  Kerogen-rich shale reservoirs will play a key role during the energy
transition, yet the effects of nano-confinement on the NMR relaxation of
hydrocarbons in kerogen are poorly understood. We use atomistic MD simulations
to investigate the effects of nano-confinement on the $^1$H NMR relaxation
times $T_1$ and $T_2$ of heptane in kerogen. In the case of $T_1$, we discover
the important role of confinement in reducing $T_1$ by $\sim$3 orders of
magnitude from bulk heptane, in agreement with measurements of heptane
dissolved in kerogen from the Kimmeridge Shale, without any models or free
parameters. In the case of $T_2$, we discover that confinement breaks spatial
isotropy and gives rise to residual dipolar coupling which reduces $T_2$ by
$\sim$5 orders of magnitude from bulk heptane. We use the simulated $T_2$ to
calibrate the surface relaxivity and thence predict the pore-size distribution
of the organic nano-pores in kerogen, without additional experimental data.
