Seismic stratigraphic study of two Oxfordian carbonate sequences, eastern Saudi Arabia

In this case study, seismic stratigraphic interpretation techniques were used to map facies changes and develop a depositional model for two Oxfordian carbonate sequences of eastern Saudi Arabia. Well data and regional geology indicate that the two sequences, R1 and R2, were deposited on a broad carbonate platform and undergo a lateral transition from grainstone to basinal mudstone within the study area. A seismic grid tied to well control was used to map facies transitions. Vertical resolution was a major factor in selecting acceptable lines from existing coverage. A high-resolution line, which tied wells on either side of the R1 and R2 transitions, was recorded and processed for stratigraphic objectives. Special seismic displays and ray trace modeling were applied as interpretive aids for this key line. The seismic interpretation procedure consisted of three steps: seismic sequence identification, seismic facies analysis, and Jurassic sea level correlation. Toplap and downlap reflection terminations defined sequence boundaries and indicated grainstone shoal progradation. Onlap terminations provided stratigraphic evidence for possible organic buildups localized on structural highs during the Oxfordian. Seismic facies descriptions involving amplitude and continuity were confined to the upper boundaries of the R1 and R2 sequences because of limited internal reflection detail. Facies maps based on these descriptions and reflection terminations established the lateral extent and transport direction of grainstones. A dip profile through the proposed regressive shoreline model shows prograd geometry that was accounted for in the seismic model and Jurassic sea level correlation. A basinward shift in coastal onlap at 147 Ma separates the two progradational cycles of the Rl and R2 sequences.