Origin and Development of a High-K Dolomitized Unit in an Early Cretaceous Carbonate Reservoir, Offshore Abu Dhabi, United Arab Emirates: Significance of Depositional and Diagenetic Controls on Reservoir Quality
Yamamoto, Kazuyuki *1; AL Zinati, Osama 1; Ottinger, Gary 1; Edwards, H. Ewart 1
(1) ZADCO, Abu Dhabi, United Arab Emirates.
A high-K dolomitized reservoir unit is well developed across the studied giant oil field. It is 3.5 feet in average thickness, extends approximately 1,500 km2 and generally has higher permeability values than the surrounding limestones. It is an important drilling target for both current and future field development. Examination of 85 cored wells shows the intensity of dolomitization is laterally variable, and related to the grain content of the depositional facies. This indicates that higher grain content in the precursor limestones had generally better permeability which resulted in increased dolomitization pathways.
The dolomitized unit occurs in the lower part of a coarsening-upward 3rd-order highstand which formed in an unrestricted ramp setting. The dolomite rarely occurs in the late highstand where facies have the highest grain content. A possible reason is that the dolomitization initiated prior to deposition of the late highstand. Additionally, some baroque dolomite and oil inclusions in the dolomite crystals suggest later burial diagenesis. Thus, at least two phases of dolomitization have impacted the reservoir unit: 1) a relatively early and short-lived dolomite nucleation event followed by 2) crystal overgrowth and recrystallization under later burial conditions.
The dolomitization generally improves the permeability as the mud-sized particles are replaced by larger dolomite crystals; however, the porosity of the dolomitized unit is generally lower than the surrounding limestones. This implies that pore-filling dolomite cementation occurred during and/or after the replacement phase. Strong cementation of dolomite is locally observed, dramatically reducing both porosity and permeability. Thus, the permeability of the dolomitized unit can be enhanced when the intensity of dolomitization is neither too strong nor too weak.
The reservoir quality can also be improved by other diagenetic factors. Burial dissolution of micrite and partial leaching of dolomite crystals are frequently observed in the crestal part of the field. A possible scenario is that corrosive fluids associated with early oil, migrated initially into the crest. This was followed by a main phase of oil charge which minimized chemical compaction during burial diagenesis, and preserved the reservoir quality more in the crest than the flank.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain