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Palynofacies indications of depositional environments and source potential for hydrocarbons: uppermost Jurassic-basal Cretaceous Sulaiy Formation, southern Iraq
Cretaceous Research (1999) 20, 359–363 Article No. cres.1999.0157, available online at http://www.idealibrary.com on
Palynofacies indications of depositional environments and source potential for hydrocarbons: uppermost Jurassic-basal Cretaceous Sulaiy Formation, southern Iraq *T. K. Al-Ameri, *F. S. Al-Musawi and †D. J. Batten *Department of Geology, College of Science, University of Baghdad, PO Box 47062, Jadiriyah, Iraq †Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK Revised manuscript accepted 26 January 1999
Three types of palynofacies have been identified in samples from the subsurface Tithonian–Valanginian Sulaiy Formation in the Mesopotamian Basin of southern Iraq. They indicate that deposition occurred in environments ranging from distal dysoxic-anoxic shelf to suboxic-anoxic basin. These conditions favoured the accumulation of large amounts of organic matter in a mainly biodegraded (amorphous) state. Burial of the sediments has been deep enough for their organic contents to be rendered thermally mature. Hence, the succession is highly rated as a source of oil with subordinate gas, not only in Iraq but also in neighbouring Kuwait and Saudi Arabia. 1999 Academic Press K W: Upper Jurassic; Lower Cretaceous; palynofacies; depositional environments; source rocks; Mesopotamian Basin; Iraq.
1. Introduction The Sulaiy Formation is one of the best source rocks in southern Iraq, Kuwait and Saudi Arabia (Beydoun, 1991). It is composed of between 196 and 304 m of mainly alternating argillaceous, dolomitic and pyrite-rich limestone units in southern Iraq. Based on occurrences of foraminifera the succession was suggested by van Bellen et al. (1959) to range in age from ?Tithonian to Berriasian. It overlies uncomformably the Upper Jurassic Gotnia Anhydrite Formation, and is in turn overlain by the Yamama Formation, which comprises a succession of limestones that are considered to be Berriasian– Valanginian in age. Sometimes it cannot be clearly differentiated from this younger formation (van Bellen et al., 1959). No attempt has been made previously to discuss the composition of the sedimentary organic matter in the Sulaiy Formation. The aim of this paper is to present palynological and geochemical observations on the succession in the Mesopotamian Basin, and to consider their implications with respect to environments of deposition and source potential for hydrocarbons. 0195–6671/99/030359+05 $30.00/0
2. Material and methods Between six and eight samples from each of the Dibdibba 1 (Di 1), Jerashan 1 (Jr 1), Luhais 2 (Lu 2), Noor 1 (No 1), Rachi 2 (Rc 2) and Ratawi 6 (Rt 6) boreholes, and 31 from the Rumaila 167 (R 167) Borehole (Figure 1) were selected for analysis. The depths and lithological units from which they were recovered are indicated in Figure 2. This is a fence diagram to show the distribution of facies within the Mesopotamian Basin. Standard palynological processing techniques were used to extract the acid resistant organic matter. Determinations of total organic carbon (TOC) and, by Rock Eval pyrolysis, of kerogen-type and the quantity of hydrocarbons (S2) generated by thermal cracking, were based on the same samples. All of the slides prepared are stored in the palynological laboratory of the Department of Geology, University of Baghdad.
3. Palynomorphs and dating of the Sulaiy Formation The palynomorph assemblages isolated from the samples examined contain numerous dinoflagellate 1999 Academic Press
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Figure 2. Fence diagram depicting generalised lithological successions of, and palynofacies occurrences in, six of the seven borehole sections through the Sulaiy Formation that have been examined. Figure 1. Map of part of southern Iraq and neighbouring countries showing the location of the boreholes on which this study is based.
cysts, with some spores and pollen grains in association. The taxa recorded include those listed on Table 1. They occur together with foraminiferal linings and fungal remains. The latter are very common in some preparations, and may have contributed to the degradation of structured plant material, converting it to amorphous organic matter (AOM). In the south (Jerashan 1 Borehole), AOM comprises up to 100% of the palynological content of the limestones. For the purpose of age determination, the palynomorphs recovered (mainly from boreholes Noor 1 and Dibdibba 1) are considered to comprise a single assemblage because no significant differences in composition were recorded through the sections examined. Comparisons were made between occurrences of the taxa encountered and records in the literature from widely scattered localities elsewhere so that the assemblage could be dated. Among the numerous publications consulted for this purpose were those by Sarjeant (1962), Millioud et al. (1974), Davey (1982), Thusu & van der Eem (1985), Thusu & Vigran
(1985), Du¨rr (1988), Ogg (1994), and Ibrahim & Schrank (1996). The published ranges of the taxa encountered suggest that the Sulaiy Formation ranges from Tithonian to Valanginian in age. 4. Palynofacies Three types of palynofacies (PF-1-3) were recognised in the palynological preparations analysed. Their occurrence and distribution in the borehole sections studied are shown in Figure 2. PF-1 Palynomorphs are more numerous in this palynofacies than in the other two, amounting to between 15 and 40% of the total recovery. The assemblage of dinoflagellate cysts is more diverse than that of the miospores (small spores and pollen grains) although, along with fungal remains and foraminiferal linings, spores are common. Most abundant are the cysts Cribroperidinium edwardsii, C.? longicorne, Oligosphaeridium complex, Spiniferites ramosus, Subtilisphaera spp., Surculosphaeridium cribrotubiferum and Systematophora daveyi. Amorphous matter is present in quantitites
Palynofacies indications of depositional environments and hydrocarbons Table 1. Characteristic dinoflagellate cysts (A) and miospores (B) recovered from the Sulaiy Formation. (A) Chlamydophorella sp. Cribroperidinium edwardsii (Cookson & Eisenack) Davey 1969 Cribroperidinium? longicorne (Downie) Lentin & Williams 1985 Cribroperidinium sp. Gochteodinia villosa multifurcata Davey 1982 Lithodinia jurassica Eisenack 1935 Oligosphaeridium complex (White) Davey & Williams 1966 Oligosphaeridium sp. Pareodinia sp. Spiniferites sp. Subtilisphaera spp. Surculosphaeridium cribrotubiferum (Sarjeant) Davey et al. 1966 Systematophora areolata Klement 1960 Systematophora? daveyi Riding & Thomas 1988 Systematophora spp. (B) Concavissimisporites variverrucatus (Couper) Brenner 1963 Cyathidites australis Couper 1953 Deltoidospora spp. Punctatisporites major (Couper) Kedves & Simoncsics 1964 Todisporites rotundiformis (Malyavkina) Pocock 1970 Toroisporis delicatus Do¨ring 1965
that range between 50 and 85% of the total organic component whereas phytoclasts comprise 5–10%. The lithological associations of this palynofacies are limestones that may be dolomitised in places. It is recorded mainly from the Noor 1 Borehole, and partly from boreholes Dibdibba 1 and Rumaila 167. PF-2 By comparison with the general aspect of PF-1, the palynomorph component of PF-2 is reduced to approximately 5–10%, and the phytoclasts to between 2 and 10% of the total organic recovery. The bulk of the palynofacies is made up of AOM. The most common dinoflagellate cysts are Cribroperidinium spp. and Systematophora areolata. Spores, fungal remains and foraminiferal linings are consistently present. The deposits with which the palynofacies is associated are mainly limestones that are marly at some levels. It dominates large parts of the Sulaiy Formation encountered in boreholes Dibdibba 1, Luhais 12 and Rumaila 167, and to a lesser extent in Rachi 2 and Ratawi 6. PF-3 In this palynofacies, the palynomorphs and phytoclasts are further diminished, by comparison with
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PF-1 and 2, to about 1–5% and 2–5% of the total organic recovery, respectively. Both of these components are also bacterially degraded, commonly to more or less an amorphous state. AOM reaches its maximum abundance in PF-3 where it typically comprises between 90 and 100% of the palynological matter. The palynofacies is associated with limestones that are clayey and/or pyrite-rich in places. It occurs throughout the Sulaiy Formation penetrated by the Jerashan 1 Borehole, and dominates much of the succession in Rachi 2 and Ratawi 6. It also prevails in the lower half of the formation in Luhais 12, but is confined to approximately the lower third and basal 40 m respectively of the sections in Dibdibba 1 and Rumaila 167. 5. Palaeoenvironmental interpretation It is well known that numerous variables are involved in the deposition of palynological matter (Traverse, 1994; Tyson, 1995; Batten 1996a). A partial analogue of the depositional conditions that led to the accumulation of the Sulaiy Formation may be off the coast of California where an anoxic layer is present at depth. This is a result of the combined effect of upwelling of fertile cold currents and high organic productivity. Dinoflagellate cysts, fungi and foraminiferal linings are abundantly preserved in gels of AOM (Cross et al., 1966; Melia, 1984; Oboh, 1992; Tyson, 1995). Many ancient successions that are rich in AOM (e.g., parts of the North Sea Jurassic: Riley et al., 1989; van der Zwan, 1990; Tyson, 1993) are known to be the end result of deposition in dysoxic-anoxic conditions during periods of sluggish circulation when conditions were suitable for the proliferation of a varied biota in the oxygenated part of the water column, particularly in the photic zone. Upwelling currents could have been generated from the southern coast of the Tethyan Ocean to the Mesopotamian Basin during the deposition of the Sulaiy Formation. At that time, there was a divergent plate boundary between the northern Gondwanan continent and the Tethys. The percentage records of palynological matter recorded from three of the boreholes discussed herein, which are representative of the succession as a whole, were summarised and plotted on a ternary diagram taken from Tyson (1993, fig. 5.2). Although established for Upper Jurassic marine shales, this is also applicable to the limestone-dominated Sulaiy Formation in the context of the development of anoxic conditions (Figure 3). All three palynofacies identified were encountered in samples from the Rumaila 167 Borehole. These may reflect changes from distal dysoxic-anoxic and dysoxic-oxic shelf to deeper-water
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Figure 3. Amorphous organic matter-phytoclastpalynomorph plot showing stylised diagnostic fields for Upper Jurassic marine shale palynofacies (slightly modified after Tyson, 1993, 1995) with the distribution of types of organic matter for three wells in southern Iraq indicated. These are associated with the following environments: VII and VIII, distal dysoxic-anoxic and dysoxic-oxic shelf; and XI, distal suboxic-anoxic basin.
distal suboxic-anoxic basin-environments (Figure 4). PF-1 and 2 were recorded less often southwards towards the Jerashan 1 Borehole, which only yielded PF-3 (Figure 2), suggesting persistent distal suboxic-
anoxic basinal conditions. The higher numbers of foraminiferal linings (up to 10%) and the abundance of AOM in these beds could possibly indicate the presence of upwelling currents during the accumulation of these sediments. Comparisons with possible analogues of the sedimentary environments in the Arabian Sea (Premuzic et al., 1982) and on the northwest African continental margin of the Atlantic Ocean (Diester-Haass, 1982) suggest that the accumulation of organic matter might have increased significantly in a dysoxic zone of water extending from depths ranging from approximately 300 to 500 m. There is minimal accumulation of organic matter in shallower, oxygenated, shelf waters today. A little below about 500 m anoxic conditions probably prevailed, leading to the preservation of even larger amounts of organic matter. It is concluded that the Sulaiy Formation comprises a succession of offshore marine-shelf deposits, although it is possible that the shelf may have been deep enough (more than 500 m) for scattered basinal facies of suboxic-anoxic character to accumulate. This conclusion is consistent with the high (c. 4:1) ratio of dinoflagellate cysts to spores and pollen grains (cf. Davey & Rogers, 1975; Melia, 1984) and the record of pyrite in the limestones (cf. Berner, 1984). 6. Source potential for hydrocarbons The predominantly dysoxic-anoxic and suboxicanoxic environments in which the sediments of the
Figure 4. Palynofacies map and block diagram for the Sulaiy Formation showing the location of the borehole sections in offshore, relatively deep-water shelf to basinal environments.
Palynofacies indications of depositional environments and hydrocarbons
Sulaiy Formation accumulated were conducive to the preservation of much organic matter, although most has been biodegraded. The total organic carbon (TOC) content is between 0.5 and 1% for the majority of the samples analysed. The means by which the degradation of the organic matter took place is partly implied by the common occurrence of fungal remains along with an inferred bacterial component. The activities of these organisms would have been responsible for the formation of abundant AOM, especially in PF-3. This material is similar in composition to oil-prone kerogen type A of Thompson & Dembicki (1986) and the mesoliptinites of Rahman & Kinghorn (1995; cf. discussion of terminology in Batten, 1996a). Rock Eval pyrolysis demonstrated kerogen type II and an oil extraction range of 200– 500 mg of hydrocarbons per gram of rock for the samples analysed. The organic matter is mature, the thermal alteration index (TAI) being 2.8 (Staplin, 1969; for comparisons with other scales, see Batten, 1996b), and hence capable of generating liquid hydrocarbons. This combination of favourable factors is the reason why the Sulaiy Formation is one of the main sources of oil in the Cretaceous reservoirs of southern Iraq and Kuwait. References Batten, D. J. 1996a. Palynofacies and palaeoenvironmental interpretation. In Palynology: principles and applications (eds Jansonius, J. & McGregor, D. C.), volume 3, pp. 1011–1064 (American Association of Stratigraphic Palynologists Foundation, Dallas). Batten, D. J. 1996b. Palynofacies and petroleum potential. In Palynology: principles and applications (eds Jansonius, J. & McGregor, D. C.), volume 3, pp. 1065–1084 (American Association of Stratigraphic Palynologists Foundation, Dallas). Beydoun, Z. R. 1991. Arabian plate hydrocarbon geology and potential—a plate tectonic approach. American Association of Petroleum Geologists, Studies in Geology 33, ix+77 pp. Bellen, R. C. van, Dunnington, H. V., Wetzel, R. & Morton, D. M. 1959. Lexique stratigraphique international. Volume III. Asie, Fasicule 10a, Iraq, 333 pp. (Centre National de la Recherche Scientifique, Paris). Berner, R. A. 1984. Sedimentary pyrite formation: an update. Geochimica et Cosmochimica Acta 48, 605–615. Buday, T. 1980. The regional geology of Iraq. Volume 1. Stratigraphy and palaeogeography, 445 pp. (GEOSERV, Baghdad). Cross, A. T., Thompson, G. G. & Zaitzeff, J. B. 1966. Source and distribution of palynomorphs in bottom sediments, southern part of Gulf of California. Marine Geology 4, 467–524. Davey, R. J. 1982. Dinocyst stratigraphy of the latest Jurassic to Early Cretaceous of the Haldager No. 1 borehole, Denmark. Danmarks Geologiske Undersøgelse, Serie B 6, 57 pp.
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Diester-Haass, L. 1982. Indicators of water depth in bottom sediments of the continental margin off West Africa. Marine Geology 49, 311–326. Du¨rr, G. 1988. Palynostratigraphie des Kimmeridgium und Tithonium von Su¨ddeutschland und Korrelation mit borealen Floren. Tu¨binger Mikropala¨ontologische Mitteilungen 5, 159 pp. Ibrahim, M. & Schrank, E. 1996. Palynological studies on the Late Jurassic-Early Cretaceous of the Kahraman-1 well, northern Western Desert, Egypt. In Ge´ologie de l’Afrique et de l’Atlantique Sud: Actes Colloques Angers 1994, pp. 611–629 (Elf Aquitaine, Pau). Melia, M. B. 1984. The distribution and relationship between palynomorphs in aerosols and deep-sea sediments off the coast of northwest Africa. Marine Geology 58, 345–371. Millioud, M. E., Williams, G. L. & Lentin, J. K. 1974. Stratigraphic range charts. Selected Cretaceous dinoflagellates. In Proceedings of a forum on dinoflagellates (ed. Evitt, W. R.), American Association of Stratigraphic Palynologists, Contributions Series 4, 65–71. Oboh, F. E. 1992. Middle Miocene palaeoenvironments of the Niger Delta. Palaeogeography, Palaeoclimatology, Palaeoecology 92, 55–84. Ogg, G. 1994. Dinoflagellate cysts of the Early Cretaceous North Atlantic Ocean. Marine Micropalaeontology 23, 241–263. Premuzic, E. T., Benkovitz, C. M., Gaffney, J. S. & Walsh, J. J. 1982. The nature and distribution of organic matter in the surface sediments of world oceans and seas. Organic Geochemistry 4, 63–77. Rahman, M. & Kinghorn, R. R. F. 1995. A practical classification of kerogens related to hydrocarbon generation. Journal of Petroleum Geology 18, 91–102. Riley, L. A., Roberts, M. J. & Connell, E. R. 1989. The application of palynology in the interpretation of Brae Formation stratigraphy and reservoir geology in the South Brae Field area, British North Sea. In Correlation in hydrocarbon exploration (ed. Collinson, J. D.), pp. 339–356 (Norwegian Petroleum Society and Graham & Trotman, London). Sarjeant, W. A. S. 1962. Upper Jurassic microplankton from Dorset, England. Micropaleontology 8, 255–268. Staplin, F. L. 1969. Sedimentary organic matter, organic metamorphism, and oil and gas occurrence. Bulletin of Canadian Petroleum Geology 17, 47–66. Thompson, C. L. & Dembicki, H. Jr 1986. Optical characteristics of amorphous kerogens and the hydrocarbon-generating potential of source rocks. International Journal of Coal Geology 6, 229–249. Thusu, B. & Vigran, J. O. 1985. Middle-Late Jurassic (Late Bathonian-Tithonian) palynomorphs. In Palynostratigraphy of North-East Libya (eds Thusu, B. & Owens, B.), Journal of Micropalaeontology 4(1), 113–129. Thusu, B. & van der Eem, J. G. L. A. 1985. Early Cretaceous (Neocomian-Cenomanian) palynomorphs. In Palynostratigraphy of North-East Libya (eds Thusu, B. & Owens, B.), Journal of Micropalaeontology 4(1), 131–149. Traverse, A. (ed.) 1994. Sedimentation of organic particles, xii+544 pp. (Cambridge University Press, Cambridge). Tyson, R. V. 1993. Palynofacies analysis. In Applied micropalaeontology (ed. Jenkins, D. G.), pp. 153–191 (Kluwer, Dordrecht). Tyson, R. V. 1995. Sedimentary organic matter. Organic facies and palynofacies, xviii+615 pp. (Chapman & Hall, London). Zwan, C. J. van der 1990. Palynostratigraphy and palynofacies reconstruction of the Upper Jurassic to lowermost Cretaceous of the Draugen Field, offshore mid Norway. Review of Palaeobotany and Palynology 62, 157–186.
Palynofacies indications of depositional environments and source potential for hydrocarbons: uppermost Jurassic-basal Cretaceous Sulaiy Formation, southern Iraq *T. K. Al-Ameri, *F. S. Al-Musawi and †D. J. Batten *Department of Geology, College of Science, University of Baghdad, PO Box 47062, Jadiriyah, Iraq †Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK Revised manuscript accepted 26 January 1999
Three types of palynofacies have been identified in samples from the subsurface Tithonian–Valanginian Sulaiy Formation in the Mesopotamian Basin of southern Iraq. They indicate that deposition occurred in environments ranging from distal dysoxic-anoxic shelf to suboxic-anoxic basin. These conditions favoured the accumulation of large amounts of organic matter in a mainly biodegraded (amorphous) state. Burial of the sediments has been deep enough for their organic contents to be rendered thermally mature. Hence, the succession is highly rated as a source of oil with subordinate gas, not only in Iraq but also in neighbouring Kuwait and Saudi Arabia. 1999 Academic Press K W: Upper Jurassic; Lower Cretaceous; palynofacies; depositional environments; source rocks; Mesopotamian Basin; Iraq.
1. Introduction The Sulaiy Formation is one of the best source rocks in southern Iraq, Kuwait and Saudi Arabia (Beydoun, 1991). It is composed of between 196 and 304 m of mainly alternating argillaceous, dolomitic and pyrite-rich limestone units in southern Iraq. Based on occurrences of foraminifera the succession was suggested by van Bellen et al. (1959) to range in age from ?Tithonian to Berriasian. It overlies uncomformably the Upper Jurassic Gotnia Anhydrite Formation, and is in turn overlain by the Yamama Formation, which comprises a succession of limestones that are considered to be Berriasian– Valanginian in age. Sometimes it cannot be clearly differentiated from this younger formation (van Bellen et al., 1959). No attempt has been made previously to discuss the composition of the sedimentary organic matter in the Sulaiy Formation. The aim of this paper is to present palynological and geochemical observations on the succession in the Mesopotamian Basin, and to consider their implications with respect to environments of deposition and source potential for hydrocarbons. 0195–6671/99/030359+05 $30.00/0
2. Material and methods Between six and eight samples from each of the Dibdibba 1 (Di 1), Jerashan 1 (Jr 1), Luhais 2 (Lu 2), Noor 1 (No 1), Rachi 2 (Rc 2) and Ratawi 6 (Rt 6) boreholes, and 31 from the Rumaila 167 (R 167) Borehole (Figure 1) were selected for analysis. The depths and lithological units from which they were recovered are indicated in Figure 2. This is a fence diagram to show the distribution of facies within the Mesopotamian Basin. Standard palynological processing techniques were used to extract the acid resistant organic matter. Determinations of total organic carbon (TOC) and, by Rock Eval pyrolysis, of kerogen-type and the quantity of hydrocarbons (S2) generated by thermal cracking, were based on the same samples. All of the slides prepared are stored in the palynological laboratory of the Department of Geology, University of Baghdad.
3. Palynomorphs and dating of the Sulaiy Formation The palynomorph assemblages isolated from the samples examined contain numerous dinoflagellate 1999 Academic Press
360
T. K. Al-Ameri et al.
Figure 2. Fence diagram depicting generalised lithological successions of, and palynofacies occurrences in, six of the seven borehole sections through the Sulaiy Formation that have been examined. Figure 1. Map of part of southern Iraq and neighbouring countries showing the location of the boreholes on which this study is based.
cysts, with some spores and pollen grains in association. The taxa recorded include those listed on Table 1. They occur together with foraminiferal linings and fungal remains. The latter are very common in some preparations, and may have contributed to the degradation of structured plant material, converting it to amorphous organic matter (AOM). In the south (Jerashan 1 Borehole), AOM comprises up to 100% of the palynological content of the limestones. For the purpose of age determination, the palynomorphs recovered (mainly from boreholes Noor 1 and Dibdibba 1) are considered to comprise a single assemblage because no significant differences in composition were recorded through the sections examined. Comparisons were made between occurrences of the taxa encountered and records in the literature from widely scattered localities elsewhere so that the assemblage could be dated. Among the numerous publications consulted for this purpose were those by Sarjeant (1962), Millioud et al. (1974), Davey (1982), Thusu & van der Eem (1985), Thusu & Vigran
(1985), Du¨rr (1988), Ogg (1994), and Ibrahim & Schrank (1996). The published ranges of the taxa encountered suggest that the Sulaiy Formation ranges from Tithonian to Valanginian in age. 4. Palynofacies Three types of palynofacies (PF-1-3) were recognised in the palynological preparations analysed. Their occurrence and distribution in the borehole sections studied are shown in Figure 2. PF-1 Palynomorphs are more numerous in this palynofacies than in the other two, amounting to between 15 and 40% of the total recovery. The assemblage of dinoflagellate cysts is more diverse than that of the miospores (small spores and pollen grains) although, along with fungal remains and foraminiferal linings, spores are common. Most abundant are the cysts Cribroperidinium edwardsii, C.? longicorne, Oligosphaeridium complex, Spiniferites ramosus, Subtilisphaera spp., Surculosphaeridium cribrotubiferum and Systematophora daveyi. Amorphous matter is present in quantitites
Palynofacies indications of depositional environments and hydrocarbons Table 1. Characteristic dinoflagellate cysts (A) and miospores (B) recovered from the Sulaiy Formation. (A) Chlamydophorella sp. Cribroperidinium edwardsii (Cookson & Eisenack) Davey 1969 Cribroperidinium? longicorne (Downie) Lentin & Williams 1985 Cribroperidinium sp. Gochteodinia villosa multifurcata Davey 1982 Lithodinia jurassica Eisenack 1935 Oligosphaeridium complex (White) Davey & Williams 1966 Oligosphaeridium sp. Pareodinia sp. Spiniferites sp. Subtilisphaera spp. Surculosphaeridium cribrotubiferum (Sarjeant) Davey et al. 1966 Systematophora areolata Klement 1960 Systematophora? daveyi Riding & Thomas 1988 Systematophora spp. (B) Concavissimisporites variverrucatus (Couper) Brenner 1963 Cyathidites australis Couper 1953 Deltoidospora spp. Punctatisporites major (Couper) Kedves & Simoncsics 1964 Todisporites rotundiformis (Malyavkina) Pocock 1970 Toroisporis delicatus Do¨ring 1965
that range between 50 and 85% of the total organic component whereas phytoclasts comprise 5–10%. The lithological associations of this palynofacies are limestones that may be dolomitised in places. It is recorded mainly from the Noor 1 Borehole, and partly from boreholes Dibdibba 1 and Rumaila 167. PF-2 By comparison with the general aspect of PF-1, the palynomorph component of PF-2 is reduced to approximately 5–10%, and the phytoclasts to between 2 and 10% of the total organic recovery. The bulk of the palynofacies is made up of AOM. The most common dinoflagellate cysts are Cribroperidinium spp. and Systematophora areolata. Spores, fungal remains and foraminiferal linings are consistently present. The deposits with which the palynofacies is associated are mainly limestones that are marly at some levels. It dominates large parts of the Sulaiy Formation encountered in boreholes Dibdibba 1, Luhais 12 and Rumaila 167, and to a lesser extent in Rachi 2 and Ratawi 6. PF-3 In this palynofacies, the palynomorphs and phytoclasts are further diminished, by comparison with
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PF-1 and 2, to about 1–5% and 2–5% of the total organic recovery, respectively. Both of these components are also bacterially degraded, commonly to more or less an amorphous state. AOM reaches its maximum abundance in PF-3 where it typically comprises between 90 and 100% of the palynological matter. The palynofacies is associated with limestones that are clayey and/or pyrite-rich in places. It occurs throughout the Sulaiy Formation penetrated by the Jerashan 1 Borehole, and dominates much of the succession in Rachi 2 and Ratawi 6. It also prevails in the lower half of the formation in Luhais 12, but is confined to approximately the lower third and basal 40 m respectively of the sections in Dibdibba 1 and Rumaila 167. 5. Palaeoenvironmental interpretation It is well known that numerous variables are involved in the deposition of palynological matter (Traverse, 1994; Tyson, 1995; Batten 1996a). A partial analogue of the depositional conditions that led to the accumulation of the Sulaiy Formation may be off the coast of California where an anoxic layer is present at depth. This is a result of the combined effect of upwelling of fertile cold currents and high organic productivity. Dinoflagellate cysts, fungi and foraminiferal linings are abundantly preserved in gels of AOM (Cross et al., 1966; Melia, 1984; Oboh, 1992; Tyson, 1995). Many ancient successions that are rich in AOM (e.g., parts of the North Sea Jurassic: Riley et al., 1989; van der Zwan, 1990; Tyson, 1993) are known to be the end result of deposition in dysoxic-anoxic conditions during periods of sluggish circulation when conditions were suitable for the proliferation of a varied biota in the oxygenated part of the water column, particularly in the photic zone. Upwelling currents could have been generated from the southern coast of the Tethyan Ocean to the Mesopotamian Basin during the deposition of the Sulaiy Formation. At that time, there was a divergent plate boundary between the northern Gondwanan continent and the Tethys. The percentage records of palynological matter recorded from three of the boreholes discussed herein, which are representative of the succession as a whole, were summarised and plotted on a ternary diagram taken from Tyson (1993, fig. 5.2). Although established for Upper Jurassic marine shales, this is also applicable to the limestone-dominated Sulaiy Formation in the context of the development of anoxic conditions (Figure 3). All three palynofacies identified were encountered in samples from the Rumaila 167 Borehole. These may reflect changes from distal dysoxic-anoxic and dysoxic-oxic shelf to deeper-water
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Figure 3. Amorphous organic matter-phytoclastpalynomorph plot showing stylised diagnostic fields for Upper Jurassic marine shale palynofacies (slightly modified after Tyson, 1993, 1995) with the distribution of types of organic matter for three wells in southern Iraq indicated. These are associated with the following environments: VII and VIII, distal dysoxic-anoxic and dysoxic-oxic shelf; and XI, distal suboxic-anoxic basin.
distal suboxic-anoxic basin-environments (Figure 4). PF-1 and 2 were recorded less often southwards towards the Jerashan 1 Borehole, which only yielded PF-3 (Figure 2), suggesting persistent distal suboxic-
anoxic basinal conditions. The higher numbers of foraminiferal linings (up to 10%) and the abundance of AOM in these beds could possibly indicate the presence of upwelling currents during the accumulation of these sediments. Comparisons with possible analogues of the sedimentary environments in the Arabian Sea (Premuzic et al., 1982) and on the northwest African continental margin of the Atlantic Ocean (Diester-Haass, 1982) suggest that the accumulation of organic matter might have increased significantly in a dysoxic zone of water extending from depths ranging from approximately 300 to 500 m. There is minimal accumulation of organic matter in shallower, oxygenated, shelf waters today. A little below about 500 m anoxic conditions probably prevailed, leading to the preservation of even larger amounts of organic matter. It is concluded that the Sulaiy Formation comprises a succession of offshore marine-shelf deposits, although it is possible that the shelf may have been deep enough (more than 500 m) for scattered basinal facies of suboxic-anoxic character to accumulate. This conclusion is consistent with the high (c. 4:1) ratio of dinoflagellate cysts to spores and pollen grains (cf. Davey & Rogers, 1975; Melia, 1984) and the record of pyrite in the limestones (cf. Berner, 1984). 6. Source potential for hydrocarbons The predominantly dysoxic-anoxic and suboxicanoxic environments in which the sediments of the
Figure 4. Palynofacies map and block diagram for the Sulaiy Formation showing the location of the borehole sections in offshore, relatively deep-water shelf to basinal environments.
Palynofacies indications of depositional environments and hydrocarbons
Sulaiy Formation accumulated were conducive to the preservation of much organic matter, although most has been biodegraded. The total organic carbon (TOC) content is between 0.5 and 1% for the majority of the samples analysed. The means by which the degradation of the organic matter took place is partly implied by the common occurrence of fungal remains along with an inferred bacterial component. The activities of these organisms would have been responsible for the formation of abundant AOM, especially in PF-3. This material is similar in composition to oil-prone kerogen type A of Thompson & Dembicki (1986) and the mesoliptinites of Rahman & Kinghorn (1995; cf. discussion of terminology in Batten, 1996a). Rock Eval pyrolysis demonstrated kerogen type II and an oil extraction range of 200– 500 mg of hydrocarbons per gram of rock for the samples analysed. The organic matter is mature, the thermal alteration index (TAI) being 2.8 (Staplin, 1969; for comparisons with other scales, see Batten, 1996b), and hence capable of generating liquid hydrocarbons. This combination of favourable factors is the reason why the Sulaiy Formation is one of the main sources of oil in the Cretaceous reservoirs of southern Iraq and Kuwait. References Batten, D. J. 1996a. Palynofacies and palaeoenvironmental interpretation. In Palynology: principles and applications (eds Jansonius, J. & McGregor, D. C.), volume 3, pp. 1011–1064 (American Association of Stratigraphic Palynologists Foundation, Dallas). Batten, D. J. 1996b. Palynofacies and petroleum potential. In Palynology: principles and applications (eds Jansonius, J. & McGregor, D. C.), volume 3, pp. 1065–1084 (American Association of Stratigraphic Palynologists Foundation, Dallas). Beydoun, Z. R. 1991. Arabian plate hydrocarbon geology and potential—a plate tectonic approach. American Association of Petroleum Geologists, Studies in Geology 33, ix+77 pp. Bellen, R. C. van, Dunnington, H. V., Wetzel, R. & Morton, D. M. 1959. Lexique stratigraphique international. Volume III. Asie, Fasicule 10a, Iraq, 333 pp. (Centre National de la Recherche Scientifique, Paris). Berner, R. A. 1984. Sedimentary pyrite formation: an update. Geochimica et Cosmochimica Acta 48, 605–615. Buday, T. 1980. The regional geology of Iraq. Volume 1. Stratigraphy and palaeogeography, 445 pp. (GEOSERV, Baghdad). Cross, A. T., Thompson, G. G. & Zaitzeff, J. B. 1966. Source and distribution of palynomorphs in bottom sediments, southern part of Gulf of California. Marine Geology 4, 467–524. Davey, R. J. 1982. Dinocyst stratigraphy of the latest Jurassic to Early Cretaceous of the Haldager No. 1 borehole, Denmark. Danmarks Geologiske Undersøgelse, Serie B 6, 57 pp.
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