Nuclear magnetic resonance (NMR) well logging tools are being commercially utilized for the characterization of various fluids confined within reservoir rocks. Conventional NMR T-2-distribution measurements are unable to quantify the actual fractions of crude oil and brine simultaneously present in the reservoir rocks. Herein, we reported NMR-responsive paramagnetic [M-EDTA] (M = Fe3+, Mn2+, Cu2+) complexes having the capability to differentiate NMR T-2-distribution signals coming from crude oil and brine by accelerating the water (H-1) relaxation. The formation of these [Fe-EDTA](-), [MnEDTA](2-), and [Cu-EDTA](2-) complexes was explored by UV-visible spectroscopy, and their chemical stability in brine solution was monitored by measuring the change in percent transmittance and backscattering intensities with time. Spin spin (T-2) relaxation signals and T-2-distribution spectra of various carbonate and sand packs were recorded. The separation of NMR T-2-distribution signals was achieved using the optimized concentration of synthesized paramagnetic complexes. The separation of NMR T-2-distribution signals with the optimum concentration of complexes and their long-term chemical stability in a brine solution suggest them to be commercially reliable contrast agents for the characterization of various rock fluids confined in oil reservoirs.