1ORIGIN OF DOLOMITE IN THE ARAB-D RESERVOIR FROM THE GHAWAR FIELD, SAUDI ARABIA: EVIDENCE FROM PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS

Abstract: A significant proportion of oil production from the Kimmeridgian aged Arab-D strata in the Ghawar field, Saudi Arabia originates from dolomitized rocks. Stratigraphic, petrographic, and geochemical data suggests that at least four episodes of dolomitization affected these sediments. The lower portion of the Arab-D, Zone 3, is only partially dolomitized, with the dolomite frequently being associated with firmgrounds. We propose that these dolomites were formed on an outer ramp setting with a maximum water depth of 50 m, during a period of non-deposition, with the dolomitization process being promoted by the oxidation of organic material and the diffusion of Mg2+ from the overlying seawater. The dolomites in Zone 2B are geochemically distinct compared to those in Zone 3 in that they have relatively positive oxygen isotopic compositions (-1 to-2 ‰ compared to-7 ‰). The relatively positive oxygen isotopic composition and the geochemical similarity of Zone 2B to the dolomites in Zone 1, that are intimately associated with the overlying evaporites, has led us to conclude that the Zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments. These hypersaline fluids bypassed Zone 2A by moving through the grain dominated sediments. Early cementation and dolomite formation made these units more susceptible t