Abstract

A significant proportion of oil production from the Kimmeridgian Arab-D strata in the Ghawar field, Saudi Arabia originates from dolomitized rocks. Stratigraphic, petrographic, and geochemical data suggest 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 nondeposition, 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 isotope compositions (-1 to -2‰ compared to -6.5‰). The relatively positive oxygen isotope composition and the geochemical similarity of Zone 2B to the dolomites in Zone 1, which 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 to later fracturing that affected the entire Arab-D formation. This fracturing allowed higher-temperature fluids to leach the dolostones, thereby removing any remaining calcite and partially resetting the oxygen isotope composition of some of the dolomites. As a result of this later dolomitization event, rocks that were only partially dolomitized were leached, creating units with extremely high permeability and porosity (super-k intervals). Dolomites in the lower Zone 3 were recrystallized during burial by the normal geothermal gradient, leading to the present negative oxygen isotope values. Zone 1 dolomites are petrographically distinct from Zone 2 dolomite in that they are mimetic and fabric preserving, although they are geochemically similar. This mimetic style of dolomitization occurs immediately adjacent to the overlying anhydrite and is interpreted to have formed very shortly after deposition from hypersaline brines.

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