Critical Practices for the Preparation and Analysis of Kerogen

Knowledge of the chemical and structural properties of kerogen is needed for accurate volumetric and economic appraisal of petroleum-bearing unconventional resources. Unfortunately, research concerning kerogen does not always follow demonstrated best practices for laboratory isolation and analysis. Here, we outline critical procedures for studies of bulk kerogen. Analyses of kerogen isolates require the dissolution of all non-kerogen components in the rock. Soluble hydrocarbons are removed first, using solvent extraction techniques. The mineral matrix is effectively removed using concentrated acids inside a closed-system, flow-through reaction cell. The acidization sequence includes HCl and HF for dissolution of carbonates and silicates, respectively, and acidified CrCl2 for dissolution of pyrite. This closed-system procedure consistently achieves recovery efficiencies exceeding 80-85% and kerogen purities exceeding 95-97%. Removal of pyrite by density-gradient centrifugation, common in traditional isolation workflows, is discouraged because of its limited efficacy. Following isolation, kerogen must be dried appropriately, using critical point drying for pore structural characterization. Oven drying should be avoided because it can collapse pores, destroying the native pore characteristics. Minimum methods for validating the purity of isolated kerogen include elemental and ash quantification by flash combustion and pyrolysis. Elemental analysis is further required for computing petrophysical properties of kerogen, guiding molecular dynamics simulations, and for correcting mineral contamination in laboratory measurements. Skeletal density is critical for organic-rich shale petrophysics and should be determined using helium pycnometry. Density-gradient centrifugation for density determinations should be avoided. Molecular structures can be assessed using several solid-state spectroscopies, including nuclear magnetic resonance (NMR), X-ray, infrared, and Raman. Using NMR as one example, it is shown how proper spectral acquisition and interpretation are both critical for quantifying basic molecular characteristics of kerogen. The practices presented here serve as a foundation for the geochemistry community for future kerogen studies.