Origin of Dolomite in the Arab-D Reservoir from the Ghawar Field, Saudi Arabia: Evidence from Petrographic and Geochemical Constraints

A significant proportion of oil production from the Kim- meridgian 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 firm- grounds. 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 Mg 21 from the overlying seawater. The dolomites in Zone 2B are geochemically dis- tinct compared to those in Zone 3 in that they have relatively positive oxygen isotope compositions ( 2 1t o22‰ compared to 26.5 ‰). The relatively positive oxygen isotope composition and the geochemical sim- ilarity 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 cemen- tation and dolomite formation made these units more susceptible to later fracturing that affected the entire Arab-D formation. This frac- turing allowed higher-temperature fluids to leach the dolostones, there- by 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 pre- sent negative oxygen isotope values. Zone 1 dolomites are petrograph- ically 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 anhy- drite and is interpreted to have formed very shortly after deposition from hypersaline brines. recovery, and their presence is inferred from a combination of production data and the diameter of the bore hole as measured using the caliper log. As a result of the importance of dolomite in the formation of the super-k intervals within the Arab-D, an understanding of the paragenesis of these zones is important not only in predicting super-k formation within the Arab-D but also as an analogue for understanding other dolomitized res- ervoirs with high permeability and porosity. Previous characterization work of dolostones from the Arab-D in the Ghawar field (Cantrell et al. 2004) has identified three types of dolomite with distinctive petrographic, stratigraphic, and geochemical characteristics: fabric-preserving, non-fabric-preserving, and baroque dolomite. Fabric-pre- serving dolomite is very finely crystalline in which petrographic details of the original limestone fabric are usually well preserved. Fabric-preserving dolomite contains low concentrations of Fe (average 247 ppm Fe) and relatively positive oxygen isotope values (average

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