NMR relaxometry interpretation of source rock liquid saturation - A holistic approach

Fluid types and saturation are essential factors in assessing oil recovery resources and potential in organic-rich mudstones. One of the methods for analyzing tight rocks' saturation is low-field nuclear magnetic relaxometry (NMR). Interpretation of NMR results, particularly the data of T-1-T-2 maps, delivers different fluid saturation and estimates hydrogen populations. Although previous research proposed a qualitative interpretation of T-1-T-2 distributions, there is still no obvious scheme that can be directly applied in routine petrophysical studies. However, current NMR-based workflows and manual interpretation routines are subjective and laborious. We attempt to fill this gap by identifying the best interpretation scheme for three distinct types of organic-rich source rock of the Bazhenov formation and modifying a well-known T-1-T-2 map interpretation scheme. Experimental results include NMR 2-MHz T-1-T-2 datasets (original and differential maps for samples in as-received, dried, and saturated states) with the corresponding Rock-Eval pyrolysis parameters and mineral composition from X-ray diffraction. The analytical part compares the existing schemes and modifies the interpretation scheme based on the rock petrophysical background. The paper reviews the features of the manual interpretation and previously established schemes and proposes a modified scheme best fitted to the petrophysical and geochemical data. In addition, we study the impact of mineral composition and geochemical properties on T-1-T-2 signal interpretation and analyze cross-correlations. The study confirms the presence of a significant amount of clay-bound water from a detailed analysis of the corresponding regions in T-1-T-2 regular and differential maps. In the paradigm, we propose a semi-automated processing of NMR T-1-T-2 data using a custom interpretation scheme composed of regions from multiple interpretation schemes and consider their modification for high-quality results.

Automated summary

The study examines NMR analysis of fluid saturation in organic mudstones and modifies interpretation schemes for better results. By comparing existing schemes, a custom interpretation scheme is proposed that is best suited for petrophysical and geochemical data. The impact of mineral composition and geochemical properties on signal interpretation is also analyzed.