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Albian-Cenomanian Palynology, Paleoecology and Organic Thermal Maturity of Well DK-B in the Dukhan Oil Field of Western Qatar

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Albian-Cenomanian Palynology, Paleoecology and Organic Thermal Maturity of Well DK-B in the Dukhan Oil Field of Western Qatar GeoArabia, Vol. 5, No. 4, 2000 Gulf PetroLink, Bahrain Albian-Cenomanian of Qatar Albian-Cenomanian Palynology, Paleoecology and Organic Thermal Maturity of Well DK-B in the Dukhan Oil Field of Western Qatar Mohamed I.A. Ibrahim and Hind H.A. Al-Hitmi University of Qatar, Doha and Suzan E. Kholeif National Institute of Oceanography and Fisheries, Alexandria, Egypt. 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 483 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/5/4/483/4559583/ibrahim.pdf by guest on 29 September 2021 Ibrahim et al. 51 E 52 27 N Pars 27 N South ARABIAN GULF N 0 50 Abu Sa’fah km Al-Rayyan Al-Shaheen Balal (Bahram) North BAHRAIN Field 26 Resalat Awali (Raklish) 26 Al-Khaleej Najwat Najem Maydan Mahzam Bul Hanin North Dome Idd Al-Shargi South Dome DK-B A-Structure North El Bundua Al-Karkara Doha 25 DUKHAN A-Structure South 25 Satah Arzanah QATAR Jarnain Bu Jufair Dalma SAUDI ARABIA Hair Dalma Bu Tini Oil field 24 Gas field Shuweihat 24 Well DK-B 51 E 52 U.A.E. Figure 1: Location map of Well DK-B, Dukhan field, Qatar. (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 * 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. 484 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/5/4/483/4559583/ibrahim.pdf by guest on 29 September 2021 Albian-Cenomanian of Qatar FORAMINIFERAL LITHOLOGIC BIOZONE DESCRIPTION (Hewaidy and Al-Hitmi, 1994) AGE STAGE GROUP FORMATION MEMBER DEPTH (ft) LITHOLOGY SAMPLE Gray limestone with thin shale Dictyoconella minima Mishrif interbeds Total Range Zone 1,800 25 1,900 Alternation of gray, white D limestone and blue to chocolate shale 2,000 Asterohedbergella 20 asterospinosa Total Range Zone 2,100 Grayish-green to C pale-brown hard shale 15 2,200 early-middle Cenomanian B Pale-buff limestone 2,300 Hensonina lenticularis A 10 Chocolate and blue shale Total Range Zone Cenomanian 2,400 Wasia White hard fossiliferous Orbitulina qatarica middle Cretaceous 2,500 Missing limestone Total Range Zone section late Alb.- late Albian late e.Cenom. 2,600 Chocolate and blue shale C and gray sandstone 2,700 Greenish gray to B Asanospira diyabi 5 bluish-brown shale 2,800 Total Range Zone early Albian early Nahr Umr Mauddud Ahmadi Blue, fine-grained sandstone, A 2,900 middle-late Albian amber and carbonaceous 1 matter are common Figure 2: Lithostratigraphic succession in Well DK-B with sample locations, concise lithologic descriptions and foraminiferal biozones of Hewaidy and Al-Hitmi (1994). 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). 485 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/5/4/483/4559583/ibrahim.pdf by guest on 29 September 2021 Ibrahim et al. HEWAIDY AND EL BEIALY AND AL-HITMI (1994) AL-HITMI (1994) PRESENT STUDY FORMATION AGE GROUP DK-A, B, C, D, E DK-C DK-B foraminifera palynology palynology D C Mishrif late Cenomanian ? Cenomanian Not studied B A early- middle middle Cenomanian early-middle Ahmadi Cenomanian early Cenomanian Cenomanian late Albian Wasia early Cenomanian Mauddud late Albian Indeterminate late Albian Albian – Cenomenian C Nahr B early Albian middle-late Albian middle-late Albian Umr A Figure 3: Age relations and correlation of the rock units of the Wasia Group based on micropaleontological studies. A, B, C, etc., are informal members of the Nahr Umr and Mishrif formations from Al-Hitmi (1994). 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.
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