A Quantitative Evaluation Method for Organic Matter in Shale Based on CPMG and FID NMR Experiments

Unconverted organic matter accounts for a high proportion in lacustrine shale oil reservoirs in China, making the quantitative evaluation of organic matter crucial for the exploration and development of shale oil. Nuclear magnetic resonance (NMR) is a rapid and non-destructive detection technique that has been widely utilized in the field of oil and gas exploration. However, the Carr-Purcell-Meiboom-Gill (CPMG) acquisition mode is limited by the shortest echo time of the instrument, making it difficult to effectively detect signals from short relaxation components such as organic matter. To address this, this paper introduces the Free-Induction-Decay (FID) acquisition mode and compares the differences between the CPMG and FID acquisition modes in shale organic matter evaluation through numerical simulations and experimental measurements. A quantitative evaluation method is proposed for shale organic matter based on both CPMG and FID measurements. The study indicates that the initial acquisition time of effective signals is crucial for evaluating shale organic matter. The FID measurement with a dwell time of 1 μs and a NMR instrument dead time of 17 μs can accurately provide the content of organic matter and fluid cpmponents, in shale, while the CPMG measurement with an echo time of 0.6 ms can only provide the content of fluid components. The difference between these two measurements shows a strong correlation with several geochemical characteristic parameters such as total organic content (TOC), cracking hydrocarbon content (S2 and (S2-1+S2-2)), S2, and (S2-1+S2-2), with correlation coefficients of 0.910 and 0.837 for TOC and (S2-1+S2-2), respectively. This proves that the method can be used for quantitative evaluation of organic matter.