Compound-Specific Hydrogen Isotope Analysis of Heteroatom-Bearing Compounds via Gas Chromatography–Chromium-Based High-Temperature Conversion (Cr/HTC)–Isotope Ratio Mass Spectrometry

The traditional high-temperature conversion (HTC) approach toward compound-specific stable isotope analysis (CSIA) of hydrogen for heteroatom-bearing (i.e., N, Cl, S) compounds has been afflicted by fractionation bias due to formation of byproducts HCN, HCl, and H₂S. This study presents a chromium-based high-temperature conversion (Cr/HTC) approach for organic compounds containing nitrogen, chlorine, and sulfur. Following peak separation along a gas chromatographic (GC) column, the use of thermally stable ceramic Cr/HTC reactors at 1100–1500 °C and chemical sequestration of N, Cl, and S by chromium result in quantitative conversion of compound-specific organic hydrogen to H₂ analyte gas. The overall hydrogen isotope analysis via GC–Cr/HTC–isotope ratio mass spectrometry (IRMS) achieved a precision of better than ± 5 mUr along the VSMOW-SLAP scale. The accuracy of GC–Cr/HTC–IRMS was validated with organic reference materials (RM) in comparison with online EA–Cr/HTC–IRMS and offline dual-inlet IRMS. The utility and reliability of the GC–Cr/HTC–IRMS system were documented during the routine measurement of more than 500 heteroatom-bearing organic samples spanning a δ²H range of −181 mUr to 629 mUr

Small-Angle and Ultrasmall-Angle Neutron Scattering (SANS/USANS) Study of New Albany Shale: A Treatise on Microporosity

Small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) techniques were applied to study the microstructure of several New Albany shales of different maturity. It has been established that the total porosity decreases with maturity and increases somewhat for post-mature samples. A new method of SANS data analysis was developed, which allows the extraction of information about the size range and number density of micropores from the relatively flat scattering intensity observed in the limit of the large scattering vector <i>Q</i>. Macropores and significant number of mesopores are surface fractals, and their structure can be described in terms of the polydisperse spheres (PDSP) model. The model-independent Porod invariant method was employed to estimate total porosity, and the results were compared with the PDSP model results. It has been demonstrated that independent evaluation of incoherent background is crucial for accurate interpretation of the scattering data in the limit of large <i>Q</i>-values. Pore volumes estimated by the N₂ and CO₂ adsorption, as well as via the mercury intrusion technique, have been compared with those measured by SANS/USANS, and possible reasons for the observed discrepancies are discussed

A Geological Model for the Origin of Fluid Compositional Gradients in a Large Saudi Arabian Oilfield: An Investigation by Two-Dimensional Gas Chromatography (GC × GC) and Asphaltene Chemistry

The heavy oil rim of a large Saudi Arabian oilfield has been shown to be in vertical and lateral equilibrium, matching predictions of the gravity term from the Flory–Huggins–Zuo equation of state for asphaltenes in the form of 5.2 nm clusters of the Yen–Mullins model. The large (10×) vertical gradient of asphaltene concentration over a very large perimeter (≫10 km) of the oilfield provided a stringent test of this equation-of-state fit. Two-dimensional gas chromatography (GC × GC) and stable isotope analysis (δ D and δ ¹³C) were used to determine the consistency of the liquid-phase components with equilibration and the effects of biodegradation or thermal maturity on the observed asphaltene gradient. These analyses confirm homogeneity of equilibrated liquid-phase components of similar chemical character and equilibrated asphaltene isotopes. Biodegradation is minimal and there is no maturity variation among the samples. Thus, the large asphaltene gradient did not result from these secondary processes and is not a remnant from how the reservoir was charged with crude oil. The results are consistent with original findings that the oil column is equilibrated. Thermodynamic equilibration over such large distances (>10 km) requires convective currents and provides constraints on fluid dynamic processes in reservoirs. A simple one-dimensional (1-D) three-component single-phase model is introduced to account for asphaltene accumulation by way of convective currents established from a diffusive gas front at the top of the oil column

Organic Reference Materials for Hydrogen, Carbon, and Nitrogen Stable Isotope-Ratio Measurements: Caffeines, <i>n</i>‑Alkanes, Fatty Acid Methyl Esters, Glycines, l‑Valines, Polyethylenes, and Oils

An international project developed, quality-tested, and determined isotope−δ values of 19 new organic reference materials (RMs) for hydrogen, carbon, and nitrogen stable isotope-ratio measurements, in addition to analyzing pre-existing RMs NBS 22 (oil), IAEA-CH-7 (polyethylene foil), and IAEA-600 (caffeine). These new RMs enable users to normalize measurements of samples to isotope−δ scales. The RMs span a range of δ²H_VSMOW‑SLAP values from −210.8 to +397.0 mUr or ‰, for δ¹³C_VPDB‑LSVEC from −40.81 to +0.49 mUr and for δ¹⁵N_Air from −5.21 to +61.53 mUr. Many of the new RMs are amenable to gas and liquid chromatography. The RMs include triads of isotopically contrasting caffeines, C₁₆ <i>n</i>-alkanes, <i>n</i>-C₂₀-fatty acid methyl esters (FAMEs), glycines, and l-valines, together with polyethylene powder and string, one <i>n</i>-C₁₇-FAME, a vacuum oil (NBS 22a) to replace NBS 22 oil, and a ²H-enriched vacuum oil. A total of 11 laboratories from 7 countries used multiple analytical approaches and instrumentation for 2-point isotopic normalization against international primary measurement standards. The use of reference waters in silver tubes allowed direct normalization of δ²H values of organic materials against isotopic reference waters following the principle of identical treatment. Bayesian statistical analysis yielded the mean values reported here. New RMs are numbered from USGS61 through USGS78, in addition to NBS 22a. Because of exchangeable hydrogen, amino acid RMs currently are recommended only for carbon- and nitrogen-isotope measurements. Some amino acids contain ¹³C and carbon-bound organic ²H-enrichments at different molecular sites to provide RMs for potential site-specific isotopic analysis in future studies