Comparing the efficacy of solid and magic-echo refocusing sequences: Applications to ¹H NMR echo spectroscopy of...

Quantitative evaluation of the solid and viscous components of unconventional shale rock, namely kerogen and bitumen, is important for understanding reservoir quality. Short transverse coherence times, due to strong ¹H-¹H dipolar interactions, motivates the application of solid state refocusing pulse sequences that allow for investigating components of the free-induction decay that are otherwise obscured by instrumental effects such as probe ringdown. This work reports on static, wide-line ¹H spectroscopy of shale rock and their extracted components, which include kerogen and bitumen, by the application of solid echo and magic echo pulse sequences. We characterize the efficiency of these cycles as a function of the radio frequency power and inter-pulse spacing. Magic echos are shown to provide superior refocusing in comparison to solid echo based experiments, as can be understood from the truncation of the Magnus expansion and ability to also refocus any I_z Hamiltonians (e.g. static field inhomogeneity). We characterize the optimal echo spacing and RF power for two shale samples of different maturity, motivating routine core and cuttings analysis and applications.