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Dynamics of Calcite Cementation in Response to Oil Charge: Carbonate Reservoirs Retain Longest Cementation History


Oil charge is thought to inhibit the growth of cements within subsurface pore systems, and so equivalent non-hydrocarbon reservoir rocks at similar burial depths often show a greater volume of cement. This has been inferred to be due to a longer history of cementation. We explore this phenomenon in eight vertically stacked reservoirs from two giant Cretaceous carbonate oil fields in U.A.E. Here, the oil-filled crest porosity ranges from 10-50% and permeability up to 830 mD, but the aquifer (water-leg/flank) porosity is reduced to 10-23% and permeability to 0.1-4 mD. Although the paragenetic sequence of the major pore-occluding calcite macrocements in the oil and aquifers follows the same succession of fringing, syntaxial, equant (< 300 μm) and blocky (> 300 μm) calcite cements, both the volumetrics and timing of cement formation as revealed by in-situ ion microprobe d18O VPDB data for individual Cathode Luminescence (CL) zones and burial depths differ markedly. In the aquifers most of the effective porosity was occluded by a large volume of inter-granular equant cement, whereas oil charge reduced the volume of inter-granular equant cement in the oil reservoirs. A similar conclusion is also reached for the blocky cement where oil charge reduced the cementation rate in the oil reservoirs and preserved porosity, whereas in the aquifer blocky cements have led to the near complete occlusion of macroporosity. These observations are corroborated by in-situ d18O VPDB data where the mean oldest cement zone d18O value in the oil leg is −1.04 ± 0.16 ‰ decreasing to −11.48 ± 0.13 ‰in the second youngest zone, returning to – 7.54 ± 0.15 ‰in the final zone. In the aquifer the mean oldest cement value is 2.54 ± 0.11 ‰ and – 8.9 ± 0.07 ‰ in the youngest zone. Both legs reveal progressively more negative d18O compositions in younger cements, interpreted to be due to an increase in pore-fluid temperature, but the more positive values of the youngest zone in the aquifer demonstrates that pore occlusion occurred earlier, and at a more shallow burial depth. These data contradict previous conclusions that have suggested continued precipitation of cements within the aquifers after oil charge into the crest in carbonate reservoirs. This has notable consequences for reservoir model formulations.