Abstract
Sedimentary facies, environment and diagenesis which control reservoir characteristics of the Lower Cretaceous Thamama Zone IVA in the Mubarras Field, offshore Abu Dhabi, United Arab Emirates, were studied by analyzing cores of about 380 feet (115.8m) from 8 wells.
Six different carbonate lithofacies have been identified in the Thamama Zone IVA. These lithofacies are (1) algal peloidal grainstone/packstone/wackestone, (2) peloidal grainstone, (3) intraclastic grainstone, (4) peloidal bioclastic packstone/wackestone, (5) bioclastic peloidal packstone and (6) orbitolinid wackestone/mudstone. Peloidal grainstones have the best reservoir characteristics (porositypermeability) among them. The Thamama Zone IVA is overlain by argillaceous bioclastic wackestone which acts as cap rock of the reservoir.
The depositional environment of the Thamama Zone IVA has been interpreted as being of lagoon to near-barrier in a shallow shelf condition. Grain-supported textures (grainstone and packstone) dominating in the northern part of the Mubarras Field suggest relatively high water energy such as near-barrier environment. On the other hand mud-supported textures (wackestone and mudstone) dominating in the southern part of the field suggest quiet water such as lagoonal environment.
The main diagenetic features in the Thamama Zone IVA are micritization, leaching, cementation, dolomitization and stylolitization. Cementation especially by ferroan calcite deteriorates reservoir characteristics. The occurrence of this cement is limited within and close to the argillaceous limestones. This phenomenon suggests that the argillaceous limestones play an important role for the cementation by ferroan calcite.
