Disentangling primary geochemical signatures and igneous-sedimentary interactions: geochemical, geostatistical and geophysical constraints on depositional conditions and thermal overprint of Devonian source rocks, Parnaíba Basin, NE Brazil

Characterization of hydrocarbon source rocks for paleoenvironmental reconstruction is often precluded by thermal overprinting, especially in sedimentary basins affected by igneous intrusions. In the Parnaíba Basin (NE Brazil), organic-rich Devonian shales of the Pimenteiras Formation were disturbed by sill emplacement related to the Central and Equatorial Atlantic magmatic provinces (Triassic-Jurassic). We applied a multiproxy geochemical approach in three wells: two wells with multiple volcanic intrusions and one well without intrusions, used here as reference of primary signatures. Total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), δ 13 C org , and δ 15 N were complemented with geophysical logs and multivariate geostatistical classification (Mahalanobis distance, calculated from TOC vs. TN and δ 13 C org vs. δ 15 N), to distinguish pristine and thermally affected intervals. The reference well exhibits a strong TOC-TN correlation (r = 0.96), whereas the thermally affected wells show no correlation (r = 0.04 and −0.11) and are marked by Contact Low-Resistivity Zones (CLRsZ), reflecting modifications on geochemical and petrophysical properties. Wells with intrusions have up to ∼80% of samples plotting outside the reference geochemical values, however, an interval (1995–2109 m) preserves pristine signatures. Within this section, six chemozones record a transgressive–regressive history. Transgressive phases are marked by dominant marine organic matter (δ 13 C org around −28‰; TOC/TN < 5), whereas regressive phases reflect stronger terrestrial input (TOC/TN > 20), both accumulated under anoxic conditions. Despite thermal overprinting, a multiproxy approach provides a reproducible workflow able to distinguish organic-rich rocks with depositional signals from magmatic alteration. This framework enables assessing source-rock quality and predicting maturation of organic matter in atypical petroleum systems.