Critical Practices for the Preparation and Analysis of Kerogen

Abstract

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 nonkerogen 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 CrCl 2 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.

Perspectives of Gas Adsorption and Storage in Kerogens and Shales

“…The helium density values for the immature and gas widow shales are consistent with studies published recently on isolated kerogens, which were extracted to remove bitumen and where the critical point drying method was used. 27,56 Various studies show that BET surface areas for shales are consistent and usually in the range of up to ∼35 m 2 g −1 . Detailed subcritical adsorption studies for kerogens are limited but higher than the corresponding shales.…”

“…29,31 However, it has been reported that sample drying may cause pore collapse and changes in the microstructure, which can be avoided using critical point drying methods. 27,28,30 3.3. Subcritical Gas Adsorption.…”

“…13,27,30,49,54 Suekuni et al discussed critical practices in the preparation and analysis of kerogens with a range of maturities [reflectance (R 0 ) range of 0.55−2.8%] from a variety of geographical sources. 27 They reported N 2 (77 K) adsorption results for a range of high-purity type II isolated kerogens prepared using the critical point drying method. The surface areas corrected for residual mineral impurities were in the range of 26−375 m 2 g −1 , and the total pore volumes (from single point adsorption at p/p 0 = 0.989) were in the range of 0.1−0.55 cm 3 g −1 .…”

2022 Pioneers in Energy Research: Oliver Mullins

“…52 Dr. Craddock from Schlumberger-Doll Research and Prof. Allgeir at the University of Kansas outlined the best practices for the preparation and analysis of crucial chemical and structural properties of kerogen in organic-rich mudrocks. 53 Dr. Gan and co-workers from CNOOC (China) and Schlumberger (United States) proposed a new mechanism of trap filling associated with solid wax formation in migration (wax-out cryo-trapping) for studying reservoir fluids with unusual properties. 54 Dr. Nelson from Woods Hole Oceanographic Institution (United States) and co-workers applied comprehensive two-dimensional gas chromatography coupled with high-resolution mass spectrometry to obtain information on reservoir connectivity by exploring the chemical complexity of hopanoid molecules in heavy oils.…”

“…Dr. Harrison and co-workers from Schlumberger-Doll Research (United States) reviewed the downhole measurement of the phase behavior of reservoir fluids and highlighted the scientific concepts and design principles . Dr. Craddock from Schlumberger-Doll Research and Prof. Allgeir at the University of Kansas outlined the best practices for the preparation and analysis of crucial chemical and structural properties of kerogen in organic-rich mudrocks . Dr. Gan and co-workers from CNOOC (China) and Schlumberger (United States) proposed a new mechanism of trap filling associated with solid wax formation in migration (wax-out cryo-trapping) for studying reservoir fluids with unusual properties .…”