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The mechanism behind the NMR surface relaxation and the large $T_1$/$T_2$ratio of light hydrocarbons confined in the nano-pores of kerogen remainspoorly understood, and consequently has engendered much debate. Towardsbringing a molecular-scale resolution to this problem, we present moleculardynamics (MD) simulations of $^1$H NMR relaxation and diffusion of heptane in apolymer matrix, where the high-viscosity polymer is a model for kerogen andbitumen that provides an organic "surface" for heptane. We calculate theautocorrelation function $G(t)$ for $^1$H-$^1$H dipole-dipole interactions ofheptane in the polymer matrix and use this to generate the NMR frequency($f_0$) dependence of $T_1$ and $T_2$ relaxation times as a function of$\phi_{C7}$. We find that increasing molecular confinement increases thecorrelation time of the heptane molecule, which decreases the surfacerelaxation times for heptane in the polymer matrix. For weak confinement($\phi_{C7} > 50$ vol%), we find that $T_{1S}/T_{2S} \simeq 1$. Under strongconfinement ($\phi_{C7} \lesssim $ 50 vol%), we find that the ratio$T_{1S}/T_{2S} \gtrsim 4$ increases with decreasing $\phi_{C7}$, and that thedispersion relation $T_{1S} \propto f_0$ is consistent with previously reportedmeasurements of polymers and bitumen. Such frequency dependence in bitumen hasbeen previously attributed to paramagnetism, but our studies suggests that$^1$H-$^1$H dipole-dipole interactions enhanced by organic nano-poreconfinement dominates the NMR response in saturated organic-rich shales,without the need to invoke paramagnetism.

Molecular Dynamics Simulations of NMR Relaxation and Diffusion of Heptane Confined in a Polymer Matrix