Abstract

Dolomitization, as part of diagenesis of carbonate rocks, may or may not significantly enhance reservoir quality of the altered sediments. In this context, a diagenesis study was performed on Upper Cretaceous reservoir of an onshore oil field, Abu Dhabi, U.A.E. The paper elaborates facies related dolomitization and its selective influence on the reservoir quality enhancements. The framework of the study is based on detailed petrographic examination and description of 850 thin sections, isotopic compositions (C, O, Sr) of selected samples; and fluid inclusion microthermometry of dolomite formed in upper and middle reservoir units.

The study concluded that dolomitization of shoal grainstones and packstones resulted in coarse crystalline dolomite, which caused significant increase in reservoir porosity and permeability in the middle resercoir unit. Conversly, dolomitization of pretidal mudstones and wackestones resulted in microcrystalline dolomite, which has caused limted increase in the micro-porosity with little impact on permeability in the upper reservoir unit. Dolomite was formed during eo- and mesodiagenesis (Th = ca 65-90° C; salinity = 15-21 wt. % NaCl). Dolomitization of pretidal mudstones is suggested to have occurred in an evaporative setting, whereas dolomitization developed in packstones and grainstones of bioclastic shoal and subtidal sediments may have been driven by deepage reflux mechanisms. Consequently, dolomitization trend maps were produced for both middle and upper reservoir units. These dolomitization trend maps were than used as constrain to build the facies model to honor the diagenetic overprint. Blind test on integrated facies model revealed high predictiability in terms over reservoir quality distribution and production behavior in both upper and middle reservoir units. This in turn has added value to optimize field development stratergies with proper well placement into the reservoir.

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