High-resolution seismic stratigraphy of the Shu'aiba and Natih formations in the Sultanate of Oman: implications for Cretaceous epeiric carbonate platform systems

Abstract

Abstract The Shu'aiba and Natih Formation carbonates are important hydrocarbon reservoirs in the Sultanate of Oman. They consist of stacked deepening and shallowing-upward depositional cycles within an extensive middle Cretaceous epeiric carbonate platform. Geological models for these units traditionally assume a layer-cake stratigraphy and a high lateral continuity of facies. This is based on the assumption that epeiric platforms consist of ramps with low depositional gradients, broad facies belts and gradual facies transitions. However, high-resolution 3D seismic data covering large areas of the platform have revealed a more complicated internal stratigraphic architecture and have led to a new geometrical model for these epeiric platform systems. The transgressive part of the cycles is dominated by a low angle ramp depositional profile with localized development of algal-dominated mounds. Differential carbonate growth led to a topography of shallow carbonate shoals and intra-platform ‘basins’ with water depths reaching several tens to 100 m. During the regressive part of the cycles these basins were progressively filled-in by prograding carbonate rudist shoal complexes with depositional slopes of 0.5° to more than 30°. Clinoform belts on seismic show a wide range of progradational geometries ranging from closely spaced, laterally continuous ‘tramlines’ to irregular wedges and noses. The cycle tops are characterized by bedrock incisions and the influx of fine clastic sediments that fill in remnants of the intra-platform basins. The seismic images show that previous stratigraphic models for these carbonates oversimplified and flattened the stratigraphy. As a result the stratigraphic trapping potential and the internal reservoir heterogeneity for these systems has been underestimated.