Magma-induced thermal alteration in Devonian organic-rich rocks (Paraná Basin, Brazil): molecular transformations and clay-catalyzed maturation

Abstract

Several studies have examined how high heating rates influence the molecular composition of organic matter, particularly in geological settings where magma interacts with organic-rich rocks. In Brazil, such interactions have been extensively documented in the Paraná Basin. However, the behavior of geochemical proxies under intense thermal stress, as well as the natural chemical products generated through the thermal cracking of organic matter, are not yet fully understood. In this study, a combination of bulk and molecular geochemical data from 233 m of Devonian strata in the Paraná Basin—intersected by two dolerite sills measuring 3.0 m and 0.86 m in thickness—was used to assess the extent and effects of magmatic heating on originally immature organic matter. The work includes a large dataset of total organic carbon and Rock-Eval pyrolysis along with thermal-maturity-related molecular geochemical proxies. The thermal evolution of organic matter was better assessed using methylated aromatic hydrocarbons, as saturate hydrocarbons are less resistant to high temperatures and may not respond well to rapid heating. The borehole profile was subdivided into four stages (I–IV) based on the response of geochemical proxies to thermal stress. The intermediate zone (Stages II and III) showed elevated MPI-1 (methylphenanthrene index) values and distorted saturate hydrocarbon ratios. Additionally, samples from this high thermal stress zone of the sills showed a significantly higher abundance of dimethyl alkanes—compared to less altered samples dominated by monomethyl alkanes. Multiple alkyl substitutions have been naturally favored in high-temperature zones by acidic clay mineral catalysis and sustained thermal stress, potentially from percolating hydrothermal fluids. The thermal alteration zone exceeds the expected halo from the two sills intersecting the borehole, which is interpreted as resulting from the percolation of hydrothermal fluids and/or a complex igneous body geometry.