaeoenvironmental interpretation of these extinct stromatoporoids.

Corals are, when compared to stromatoporoids, poorly represented in the Hanifa. The relative low abundance of corals has been attributed by Leinfelder et al. (2005) to significantly higher temperatures over Arabia, based on the palaeoclimate model of Sellwood et al. (2000). The inferred presence of considerably warmer waters within a climatic belt with annual mean surface water temperatures exceeding 28°C may explain the occurrence of pure stromatoporoid assemblages, as they are considered to have a greater tolerance to warmer waters; their tolerance to hypersaline conditions is not fully understood. As filter feeders with minimal photosynthetic requirements, the inferred ability of stromatoporoid sponges to occupy muddier waters beyond the tolerance range of corals may be another significant factor worthy of consideration. It is of interest to speculate on the effects of the mid-Oxfordian thermal minimum and the warming stage of 3–4°C during mid- to Late Oxfordian, as described by Lecuyer et al. (2003). Such data require reconciliation with the evidence for glacial conditions in the Late Callovian as suggested by Dromart et al. (2003), Cecca et al. (2005) and Tremolada et al. (2006).

Biofacies E (Plates 6 and 7) is characterised by the presence of smaller foraminifera species that are normally considered to occupy deeper-marine environments. These include species resembling Lenticulina sublenticularis (Schwager) and Astacolus vacillantes Espitalie and Sigale, Nodosaria spp., various polymorphinids and agglutinated forms such as Bigenerina spp. and the ubiquitous Kurnubia palastiniensis (Henson). Rare valves of the pelagic bivalve Bositra buchi (Roemer) are also present within this biofacies.

In addition, this fine-grained, mudstone and wackestone lithofacies is characterised by the presence of common to locally abundant sponge spicules that include monaxon, triaxon and tetraxon types (Shrock and Twenhofel, 1953; Laubenfels, 1955), of which the triaxon forms belong to the Class Hexactinellidae and are preferentially representative of deeper-marine spicule-secreting sponges. Monaxon and tetraxon forms are typical of Class Demospongia that occupy a wide palaeobathymetric range. This biofacies is confined to the central part of the study area. Barren mudstones are also included within this biofacies.

Each of the five biofacies (A to E) represented in Figure 1a have been identified from the various study wells, and their distribution is controlled by coloured dots depicting each biofacies. With this very scattered distribution of reference points for each biofacies, delimitation of the five palaeoenvironmental provinces associated with each biofacies must remain tentative. Where no evidence exists for a particular biofacies, the presence of its neighbouring biofacies, as suggested by Figure 1b, has been used to guide the position of the palaeoenvironmental province. This is especially the case in the southwestern part of the study area. The east flank of the basin is poorly constrained, and control relied upon the published work by Ayoub and En Nadi (2000) and de Matos and Holstrand (2005) and Strohmenger et al. (2004). For the region adjacent to Qatar, Droste (1990) described the Hanifa as consisting predominantly of organic-rich mudstones with minor evaporites that suggests that deep, restricted-marine conditions prevailed. For the north, the basin margin interpretation by McGuire et al. (1993) was used.

The resulting map (Figure 1a) displays an inverted anvil-like shape for the intra-shelf basin that was flanked by a stromatoporoid bank complex and extended to the open-marine setting to the northeast in the region of Qatar. The stromatoporoid bank and back-bank complex was rather narrow along the southern and southwestern basin margin, but tended to widen along the northwestern flank where its width seems to increase to the north. The width of the stromatoporoid complex is a function of the combination of Biofacies C and D, because the separation of the domed-stromatoporoid grainstone facies and the back-bank Cladocoropsis grainstone and packstone facies is expected to be transitional and difficult to locate without closer well spacing. The width of the combined branched and domed/encrusting-stromatoporoid belt is remarkably wide along the northwest flank of the basin complex, and contrasts markedly with the interpreted narrow belt that flanks the southwesterly corner of the interpreted elongate arm of the basin.

For the first time, a broad interpretation of the palaeoenvironmental variations of the upper part of the Oxfordian Hanifa Formation within Saudi Arabia has been generated using rock-based evidence. There is, of course, much room for future refinement of this depositional model by insertion of additional wells and possibly by using micropalaeontology of cuttings samples. The depositional model provides considerable information for the palaeogeographers as well as for the hydrocarbon explorationists.

The overall picture is that the study area consisted of an intra-shelf basinal complex that was flanked by shallow lagoons on the western and southern sides (Figure 1a). The margin of the intra-shelf basin complex is defined by the change from deep-marine wackestones and mudstones to stromatoporoid grainstones that accumulated on the high-energy shoal complex. This map can be cautiously used to delimit the location of stromatoporoid-bank grainstones that would be expected to have good porosity and permeability and may provide good hydrocarbon reservoirs. Their juxtaposition to an intra-shelf basin with known source-rock facies, and the presence of an overlying mud-rich succession of the basal Jubaila Formation, adds to the highly advantageous combination of the required elements necessary to high-grade these locations for hydrocarbon exploration. Storm-derived allochthonous shallow-marine grainstones within the upper Hanifa and the overlying Jubaila Formation would be expected to flank the basin sides of these banks, and may provide stratigraphically-trapped debris-flow reservoirs within the basinal muds.

The map could be improved by additional wells, and a similar exercise could be carried out for each Saudi Arabian lithostratigraphic unit. Significant insights to the regional biofacies, lithofacies and palaeoenvironmental distribution would be realised with potentially tremendous leads for reconnaisance seismic and exploration target delimitation.

Subsequent to this study, 3-D seismic surveys have been completed over parts of the study area to test the validity of the biofacies-derived map. The objective was to map and confirm the suggested existence of basin-margin stromatoporoid bank complexes of potential reservoir value. The preliminary data is encouraging as amplitude differences have been seen that can be related directly to the suggested palaeoenvironments (Figure 14). Such seismic delineation of the grainstone facies belt with reservoir potential can now guide the search for exploration targets. Additional potential targets are offered by the possibility of stratigraphically trapped, allochthonous, basin-margin grainstones along the deeper flanks of the Hanifa intra-shelf basins, as described by Mullins and Cook (1986). Locating the Hanifa basin margin also provides a guide to the possible location of basin-margin grainstones of the underlying Upper Fadhili and Dhruma reservoir facies of the Tuwaiq Mountain Limestone. The presence of mud-dominated intra-shelf basin facies will allow geochemists to delimit possible source-rock accumulation locations, as the Hanifa Formation and the underlying Tuwaiq Mountain Limestone are known to have considerable source-rock potential (Carrigan et al., 1995).

Despite the very limited database, use of detailed, core-based micropalaeontology and modern palaeoenvironmental interpretations of the biofacies, combined with published palaeoenvironental data of adjacent regions, has enabled definition of the basin margin grainstone belt trend and location of intra-shelf basins. Relatively low-cost, high-definition micropalaeontological analysis has proved its ability to support exploration and significantly extends the application of micropalaeontology for regional and local intra-reservoir studies, in addition to the conventional use for age control. Microfaunal variations are sufficiently varied that consideration should be given for their use to assist biosteering in reservoirs where under-balanced coiled tube development activities are planned.

The results presented here are based on a Saudi Aramco project, and were presented as a poster display at the GEO 2006 Conference, Bahrain, 2006, at the International Congress on the Jurassic System, Krakow, 6–18 September, 2006, and at the 7^th Saudi Society for Earth Sciences Conference in Riyadh, May 12–13, 2007. Valuable geological discussions with Ravi Singh, Saudi Aramco Area Exploration Department, are acknowledged. Figure 14 is based on data provided by P. Lawrence and A. Gregory, Saudi Aramco. Thanks are given to Saudi Aramco and the Saudi Arabian Ministry of Petroleum for their kind permission to publish this paper. Valuable editorial comments by A. Afifi and M. Miller, Saudi Aramco, are gratefully acknowledged, in addition to those made by GeoArabia’s reviewers. The final design and drafting by GeoArabia Graphic Designer Nestor Niño Buhay is appreciated.

Geraint Wyn ap Gwilym Hughes is Senior Geological Consultant in the Carbonate Systems and Micropalaeontology Unit in Saudi Aramco’s Geological Technical Services Division. He gained BSc, MSc, PhD and DSc degrees from Prifysgol Cymru (University of Wales) Aberystwyth and in 2000 he received the Saudi Aramco Exploration Professional Contribution award, in 2004 the best paper award, and in 2006 the GEO 2006 best poster award. His biostratigraphic experience, prior to joining Saudi Aramco in 1991, includes 10 years with the Solomon Islands Geological Survey, and 10 years as Unit Head of the North Africa-Middle East-India region for Robertson Research International. Wyn’s professional activities are focused on integrating micropaleontology with sedimentology to enhance the sequence stratigraphically-influenced understanding of Saudi Arabian hydrocarbon intra-reservoir bio- and lithofacies distribution. He maintains links with academic research as an Adjunct Professor of the King Fahd University of Petroleum and Minerals, Dhahran. He is an editor for the AAPG, GeoArabia reviewer, and a member of the British Micropaleontological Society, the Grzybowski Agglutinated Foraminiferal Society, the Dhahran Geoscience Society, the International Fossil Alga Association and the Cushman Foundation for Foraminiferal Research.

[email protected]

Osman Varol gained his BSc and MSc from the University of Istanbul, followed by a PhD in calcareous nannopalaeontology from the University College London. He was a nannopalaeontological biostratigrapher for international projects in the Singapore and North Wales offices of Robertson Research from 1981 until 1991. In 1991, he formed Varol Research, a biostratigraphic consultancy specialising in the use of calcareous nannofossils, ascidian spicules and siliceous fossils for worldwide application for exploration and development. Osman has published extensively in both the taxonomic and applied use of nannofossils, and has developed new regional nannofossil-based zonation schemes to improve sequence stratigraphic interpretations.

osman@varol. com

Mokhtar Al-Khalid graduated with a BSc in geology from King Abdul Aziz University, Jeddah, and joined Saudi Aramco in 1995. He is a carbonate sedimentologist with the Geological Technical Services Division and currently studying towards his MSc in petroleum geology at King Fahd University of Petroleum and Minerals.

[email protected]