Geochemical variability of organic matter composition in marine deposits affected by heat of igneous intrusions, Paraná Basin – southern Brazil

Depositional conditions, thermal evolution, and post-depositional alteration processes control organic geochemical signatures preserved in sedimentary rocks. This study investigates the geochemical variability of organic matter and hydrocarbon composition in shale, marlstone, and siltstone influenced by the thermal effects of igneous intrusions within the Permian succession of the Paraná Basin, Brazil. This interval corresponds to the most significant anoxic event recorded in the basin and represents an important organic-rich succession within the Gondwana stratigraphic record. The study area is located along the southeastern margin of the basin, near the rifted South Atlantic margin. Core samples from borehole CBM-003-ST-RS were collected at varying distances from an approximately 18-m-thick diabase sill and analyzed using an integrated approach that combined random vitrinite reflectance (VRr), open-system pyrolysis, and molecular biomarker analyses. Based on lithology, organic matter characteristics, and distance from the intrusion, four sample zones (A, B, C, and D) were defined. The results reveal a clear thermal gradient consistent with conductive heat transfer from the sill. Zones A and B, located within the first 17 m from the contact, exhibit strong thermal alteration of organic matter, with VRr values of 1.02–3.11% and T max values of 437–470 °C. These samples display low Total Organic Carbon (TOC) contents (< 1 wt%) and very low S 1 and S 2 values (< 0.5 mg HC/g rock), indicating severe depletion of hydrocarbon-generating potential. Molecular parameters also indicate intense thermal stress, including the absence of pristane and phytane, marked alterations in biomarker ratios, and significant changes in polycyclic aromatic hydrocarbon (PAH) distributions, characterized by the predominance of fluoranthene and pyrene. Variations in phenanthrene and methylphenanthrene isomers further indicate advanced thermal maturity. In contrast, Samples located 17.65–33.35 m from the intrusion (Zones C and D) show limited thermal influence. These intervals display VRr values of 0.7–0.8%, consistent with the onset of the hydrocarbon generation window, and preserve relatively stable biomarker ratios and PAH distributions. Sterane distributions (C₂₇–C₂₉) and the presence of gammacerane indicate a predominantly marine-transitional depositional environment under moderate salinity conditions. Elevated GAM/C₃₀ hopane in some samples from Zone C suggests deposition in relatively shallow-water environments where carbonates were interbedded with organic-rich shales. These results are consistent with biomarker data reported from other sectors of the basin where equivalent strata were not affected by igneous heating, indicating that key depositional signals may remain preserved despite localized thermal overprinting. The Irati Formation, therefore, represents an example of molecular resilience in organic-rich sediments affected by magmatic activity. Unlike other volcanic-influenced basins, such as the Karoo Basin (South Africa), Raton Basin (USA), and Newark Basin (USA), where magmatic heating commonly has destroyed primary geochemical signatures, the Irati Formation succession preserves diagnostic proxies, such as gammacerane, even under moderate thermal stress. These observations support a “dual control” model in which original depositional signals can be distinguished from the thermal overprint generated by igneous intrusions.