Anticline crest and flank of an upper jurassic carbonate reservoir, Abu Dhabi, United Arab Emirates

This petrographic, stable isotopic and fluid inclusion microthermometric study revealed that the diagenetic and related reservoir quality evolution of the Upper Jurassic limestones across the studied field onshore Abu Dhabi, UAE, occurred during three phases, including: (i) near-surface to burial diagenesis, during which reservoir quality across the field was either deteriorated or preserved by calcite cementation presumably derived from marine or evolved marine pore waters. Improvement of reservoir quality was due to the formation of micropores by micritization of ooids and skeletal fragments and moldic/intragranular pores by subsequent dissolution of the peloids. (ii) Obduction of Oman ophiolites and formation of the anticline, which included gas migration and cementation of the crest and flank limestones by saddle dolomite and coarse calcite spar. High homogenization temperatures and salinity of the fluid inclusions, negative δ18OVPDB values, saddle shape of dolomite, and the presence of exotic cements suggest that these carbonates were formed by flux of hot basinal brines, possibly related to the tectonic compression event. (iii) Rapid subsidence subsequent to the obduction, which resulted in extensive stylolitization and cementation by calcite. This calcite cement occluded most of the remaining pores of the flank limestones (water zone). The results suggest that gas emplacement: (i) halts deep-burial diagenesis but less diagenesis related to flux of hot basinal brines, and (ii) accounts for the better carbonate reservoir quality in the crest than flanks of the anticline. These findings, which have important implications for the fluid flow and hydrocarbon migration, relates for first time diagenesis and reservoir quality evolution to the burial and tectonic events of foreland basins in the Arabian Gulf area.