In this study, we investigate the utilization of nuclear magnetic resonance (NMR) in unconventional formations with NMR relaxation signals and relaxation mechanisms. We report a novel Spliced NMR inversion method to separate liquid-like components with an exponential decay (T2e) in transverse magnetization from solid-like components with a Gaussian decay (T2G). The T1–T2 maps clearly distinguish liquid-like signals (including micro/meso-macro pore fluids, heptane dissolved in bitumen, and clay-bound water) from solid-like signals (including kerogen, bitumen, and clay hydroxyls) in the organic-rich chalks. This novel method potentially enhances the analysis of fluid typing and saturation from liquid components and is used for clay mineral identification and determination of kerogen content from solid components.

Following this novel Spliced NMR signal inversion method, the quantity of Kerogen is further investigated using the “2D splice NMR” method consisting of T1 with solid-echo (T2G∗) and spin-echo train (T2e). This 2D splice NMR is integrated with Rock-Eval analysis to study the organic matter in Type II-S organic-rich chalk as a function of maturity (i.e., depth), from immature to oil-window. The readily extractable bitumen is distinguished from the remaining bitumen after solvent extraction as a function of depth. Further, the elemental H/C ratio, kerogen swelling, kerogen nano-pore size, and compaction effects on macro pores are studied as a function of depth.

One interesting phenomenon is the narrowing T1,2 distribution observed on the unconventional formations due to the cross-relaxation effect (a.k.a spin diffusion). We investigate the effect of 1H NMR cross-relaxation σ1 (a.k.a., spin diffusion), which manifests itself as a narrowing in the T1 distribution using a proposed metric |σ1|/R1 for the relative strength in cross-relaxation. These insights into the 1H NMR relaxation offer valuable information about the molecular dynamics of viscous fluids, proving beneficial in both the medical and energy fields without invoking the physics of paramagnetism.

Another intriguing observation is the mild increase in T1_Ker with depth for kerogen in the organic-rich chalk. The decrease in the second moment Δω2 (∝H/C) with maturity can partially account for it. We further utilize molecular dynamic (MD) simulations of realistic kerogen models with varying maturity to compute the NMR 1H-1H dipole-dipole autocorrelation function. MD simulations reveal new insight into the intramolecular versus intermolecular NMR relaxation in bulk kerogen molecules with varying maturity. We combine the MD simulation with the Plateau model to predict the NMR relaxation of bulk kerogen molecules in the slow-motion regime. A consistent trend between the simulated T2G (and T1) versus H/C, and the trend found from NMR measurements of Type II-S organic-rich chalk as a function of maturity using a solid-echo pulse sequence to detect the solid kerogen.