Stratigraphic Evolution and Reservoir Quality of the Tuwaiq Mountain and Hanifa Formations, Onshore and Offshore Abu Dhabi

Abstract

Abstract The stratigraphic evolution and reservoir quality development of the Upper Jurassic Tuwaiq Mountain and Hanifa formations (Sila Group) has been investigated through a regional core-based study covering both onshore and offshore Abu Dhabi. This follows recent renewed exploration interest that aims to fully assess the conventional potential of these two formations. Based on the core description of 13 wells across a distance of 280 km, the lateral and vertical stratigraphic evolution of the proximal settings of the Tuwaiq Mountain and Hanifa formations has been assessed at a regional scale. Each formation describes a large scale, 3rd-order shallowing-upward trend, respectively from muddy basinal/slope deposits to grainstone-dominated inner ramp sediments in the Tuwaiq Mountain Formation, and from mud- to packstone-dominated inner ramp deposits to grainstone-dominated fabrics in the Hanifa Formation. Large facies belts likely characterised the depositional environment of both formations, with an increasing predominance of proximal inner ramp settings over time due to the overall westward progradation of the carbonate system. Of interest is the preferential deposition of oolitic and/or skeletal grainstone deposits towards the edge of the rim where they form kilometre- to decakilometre-scale high-energy shoals broadly orientated N-S (parallel to the slope). Special emphasis has been placed on assessing the regional thickness variations of both the Tuwaiq Mountain and Hanifa formations, which are not yet fully understood. Several assumptions, based on core data and/or 3D seismic data, have been made to improve consistency and refine the picking of the boundary between the Tuwaiq Mountain and Hanifa formations. The description of 200 representative thin-sections, along with SEM and modal/textural analyses performed on selected samples, confirmed that the development of reservoir quality in both formations is primarily controlled by depositional parameters. The best reservoir potential is hosted within the high-energy grainstone deposits in which a vast amount of primary interparticle macropores formed at deposition. The abundance of latter macropores is a function of grain sorting and grain size, these two parameters being quite variable throughout the Tuwaiq Mountain and Hanifa formations. Subsequent post-depositional preservation of these primary macropores is explained by limited cementation coupled with successive pore-enhancing events (ie. dissolution events), which re-opened primary macropores and created secondary macropores, as well as enhanced microporosity. The uppermost parts of the Tuwaiq Mountain and Hanifa formations constitute the best reservoirs, as a result of the widespread deposition of grainstone in the study wells. By assessing the lateral and vertical stratigraphic evolution of these underexplored formations and identifying their reservoir potential, this study provides valuable insights that could help guide future exploration and production activities in the region.