Reservoir characterization of the Shuaiba Formation (Lower Cretaceous) Abu Dhabi, United Arab Emirates and Jebel Akhdar, Sultanate of Oman
The Lower Cretaceous (Aptian) Shuaiba Formation, one of the most productive reservoirs in the Middle East, was studied in six outcrops and four subsurface areas of the United Arab Emirates and the Sultanate of Oman to resolve small-scale vertical heterogeneity in lithofacies, biostratigraphic assemblages, and reservoir quality. Geological heterogeneity, such as texture and faunal assemblage, is uniquely associated with reservoir quality, whereby porosity and permeability vary inversely for rock types comprising two rudist families---caprotinids and caprinids. Caprotinid floatstones have 25-30% porosity and 10 mD permeability, whereas, caprinid floatstones-rudstones have 15-20% porosity and 100-1,000+ mD. Caprinid debris rudstones with permeability of Darcies+ resulted from burial compaction fracturing of a fragile framework of early marine fringing cements supporting vugs from dissolved aragonitic debris. Using cathodoluminescence petrography, the Shuaiba has undergone at least two phases of exposure to meteoric-derived pore fluids. One dissolution phase, likely an early burial effect of deeper driven meteoric-derived pore fluids, was pervasively neomorphosing and may have created the chalky textures in aragonite-rich matrix. Comparison of the diagenetic history between outcrop and subsurface reveals substantial diagenetic contrasts and shows that dissimilar but laterally linked paleohydrological regimes can overprint identical facies with very different reservoir characteristics. A proposed exploration play of regional, stratigraphically trapped hydrocarbon potential is identified in a transitional diagenetic environment where downward-flowing, meteoric-derived pore fluids encounter upward-moving formation fluids of marine origin.