Reservoir Quality of Mixed Carbonate/Evaporite Sucessions of the Upper Jurassic Arab Formation, Offshore Abu Dhabi
Carbonates and evaporites of the Upper Jurassic were deposited on a large carbonate platform spanning the Arabian plate. Two shallow intra-platform basins, which were in part dominated by carbonate and evaporate deposition, were located in the area of eastern Saudi Arabia and the United Arab Emirates. Some of the world’s giant oil fields are located along these basins and produce from these mixed carbonate/evaporite reservoirs.
The Upper Jurassic carbonate and evaporite succession in Abu Dhabi is subdivided into four reservoir intervals. The lowest reservoir is dominated by shoaling-upward sequences and does not contain any evaporates in the studied wells. The other three reservoir intervals are dominated by carbonates and evaporate cycles and sequences. The amount of evaporates in the succession, either as cement and nodules plugging the pore space or as individual layers, increases upward in the succession. Within individual shallowing-upward carbonate/evaporite cycles, the anhydrite is interpreted to have formed in a sabkha environment. Continuous and lateral extensive anhydrite layers separating individual reservoir units are interpreted to have formed in restricted sub-aqueous lagoons. The succession is capped by the thick anhydrites, which forms the top seal for the Upper Jurassic reservoirs. Most of the carbonate succession is dolomitized; however, limestone layers with poor reservoir quality exist in the lower part of one reservoir interval where no anhydrite is present. The highest reservoir quality is characterized by dolomitized grainstones with vuggy porosity and/or euhedral dolomites with high intercrystalline porosity. The reservoir quality decreases with increasing amount of anhydrite cementation and replacement in the dolomites. Therefore, understanding the distribution of anhydrites within the reservoir section as well as cementation and replacement processes of the dolomites by anhydrite is critical for predicting reservoir quality in these mixed carbonate-evaporite reservoirs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009