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AAPG Bulletin, V.
Stratigraphic organization of carbonate
ramps and organic-rich intrashelf basins: Natih Formation (middle
Cretaceous) of northern Oman
1Institut FranÁais du PÈtrole
(IFP), Geology and Geochemistry Division, BP 311, 92.506 Rueil-Malmaison
Cedex, France; email: [email protected]
2Bureau de Recherche GÈologique et MiniËre (BRGM), OrlÈans, France; current address: Centre EGID, University of Bordeaux III, 1 allÈe Daguin, 33.607 Pessac cedex, France; email: [email protected]
3TotalFinaElf, Pau, France; current address: Center for Carbonate Studies, Sultan Qaboos University, PO Box 36, Post code 123 Al-Khod, Sultanate of Oman; email: [email protected]
4Petroleum Development Oman (PDO), Muscat, Oman; current address: Preussag Energie GmbH, Waldstrasse 39, D-49808 Lingen (Ems), Germany; email: [email protected]
5UniversitÈ de ProvenceñCentre St. Charles (Aix-Marseille I), Centre de SÈdimentologie-PalÈontologie, Place Victor Hugo, 13.331 Marseille Cedex 03, France; email: [email protected]
Frans van Buchem is senior research scientist at the Geology-Geochemistry Division of the Institut FranÁais du PÈtrole (IFP). Since 1990 he has studied the sedimentology, (organic) geochemistry, and sequence stratigraphy of carbonate systems and source rocks through time in outcrop analogs and the subsurface of the petroleum provinces of western Canada, the western United States, northern Africa, and the Middle East. He received a Ph.D. in geology (1990) from the University of Cambridge, United Kingdom.
Philippe Razin received his Ph.D. in sedimentology and tectonics in 1989 from the University of Bordeaux, France. He joined the French Geological Survey (BRGM) in 1990 where he was involved in basin analysis projects in the Peritethys area (France, Morocco, Arabian Peninsula). He is currently an assistant professor at the University of Bordeaux continuing his research on platform-basin relationships in various tectonic settings.
Peter Homewood is professor of carbonate geology and director of the Shell-endowed Carbonate Research Center at Sultan Qaboos University, Oman. He was previously senior advisor for sedimentology at Elf EP and then TotalFinaElf. Previously, he taught at universities in Switzerland and in France. He served as editor of Sedimentology (1986ñ1990), International Association of Sedimentologists Publications secretary (1990ñ1994), and AAPG European Distinguished Lecturer (1998ñ1999), and he received the Elf Science Prize (1995) and the TotalFinaElf communications award (2000).
Heiko Oterdoom currently is an exploration geologist for Preussag Energie. From 1981 to 1999 he worked as up-front exploration geologist for Shell International E & P, in Holland, Morocco, Thailand, Norway, and Oman. From 2000 to 2001 he worked as a multiclient seismic survey generator for Fugro Geoteam AS in Oslo. He holds a Ph.D. in petrography from the Eidgenssische Technische Hochschule, Z¸rich. He is a licensed guide of the Dutch tidal flats.
Jean M. Philip is a professor of geology at the UniversitÈ de Provence in Marseille, France. He received his Doctoral of Sciences degree in 1970. His main domains of research concern the Upper Cretaceous carbonate platforms of the Tethyan area, including stratigraphic, paleontologic (rudists), and sedimentological aspects. He has collaborated with geologists of petroleum companies (Total, Elf, Institut FranÁais du PÈtrole, and Petroleum Development Oman) and the Bureau de Recherches GÈologiques et MiniËres (BRGM) for field investigations in Tunisia, Algeria, Italy, Oman, and Saudi Arabia.
The data acquisition for this study has been a group effort involving numerous colleagues; thus, we would like to thank T. Boisseau, S. Cantaloube, B. Doligez, G. Dromart, G. Eberli, R. Eschard, J. P. Leduc, P. Masse, C. Muller, J. P. Platel, B. Pradier, J. Roger, A. Schwab, and F. Walgenwitz. We also benefited from discussions with and support from Petroleum Development Oman (PDO) geologists H. Droste, N. Kharusi, M. Partington, and M. van Steenwinkel. Both PDO and the Ministry of Oil and Gas of the Sultanate of Oman kindly provided access to subsurface data. Comments by AAPG Bulletin reviewers T. Burchette, R. Scolaro, and an unknown reviewer helped to improve the article. We are indebted for draft work to Y. Calot, N. Doizelet, and A. Nakou and for logistical support to BRGM Oman and ELF Petroleum Oman. This project was financed by the Fonds de Soutien des Hydrocarbures (FSH), ELF EP, IFP, and BRGM. Publication of this article is by kind permission of BRGM (Recherche), IFP, PDO, TotalFinaElf, and the Ministry of Oil and Gas of the Sultanate of Oman.
The middle Cretaceous carbonate deposits in the Middle East are among the most productive oil-bearing stratigraphic intervals in the world, containing numerous giant fields in, for instance, the United Arab Emirates (Mauddud and Mishrif formations), Iran (Sarvak Formation), and Oman (Natih Formation). One of the main reasons for this concentration of hydrocarbons is a geological factor: the coexistence of both reservoir facies and source rocks in the same depositional sequences due to the repeated creation of organic-rich intrashelf basins. This is demonstrated in a high-resolution sequence stratigraphic study of the Natih Formation in Oman, which shows distinct and predictive patterns in the distribution and geometries of reservoir, source rock, and seal facies. The sequence stratigraphic model presented here may serve as a reference for time-equivalent deposits in the Middle East.
The sedimentological analysis showed that the Natih Formation was formed by the alternation of two types of depositional systems: (1) a flat-bedded, mixed carbonate-clay ramp, dominated by benthic foraminifera, and (2) a carbonate-dominated ramp bordering an intrashelf basin, with abundant rudists in the mid-ramp environment and organic-rich basinal facies.
Three fully developed third-order sequences are distinguished, showing a similar evolution of the depositional system, with a mixed carbonate-clay ramp system at the base, followed by a carbonate-dominated ramp system in the upper part. Variations occur on this pattern, however, depending on the relative influence of eustasy, environmental factors, and tectonism. The late Albianñearly Cenomanian sequence I shows an evolution from a mixed, flat ramp to a carbonate-dominated ramp and organic-rich intrashelf (Begin page 22) basin, and sedimentation is predominantly controlled by eustatic sea level. In the middle Cenomanian sequence II, the evolution from a mixed ramp to a carbonate ramp is also observed, but no intrashelf basin topography was developed in the studied area. This may be due to the high influx of clay that influenced the environment in this sequence, inhibiting the carbonate production, probably in combination with the lack of sufficient creation of accommodation space. The late Cenomanianñearly Turonian transgressive part of sequence III shows a similar evolution to that observed in sequence I, with the development of an organic-rich intrashelf basin. During highstand, however, a tectonically controlled sedimentation pattern is observed, with the development of forced regressive wedges (due to the flexural bulge of the foreland basin).
Intrashelf basin formation occurred twice in the transgressive part of the third-order depositional sequences of the Natih Formation. Our study shows that this is mainly the result of differential sedimentation rates, that is, the dynamics of the carbonate sedimentary system itself in response to (rapid) rises in relative sea level, probably of eustatic origin. Tectonism was only a minor factor in the creation of the basin topography, possibly through the creation of small initial relief. The accumulation of the organic matter is not only a result of the creation of a sufficiently deep-water column to guarantee dysaerobic conditions for its preservation. The late Albian and late Cenomanianñearly Turonian were also periods of generally favorable conditions worldwide for high organic matter productivity.
The time lines and stratigraphic architecture of the third-order sequences presented here have an application potential at the scale of the Arabian plate. The general sedimentation pattern is predicted by our model, but modifications due to different local conditions are likely to occur.
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