Reservoir Quality Impact on Identifying Unayzah Prospective Areas in Abu Dhabi

Abstract

Reservoir quality in the Unayzah is largely controlled by depositional environment early clay diagenesis, burial- compaction and quartz cementation. Present day reservoir quality can be predicted by modeling the porosity evolution of simplified model rocks with representative depositional and early diagenetic attributes through their burial history. In Abu Dhabi Unayzah fluvial sands are predicted to have consistently low reservoir quality due to poorly-developed, early grain coats that have allowed abundant quartz cement to form. In the Unayzah C, open fractures observed in fluvial samples may provide useful high permeability streaks within these intervals. Fluvial sands are predicted to have higher porosities in areas with low thermal maturity, where quartz cementation would be less advanced. Unayzah sandy sabkha reservoir quality is predicted to be uniformly poor as a result of fine grain size, poor sorting, and early diagenetic clays that filled pores. Unayzah Aeolian sand quality is variable, depending on the type of clays in pores. A zone of infiltrated clay coats is observed in well H-1. this zone is characterized by clean, effective macroporosity, resulting in good permeabilities and low connate water saturations. Below this zone, all Unayzah aeolian sands contain diagenetic clays. Where finer grain size or poorer sorting results in small pores, the diagenetic clays bridge much of the porosity and significantly reduce effective macroporosity. However, in well-sorted, coarser-grained dune sands, pores are large enough to be lined but not filled with diagenetic clays. Reservoir quality prediction approach is based on conceptual model of sandstone porosity evolution, with burial derived from observation in multiple reservoirs. In this model, the main controls on the pore system evolution are mechanical compaction and cementation.