The invention provides a method for the secondary recovery of oil from an oil-bearing stratum having a zone of high fluid permeability, in which this zone is plugged by introducing into the stratum a gelable solution comprising an alkaline material, a polyphenolic vegetable material such as mimosa tannin extract and formaldehyde, the last compound being present as free formaldehyde, paraformaldehyde, a phenol-formaldehyde resole, hexamethylenetetramine or urea-formaldehyde concentrate. The gelable solution has a pH of at least 9.5 and may optionally include a viscosifier. The gelable solutions are relatively insensitive to brines or temperature and gel to produce mechanically strong gels, even in the presence of oil-wet minerals. If desired, the alkaline material can be an alkali metal carbonate, and the use of such carbonates permits the permeability of the plugged high permeability zone to be adjusted after formation of the gel by treating the gel with acid.