Palaeogeographic Reconstruction of a Tide-Dominated Oolite Shoal Complex in the Lower Arab Formation, Onshore UAE

Abstract

AbstractThe Late Jurassic Arab Formation is being developed in a major gas accumulation located onshore southern UAE. The lower Arab Formation consists of wackestones to mudstones of mid ramp to basinal setting overlain by foreshoal and oolitic grainstone shoal deposits. Despite the number of well penetrations, several published models have been proposed for the palaeogeographic evolution of the shoal complex.The grainstone-dominated interval comprises skeletal and ooid-rich wackestones and packstones grading upwards into oolitic grainstones interpreted as the result of progradation of foreshoal to ooid shoal environments. A key element to understanding reservoir architecture has been the integration of core descriptions with borehole image logs, permitting recognition and re-orientation of the main bounding surfaces and cross-bedding sets. Existing depositional models for grainstone shoals are highly schematic and not useful for defining reservoir architecture.Two key surfaces constrain correlation and reservoir architecture: a basal hardground typified by dolomitised burrows and an upper erosion surface which terminates grainstone deposition. Shoal initiation was in the southwest of the study area with progradation of bioturbated foreshoal deposits with isolated planar cross-sets. Tidal reversals become more frequent upwards, with deposition mainly in offshore flood-oriented ooid sand ridges.The main part of the grainstone interval is dominated by a stacked parabolic sand shoal complex comprising cross-bedded oolitic and skeletal grainstones. Initial primary macroporosity in this facies association is pervasively occluded by calcite cements. Behind the shoal complex, planar laminated oolitic grainstones accumulated in flood-tidal deltas in a broad lagoon. Progressive infilling of the lagoon by sediment supplied from the shoal resulted in basinward (northeasterly) shifting of the shoreline and development of linear beach ridges. Bioturbated to massive grainstones capping shoal complexes are clean and only patchily cemented retaining a well-connected primary macropore network.Geometries, scales of cyclicity and facies distributions indicate analogs to modern tidally-influenced ooid ridges and parabolic bars of the Bahamian system.