Dynamics of cementation in response to oil charge: Evidence from a Cretaceous carbonate field, U.A.E.

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Oil charge is thought to inhibit the growth of cements within subsurface pore systems. We explore this phenomenon in a giant Cretaceous carbonate field from U.A.E., where the oil-filled crest porosity ranges from 10 to 50% and permeability from 0.08 to 830 mD but coeval water leg porosity is reduced to 10 to 23% and permeability to 0.1 to 4 mD. Only 5% of primary interparticle pores (> 30 μm diameter) in the crest are fully cemented, compared to 99% of pores in the water leg. Syntaxial calcite burial cements (> 10 μm diameter) in the oil leg show 12 cathodoluminescence zones with oil inclusions ( n = 27) occurring in four of the five final zones. Mean in-situ ion microprobe δ 18O VPDB data from the oil leg cements range from −1.2‰ in the oldest zone decreasing to −10.3‰ in zone 11, returning to −7.7‰ in the final zone. The oldest distinguishable cement zone in the water leg shows highly variable δ 18O from −3.6‰ to −9.3‰ with a mean of −7.3‰, and with subsequent zones decreasing to a mean value of −9.4‰ for the youngest cement zone. Decreasing δ 18O values are interpreted as indicating increasing temperature reflecting burial and the evolution of pore water composition: broadly similar trends in the oil and water legs suggest precipitation under the same general conditions. Unlike the oil leg cements, the final zone in the water leg occludes nearly all remaining pore space. The δ 18O VPDB of bulk micrite from the water leg shows an average of −7.4‰ ( n = 9) compared to −6.2‰ ( n = 10) from the oil leg, suggesting the precipitation of further micrite cement at greater burial depths. We infer that burial cementation slowed in the presence of oil due to a reduction of potential nucleation sites as well as porewater and solute movement within weakly oil-wet pores, whereas continued flow and solute movement through all pores including the micropores (< 10 μm diameter) enabled extensive cementation in the water leg.