Surfactant–polymer (SP) flooding in carbonate reservoirs under high-temperature, high-salinity (HTHS) conditions is governed not only by interfacial and rheological properties but also by carbonate geochemical re-equilibration. This paper provides an integrated experimental analysis of chemical propagation, dynamic retention, and effluent-ion evolution to clarify the geochemical mechanisms that influence surfactant and polymer transport under harsh conditions. To isolate carbonate–brine–chemical interactions, three sequential single-phase, oil-free corefloods were conducted on Indiana limestone at 90 °C. Each coreflood progressed through two cycles, including preflush, chemical-slug, and postflush stages, first in seawater (SW) cycle and then in ten-times diluted seawater (10dSW) cycle. Two of these corefloods used the zwitterionic surfactant oleyl polyoxyethylene amidopropyl carboxybetaine (OPAC) alone at 0.5 and 0.1 wt%, while the third coreflood was a surfactant–polymer (SP) case, 0.5 wt% OPAC with a 2000 ppm acrylamide-tertiary-butyl sulfonate (ATBS)-based polymer. Bulk rheology showed that 2000 ppm polymer delivered practical viscosity in both brines, while adding 0.5 wt% OPAC caused only a slight viscosity penalty (∼5% on average). In corefloods, surfactant-only solutions behaved essentially as brine, whereas the SP bank provided strong mobility control, with resistance factors of 3.46 in SW and 11.14 in 10dSW and low residual resistance factors of 1.36 and 1.29, respectively. Lowering salinity from SW to 10dSW reduced surfactant retention from 0.284 to 0.198 mg/g-rock at 0.5 wt% OPAC and from 0.191 to 0.084 mg/g-rock at 0.1 wt% OPAC. At 0.5 wt% OPAC, polymer coinjection further reduced surfactant retention to 0.202 mg/g-rock in SW and 0.086 mg/g-rock in 10dSW, while polymer retention remained low at 0.063 and 0.044 mg/g-rock, respectively. To understand the origin of these salinity- and polymer-dependent retention trends, the effluent inorganic chemistry was examined throughout the flooding sequence. During seawater-salinity surfactant flooding, pronounced deviations of effluent calcium (Ca) and magnesium (Mg) from the injected brine concentrations may indicate active redistribution of divalent ions among the flowing brine, carbonate surface complexes, and surfactant-associated interfacial species. Additionally, upon switching from SW to 10dSW, small surfactant and polymer remobilization peaks coincided with sulfate depletion, suggesting that salinity reduction partly released weakly retained chemicals from the preceding SW cycle while reopening positively charged calcite sites for renewed sulfate uptake during low-salinity re-equilibration. Furthermore, upon arrival of the 10dSW SP bank, a distinct Ca–Mg pulse revealed rapid divalent-ion complexation by the injected chemicals, compensating for release from carbonate surface complexes, and possible mild calcite dissolution. These results suggest that an SW-to-10dSW sequence may be mechanistically attractive for harsh-condition carbonate SP flooding, with the first cycle helping establish favorable interfacial conditioning and the diluted second cycle supporting lower chemical loss and stronger propagation of the SP bank.