Seismic Geomorphology of Cretaceous Epeiric Platform Carbonates in Oman

Henk Droste
Shell, Mina Al Fahal, Oman

Epeiric carbonate platforms are often visualized as carbonate ramps with low depositional gradients, broad facies belts and gradual facies transitions. As a result, reservoir geological models usually assume flat time lines and a high lateral continuity of facies.

Recent studies, using high-resolution 3-D seismic data covering large areas of the Cretaceous platform interior carbonates of Oman, show a complex internal architecture with bedrock incisions and abundant inclined and mounded stratal geometries. Clinoform belts range from closely-spaced, laterally continuous ‘tramlines' to broad and diverging. In places they form irregular wedges and noses. Clinoform angles, with dips of 0.5 to more than 30 degrees, are much steeper than suggested in conventional correlations based on well data only. These different morphologies are controlled by relative sea level changes, palaeo-current and wind direction, carbonate grain size and influx of siliciclastics.

Individual lithostratigraphic units on the platform, such as the Aptian Shu'aiba Formation or the Albian to Cenomanian Natih-e and a/b members appear to consist of a number of merged intra-shelf platforms that developed from scattered (probably structurally controlled), small carbonate buildups that started to grow following major flooding events in the platform interior.

The observations from seismic images show that the previous stratigraphic models for these carbonates, based on the correlation of 1D well and core data, oversimplify and flatten the stratigraphy, even between closely-spaced wells, as individual markers/log picks tend to be pushed through as far as possible. As a result the stratigraphic trapping potential as well as the internal reservoir heterogeneity for these systems tends to be underestimated.