When Carbonate Platforms Emerge: Complicating Reservoir Heterogeneity in Natih Fm, Cretaceous, Oman

By

Peter W. Homewood¹, Omar Al-Ja'Aidi², Philippe Razin³, Anne M. Schwab⁴

(1) Sultan Qaboos University, Al-Khod, Oman (2) Petroleum Development Oman, Muscat, Oman (3) Bordeaux University, Bordeaux, France (4) Aberdeen University, Aberdeen, United Kingdom

 The Natih formation comprises three major transgressive-regressive sequences (I-III). Higher frequency cycles, which may be correlated regionally over several hundreds of kilometers, are shown to provide best fits between stratigraphic subdivisions and reservoir flow units.

The Natih platform system emerged at the turnaround from sequence I to II, and at the top of the Natih. Complex heterogeneity developed from incisions within the upper 20 metres of the Natih E. Incision fills are made up of several genetic units, with a variety of dolomitised and carbonate facies composing smaller scale channel forms and infills or wider reaching marl-limestone cycles.

3D seismic from North Oman shows complex channeling of the surface of the Natih. Several hundred metres wide channels, fairly straight to slightly sinuous in pattern but with significant depths, contrast with narrow, highly sinuous channels showing meander loops and cutoffs. Natih E features, commonly ascribed to pull-down artifacts from Natih A channels, have been assessed by seismic forward modeling.

The data from field exposures alone are not sufficient to allow interpretation of channel patterns. However, the seismic is not sufficiently detailed to show the nature of the channel fills. The combination of seismic images with outcrop data, at the scale of facies and genetic units, allows reconstruction of the heterogeneity that affects the upper 20m of the Natih E. Understanding and predicting this heterogeneity will be crucial for the choice of drive mechanisms (water flood or GOGD) in future EOR field development planning.