Hydraulic fracturing of a subterranean hydrocarbon reservoir is carried out using an aqueous wellbore fluid which is an aqueous solution of a surfactant which has the formula (R1—X)nZ, where R1 is an aliphatic group comprising a C10-C25 principal straight chain bonded at a terminal carbon atom thereof to X, and comprising at least one C1-C6 side chain. X is a charged head group, Z is a counterion, and n is an integer which ensures that the surfactant is charge neutral. The surfactant reversibly thickens the aqueous solution such that the wellbore fluid is a viscoelastic gel. The viscoelastic gel breaks and undergoes a reduction in viscosity within the reservoir and surfactant from the broken gel mixes with hydrocarbon in the reservoir.