Double tapered surfactant waterflood oil recovery process

In a method of recovering petroleum from a subterranean, petroleum-containing formation, said formation being penetrated by at least one injection well and by at least one spaced-apart production well, both wells being in fluid communication with the formation, said formation containing water of known or determinable salinity greater than 20,000 parts per million total dissolved solids, comprising injecting an aqueous, saline surfactant-containing fluid into the formation, said fluid containing at least one primary anionic surfactant comprising an organic sulfonate and at least one solubilizing co-surfactant selected from the group consisting of alkylpolyethoxy sulfates, alkylarylpolyethoxy sulfates, alkypolyalkoxyalkylene sulfonates, alkylarylpolyalkoxyalkylene sulfonates, and mixtures thereof, and following said surfactant fluid with an aqueous fluid containing a viscosifying amount of a hydrophilic polymer, the salinity of the polymer-containing fluid being less than 50,000 parts per million total dissolved solids, wherein the improvement comprises

• decreasing both the salinity and concentration of solubilizing co-surfactant of the surfactant fluid from a value at the leading edge of the surfactant fluid at which the salinity is from 50 to 150 percent of the salinity of the water present in the subterranean formation, to a final value at the trailing edge of the surfactant fluid at which the salinity is from 75 to 300 percent of the salinity of the hydrophilic polymer-containing fluid, and maintaining the concentration of solubilizing co-surfactant at a value which is at least sufficient to render the petroleum sulfonate partially soluble in the fluid at the salinity of each portion of the surfactant fluid.