The Mid-Cretaceous Natih Formation in northern Oman: a model for platform-intrashelf basin depositional systems and associated petroleum habitat

Abstract

The Late Albian – Early Turonian Natih Formation in Oman and its stratigraphic equivalents in the Middle-East correspond to a wide carbonate platform that developed on the Arabian plate along the passive margin of the Neotethys Ocean. It is composed of rudist-rich shallow marine deposits grading laterally to organic-rich deposits accumulated in intrashelf basins of relatively limited water depth. Consequently, this formation corresponds to a prolific petroleum system isolated below and above by effective regional seals. This formation, largely exploited in subsurface in the Middle-East, is spectacularly exposed in the Oman mountains thanks to the tectonic deformation of the Neotethys palaeomargin. These outcrops provide an excellent data set to construct a high-resolution sequence stratigraphic model for this type of carbonate system. The stratal architecture and the facies distribution clearly reveal how the platform evolution can be related to the ratio between accommodation rate and carbonate production during cycles at different scales. The cyclic evolution of this ratio is responsible for the gradual transition from a very flat platform with minor lateral facies and thickness variations to a low angle depositional profile from platform to intrashelf basin illustrated by clinoform geometries and pronounced lateral facies changes. Tectonic deformations have minor if no effect on the development of these intrashelf basins. Numerous additional interpretative key points will be addressed during this field trip: the location of shale deposits restricted to very shallow marine environment and implications on the inner platform sequence interpretation, the strong asymmetry of these transgressive sequences related to volumetric partitioning, the lack of pronounced rudist build-ups, the heterogeneities associated to subaerial exposure surfaces; and the hierarchy and fractal character of depositional sequences. This depositional model of the Natih Formation can be useful for the prediction of reservoir and source rock distribution and heterogeneity in this type of carbonate system.