ABSTRACT

A palynological investigation of the Nahr Umr, Mauddud and Ahmadi formations of the middle Cretaceous Wasia Group in Well DK-B in the Dukhan oil field of Qatar, yielded 30 species of dinoflagellate cysts, 18 of pteridophytic spores, 14 of gymnosperm pollen, and 16 of angiosperm pollen. Based on the investigation, the age of the Nahr Umr Formation is middle to late Albian. The basal part of the Formation was deposited in a marine prodelta or shallow shelf environment, whereas sedimentation of the upper part took place in normal marine conditions of an inner to middle shelf at depths of 10 to 80 meters. The carbonates of the Mauddud Formation are of late Albian (Vraconian) to early Cenomanian age and were deposited in an inner-middle shelf environment (20–100 meters). The shales and limestones of the Ahmadi Formation are of early to middle Cenomanian age and accumulated in open-marine conditions within an outer-shelf environment (100–200 meters). Two regressive pulses or lowstand system tracts can be detected in the lower and middle parts of the Ahmadi Formation that are consistent with published short-term global eustatic curves. Rocks of the Wasia Group studied in Well DK-B are in general enriched in kerogen type II (oil-prone material) except for the lower part of the Nahr Umr Formation that can be attributed to kerogen type III (gas-prone material). The thermal alteration index range of 2, 2+ to 3− suggests that the sediments of the Ahmadi Formation are immature whereas those of the Mauddud and Nahr Umr formations are slightly mature. The Albian-Cenomanian palyno-assemblage of western Qatar is referable to the African-South American Phytogeoprovince as shown by the presence of Crybelosporites pannuceus, ephedroid pollen, elater-bearing pollen, Afropollis, Stellatopollis, Reyrea and Cretacaeiporites species. Arid to semi-arid (tropical to subtropical) climatic conditions prevailed in the African-South American province at this time. Dinoflagellate cysts suggest a Tethyan connection.

INTRODUCTION

The Qatar Peninsula is an area of Tertiary outcrops that extends into the shallow waters of the central Arabian Gulf. It is part of the regional north-northeast-trending Qatar-South Fars arch that is flanked by the western Gulf basin to the northwest and the eastern Gulf basin to the southeast (Schlumberger, 1981). The Anglo-Persian Company began exploration for oil in Qatar in 1935 having been granted a concession covering the whole Qatar Peninsula. A subsequent geological survey in 1937–38 confirmed surface evidence of an anticlinal structure (the Dukhan structure) along the western coast of the Peninsula. The material on which this palynological study is based was taken from well Dukhan-B (DK-B) at latitude 25°25′16.2″N, longitude 50°47′0.6″E (Figure 1). It was the first well to be drilled on the Dukhan structure, and oil was discovered at a depth of 5,595 ft in 1940.

Only a few paleontological and palynological studies have been made of the Cretaceous rocks of the Dukhan oil field. One study was the pioneering work of Henson (1947) in which upper Cretaceous foraminifera were described. Subsequently, he described 18 larger imperforate benthonic foraminiferal genera of which 12 new species were recorded from the middle and upper Cretaceous of Well DK-B (Henson, 1948). Recently, Hewaidy and Al-Hitmi (1993 a,b,c; 1994; 1999) and Al-Hitmi (1994) studied the smaller benthonic and planktonic foraminifera, biostratigraphy and paleoecology of the Cretaceous to Lower Eocene rocks of five wells in the Dukhan field, including DK-B.

With regard to the palynology of rocks from western Qatar, the only previous work was by El Beialy and Al-Hitmi (1994) who made a reconnaissance study of the foraminifera and palynology of the lower Cretaceous Thamama *and middle Cretaceous Wasia groups* in Well DK-C of the central Dukhan field. This present work provides precise age determinations by means of palynomorphs. In addition, it is the first attempt at using palynofacies to deduce the paleoenvironmental parameters and paleoclimatic conditions that prevailed during the deposition of the Nahr Umr, Mauddud and Ahmadi formations. Kerogen typing and the organic thermal maturity of the succession were also determined.

GENERAL STRATIGRAPHY

The nomenclature of the middle Cretaceous rocks of Qatar is based on that of Saudi Arabia, Kuwait and Iraq, as published by Sugden and Standring (1975) in the Stratigraphic Lexicon for the Qatar Peninsula. The Albian-Cenomanian succession in the Dukhan oil field belongs to the Wasia Group that unconformably overlies the lower Cretaceous Thamama Group and is unconformably overlain by the upper Cretaceous Aruma Group. The Wasia Group is composed (in ascending order) of the Nahr Umr, Mauddud, Ahmadi and Mishrif formations (Figure 2). The Mishrif Formation was not part of the present study because of the lack of samples. Figure 3 shows the age relations based on micropaleontological studies.

Nahr Umr Formation

Name: After Nahr Umr River in southern Iraq.

Author:Owen and Nasr (1958).

Type section: Nahr Umr-2 in southern Iraq.

Reference section in Qatar: Dukhan-26, between drilled depths 3,278 and 3,828 ft.

Contacts: The Nahr Umr Formation unconformably overlies the Aptian Orbitolina limestone of the Shu’aiba Formation and conformably underlies the Mauddud Formation.

Depth and thickness (Well DK-B): 2,589 to 3,038 ft; 449 ft thick.

Lithology (Well DK-B): Generally siliciclastic sediments subdivided into three units A, B and C (Al-Hitmi, 1994), consisting of sandstone, shale and glauconitic sandstone, respectively.

Age: Attributed to the early Albian Asanospira diyabi foraminiferal Total Range Zone (TRZ) (Hewaidy and Al-Hitmi, 1994), but palynological evidence is for a middle to late Albian age (Figure 3).

Mauddud Formation

Name: After Ain Mauddud, a locality near Gebel Dukhan, Qatar.

Author: Henson (1940, unpublished Qatar Petroleum Corporation report).

Type section: Dukhan-1 (= DK-B, studied well).

Contacts: The Mauddud Formation conformably overlies the clastics of the Nahr Umr Formation and is conformably overlain by the Ahmadi Formation.

Depth and thickness (Well DK-B): 2,408 to 2,589 ft; 181 ft thick.

Lithology (Well DK-B): White, hard fossiliferous limestone with calcite veins, sometimes termed the Orbitolina limestones, as found in many parts of the Middle East.

Age: Attributed to the late Albian Orbitolina qatarica foraminiferal TRZ (Hewaidy and Al-Hitmi, 1994). but palynological evidence is for a late Albian (Vraconian) to early Cenomanian age (Figure 3).

Ahmadi Formation

Name: After the town of Ahmadi in southeast Kuwait.

Author:Owen and Nasr (1958).

Type section: Burgan-62 in Kuwait.

Reference section in Qatar: Dukhan-28 between drilled depths 2,348 and 3,035 feet.

Contacts: The Ahmadi Formation is conformable on the Mauddud Formation and is conformably overlain by the Mishrif Formation.

Depth and thickness (Well DK-B): 1,820 to 2,408 ft; 588 ft thick.

Lithology (Well DK-B): The Ahmadi Formation is composed mainly of an interbedded shale-limestone succession. It was subdivided into units A-D by Al-Hitmi (1994).

Age: Unit A is attributed to the early Cenomanian Hensonia lenticularis foraminiferal TRZ, and unit B, C and D to the Cenomanian Asterohedbegella asterospinosa foraminiferal TRZ (Hewaidy and Al-Hitmi,1994). This is in broad agreement with an early to middle Cenomanian palynological age (figure 3).

PALYNOLOGICAL STUDIES

Materials and Method

A total of 26 core samples from the Albian-Cenomanian Nahr Umr, Mauddud and Ahmadi formations of the Wasia Group were palynologically analyzed. The sample preparation technique was based on procedures described by Ibrahim (1996) and Ibrahim et al. (1997) and involves HCl-HF-HCl treatment. One portion of the residue was wet sieved with 15 μm polyester sieves and mounted in glycerin jelly. Three slides of each sieved residue were microscopically examined for sporomorphs and dinoflagellates, and the first 200 palynomorphs were counted. In addition, one slide of each unsieved residue was examined for palynofacies and for the definition of particulate organic matter. All slides, processed residues and unprocessed portions of the samples are stored in the collections of the first author in the Department of Geology, Faculty of Science, University of Qatar.

Palynostratigraphy

Because reliable, detailed, and universally valid palynomorph zones from the mid-Cretaceous Period are not yet formalized, age determination are based on comparison with dinoflagellate cyst assemblages and index miospores from dated stratigraphic sections. The TAXON palynological database of Ravn (1996) was consulted for comparison purposes but the studied intervals also have independent age control based on foraminifera (Al-Hitmi, 1994; Hewaidy and Al-Hitmi, 1994). The palynological analysis of the Nahr Umr, Mauddud, and Ahmadi formations led to the recognition of at least 80 species of spores, pollen grains, dinoflagellate cysts and other phytoplanktons. Figure 3 indicates the age relationships and correlation of the rockunit of the Wasia Group, and Figure 4 shows the quantitative distribution of major palynomorph elements in Well DK-B.

The distribution and stratigraphic occurrences of palynomorphs in Well DK-B are depicted in Tables 1 and 2. Plates 1 to 4 illustrate the most important palynomorph species and Plate 5 includes some of the palynofacies and particulate organic matter obtained from the studied material. (Note that reference to Plates is in the form, 1.13; i.e. Plate 1, illustration 13).

The palynological characteristics of the three formations are as follows:

Nahr Umr Formation

(Core samples 1–6; 2,589–3,038 ft)

Miospores recorded in the Nahr Umr Formation are Camarozonosporites insignis (1.6), Crybelosporites pannuceus (1.8), Uesuguipollenites cf. callosus (1.11), Ephedripites jansonii (1.14), Rhoipites sp. sensu Schrank, 1987 (2.14), Stellatopollis cf. barghoornii (1.19/1.20/2.4/2.8), Foveotricolpites concinnus (2.11), Brenneripollis peroreticulatus, and Afropollis jardinus (2.5). This is the first record of the genus Uesuguipollenites from the Arabian Gulf area. It had previously been found in the late Aptian to early Albian of northeastern Brazil (Dino, 1994) and in the Aptian Cocobeach Group of northern Gabon (Wood et al., 1997).

Samples 1, 2 and 4 are barren of dinoflagellate cysts, whereas samples 3, 5 and 6 contain many examples. The stratigraphically significant species are Odontochitina ancala (3.3), Senegalinium aenigmaticum (3.7), Cyclonephelium vannophorum, Xenascus ceratoides (3.4), Florentinia cooksoniae (4.7), F. laciniata (4.6/4.2), Dinopterygium cladoides, D. tuberculatum (3.13/3.14) and Microdinium setosum.

Rhoipites sp. sensu Schrank, 1987 (2.14) is a tricolporate species recorded from the late Albian to early Cenomanian from the Ammonite well of central Egypt (Schrank, 1987). Foveotricolpites concinnus (2.11) is a middle to late Albian marker species recorded from many areas in North America, including northwestern Alberta (Singh, 1971), southern Oklahoma (Wingate, 1980), and Kansas (Ward, 1986).

Odontochitina ancala (3.3) is reported from the middle to late Albian of Kansas (Bint, 1986) and western Qatar (El Beialy and Al-Hitmi, 1994) and may extend upward into the early Cenomanian in northwestern Egypt (Schrank and Ibrahim, 1995) and northeastern Brazil (Lana, 1997). Senegalinium aenigmaticum (3.7) is another late Albian marker species as recorded from Gabon and northwestern Egypt (Boltenhagen, 1977; Schrank and Ibrahim, 1995). Dinopterygium cladoides and D. tuberculatum (3.13/3.14) occur mostly in the middle to late Albian. However, they may extend upward into the Cenomanian as recorded from Australia (Eisenack and Cookson, 1960), France (Davey and Verdier, 1973; Courtinant et al., 1991), worldwide (Williams and Bujak, 1985), Egypt (Omran et al., 1990), and western Qatar (El Beialy and Al-Hitmi, 1994). Cyclonephelium vannophorum has been recorded from Albian and/or Cenomanian successions in France (Davey and Verdier, 1973), northwestern Alberta (Singh, 1983), northeastern Libya (Batten and Uwins, 1985; Uwins and Batten, 1988), and northwestern Egypt (Schrank and Ibrahim, 1995).

The assemblage gives a middle to late Albian age for this part of the Nahr Umr Formation. Similarly, El Beialy and Al-Hitmi (1994) assigned a middle to late Albian palynological age for this formation in the Dukhan-C well. However, by using foraminifera, Hewaidy and Al-Hitmi (1994) assigned the studied interval to the Asanospira diyabi Zone and dated it as being early Albian (Figure 2).

In offshore Abu Dhabi, Athersuch (1987) dated marine shales of the Nahr Umr Formation as Albian based on nannofossils Prediscospharea cretacea, Parhabdolithus achlyosturion and the palynomorphs Subtilisphaera cheit, Xiphophoridium alatum, and Dinopterygium cladoides. He also noted that the characteristic ostracod species for this formation are Glenocythere reticulata, Veeniacythereis streblolophata, and Metacytheropteron sp.

Mauddud Formation

(Core samples 7 and 8; 2,408–2,589 ft)

Sample 8 contains palynomorphs but Sample 7 is of barren, highly crystallized limestone. Most palynomorphs recorded from the underlying Nahr Umr Formation are also found in the Mauddud Formation (Tables 1 and 2, and Figure 3). In addition, some dinoflagellate cyst species, such as Odontochitina cribropoda (3.2), O. coststa, Florentinia radiculata (4.4), F. resex (4.8) and F. berran (4.5), first occur in the Mauddud Formation.

Odontochitina cribropoda (3.2) has been recorded from the Cenomanian/?Maastrichtian of South Africa (McLachlan and Pieterse, 1978) and from the late Albian of offshore northwestern Africa (Below, 1984). Odontochitinia costata was reported from the late Albian (Vraconian) to Campanian, as recorded from France by Davey and Verdier (1973), worldwide by Williams and Bujak (1985), and from northeastern Libya by Uwins and Batten (1988).

The co-existence of Florentinia radiculata (4.4), F. resex (4.8) and F. berran (4.5) in the same sample also indicates a late Albian to early Cenomanian age for the Mauddud Formation. This is based on the age range of the Florentinia species determined in France (Davey and Verdier, 1976; Tocher and Jarvis, 1996), offshore northwestern Africa and Morocco (Below, 1982; 1984), Alberta (Singh, 1983), northeastern Libya (Uwins and Batten, 1988), and northwest Egypt (Schrank and Ibrahim, 1995).

Based on the palynological evidence, a late Albian (Vraconian) to early Cenomanian age is given to the Mauddud Formation. Similarly, in southern Iraq, the age of the Mauddud Formation is late Albian to early Cenomanian according to Sayyab and Mohammed (1984). However Hewaidy and Al-Himi (1994) suggested that the equivalent interval dated as late Albian belonged to the late Albian Orbitolina qatarica TRZ (Figure 2).

Ahmadi Formation

(Core samples 9–26; 1,820–2,408 ft)

Eighteen samples (Samples 9–26) were analyzed of which samples 11, 12, 17, 18, 21, 24 and 25 are barren and the rest yielded a rich palynomorph microflora. The palynomorph assemblage of the Ahmadi Formation contains several distinctive species, most of which are long ranging. A few, however, are more indicative of a Cenomanian age and are considered here to be of stratigraphic significance. These are the angiosperm species Dichastopollenites cf. ghazalatensis (2.2/2.3), Rousea cf. georgensis (2.9/2.10) and Rousea spp. (2.12), and the dinoflagellate species Subtilisphaera cheit, S. hyalina (3.5), S. senegalensis (3.6), Hystrichosphaerina turonica (3.12), Florentinia deanei, and F. clavigera (4.10).

Genus Dichastopollenites is possible angiosperm pollen known only from the Cenomanian of Utah and Arizona (D. reticulatus, May, 1975), the middle Cenomanian of northern Alberta (D. dunviganensis, Singh, 1983), and from northwestern Egypt (D. ghazalatensis, Ibrahim, 1996). Therefore, the presence of Dichastopollenites cf. ghazalatensis (2.2/2.3), together with elaterate pollen-like Elatersporites klaszii (1.15), Elateroplicites africaensis and Elaterocolpites castelainii (1.16) suggests an early to middle Cenomanian age for the Ahmadi Formation.

Singh (1983) first described Subtilisphaera hyalina (3.5) from the Cenomanian of northwestern Alberta and Uwins and Batten (1988) later reported it from the Vraconian-Cenomanian of northeastern Libya. Florentinia deanei has been reported from the middle to late Cenomanian of England and France (Davey, 1969), and from the early to middle Cenomanian of the Paris Basin (Tocher and Jarvis, 1996). F. clavigera (4.10) has a basal occurrence in the Cenomanian and may extend into the latest Cretaceous according to Srivastava (1991).

It is concluded that the Ahmadi Formation in well DK-B is of early to middle Cenomanian age. This is consistent with the work of Hewaidy and Al-Hitmi (1994) who subdivided the interval into the two foraminiferal zones of the Hensonina lenticularis TRZ in the basal part of Ahmadi Formation and the Asterohedbergella asterospinosa TRZ in the upper part (figure 2). El-Naggar and El-Nakhal (1987) assigned the Ahmadi Formation in Kuwait to the Cenomanian (Favusella hiltermanni Zone).

PALYNOFACIES AND PALEOENVIRONMENTAL INFERENCES

Qualitative and semi-quantitative studies of the palynofacies allow for the determination of regressive and transgressive intervals, and can yield useful information on depositional paleoenvironments and the hydrocarbon source rock potential. Figure 5 shows the trend curves of various palynological parameters of the Nahr Umr, Mauddud and Ahmadi formations.

Basal part of the Nahr Umr Formation

Depositional Environment

The overall composition of palynomorphs and particulate organic matter in the lower part (member A, Figure 2) of the Nahr Umr Formation (samples 1, 2, 4) is characterized by the predominance of miospores, phytoclasts and tabular inequidimensional opaque fragments. This palynofacies is similar to palynofacies 3 in the classification of Ibrahim et al. (1997). The abundance of sporomorphs and phytoclasts in the lower part of the Formation implies oxidizing conditions and proximity to a terrestrial source and to an active fluvio-deltaic environment (Fisher, 1980; Tyson, 1995; Ibrahim et al. 1997). The large fragments of cuticles and leaf parts (5.3–5.8) are characteristic of prodelta facies (Batten, 1974; Ibrahim et al., 1997).

The common occurrence of Botryococcus was noted in samples 1, 2 and 4. Botryococcus is a unicellular alga with cup-shaped individual cells that usually occurs in colonies (4.13). Fossils of Botryococcus spp. are found primarily in freshwater-lacustrine, fluvial, lagoonal and deltaic environments (Batten and Grenfell, 1996; Guy-Ohlson, 1992; Tyson, 1995; Wood and Miller, 1997). Studies of present-day Botryococcus show that it is transported offshore by rivers and may be deposited in marine prodeltas and on the adjacent shelves (Caratini et al., 1981; Wood and Miller, 1997). Accordingly, the basal part of the Nahr Umr Formation (intervals represented by samples 1, 2 and 4) is equated with deposition in such a marine prodelta or shallow-shelf environment. This is in agreement with Alsharhan and Nairn (1997). They concluded that at the time of deposition of the Nahr Umr Formation, Qatar was in a transitional zone between continental, depositional conditions (fluvial and lower coastal plain) in the west and a shallow-marine environment with shale and minor sandstone in the east.

Kerogen Type

The large amount of phytoclasts with abundant sporomorphs and the rare occurrence of amorphous organic matter (AOM) indicate the presence of gas-prone, type III kerogen (Tyson, 1993, 1995; Ibrahim et al., 1997). However, Botryococcus that is highly aliphatic and related to type I kerogen (Tyson, 1995; Wood and Miller, 1997), has been recovered in small amounts from the core samples.

Upper part of the Nahr Umr Formation

Depositional Environment

The upper part of the Formation (members B and C, Figure 2) is represented by samples 3, 5 and 6 and characterized by an abundance of dinoflagellate cysts (72–88%) and palynoforaminiferal test linings. The dominant taxa of Cribroperidinium, Oligosphaeridium, Cyclonephelium, Spiniferites and Florentinia are evidence of an exclusively marine environment. Davey (1970) and Tyson (1993; 1995) concluded that the high dinoflagellate concentration indicates deposition from the inner part of the shelf to the upper part of the continental slope, and that the deposits have a high primary hydrocarbon potential.

Sample 3 is enriched in AOM that typically indicates a new transgressive phase, reducing conditions, and a distal dysoxic-anoxic shelf (Batten, 1983; Tyson, 1993; Ibrahim et al., 1997). The upper part of the Nahr Umr Formation is therefore interpreted as having been deposited under normal marine conditions of the inner to middle shelf at depths of from 10 to 80 m.

Kerogen Type

The main constituents are dinoflagellate cysts. Inconsistent particulate organic matter is present in samples 3, 5 and 6 (Figure 5). Sample 5 was also enriched in opaques whereas sample 6 contains moderate amounts of phytoclasts, opaques and AOM. On the basis of this evidence, there exists a potential for oil-prone kerogen type-II material.

Mauddud Formation

Depositional Environment

The Mauddud Formation is characterized by the abundance of dinoflagellate cysts such as the skolochorate forms Florentinia spp. (4.3), the spiniferate Spiniferites ramosus (3.19), and the cornucavate Odontochitina spp. In addition, microforaminiferal test linings are abundant. AOM and opaques are the dominant particulate organic matter (Figure 5).

The abundance of thin-walled delicate cysts with elaborate processes (chorate taxa) is believed to characterize offshore, open-marine environments (Vozzhennikova, 1965; Tyson, 1995). Moreover, Davey (1970) and Davey and Rogers (1975) interpreted the abundance of chorate morphotypes as an adaptation to warm, oceanic water masses. The presence of Florentinia spp. (which is robust and may be more than 80 μm long, implies deposition in open-marine conditions (Uwins and Batten, 1988). The occurrence of microforaminiferal test linings is characteristic of normal marine conditions within the area of the continental shelf. In the samples studied, the opaque fragments are small, equidimensional and diluted by overall AOM and probably represent wind-blown material (Habib, 1982; Masran, 1984).

The palynofacies parameters and the abundance of skolochorate spinose dinoflagellate cysts, suggests that the deposition of the Mauddud Formation carbonates took place in an inner-middle shelf environment at depths of between 20 and100 m. This is in accordance with Hewaidy and Al-Hitmi (1994) who concluded that the Mauddud Formation in the Dukhan field represents a quiet phase of widespread shallow-shelf carbonate deposition.

Kerogen Type

An abundance of dinoflagellate cysts (Figure 5) with AOM and opaques indicates kerogen type II, oil-prone material. Alsharhan and Nairn (1997) concluded that the organic matter in the Mauddud Formation is kerogenous to mainly kerogenous and an excellent potential source of oil.

Ahmadi Formation

Marine phytoplankton is more common than non-marine and terrestrial varieties in the Ahmadi Formation, with the exception of samples 10 and 15 (Figure 5).

The predominant dinoflagellate cysts are the peridinioid cavate forms like Senegalinium, Subtilisphaera, cavate cysts such as Xenascus, and the long-process and chorate forms such as Florentinia, Cleistosphaeridium, Spiniferites, Coronifera and Dinopterygium. Scott and Kidson (1977) recorded peridinioid cavate cysts as being typical of plankton-dominated assemblages from high-energy, near-shore carbonate facies in the Albian-Cenomanian of western Texas. Conversely, the peridinioid association is also seen in ancient upwelling facies, especially from the mid-Cretaceous to the Paleogene (Schrank, 1984; Rauscher et al., 1986). Organic debris in the samples from the Ahmadi Formation is characterized by the abundance of AOM and minor opaque fragments.

The shales and limestones of the Ahmadi Formation were deposited in open-marine conditions on the outer shelf at depths of from 100 to 200 m. The presence of Micrhystridium stellatum (4.11) (spinose acritarchs) in some parts of the Formation is consistent with this interpretation (Uwins and Batten, 1988, El Beialy and Al-Hitmi, 1994).

There is evidence of two regressive pulses or lowstand system tracts having occurred during the deposition of the Formation. The first one at a depth of 2,360 ft (sample 10; Figure 5), is recognizable by an increase in terrestrial sporomorphs to about 23 percent. The second regression phase is seen in sample 15 in which the terrestrial elements constitute about 80 percent. These pulses of regression during the Cenomanian in the area of the Arabian Gulf are consistent with the global short-term eustatic curve of Haq et al. (1987, 1988).

Kerogen Type

Abundant dinoflagellate cysts with AOM indicate kerogen type II, oil-prone material.

ORGANIC THERMAL MATURATION

The color of spores and palynodebris can be used to determine the level of organic thermal maturation (Staplin, 1969; Firth, 1993; Ibrahim et al., 1997). The increase in temperature that results from deep burial is thought to be the essential parameter of the change of spore color. The thermal alteration index (TAI) is deduced from the changes in spore color with depth.

There are no major changes in spore color in the sediments studied. The spore color ranges from yellow (Ahmadi Formation) to orange (Mauddud and Nahr Umr formations) and grades to pale-brown in samples 1 and 2 of the Nahr Umr Formation. The color changes indicate that the TAI ranges from 2, 2+ to 3− according to the scale of the pollen/spore color chart of Pearson (1990). The TAI for Dukhan-B suggests that the sediments of the Ahmadi Formation are immature whereas those of the Nahr Umr and Mauddud formations are slightly thermally mature. However, this result is at variance with the statement by Alsharhan and Nairn (1997) that the Cretaceous source rocks are immature throughout Qatar.

THE AFRICAN-SOUTH AMERICAN MICROFLORAL PHYTOGEOPROVINCE

The Albian-Cenomanian microfloral assemblage of the Dukhan oil field contains forms that are characteristic of the African-South American (ASA) Microfloral phytogeoprovince (Figure 6) of Herngreen (1974). It is also coeval with the Northern Gondwana Province (Brenner, 1976; Herngreen and Chlonova, 1981), Galeacornea paleophytogeoprovince (Srivastava, 1978), Elaterosporites phytogeoprovince (Srivastava, 1981), Northern Gondwanan Realm (Batten and Li Wenben, 1987), and the mid-Cretaceous elater-bearing phytogeoprovince (Srivastava, 1994).

The Albian-Cenomanian assemblage of western Qatar of this paper is referable to the ASA microfloral phytogeoprovince by virtue of the following characteristics:

  • Rarity of cosmopolitan fern genera. Most spores belong to the psilatrilete group, Cicatricosisporites, Leptolepidites and Perotriletes.

  • Presence of the characteristic spore Crybelosporites pannuceus (1.8).

  • Absence of bi- and tri-saccate gymnospermous pollen grains.

  • Presence of ephedroid pollen such as Ephedripites jansonii (1.14). The absence of Equisetosporites, Gnetaceaepollenites and Steevesipollenites may be the result of environmental controls.

  • Presence of elater-bearing pollen such as Elaterosporites, Elateroplicites and Elaterocolpites.

  • The Albian-Cenomanian angiospermous pollen grains such as Afropollis, Stellatopollis, Reyrea and Cretacaeiporites are endemic to the ASA (Figure 6).

The climatic conditions that prevailed in the ASA during Albian-Cenomanian times have usually been described by palynologists as arid to semi-arid (tropical to subtropical) and warm (Brenner, 1976; Boltenhagen and Salard-Cheboldaeff, 1980; Batten and Li Wenben, 1987, and others). The presence of ephedroid pollen, as well as elater-bearing species in the present assemblage, supports this climatic inference.

On the other hand, the overall composition of the recovered dinoflagellate cysts from western Qatar reflects a Tethyan realm. The abundance of the chorate forms Florentinia and Spiniferites, cornucavate Odontochitina, and cavates Subtilisphaera and Senegalinium characterize the dinoflagellates. These forms are exclusively enriched in the Mediterranean region. For example, they are present in northern Egypt (Omran et al., 1990; Ibrahim et al., 1995; Schrank and Ibrahim, 1995), northeastern Libya (Batten and Uwins, 1985; Uwins and Batten, 1988), Morocco and offshore northwest Africa (Below, 1982, 1984), and France (Davey, 1969; Davey and Verdier, 1973, 1976; Courtinat et al., 1991; Tocher and Jarvis, 1996).

CONCLUSIONS

The Albian-Cenomanian succession of the Wasia Group (excluding the Mishrif Formation) in Well DK-B yields a well-preserved and diversified assemblage of dinoflagellate cysts, pollen grains and pteridophytic spores.

The siliciclastic sediments of the Nahr Umr Formation are dated as middle to late Albian on the basis of the index sporomorphs Crybelosporites pannuceus (1.8), Afropollis jardinus (2.5), Foveotricolpites concinnus (2.11) and Rhiopites sp. (2.14). Additional characteristic dinoflagellate cysts include Odontochitina ancala (3.3), Senegalinium aenigmaticum (3.7), Cyclonephelium vannophorum, Xenascus ceratoides, Florentinia cooksoniae (4.7), F. laciniata (4.6), Dinopterygium cladoides, D. tuberculatum (3.13/3.14) and Microdinium setosum. The basal part of the Nahr Umr Formation (as represented in Well DK-B) accumulated in a marine prodelta or shallow-shelf environment, whereas the upper part was deposited in deeper, normal marine conditions of the inner-middle shelf in water depths of from 10 to 80 m. Kerogen type III, gas-prone material and kerogen type II, oil-prone material is indicated for the lower and upper parts respectively of the Nahr Umr Formation.

The carbonate facies of the Mauddud Formation are assigned a late Albian to early Cenomanian age by the presence of the marker dinoflagellate cysts Odontochitina cribropoda (3.2), O. costata, Florentinia radiculata (4.4), F. resex (4.8) and F. berran (4.5). The Formation was deposited in an inner to middle shelf marine environment in waters from 20 to 100 m deep. Amorphous organic matter and opaques are the main palynofacies elements together with dinoflagellates, which implies that kerogen of the Mauddud Formation is type II, oil-prone material.

The age of the Ahmadi Formation is identified as early to middle Cenomanian from the palynomorph assemblage that includes the angiosperm species Dichastopollenites cf. ghazalatensis (2.2), Rousea cf. georgensis (2.9/2.10), and Rousea spp. (2.12), and the dinoflagellate species Subtilisphaera cheit, S. hyalina (3.5), S. senegalensis (3.6), Hystrichosphaerina turonica (3.12), Florentinia deanei, and F. clavigera (4.10).

The shales and limestones of the Ahmadi Formation were deposited in open-marine conditions on the outer shelf at depths of between100 and200 m. The changing proportions of marine to non-marine microflora indicate two regressive pulses, or lowstand system tracts, at sample depths of 2,360 and 2,190 ft (Figure 5). The organic material of the Ahmadi Formation is interpreted as kerogen type II, oil-prone material. The pollen/spore color reveals that of the sediments studied in Dukhan-B, those of the Ahmadi Formation are immature whereas the Nahr Umr and Mauddud sediments are slightly thermally mature.

The Albian-Cenomanian sporomorph assemblage can be referred to the African-South American Microfloral Phytogeoprovince of Herngreen (1974) in which arid to semi-arid (tropical to subtropical) climatic conditions prevailed. However, the dinoflagellate cysts from western Qatar reflect a Tethyan realm.

ACKNOWLEDGMENTS

The authors are indebted to the Qatar General Petroleum Corporation for providing the material on which this study is based. We are much obliged to the three anonymous referees and to the Geoscience Editor of GeoArabia for many helpful comments and suggestions that improved an earlier version of the manuscript. We also thank GeoArabia’s staff for designing the final graphics.

ABOUT THE AUTHORS

Mohamed I.A. Ibrahim has recently rejoined the staff of Alexandria University, Egypt (Department of Environmental Sciences) following secondment to the Geology Department of Qatar University as an Assistant Professor from October 1996 to mid-2002. Mohamed has a BSc (Honors, 1981) in Special Geology and a MSc (1986) in Palynology and Micropaleontology from Alexandria University. He obtained his PhD in Micropaleontology and Paleoecology in 1993 from Alexandria University and the Technical University of Berlin through a combined channel program. He taught Micropaleontology and Paleoecology from 1993 to 1995 in the Geology Department of Alexandria University and was a Palynostratigraphic 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.

Hind H.A. Al-Hitmi is a Lecturer in Micropaleontology and Biostratigraphy in the Geology Department of Qatar University. She has a BSc (Honors, 1979) in Geology and Chemistry (double major) from Qatar University and a MSc (1987) and PhD (1994) in Micropaleontology and Biostratigraphy from Ain Shams University, Egypt. She was a Demonstrator (1979–87) and Assistant Lecturer (1987–94) in the Geology Department of Qatar University, and was appointed Lecturer in 1994. She is the author of several research articles on the micropaleontology of Qatar.

Suzan E. Kholeif received her Bachelor’s degree in Geology from Alexandria University in 1983 and worked for Alexandria Petroleum Company until 1995. She completed her MSc (1989) and PhD (1999) from Mansoura and Cairo Universities respectively, in palynology, palynofacies and paleoecology. In 1999, she joined the Institute of Oceanography and Fisheries in Alexandria as a Lecturer in Micropaleontology. Suzan is the author of several research articles on the palynology and paleoecology of Egypt. She is a member of the IGCP 831, South Atlantic Mesozoic Correlation program, and a member of GSE and ESQUA.

*
In the Middle East. the Cretaceous is generally divided into early/lower (Berriasian–Aptian), middle (Albian–Turomanian) and late/upper (Coniacian–Maastrichtian). This is convenient as it corresponds to the unconformity-bounded Thamama, Wasia and Aruma units. However, as a tripartite divison of the Cretaceous is not internationally recognized, the terms “early”/“lower”, “middle” and “late”/“upper” Cretaceous are used infomally in this paper and their initial letters are not capitalized.