The Upper Araej Member: Stratigraphic Complexity and Reservoir Insights from Offshore Abu Dhabi

Abstract Extensive offshore exploration has targeted the Upper Araej Member (Middle Jurassic), representing the topmost part of the Araej Formation, and numerous hydrocarbon accumulations have been discovered and developed. This study is an integrated analysis of c.320 ft of cored wells within an offshore block, focusing on the depositional architecture and factors influencing reservoir quality to reveal the Upper Araej Member's potential. The Upper Araej Member, with an average thickness of c.300 ft in the studied wells, contains two main carbonate reservoir intervals. These are characterised by stacked beds of dominantly massive, locally dolomitised, skeletal and peloidal pack-grainstone or grainstone deposited in high-energy, proximal settings. These are organised into repeated, ft-scale coarsening-upward trends. Medium- to coarse-grained peloids are abundant throughout the succession, whereas coarse-grained ooids and coated grains are mostly present within units showing parallel or cross-bedding. The local development of laminated features indicates shoal complexes with a higher environmental energy compared to the intervals where grainy sediments do not exhibit lamination. The common hardgrounds and firmgrounds in conjunction with the local reddened nature of the sediments indicate episodic short exposure of the shoal complexes and consequent erosion/cementation and/or oxidation of the sediments. The two reservoir units are separated by an interval associated with a marked increase in gamma-ray values, interpreted as possible argillaceous and/or organic-rich, distal deposits acting as a barrier at the scale of the study area. The Upper Araej Member exhibits a broad range of porosity and permeability values. Grain-supported textures are notably associated with higher reservoir quality compared to mud-supported textures. Additionally, grain size emerges as a pivotal factor affecting permeability; textural analysis of selected thin sections reveals that peloid grain size dictates the size and connectivity of interparticle macropores. Specifically, a shift from fine- to coarse-grained peloids correlates with an increase in permeability for a given porosity level, due to the formation of larger pore throat radii and enhanced connectivity. Moreover, diagenetic processes like cementation and dolomitization play crucial roles in shaping reservoir characteristics. Cementation tends to decrease porosity and permeability by filling pore spaces, whereas dolomitization can improve these properties by creating secondary porosity and improving pore connectivity throughout the Upper Araej Member. This core- and log-based integrated work not only addresses key uncertainties regarding the Upper Araej Member in Abu Dhabi but also establishes a foundation for future offshore exploration and development strategies.