Organic response to igneous intrusion in Irati Formation (Southern Paraná Basin, Brazil): Biomarkers, open-system pyrolysis, vitrinite reflectance

Most petroleum systems are associated with organic maturation driven by burial depth. However, in the Paraná Basin (Brazil), atypical systems are driven by heat flow from igneous intrusions. Therefore, this study investigates organic geochemical variations in the Irati Formation induced by a diabase intrusion. A total of 36 rock samples were  collected from a borehole profile (CBM-002-CL-RS, 387.85-426.40 m), intersected by a 0.7 m thick diabase intrusion (406.15-406.85 m), recording the stratigraphic sequence from the basal Taquaral to the upper Assistência members. Open-system pyrolysis was performed on all samples, with 18 selected for random vitrinite reflectance (VRr) and biomarker analysis. Samples were extracted with dichloromethane to obtain extractable organic matter (EOM) and fractionated for gas chromatography-mass spectrometry (GC-MS). Pyrolysis results show total organic carbon (TOC) values from 0.36 to 13.24 wt% and EOM yields up to 4.23 mg/g rock. Hydrogen Index (HI) decreases to 155 mg HC/g TOC in the immediate contact (0.25 m), while distal samples preserve values up to 387 mg HC/g TOC (T_max reaching 449 °C). VRr ranges from 0.54% to 1.17% (higher values near the igneous intrusion). Saturated biomarkers persist near the contact due to flash heating, showing a distribution from n-C₁₁ to n-C₃₁ with prevalent short-chain compounds (n-C₁₁ to n-C₁₈). Pristane/phytane ratios vary from 1.75 in the overlying upper Assistência Member (deeper marine conditions) to 0.05 in its base (shallower marine conditions). Terpanes (C29-C30 hopanes) follow a thermal evolution marked by Ts/(Ts+Tm) and stable C32 22S homohopanes (~0.58), with a gammacerane index increase (GAM/H30 > 2.0) in the lower Assistência Member. Regular steranes reveal clear facies zonation: the upper Assistência Member is dominated by C₂₉ steranes; the lower Assistência Member shows a complex mixture of C₂₇, C₂₈, and C₂₉ steranes coinciding with maximum saline stratification; and the Taquaral Member records a basal transition with C₂₉ steranes decreasing from 56% to 36% toward the base of the unit. Sterane isomerization ratios [20S/(20S+20R) and ββ/(αα+ββ)] show an atypical thermal response, remaining lower at the contact (0.40 and 0.21) while peaking (0.57 and 0.32) in distal samples). Finally, the methylphenanthrene ratio (MPR) increases from 1.69 to 2.34 and the methyldibenzothiophene ratio (MDBTR) varies from 1.50 to a peak of 7.43 toward the intrusion. Notably, the highest retene abundance in the lower Assistência Member coincides with maximum gammacerane, evidencing terrestrial input synchronous with hypersalinity, due to deposition in a shallower shelf. Various thermogenic PAHs were identified in the aureole. It is concluded that the thermal impact of the intrusion in the Irati Formation rocks is restricted to ~1.0 m distance, both above and below it, allowing the original geochemical signature to prevail over magmatic alteration.