Experimental Evaluation of Low-Temperature Catalytic Reactions of Steroidal Biomarkers in Shales

The heterogeneity of organic matter and mineral content of shales complicates the process of determining controls on the pathways of molecular transformations in sedimentary archives. Efforts to characterize chemical reactions occurring in shallow subsurface sediments over geologic timescales typically employ laboratory heating experiments to model these reaction processes. To compensate for the longevity of geological time, these experiments, with notable exceptions, often use anomalously high temperatures (e.g., >300 °C) that are not representative of conditions encountered in sedimentary archives. Such artificially high temperatures fundamentally change the reaction kinetics of natural systems, and thereby fail to accurately replicate them. In addition, they generate isotopic signatures unrepresentative of lower temperature reactions confirming the temperature dependency of pathways and reactions. Consequently, simulations that utilize short-term heating experiments (hours to 1 month) at low-temperatures (120 - 200 C) are more representative of the diagenetic transformations involving organic carbon and clay mineral facies occurring in organic-rich marine shales under natural conditions in the subsurface. Experiments utilized 5(H)-cholestane as the model organic substrate under anhydrous and hydrous conditions to assess the influence of water and in the presence of either natural clays (smectite, illite) or synthetic catalysts (ZSM-5, FeZSM-5) with elemental composition (Al, Si, O, Fe, Na) comparable to natural clays. The synthetic catalysts have a tunable elemental composition that provides the ability to alter metal content, metal oxidation state, and Lewis acidity of the host, which govern reaction outcomes. Only a small fraction of 5(H)-cholestane was transformed in all experiments, as expected given the short incubation time. However, substantive differences in the experimental products provide evidence for multiple reaction pathways including isomerization, desaturation, demethylation, structural rearrangement, aromatization, and side chain degradation. In addition, the varied composition of products confirms how differences in sedimentary conditions can affect the diagenetic products from a given substrate.