Effects of thermal maturation on organic hydrogen-2/hydrogen-1 ratios and hydrogen isotopic exchangeability in Paleozoic marine kerogens (type-II)

The influence of thermal maturity on 2H/1H stable isotope ratios of nonexchangeable organic hydrogen and on hydrogen isotopic exchangeability (expressed as Hex, in % of total organic hydrogen) was investigated in two suites of type-II kerogens (the insoluble fraction of organic matter in marine sediments). Kerogens were obtained from Late Devonian to Early Mississippian black shales from the New Albany Shale (Illinois Basin) and from the Exshaw Formation (Alberta Basin). The precursor organic matter of the investigated kerogens was deposited in comparable near-equatorial marine environments and the original 2H/1H ratios were presumably quite similar. Different degrees of thermal maturity and different 2H/1H ratios of formation waters introduced isotopic shifts. Parallel studies via Nuclear Magnetic Resonance, Fourier Transform Infra-Red spectroscopy and pyrolysis-gas chromatography/mass-spectrometry confirmed that observed hydrogen isotopic differences between the two suites of kerogen are not related to differences in maturation pathways. With increasing thermal maturity, Hex initially decreases to reach a minimum at vitrinite reflectance (Ro) ∼0.8 to 1.1 % due to elimination of chemical functional groups that contain exchangeable hydrogen. A subsequent substantial rise in Hex at higher thermal maturity correlates with increasing aromaticity and abundance of exchangeable aromatic hydrogen. In both kerogen suites, 2H/1H ratios increase with maturity up to an Ro ∼1.5 %, then level out. Increasing 2H/1H ratios suggest (i) isotopic transfer of relatively 2H-enriched hydrogen from water into the pool of organic non-exchangeable hydrogen, and/or (ii) preferential loss of 1H-enriched organic chemical moieties from kerogen during maturation. More extensive 2H-enrichment with increasing maturity in New Albany Shale kerogens relative to Exshaw Formation kerogens is apparently driven by a higher 2H-abundance in New Albany Shale formation waters and a larger water-kerogen isotopic difference. To date, this is the strongest direct evidence for hydrogen isotopic transfer between naturally maturing kerogen and water hydrogen. Thermally induced changes in 2H/1H ratios alter the original hydrogen isotopic paleoenvironmental signal in kerogen, albeit in a systematic fashion. The specific 2H/ 1H response of each kerogen suite through maturation correlates with H/C elemental ratios and can therefore be corrected to yield paleoenvironmentally relevant information for a geochemically and isotopically constrained system.