Experimental Investigation of Solid Organic Matter with a 2D NMR T1-T2 Map

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.