ABSTRACT

The First Eocene reservoir at the Wafra Field produces heavy oil from very porous dolomites at depths of 1000  to  1300  ft (300 to 400 m) in the Paleocene–Eocene Umm Er Radhuma Formation. Porosity is commonly 30–50%, permeability is commonly 100–2000 md, and those reservoir characteristics were determined largely by diagenesis. Early diagenesis is dominated by dolomitization, dissolution associated with dolomitization, and precipitation of sulfates. Petrographic and stable isotopic characteristics support dolomitization and sulfate precipitation in evaporated (refluxing) seawater during shallow burial. The highest permeabilities occur in subtidal facies. Low-permeability tidal-flat facies stratify the reservoir. Heavy oil preferentially filled high-permeability dolomites; whereas, low-permeability tidal-flat facies are commonly filled with water because their pore throats are too small to allow migration of viscous oil into the rock. This reservoir’s very high porosity is probably related to its shallow burial and early oil emplacement.

Late-stage diagenesis is dominated by bacterial sulfate reduction (BSR) that caused dissolution of sulfate nodules, calcite cementation, sulfur precipitation, and oil biodegradation. The BSR is indicated by very low δC13 compositions of calcite cements (17.1 to 34.9, Peedee Belemnite standard), which require an organic carbon source; probably oil. The oxygen isotopic compositions of the calcites support precipitation from formation waters similar to those in the reservoir now. The BSR probably started during initial oil emplacement and continues to the present. The BSR was heterogeneous resulting in produced oils with gravities of 14–21° API. Even heavier oils are present that could not flow during primary production. Primary production was likely greatest in areas and intervals with lighter, less viscous oil.

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