Dolomitization of the Eocene Rus Formation, Dammam Dome, Eastern Saudi Arabia – An Example of Mixed-Water/reflux Model of Dolomitization in a Mudflat/Lacustrine Environment

The Dammam Dome, the structure which marks the first oil discovery of the Kingdom of Saudi Arabia, comprised a succession of dominantly carbonate rocks ranging in age from Paleocene to Middle Eocene. The Eocene age Rus Formation, which comprised an approximately 56m thick sequence of alternating evaporite and carbonate rocks is the oldest formation exposed at the dome. A detailed petrographic analysis involving thin-section petrography, XRD, SEM, and EDX-based ionic mapping of the carbonate intervals recognized two distinct generations of dolomite with contrasting color and size differences. The group representing smaller size dolomite crystals comprised well-developed rhombohedra ranging in size from 5 mµ to 15 mµ. The coarser crystals, which comprised rhombohedra ranging in size from 50µm to 80 µm, often show a core of smaller rhombohedra with contrasting color. Based on the nature and distribution of the dolomite, an abundance of directly precipitated polygorskite clays (indicated by the preservation of interwoven fibrous mats), in the Rus and younger formations in the area including Dammam and Hofuf formations, and overall depositional environment of the region during the closing phase of the Neo-Tethys, a mudflat to lacustrine depositional environment is interpreted for the deposition of these formations. Such a setting is likely to favor a mixed-water and/or brine reflux model for the dolomitization of the Rus carbonates