Diagenesis of a limestone reservoir (Lower Cretaceous), Abu Dhabi, United Arab Emirates: Comparison between the anticline crest and flanks

Petrographic, stable-isotope and fluid-inclusion analyses were conducted on a Lower Cretaceous limestone reservoir, onshore Abu Dhabi, United Arab Emirates in order to compare the diagenetic processes and products in the oil zone (i.e. crest) versus water zone (i.e. flanks) of a giant oilfield anticline. The near-seafloor, shallow and intermediate burial (< 1 km) diagenetic processes across the anticline include micritization of allochems, mechanical compaction, cementation by calcite (rim, syntaxial overgrowths and equant spar) and rhombic dolomite, peloids dissolution, partial dolomitization, and incipient stylolitization. Diagenetic processes during tectonic compression of the foreland basin in Late Cretaceous and concomitant oil migration were mediated by basinal brines. These processes, which are more extensive in the flanks than the crest, include cementation by calcite, subordinate saddle dolomite, and minor dickite, fluorite, and sphalerite. Additional diagenetic processes subsequent to the main tectonic compression phase and oil migration and emplacement in the crest have occurred during deep burial (i.e. the flanks; present-day depth 2.5–3.5 km), and include extensive stylolitization and cementation by blocky calcite. These processes account for the poorer reservoir quality of limestones in the water zone in the flanks than the oil-saturated limestones in the crest. This paper demonstrates that variations in the role of diagenesis on distribution and evolution of reservoir quality across anticlinal structures of oilfields can be better understood in the light of: (i) the timing of generation, migration and emplacement/saturation of oil, and (ii) burial-tectonic evolution of the basin and related geochemical evolution of formation waters and flux of basinal fluids.