The permeability of high-permeability zones in a oil reservoir can be reduced by using a gellable aqueous composition with delayed gelation. The composition comprises:a) water;b) a high-molecular weight natural or synthetic water-soluble polymer cross-linkable by means of Cr.sup.3+ ions;c) a water-soluble inorganic Cr.sup.3+ salt;d) a gelation-retardant organic ligand definable by the formula: ##STR1## where: R is a direct single bond, or a saturated or unsaturated linear or branched aliphatic hydrocarbon radical with from 1 to 6 carbon atoms, or a cycloalkyl radical with from 3 to 7 carbon atoms, or an aryl radical;X is OH or OR' (where R' is an alkyl radical with from 1 to 5 carbon atoms), or NR"R"' (where R" and R"', which may be equal or different, are H or an alkyl radical with from 1 to 5 carbon atoms); and relative water-soluble salts;the polymer b) being present in a concentration of between 1000 and 50,000 ppm; the salt c) being present in a concentration of between 10 and 5000 ppm evaluated as metal; the polymer/Cr.sup.3+ weight ratio varying from 1:1 to 1000:1; the molar organic ligand/Cr.sup.3+ ratio varying from 0.5 to 500:1; the composition having a pH equal to or less than about 7.0.