One of the much debated mysteries in 1 H NMR relaxation measurements of bitumen and heavy crude oils is the departure from expected theoretical trends at high viscosities, where traditional theories of 1 H− 1 H dipole−dipole interactions predict an increase in T 1 with increasing viscosity. However, previous experiments on bitumen and heavy crude oils clearly show that T 1LM (i.e., log-mean of the T 1 distribution) becomes independent of viscosity at high viscosities; in other words, T 1LM versus viscosity approaches a plateau. We report 1 H NMR data at ambient conditions on a set of pure polymers and polymer−heptane mixes spanning a wide range of viscosities (η = 0.39 cP ↔ 334 000 cP) and NMR frequencies (ω 0 /2π = f 0 = 2.3 MHz ↔ 400 MHz) and find that at high viscosities (i.e., in the slow-motion regime) T 1LM plateaus to a value T 1LM> ∝ ω 0 independent of viscosity, similar to bitumen. More specifically, on a frequency-normalized scale, we find that T 1LM> × 2.3/f 0 ≃ 3 ms (i.e., normalized relative to 2.3 MHz), in good agreement with bitumen and previously reported polymers. Our findings suggest that in the high-viscosity limit T 1LM> and T 2LM> for polymers, bitumen, and heavy crude oils can be explained by 1 H− 1 H dipole−dipole interactions without the need to invoke surface paramagnetism. In light of this, we propose a new relaxation model to account for the viscosity and frequency dependences of T 1LM and T 2LM , solely based on 1 H− 1 H dipole−dipole interactions. We also determine the surface relaxation components T 1S and T 2S of heptane in the polymer−heptane mixes, where the polymer acts as the "surface" for heptane. We report ratios up to T 1S /T 2S ≃ 4 and dispersion T 1S (ω 0 ) for heptane in the mix, similar to previously reported data for hydrocarbons confined in organic matter such as bitumen and kerogen. These findings imply that 1 H− 1 H dipole−dipole interactions enhanced by nanopore confinement dominate T 1S and T 2S relaxation in saturated organic-rich shales.

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Organic matter (OM), as the source of the hydrocarbon in shale plays, should be studied extensively from various perspectives. In this regard, using a 2D NMR T 1–T 2 map is becoming a popular method that can be used to distinguish proton populations of different sources, including the OM in shale samples. In this study, 17 shale samples with different maturities from the Bakken Formation in the United States were collected and analyzed by using the 2D NMR T 1–T 2 map method to assess the physicochemical properties of the OM. The results showed that the OM in these samples belongs to kerogen type II and is in the immature to early oil generation window stages. While the 2D NMR T 1–T 2 maps were separated into four different regions representing different sources of protons, the focus of the study was only on the signal for region 2 that is indicative of the solid OM. Based on detailed analysis of the spectrum from this region, two new parameters named T 1p and T 2p, respectively, representing the peak of the T 1 and T 2 relaxation time distributions, were proposed. The detailed analysis demonstrated that T 2P did not vary much with maturity, while the T 1P value increased as thermal maturity advanced. This resulted in an overall increase of the T 1P/T 2P ratio versus thermal maturity. T 1 relaxation time distribution more clearly exhibits the effect of thermal maturity on OM compared to T 2 relaxation time distribution in region 2. Finally, the fractal analysis of the T 1 relaxation time spectrum truncated of region 2 showed that as the maturity increased, the fractal dimension declined, which suggests that the T 1 relaxation time attributes a more chemically homogeneous structure to the OM.