Influence of maceral composition on geochemical characteristics of immature shale kerogen: Insight from density fraction analysis

Variations in the relative proportions of individual macerals in shales can significantly influence the geochemical characteristics of bulk organic matter. Density fractions of kerogen from the thermally immature New Albany Shale (Devonian and Mississippian) with contrasting maceral compositions exhibit strong geochemical differences. The parental shale is characterized by a vitrinite reflectance (Ro) of 0.45%, a total organic carbon content of 13 wt.%, and a sulfur content of 6.2 wt.%. Organic matter is dominated by amorphinite and alginite, with vitrinite and inertinite accounting only for 1% by volume. Alginite-dominated density fractions (density ca. 1.0–1.15 g/cm3) contain significantly more aliphatic hydrogen, a stronger carboxyl/carbonyl contribution, and reduced Fourier transform infrared spectroscopy absorbance in the 1000 to 1100 cm− 1 region assigned to ether bonds (COC), as compared to the amorphinite-dominated density fraction (density ca. 1.2–1.6 g/cm3). Aromaticity generally increases from alginite-dominated to amorphinite-dominated fractions. Density fractions dominated by amorphinite are more deuterium-depleted (δDn values of nonexchangeable hydrogen up to − 105‰) than alginite-rich density fraction (δDn values reach − 90‰). In contrast, changes in relative proportions of alginite and amorphinite in the New Albany Shale do not significantly affect the amount of isotopically exchangeable hydrogen in total hydrogen (i.e., hydrogen exchangeability). Alginite is relatively 13C-enriched, whereas density fractions having a high content of amorphinite are relatively 13C-depleted. Our results suggest that even small bulk geochemical and isotopic differences can gain relevance after deconvolution from maceral-related variability. The masking influence of maceral abundance patterns must be considered when interpreting bulk geochemical data as paleoenvironmental proxies. The findings of this study on shale and Type II kerogen are relevant for all types of kerogens in sediments and rocks.

► Density of kerogen in shales depends on maceral composition.
► Maceral composition influences chemistry and isotopic compositions of kerogen.
► Isolation procedure of bulk kerogen must avoid bias against any organic density fractions.