Intrashelf Basin Development in the Cenomanian/Turonian Carbonate Platforms of Iran and Oman (Sarvak and Natih Formations)

Van Buchem, Frans¹, Farid Taati², Philippe Razin³ (1) IFP, Paris, France (2) National Iranian Oil Company, Tehran, Iran (3) Bordeaux University, Bordeaux, France

The stratigraphic architecture of the shallow water carbonate platforms of Cenomanian/ Turonian age on the Arabian plate is characterised by an organisation at different scales of depositional sequences, and the presence of intrashelf basins within these widespread plat­forms. The results demonstrate the presence of four comparable 3rd order sequences in both sites, as well as the dominant role of differential sedimentation rate in the creation of the intrashelf basin topography. The studied site in Iran is located in the high Zagros (Kuh-e-Landareh), where, along a 10 km long and 300 meters high transect, two margins of an intrashelf basin are exposed. In combination with information of other outcrops in the area a depositional model is proposed and a regional map of the intrashelf basin is constructed. This intrashelf basin was formed on top of a regional platform, during the last 3rd order depositional sequence of late Cenomanian age. An overall backstep occurred to well into the Turonian times, followed by a tectonically forced progradation topped by a laterite soil (Laffan Formation). A comparable stratigraphic organisation has been observed in the Natih formation exposed in the foothills and Jabal Akhdar area in northern Oman. Here, intrashelf basin development took place twice, in the first and in the last Cenomanian 3rd order sequences. Margin geometries are less well exposed, but the geometries are beautifully illustrated in seismic lines. The similar timing and comparable mechanisms for the intrashelf basin creation observed in both locations, suggest that we are dealing here with a charac­teristic feature of cenomanian/turonian carbonate systems of, at least, the arabian plate. This model is immediately applicable to the petroleum reservoir in this stratigraphic interval, while the proposed mechanism may have an even wider application, in other time intervals and geographic locations.