ABSTRACT

The Natih Formation in Oman is an exceptionally well–exposed and studied example of a large (>1000 km [621 mi]) epeiric, shallow marine, tropical carbonate platform system with adjacent organic-rich intrashelf basins. It serves as an outcrop analog for many of the mid-Cretaceous giant oil reservoirs as well as for unconventional reservoirs in the Mesozoic of the Arabian plate. A key feature of the Natih Formation is its strict hierarchical stratigraphic organization at multiple orders of depositional sequences (third to fifth), which was first demonstrated in outcrop and subsequently adopted in the seismic and log data sets of the nearby subsurface. It has provided a high-resolution time framework for the robust sequence stratigraphic model that has been developed for an area of 6000 km2 (2317 mi2). Deposited in a calm tectonic setting, the Natih Formation provides an excellent example to demonstrate the principles of carbonate systems’ response to eustatic sea level change and some fundamental differences with siliciclastic systems. Key aspects that are addressed include the influence of the rate of sea level rise on the transition from ramp to platform, the creation of organic-rich intrashelf basins through differential accumulation rates, changing facies models within third-order sequences, and the importance of different platform top channel types. The economic relevance of this case study is the improved prediction of reservoir, seal, and source rock facies distribution and the associated early diagenetic overprint, insights that have been applied in the reservoir modeling of nearby oil fields as well as in exploration-scale studies.

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