The Middle-Triassic Gulailah Formation, also known as the Jilh Formation, was poorly understood in Abu Dhabi due to lack of data integration. A recent study involving mapping of the internal architecture of the formation on the basis of regional seismic data and all available wells across Abu Dhabi shows the evolution of an intra-shelf basin during deposition of the Gulailah Formation which was overlooked during previous studies.
Interpretation of regional mud logs and updated isopach maps show a deepening trend from a shallow shelf in the northwestern offshore Abu Dhabi to a depo-center in the southern onshore. The isopach contours imply an increase in water-depth toward the basin center and mark the outline of the Gulailah intra-shelf basin as a restricted depression on the Middle-Triassic to Middle-Jurassic shallow-water carbonate shelf. The basin margin is characterized by a shallow-water shelf break, where the sudden change in slope leads to a higher thickness gradient. The regional variation of seismic reflection patterns from prograding and thickening events around the shelf-break to parallel reflections at the depo-center characterize the general geometry of the Gulailah intra-shelf basin from basin margin to basin center. Detailed well correlations based on GR cycles corresponding to 3rd-/4th-order sequences show a layer-cake model at the bottom followed by a prograding sequence towards the top of the Gulailah, representing the evolution from a gentle ramp to an intra-shelf basin.
In the Lower Gulailah, laminated algae-related dolo-mudstone with desiccation cracks and intercalated anhydrite layers indicate a low-energy tidal flat environment in the northwest. The increase of sub-tidal lime-mudstone to the east and to the south indicates a deepening of the carbonate ramp system in those directions. Influenced by the Late-Triassic uplift of the Qatar Arch, the Upper Gulailah subsequently developed into an intra-shelf basin with a more confined accommodation space, where carbonate sediments gradually filled-up the basin as low-angle prograding ramps from the northwestern offshore to the southern onshore. At the base of the Upper Gulailah, low-GR grainy facies extend from the basin margin to the basin center, indicating a shoal-related depositional setting. Above this interval, prograding ramps are dominated by muddy facies, indicating a more restricted environment. Micro-conglomerates and breccia intercalated with thin layers of grainy shoal sediments reflect a peritidal dominated environment between the shelf-break and the basin center. Tight limestones with thin beds of shale representing a deep sub-tidal environment were deposited in the basin center.
Different from the well-known reef-originated Bab basin, the Gulailah intra-shelf basin is a consequence of tectonic activities. High-relief reefs are not developed due to highly-frequent clastic influxes and high salinity. The basin is characterized by low-angle prograding ramps, thickening towards the basin center, and gradually filling-up the shallow basin during a period of relatively stable sea-level.