A method for producing mineral oil from underground mineral oil deposits, in which an aqueous saline surfactant formulation comprising a surfactant mixture, for the purpose of lowering the interfacial tension between oil and water to < 0.1 mN/m at deposit temperature, is injected through at least one injection well into a mineral oil deposit and crude oil is withdrawn through at least one production well from the deposit, wherein a) the mineral oil deposit has a deposit temperature of 55°C to 150°C, a crude oil having more than 20° API and a deposit water having more than 100 ppm of divalent cations; and b) the surfactant mixture comprises at least one anionic surfactant of the general formula R 1 -O-(CH 2 C(R 2)HO) x -(CH 2 C(CH 3)HO) y -(CH 2 CH 2 O) z -CH 2 CO 2 M and at least one nonionic surfactant of the general formula R 1 -O-(CH 2 C(R 2)HO) x -(CH 2 C(CH 3)HO) y -(CH 2 CH 2 O) z -H, where a molar ratio of anionic surfactant to nonionic surfactant of 51:49 to 92:8 is present in the surfactant mixture on injection and the nonionic surfactant serves as starting material for the anionic surfactant, where R 1 is a primary linear or branched, saturated or unsaturated, aliphatic hydrocarbyl radical having 10 to 36 carbon atoms; and R 2 is a linear saturated aliphatic hydrocarbyl radical having 2 to 14 carbon atoms; and M is H, Na, K or NH 4; and x is a number from 0 to 10; and y is a number from 0 to 50; and z is a number from 1 to 35; where the sum total of x + y + z is a number from 3 to 80 and the x+y+z alkoxylate groups may be arranged in random distribution, in alternation or in blocks; and where the sum total of x + y is a number > 0 if R 1 is a primary linear, saturated or unsaturated, aliphatic hydrocarbyl radical having 10 to 36 carbon atoms; and c) the concentration of all the surfactants together is 0.05% to 0.49% by weight, based on the total amount of the aqueous saline surfactant formulation. a) the mineral oil deposit has a deposit temperature of 55°C to 150°C, a crude oil having more than 20° API and a deposit water having more than 100 ppm of divalent cations; and b) the surfactant mixture comprises at least one anionic surfactant of the general formula R 1 -O-(CH 2 C(R 2)HO) x -(CH 2 C(CH 3)HO) y -(CH 2 CH 2 O) z -CH 2 CO 2 M and at least one nonionic surfactant of the general formula R 1 -O-(CH 2 C(R 2)HO) x -(CH 2 C(CH 3)HO) y -(CH 2 CH 2 O) z -H, where a molar ratio of anionic surfactant to nonionic surfactant of 51:49 to 92:8 is present in the surfactant mixture on injection and the nonionic surfactant serves as starting material for the anionic surfactant, where R 1 is a primary linear or branched, saturated or unsaturated, aliphatic hydrocarbyl radical having 10 to 36 carbon atoms; and R 2 is a linear saturated aliphatic hydrocarbyl radical having 2 to 14 carbon atoms; and M is H, Na, K or NH 4; and x is a number from 0 to 10; and y is a number from 0 to 50; and z is a number from 1 to 35; where the sum total of x + y + z is a number from 3 to 80 and the x+y+z alkoxylate groups may be arranged in random distribution, in alternation or in blocks; and where the sum total of x + y is a number > 0 if R 1 is a primary linear, saturated or unsaturated, aliphatic hydrocarbyl radical having 10 to 36 carbon atoms; and R 1 is a primary linear or branched, saturated or unsaturated, aliphatic hydrocarbyl radical having 10 to 36 carbon atoms; and R 2 is a linear saturated aliphatic hydrocarbyl radical having 2 to 14 carbon atoms; and M is H, Na, K or NH 4; and x is a number from 0 to 10; and y is a number from 0 to 50; and z is a number from 1 to 35; c) the concentration of all the surfactants together is 0.05% to 0.49% by weight, based on the total amount of the aqueous saline surfactant formulation.