Abstract: Hydraulic cement slurry for use in cementation of oil wells containing hydraulic cement, 5-100% microsilica based on the weight of cement, 2-200% light weight aggregate with a real particle density between 0.1 and 1.5 g/cm.sup.3 based on the weight of the cement, 0-5% thinner (dry weight) based on the weight of the cement, 0-10% fluid loss additive (dry weight) based on the weight of the cement, and water in such an amount that the cement slurry has a density between 0.8 and 2.0 g/cm.sup.3.The cement slurry according to the present invention is gas tight and can thereby be used for cementation of oil well in fields having gas containing formations.