ABSTRACT

Strontium isotope, palynological, and total organic carbon (TOC) analyses were made on core samples from the Izhara, Araej, and Hanifa formations of three wells in onshore Qatar. Eleven samples were analyzed for their 87Sr/86Sr ratios. The results gave a chronostratigraphic range of Early Jurassic (Hettangian) to Late Jurassic (Oxfordian), with an overall age range of 202.4 Ma to 157.8 Ma. Maximum flooding surfaces MFS J10 to MFS J50 occur in the Izhara, Araej, and Hanifa formations.

The organic matter in the carbonate sediments of the Izhara, Araej, and Hanifa formations was almost exclusively of marine algal origin dominated by marine microplankton. Organic matter obtained from the Izhara Formation was mature to over-mature kerogen type-III to IV, gas-prone to inert material (mean TOC 0.31%). In the Araej Formation, mature oil-prone and gas-prone kerogen type-II and type-III organic matter occurred in both the lower and upper Araej members (TOC 0.13-0.69%). The stylolitic limestone of the Uwainat member of the Araej Formation has poor potential as a source rock for petroleum (TOC 0.03-0.13%), but is considered to be the main reservoir rock of the Middle Jurassic in Qatar. Mature, highly oil-prone organic matter (type-I and type-II kerogen) was present in black limestone of the Hanifa Formation (mean TOC 0.60%; maximum 0.93%). The limestone is considered to be the most likely petroleum source rock for the underlying and overlying limestone reservoirs of the Uwainat member of the Araej Formation (Bathonian) and the Arab-D member of the Arab Formation (Kimmeridgian) units, respectively.

INTRODUCTION

Qatar is part of the Interior Platform of Arabia over which thick sedimentary sequences have accumulated since the Paleozoic; a thickness estimated to be about 10 km (Alsharhan and Nairn, 1997). The Jurassic rocks in the subsurface of Qatar have attracted the attention of oil companies as they contain most of the hydrocarbon reservoirs and source rocks. Dominguez (1965), Sugden and Standring (1975), Owen (1975), Beydoun (1988), and Alsharhan and Nairn (1994, 1997) summarized the history of hydrocarbon exploration and production in onshore and offshore Qatar.

The aims of our study were as follows: (1) chronostratigraphic determination of the Izhara, Araej and Hanifa formations; (2) palynofacies analyses of the Lower to Middle Jurassic successions; (3) determination of kerogen type and source-rock potential; (4) determination of the degree of organic thermal maturation; and (5) correlating the results with previously obtained data from Qatar and neighboring countries.

Previous Studies

Few Jurassic biostratigraphic studies have been made of Jurassic rocks from Qatar. Smout and Sugden (1961) described some species of the genus Pfenderina from the Uwainat member of the Araej Formation. They introduced a new species (Pfenderina trochoidea) and placed it in a new family Pfenderinidae. Sugden and Standring (1975) discussed the main stratigraphic and paleontologic characteristics of the Jurassic and Triassic Araej, Izhara, Hamlah, and Gulailah formations in Qatar. They also listed some elements of the faunal contents of these formations of which the genus Pfenderina and allied forms are important.

Al-Saad et al. (1992) studied the lithofacies and depositional setting of the Araej Formation in the Dukhan oil field. Based on lithologic and microfacies analysis, they recognized 31 microfacies of mainly shallow-water carbonates in the Araej and suggested that it was deposited in a subtidal to shoal setting. Hewaidy and Al-Saad (1993) identified the foraminiferal content of the Araej in the Dukhan field. They divided the Formation into five biostratigraphic zones and assigned it a Bathonian to early Callovian age. They also attempted to correlate their zones with those of the Dhruma Formation in Saudi Arabia.

Hewaidy and Al-Saad (2000) studied the foraminifera of the Lower to Middle Jurassic exposures near Riyadh in central Saudi Arabia, and in ten wells drilled in the Qatar Peninsula. Biostratigraphic schemes were proposed for each area. In central Saudi Arabia, the Middle Jurassic was divided into eight biostratigraphic zones, and into seven zones in Qatar. Important and related studies on the neighboring countries included those of Droste (1990), de Matos et al. (1994), de Matos and Hulstrand (1995), and Al-Suwaidi and Aziz (2002). Recently, Sharland et al. (2001) in their regional work on the sequence stratigraphy of the Arabian Plate defined 10 key maximum flooding surfaces (MFS J10 to J110). These were given ages that ranged from middle Toarcian to middle Tithonian (185–147 Ma), based on lithological, biostratigraphic, sedimentological, sequence stratigraphic, and wireline log criteria.

MATERIALS AND METHODS OF INVESTIGATION

The investigation was based on the analysis of core samples from exploration wells Q-5, Q-8 and Q-9 drilled in western, central and northern Qatar (Figure 1). A total of 83 core samples were taken from the Izhara, Araej and Hanifa formations. The particulars of each well and the number of core samples are shown in Table 1.

Selected samples from the base, middle, and top of each formation were analyzed for their strontium isotope contents (86Sr and 87Sr). The material used had to be unaltered calcite or aragonite from marine fossils that incorporated Sr during life. The 87Sr content was determined by its rate of decay from radioactive 87Rb, and the 87Sr/86Sr ratio compared with the global seawater Sr curve (database of Howarth and McArthur, 1997). The total organic carbon (TOC) content of 41 samples was also analyzed using the modified Walkley-Black titration method of Gaudette et al. (1974).

In addition, all 83 core samples were processed using standard palynological techniques. The samples were treated with dilute HCl, HF, and concentrated HCl acids to remove carbonates, silicates, and fluorides respectively. The residues were then split into two portions–one was used to prepare kerogen slides, and the other was filtered using wet sieving with 15 μm polyester sieves. Epoxy resin was used as the mounting medium for the slides. All slides were microscopically examined and at least two slides of each sample were examined in detail. The first 200 palynomorphs were counted for statistical analysis. Species of interest were photographed using an Olympus microscope. The palynological analysis consisted of qualitative and quantitative analysis of organic particles, assessment of palynofacies type, assessment of kerogen type, and determination of spore color and thermal alteration index (TAI).

STRATIGRAPHIC SETTING

The Izhara, Araej, and Hanifa formations in the three wells (Figures 2 and 3) represent the Jurassic sequence from base to top.

Izhara Formation

Sugden introduced the Formation name from a locality near Kharaib in central Qatar in an unpublished report. Sugden and Standring (1975) described the Formation, the type section of which is in the Kharaib-1 well in central Qatar. The overall thickness of the Izhara Formation in central Qatar ranges from 180 to 468 ft, whereas in the Dukhan field (Khatiyah sector) it is from 402 to 468 ft thick (Al-Saad, 1996).

Lithologically, the Formation consists of dark-gray to black argillaceous limestone in its upper and lower parts, and dolomitic limestone in the middle. Generally, the limestone is intercalated with thin lamina of dark-gray shale, marl, claystone, and anhydrite in its lower part. Petrographic analysis of thin sections showed that the Formation is dominated by wackestone/packstone that are intercalated with thin beds of silt in the basal and topmost units. Grainstones are well developed at the middle of the Formation. The grainstone facies are mainly oncoidal with some pyritic remains.

The Formation was assigned an age range of Bajocian to possibly early Bathonian (Middle Jurassic) (Sugden and Standring, 1975; Al-Husseini, 1997; Alsharhan and Nairn, 1997; Hewaidy and Al-Saad, 2000), and was regarded as being equivalent to the lower Dhruma Formation of Saudi Arabia. However, we consider the Lower Jurassic Marrat Formation in Saudi Arabia and Bahrain as being equivalent to the lower part of the Izhara Formation (Figures 2 and 4), as was concluded by de Matos et al. (1994) in their study of the subsurface biostratigraphy from Abu Dhabi to Qatar. Accordingly, the Izhara Formation is assigned an age range of Early to Middle Jurassic. A major hiatus occurred in the middle part of the Izhara Formation (Al-Husseini, 1997, p. 366, Figure 4). It unconformably overlies the Gulailah Formation and the Hamlah Formation in central and western Qatar respectively, and is conformably overlain by the Araej Formation (Figure 4).

Araej Formation

The Formation was introduced by Sugden (cited in Sugden and Standring, 1975) and takes its name from Jebel Araej in central Qatar. The type section (590 ft thick) was established in the Kharaib-1 well, between the drilled depths of 7,010 and 7,600 ft. In other wells, the Araej has a thickness of between 590 and 656 ft. It is composed of a succession of tight argillaceous limestone in its upper and lower parts, and a middle part that consists of mainly clean limestones. These facies are associated with nodules of anhydrite and numerous stylolitic streaks.

It is divided into three members (Sugden and Standring, 1975; Al-Saad, 1996; Alsharhan and Nairn, 1997) from top to bottom as follows:

Upper Araej member

This member is composed of dark-gray to black or dark-reddish pyritic shaly/argillaceous limestone. Texturally, it is characterized by wackestone interbedded with thin units of packstone/grainstone. In the upper part of the member, these facies gradually change into dense lime mudstone containing small amounts of terrigenous material, including detrital quartz and clay. The upper Araej member is the thinnest member in the Araej Formation, and ranges in thickness from 95 to 250 ft. The contact between the dark shaly limestone of the upper Araej member and the olive limestone of the underlying Uwainat member is sharp and clear.

Uwainat member

The Uwainat member took its name from a locality near the Dukhan oil field. This middle member of the Araej Formation is composed of gray or olive-colored, oil-stained, compact stylolitic fossiliferous limestone. It is characterized by its light color, as compared with the members above and below. Based on the dominant facies, the upper part of the member is composed of wackestone interbedded with thin beds of packstone containing foraminifera and bivalves and rare stylolites. The middle part of the member is characterized by fining-upward facies that are organic-rich and formed of bituminous grainstone/wackestone, whereas the lower part of the member is mainly shaley wackestone. The Uwainat member is from 162 to 195 ft thick.

Lower Araej member

This is the thickest member of the Araej Formation being from 310 to 385 ft thick. It is composed of dark-gray, black and dark-red pyritic and argillaceous limestone. Texturally, the lower Araej consists of silty wackestone/packstone interbedded in places with pelletal grainstone. It has a well-defined contact with the overlying olive-colored limestone of the Uwainat member.

Sugden and Standring (1975) assigned a Bathonian to Oxfordian age to the Araej Formation, and thus equated it with the middle and upper parts of the Dhruma Formation in the Tuwayq escarpment of central Saudi Arabia (Al-Husseini, 1997). It is overlain disconformably by the Hanifa/Diyab formations and is conformable on the underlying Izhara Formation (Sugden and Standring, 1975; Al-Saad, 1996; Alsharhan and Nairn, 1997).

Hanifa Formation

The Hanifa Formation in Qatar has a significant lateral variation in both thickness (50 to 300 ft) and lithology. It consists of a basal bituminous, thin-bedded black limestone that grades upward into basin-fill shelf carbonates, capped by anhydrite of the basal Jubaila Formation. The lower part of the Formation is mainly composed of tight limestone that grades into bioturbated wackestone and peloidal packstone in the middle part; the upper part consists of fossiliferous dolomitic packstone. Thin streaks of anhydrite are scattered throughout the Formation.

The Hanifa Formation is equivalent to the Diyab Formation of Sugden and Standring (1975). It was dated as late Oxfordian to possibly early Kimmeridgian (Le Nindre et al., 1990). In Saudi Arabia and Bahrain, the Hanifa Formation consists of shales in the lower and middle part, and limestone in the upper part. The Hanifa shales are the main Jurassic source rocks, and the limestones form the Hanifa reservoir (Ayers et al., 1982).

STRONTIUM ISOTOPE ANALYSIS AND AGE DATING

Strontium (Sr) isotope stratigraphy is a valuable tool to complement biostratigraphic dating or to use where biostratigraphy is absent or is producing broad biozones. However, it is easier to express radiogenic 87Sr (formed from the radioactive decay of 87Rb) by means of its ratio to the stable isotope 86Sr. The amount of radiogenic strontium is principally controlled by its input from continental runoff, exhalations at oceanic ridges, and meteoric waters. Variations between these major factors produces unique 87Sr/86Sr values for much of the Phanerozoic. Because strontium has a long residence time in oceanic waters, there is time for it to become thoroughly mixed so as to produce a uniform 87Sr/86Sr ratio for a given time slice. However, if water circulation within the basin is restricted, either by an offshore barrier or by the basin being totally enclosed, the strontium isotopes will not be mixed efficiently and the resulting ratios will reflect an internal basin stratigraphy and not the seawater strontium curve. It is then possible to construct a local strontium stratigraphy that can be used for intrabasin correlation.

The main sink for strontium is carbonate-bearing or phosphatic sediments. However, this does not restrict the technique to such sediments as mudstones and sandstones may also contain carbonate or phosphatic fossils. Preferably, the material used should be unaltered marine fossil calcite or aragonite into which radiogenic 87Sr was incorporated during the life of the organism. The 87Sr/86Sr ratio is compared to the global seawater strontium curve of Howarth and McArthur (1997), the original database for the strontium curve was by Smalley et al. (1994). There is a 95 percent chance that the true age and the mean relative age are within the time interval designated. The use of fossil material has been standard for all academic work involving strontium isotopes.

Strontium Age Results

A total of 11 core samples were selected for strontium isotope analysis from the Izhara, Araej, and Hanifa formations in wells Q-5 and Q-8. The samples, composed mainly of carbonates, were analyzed for their 87Sr/86Sr ratios and the results are given in Table 2. Strontium isotopes do not provide an absolute age, as do true radiometric techniques based on the decay of unstable radioactive isotopes. Rather, the value of the ratio of the two strontium isotopes can be interpreted in terms of a stratigraphic age by comparing the ratio with the published global seawater strontium curve; a numerical age is derived from a corresponding timescale. The strontium isotope values were plotted as Figure 5 and the preferred age profile plotted against depth in Figure 6. The ages are given in terms of the Gradstein et al. (1994, 1995) timescale for the Mesozoic.

Table 2 gives the 87Sr/86Sr results in terms of Stages based on the timescales of Gradstein et al. (1994, 1995) and Haq et al. (1988). Significant differences are apparent between the two timescales; for example, the top of the Oxfordian is at 154.1 Ma on the Gradstein scale but at 152 Ma on the Haq scale. The age of the top of the Bathonian/base Callovian is 164.4 Ma according to Gradstein but 157 Ma on the Haq scale. Similarly, the top of the Bajocian is at 169.2 Ma on the Gradstein scale but 165 Ma on the Haq scale.

Age Conclusions

The well samples ranged in age from Late Jurassic (Oxfordian) at KB4 (6,773 ft) to Early Jurassic (Hettangian) at IZ42 (7,569 ft). Numerical ages were from 157.8 Ma to 202.4 Ma. The age breakdown is as follows:

Mid-Hanifa Formation (well Q-8)

One sample was dated as Oxfordian at 6,773 ft (157.8 Ma). Sharland et al. (2001) assigned a middle Oxfordian age to MFS J50 of the “condensed organic-rich limestones within the lower Hanifa Formation and Diyab Formation (Droste, 1990; Alsharhan and Nairn, 1997)”, and dated it at 156 Ma based on the timescale of Gradstein and Ogg (1996). Al-Suwaidi and Aziz (2002) dated the Hanifa Formation in well-J of offshore Abu Dhabi as Oxfordian (154.9 Ma, Depositional Sequence Jurassic 50-DS J50) based on strontium isotopes.

Araej Formation (well Q-8)

Upper Araej member: Bathonian/Callovian (164.70 Ma, sample depth 7,005 ft) and late Bathonian (165.0–165.2 Ma, sample depths 7,029 and 7,095 ft). However, Sharland et al. (2001) placed MFS J40 in the “shale at the base of the upper Araej Formation (Murris, 1980; Alsharhan and Nairn, 1997)”, and dated it as middle Callovian at 162 Ma based on the timescale of Gradstein and Ogg (1996).

Uwainat member: Bathonian (167.7 Ma, sample depth 7,200 ft).

Lower Araej member: Bajocian/lower Bathonian (170.1 Ma, sample depth 7,470 ft). Sharland et al. (2001) placed MFS J30 in the “argillaceous limestone within the lower Araej Formation (Alsharhan and Nairn, 1997)”, and dated it as early Bathonian at 168 Ma (Gradstein and Ogg, 1996).

Izhara Formation (wells Q-8, Q-5)

The top of the Formation was assigned a Middle Jurassic, Bajocian age (175.0 Ma in well Q-8 and 173.4 Ma in well Q-5), and its base was dated as Sinemurian (195.6 Ma) in well Q-8 and Hettangian (202.4 Ma) in well Q-5. However, according to Grabowski and Norton (1995) the base of the Izhara Formation is Pliensbachian. A disconformity, and the absence of the Pliensbachian, ?Toarcian, and ?Aalenian, was detected between the lower and upper parts of the Formation in well Q-8 between samples 101 (195 Ma) and 99 (175 Ma) (Figure 3). Al-Husseini (1997) interpreted major hiatuses in the middle of the Formation and at its base. Thus, the Izhara Formation has a complex history, and without detailed dating its base could be interpreted as being almost anywhere within the Lower Jurassic.

JURASSIC PALYNOSTRATIGRAPHY

Palynology is widely used in the petroleum industry but there are no publications on this subject concerning the Jurassic of Qatar, and few for the Arabian Peninsula in general. The present investigation represents the first palynological study of the subsurface Izhara, Araej, and Hanifa formations in wells Q-5, Q-8, and Q-9.

The stratigraphic distribution and occurrence of the palynomorphs led us to propose five dinoflagellate cyst zones and one barren zone within the Hettangian to Oxfordian interval. Detailed biozonation is beyond the scope of the present paper and the results will be published separately (Ibrahim et al., in press). Table 3 summarizes the dinoflagellate biozones.

Most of the 83 samples analyzed yielded moderately rich and well-preserved, but not always diversified, dinoflagellate cyst assemblages. This was particularly so for the Araej Formation, whereas poor preservation and a scarcity of palynomorphs was a characteristic feature of samples from the upper Izhara and Hanifa formations in wells Q-5 and Q-8, respectively. Spores and pollen were determined quantitatively to the specific level but were present only rarely in the studied intervals. Seventy dinoflagellate cyst species belonging to 32 genera, 4 acritarch taxa, and 18 miospore species related to 13 genera, were identified (see Appendix); some of the dinoflagellate species are shown in plates 1 and 2.

JURASSIC PALYNOFACIES, SOURCE ROCK POTENTIAL AND ORGANIC THERMAL MATURATION

Combaz (1964) was the first to apply the term “palynofacies” to describe the quantitative and qualitative palynological study of the total particulate organic matter assemblage. Palynofacies analysis involves the identification of individual palynomorphs, plant debris, and amorphous components, their absolute relative proportions, size spectra, and preservation states. Plate 3 illustrates palynomorphs from the Izhara, Araej, and Hanifa formations. Palynofacies analysis is an interdisciplinary technique that forms the natural interface between palynology, sedimentology, and organic geochemistry (Tyson, 1993, 1995). It represents the aspect of the organic facies that may be determined by transmitted light microscopy or, in other words, the petrographic recognition of kerogen parameters and types.

The Jurassic rocks in Qatar, as in the Gulf region in general, have high petroleum potential. The principal exploration objectives are formations of Middle and Late Jurassic age as these contain the near-ideal association of source rocks and high-porosity/permeability limestone reservoirs below excellent cap rocks. The rocks form broad open structures that hold very large accumulations of petroleum (Alsharhan and Nairn, 1997).

The methods used to assess the hydrocarbon potential of source rocks are of two main types: optical (palynofacies) and geochemical. Batten (1996) concluded that there was a reasonable correlation between optical and chemical data in terms of assessing the petroleum potential.

One of the most widely used methods of determining the amount of organic matter in rocks is to measure the TOC content expressed as a weight percent of the sediments. The TOC content is usually considered in the context of kerogen types and macerals, but some correlations with palynofacies have been reported. Most weight-percent values obtained link well with the abundance of amorphous organic matter (AOM) recorded from fine-grained sediments that accumulated in anoxic environments. It is generally accepted that for a rock to be a source of hydrocarbons it has to contain sufficient organic matter to allow significant generation and expulsion (Tissot and Welte, 1978). There is an agreement among organic geochemists that a rock can be considered as having source potential if it contains more than 0.5 percent TOC for shales, and at least 0.3 percent for carbonates (Tissot and Welte, 1978; Hunt, 1979; Tyson, 1995; Batten, 1996).

Relationships between palynofacies and source potential have been reported by palynologists and geochemists (see Staplin, 1969; Correia, 1971; Batten 1981, 1996; Thompson and Dembicki, 1986; Nøhr-Hansen, 1989; Firth, 1993; Wood et al., 1997; and Ibrahim et al., 1997). The major oil-prone classes of organic matter are the algal and amorphous material, prasinophyte, chlorococcales, and cyanobacteria. Conversely, structured woody material and other plant debris are commonly regarded as significant source of gas when abundant in sediments. Opaque fragments (charcoal) have no oil or gas potential.

The type and amount of organic matter in the source rock and its organic maturation level, essentially control the generation of petroleum. Methods used to interpret and document the past thermal history of particulate organic matter include palynomorph color, spore translucency, vitrinite reflectance, and degree of fluorescence (Table 4). In the present study, the thermal alteration index (TAI) was determined using the changes in color of smooth pteridophytic spores (e.g., Cyathidites, Dictyophyllidites, and Deltoidospora) and gymnosperm pollen (Classopollis and Araucariacites) through the studied intervals. TAI was determined by comparing the spore/pollen color with the color chart of Pearson (1990). In general, the TAI values increase with depth from 2+, 3-, 3 to 3+, which indicates a mature to over-mature state for the organic matter.

Izhara Formation

The palynofacies of the Hettengian-Bajocian limestone and claystone of the Izhara Formation in wells Q-5 and Q-8 were well represented by opaque fragments (Figure 7 and Plate 3a) that resulted from oxidation of vitrinitic particles (phytoclasts). Phytoclasts and AOM of terrestrial origin were present in subordinate amounts. TOC values ranged from 0.02 to 0.68 percent (mean of 14 samples = 0.31%) (Figure 8 and Table 4). The spore/pollen color ranged from medium to dark-brown (Plate 1a–1c). The determined TAI was 3 and 3+, and its equivalent vitrinite reflectance (Ro) ranged from 1.0 to 1.3 and indicated the onset of gas generation (Table 4).

Accordingly, the organic matter of the Izhara Formation was determined as mature to over-mature gas-prone to inert type-III to type-IV kerogen. However, based on the analysis of two core samples from well DK-5 in the Dukhan oil field, Khaled et al. (1999) concluded that the Izhara Formation reflected mature type-II kerogen, but had low genetic potential.

Araej Formation

The particulate organic matter in the lower and upper members of the Araej Formation showed no distinct abundance and consisted of AOM, opaques, and phytoclasts in similar proportions (Figure 7). The AOM in the Uwainat member was the lowest measured during the study and the TOC had a mean of only 0.13 percent (Figure 8a and Table 4). In contrast, the lower and upper Araej members had a mean value TOC content of 0.39 percent. The spore/pollen color was a light-medium brown that indicated a TAI of 3- to 3, and the estimated Ro ranged from 0.8 to 0.9 indicating peak oil generation.

In conclusion, organic matter in the lower and upper Araej members is mature type-II and type-III kerogen (oil and gas-prone material). The stylolitic limestone of the Uwainat member has poor potential as a source rock for oil and gas, but is considered to be the main reservoir rock in the Middle Jurassic of Qatar (Alsharhan and Nairn, 1994, 1997). In contrast, Khaled et al. (1999) emphasized that the Uwainat member of the middle Araej Formation contained relatively highly mature organic matter of type-I kerogen, but this assumption was based on the analysis of only one sample from well DK-24.

Hanifa Formation

The Oxfordian carbonate facies of the Hanifa Formation has a high AOM (Figure 7) and resin content (Plate 3d), and a high TOC content (Figure 8a; Table 4). The AOM is almost exclusively of marine algal origin, raised up to 80 percent in the upper part of the Formation by the abundance of pyrite. The TOC content has a maximum value of 0.93 percent and a mean of 0.60 percent (nine samples). In general, the spore/pollen color was orange to orange-brown, indicating a TAI of from 2+ to 3-, and the estimated Ro ranged from 0.7 to 0.8 (peak oil generation).

The organic matter in the black limestone of the Hanifa Formation was probably mature (highly oil prone) type-I and type-II kerogen. It is considered to be the source rock for the underlying and overlying limestone reservoirs of the Uwainat member (Bathonian) and the Arab-D member (Kimmeridgian), respectively. This conclusion was in accordance with the findings of Alsharhan and Nairn (1994, 1997). They concluded that the Hanifa Formation in Qatar was a source rock for the Late Jurassic formations and that it contains sapropelic, liptodetrinitic and algal organic matter with a TOC content of from 1 to 6 percent.

Ayers et al. (1982) identified the organic-rich source rock in the Jurassic of Saudi Arabia as being thermally mature and containing amorphous alginite mostly derived from blue-green algae. The organic-rich sediments accumulated in an intrashelf basin during Callovian to Oxfordian times to form the Tuwaiq Mountain Limestone and the Hanifa Formation.

CONCLUSIONS

Dating of the Jurassic sediments of Qatar was by means of strontium isotope rations (87Sr/86Sr) linked to the Howarth and McArthur (1997) global strontium seawater curve and related to the Gradstein et al. (1994, 1995) timescale. The results of the study show that the lower Izhara Formation (equivalent to the Marrat Formation of Saudi Arabia) has an age range of Hettangian to Sinemurian (202.4 to 195.6 Ma), whereas its upper part is Bajocian (175 Ma). The Araej Formation is dated as Middle Jurassic (late Bajocian/Bathonian to early Callovian; 164.7 to 170 Ma). The lower-middle Hanifa Formation is Upper Jurassic (Oxfordian; 157.8 Ma). The studied sequences correspond to the maximum flooding surfaces MFS J10 to MFS J50 of Sharland et al. (2001).

A major hiatus in the middle part of the Izhara Formation is shown by the absence of rocks of Pliensbachian, ?Toarcian and/or ?Aalenian age. Missing late Callovian-age sediments between the top of the Araej Formation and the base of the Hanifa Formation indicate another sedimentary break.

Palynological analysis of the Izhara, Araej, and Hanifa formations have led to the identification of 70 dinoflagellate species, 4 acritarch taxa and 18 miospore species. One barren zone (Hettangian-Toarcian), and five dinoflagellate assemblage biozones are proposed for the Hettangian-Oxfordian succession of the Izhara, Araej, and Hanifa formations. The biozones are zone I (early Bajocian), zone II (late Bajocian), zone III (Bathonian), zone IV (late Bathonian-early Callovian), and zone V (early Oxfordian).

The Izhara Formation may have been deposited in a shallow-marine inner oxic shelf environment. Organic matter of the Izhara Formation is mature to over-mature type-III to type-IV kerogen (gas-prone to inert material) with a mean TOC of 0.31 percent.

The carbonate sediments of the Araej Formation were deposited under shallow, normal marine conditions, most probably on the middle shelf in dysoxic conditions. The organic matter of both the lower and upper Araej members is mature type-II and type-III kerogen (oil and gas-prone material) with a TOC content of between 0.13 percent and 0.69 percent. The stylolitic limestone of the Uwainat member has poor source potential for petroleum (TOC between 0.03% and 0.13%) but is considered to be the main reservoir rock of the Middle Jurassic in Qatar.

The black limestone of the Hanifa Formation probably accumulated in shallow water of the middle shelf in anoxic bottom-water conditions. The organic matter is mature, highly oil-prone type-I and type-II kerogen (mean TOC 0.60%, maximum 0.93%). It is considered to be the main source rock for the underlying (Bathonian Uwainat) and overlying (Kimmeridgian Arab-D) limestone reservoirs.

ACKNOWLEDGMENTS

We thank Qatar Petroleum for the provision of core samples necessary for this study. The authors are indebted to Dr. Homaid Al-Madfa, Director of the Scientific and Applied Research Centre (SARC) of the University of Qatar, for his kind support and assistance. This research was supported in part by funds from SARC Project No. 10/2000 and Chevron Overseas Petroleum (Qatar) Ltd. for which the help of Robert J. Rossetter and Gary W. Jacobs is gratefully acknowledged. We acknowledge the use of Howarth and McArthur (1997) Sr-isotope database by A.C. Higgins (StrataData). We thank the three anonymous reviewers for their constructive comments and GeoArabia editors David Grainger and Moujahed Al-Husseini for helpful suggestions. The design and drafting of the final graphics was by Gulf PetroLink.

APPENDIX

Taxonomic Species

The palynomorphs found in the present study are listed below. The dinoflagellate-cyst taxonomy used follows that of Williams et al. (1998).

  • Dinoflagellate Cysts

  • Aldorfia sp.

  • Barbatacysta pilosa (Ehrenberg) Courtinat, 1989

  • Barbatacysta sp.

  • Batiacasphaera spp.

  • Chlamydophorella ectotabulata Smelror, 1989

  • Chlamydophorella sp.

  • Chytroeisphaeridia chytroeides (Sarjeant) Downie and Sarjeant, 1965, emend. Davey, 1979

  • Chytroeisphaeridia spp.

  • Cribroperidinium crispum (W. Wetzel) Fenton, 1981

  • Cribroperidinium spp.

  • Ctenidodinium continuum Gocht, 1970

  • Ctenidodinium ornatum (Eisenack) Deflandre, 1939

  • Ctenidodinium spp.

  • Diacanthum sp.

  • Dichadogonyaulax sellwoodii Sarjeant, 1975

  • Dichadogonyaulax spp.

  • Dissiliodinium cf. volkeimeri Quattrocchio and Sarjeant, 1992

  • Dissiliodinium cf. willei Bailey and Partington, 1991

  • Dissiliodinium sp.

  • Ellipsoidictyum cinctum Klement, 1960

  • Ellipsoidictyum sp.

  • Escharisphaeridia pocockii (Sarjeant) Erkmen and Sarjeant, 1980

  • Escharisphaeridia psilata Kumar, 1986

  • Escharisphaeridia rudis Davies, 1983 emend Prauss, 1989

  • Escharisphaeridia spp.

  • Gonyaulacysta jurassica adecta longicornis (Deflandre) Downie and Sarjeant, 1965, emend. Sarjeant, 1982

  • Gonyaulacysta jurassica (Deflandre) Norris and Sarjeant, 1965, emend. Sarjeant, 1982

  • Gonyaulacysta pectinigera (Gocht) Fensome, 1979

  • Gonyaulacysta spp.

  • Hystrichosphaerina sp.

  • Impletosphaeridium varispinosum (Sarjeant) Islam, 1993

  • Kallosphaeridium sp.

  • Korystocysta gochtii (Sarjeant) Woollam, 1983

  • Korystocysta kettonensis (Sarjeant) Woollam, 1983

  • Korystocysta spp

  • Korystocysts pachyderma (Deflandre) Woollam, 1983

  • Liesbergia liesbergensis Berger, 1986

  • Lithodinia callomonii (Sarjeant) Gocht, 1976

  • Lithodinia caytonensis (Sarjeant) Gocht, 1976

  • Lithodinia jurassica Eisenack, 1935, emend. Gocht, 1975

  • Lithodinia reticulata (Dodekova) Gocht, 1976

  • Lithodinia spp.

  • Mancodinium cf. semitabulatum Morgenroth, 1970, emend. Below, 1987

  • Mancodinium semitabulatum Morgenroth, 1970, emend. Below, 1987

  • Mancodinium sp.

  • Mendicodinium groenlandicum (Pocock and Sarjeant) Davey, 1979

  • Mendicodinium microscabratum Bucefalo Palliani, Riding and Torricelli, 1997

  • Mendicodinium sp.A

  • Mendicodinium spinosum subsp. spinosum Bucefalo Palliani, Riding and Torricelli, 1997

  • Mendicodinium spp.

  • Nannoceratopsis gracilis Alberti, 1961, emend. Evitt, 1962, emend. Van Helden, 1977

  • Pareodinia ceratophora Deflandre, 1947, emend. Gocht, 1970

  • Pilosidinium echinatum (Gitmez and Sarjeant) Courtinat, 1989

  • Prolixosphaeridium sp.

  • Rhynchodiniopsis cladophora (Deflandre) Below, 1981

  • Rhynchodiniopsis? cf. regalis (Gocht) Jan du Chêne et al., 1985

  • Rhynchodiniopsis? regalis (Gocht) Jan du Chêne et al., 1985

  • Scriniodinium sp.

  • Sentusidinium rioultii (Sarjeant) Sarjeant and Stover, 1978, emend. Courtinat, 1989

  • Sentusidinium sparsibarbatum Erkmen and Sarjeant, 1980

  • Sentusidinium spp.

  • Systematophora areolata Klement, 1960 Systematophora spp.

  • Tubotuberella dangeardii (Sarjeant) Stover and Evitt, 1978, emend. Sarjeant, 1982

  • Valensiella ovulum (Deflandre) Eisenack, 1963, emend. Courtinat, 1989

  • Valensiella sp. A.

  • Valensiella sp. B

  • Valensiella spp.

  • Wallodinium sp.

  • Wanaea acollarris Dodekova, 1975

  • Other Microplankton

  • Cymatiosphaera sp.

  • Leiosphaeridia sp.

  • Micrhystridium sp.

  • Microforaminiferal linings

  • Pterosphaeridia cf. volkeimerii Quattrocchio, 1980

  • Spores and Pollen

  • Apiculatasporites sp.

  • Araucariacites australis Cookson, 1947 ex Couper, 1953

  • Callialasporites dampieri (Balme) Sukh Dev, 1961

  • Callialasporites trilobatus (Balme) Sukh Dev, 1961

  • Callialasporites spp.

  • Cibotiumspora jurienensis (Balme) Filatoff, 1975

  • Circulina parva Brenner, 1963

  • Classopollis classoides Pflug, 1953

  • Cyathidites major Couper, 1953

  • Cyathidites minor Couper, 1953

  • Cycadopites spp.

  • Deltoidospora hallii Miner, 1935

  • Dictyophyllidites harrisii Couper, 1958

  • Dictyophyllidites spp.

  • Inaperturopollenites spp.

  • Matonisporites crassiangulatus (Balme) Dettmann, 1963

  • Staplinisporites caminus (Balme) Pocock, 1962

  • Staplinisporites perforatus Dettmann, 196

ABOUT THE AUTHORS

Mohamed I.A. Ibrahim rejoined the staff of the University of Alexandria, Egypt, in mid-2002 after secondment to the Geology Department of Qatar University as an Assistant Professor since October 1996. Mohamed has a BSc (Honors, 1981) in Special Geology and an MSc (1986) in Palynology and Micropaleontology from the University of Alexandria. He obtained his PhD in Micropaleontology and Paleoecology in 1993 from the University of Alexandria and the Technical University of Berlin through a combined channel program. He taught Micropaleontology and Paleoecology from 1993 to 1995 at the University of Alexandria and was a Palyno-stratigraphic Consultant for GEOEX-Egypt (1994–96). He is the author of about 25 research articles on the palynology, micropaleontology, and paleoecolgy of Egypt, Libya, and Qatar. He is National Coordinator and Member of the IGCP 831, ‘South Atlantic Mesozoic Correlation Program’, and a member of AASP, BMS, GSE, and ESQUA.

mohamedibrahim59@hotmail.com

Hamad Al-Saad is Chairman of the Geology Department at the University of Qatar. He has an MSc in Geology from the University of South Carolina, USA, and a PhD from Ain Shams University, Egypt, 1997. His publications have concentrated on the Middle Jurassic of Qatar. Hamad’s current research interests include the sequence stratigraphy of the Jurassic of eastern Arabia. He is a member of SPE and BMS.

hamadsaad@qu.edu.qa

Suzan Kholeif is a Lecturer in Micropaleontology at the National Institute of Oceanography and Fisheries in Alexandria, Egypt. She received a BSc in Geology from the University of Alexandria in 1983 and worked for Alexandria Petroleum Company until 1985 when she returned to university. She was awarded an MSc (1989) and PhD (1999) from Mansoura and Cairo universities, respectively, for work on palynology, palynofacies, and paleoecology. She joined the National Institute in 1999. Suzan is the author of research articles on the palynology and paleoecology of Egypt and the Gulf region. She is a member of IGCP 831, ‘South Atlantic Mesozoic Correlation Program’, and a member of the Geological Society of Egypt and ESQUA.

suzan_kholeif@hotmail.com