Abstract The Irati Formation (Paraná Basin) is a mixed carbonate and organic-rich shale sequence intruded by Jurassic-Cretaceous basic rocks, featuring Brazil’s most important oil shale deposits with different maturity levels. For the first time, the distribution of oil shale biomarkers from an outcrop section (quarry) of the Irati Formation in the northernmost Paraná Basin was analyzed by GC-MS and GC-MS/MS to determine the thermal evolution, organic matter origin and the depositional paleoenvironment. The organic-rich shale at the northernmost border of the basin has high similarity with the central and southernmost areas, indicating a primary control able to induce cyclic sedimentation in a broad (106 km2) and restricted environment. PCA and HCA analysis of bulk and molecular parameters showed changes in the organic matter composition and paleoenvironmental conditions throughout the stratigraphic column. Nonetheless, there are significant differences compared to the central-eastern and southern areas of the basin. Contrasting with the southern region, the north, predominates biphytane, low and medium gammacerane index. Pr/n-C17, Ph/n-C18, HI and OI values suggest type II/III kerogen from marine organic matter with freshwater input. Among the steranes, those of stereochemistry ααα 20R predominate over ααα 20S, and the presence of βTm indicates the shales are less thermally evolved. organicrich rich JurassicCretaceous Jurassic Cretaceous rocks Brazils Brazil s levels time quarry (quarry GCMS GC MS GCMS/MS GCMSMS MS/MS evolution paleoenvironment 106 (10 km2 km environment column Nonetheless centraleastern eastern region north biphytane index Pr/nC17, PrnC17 PrnC Pr/n C17, Pr n C17 C Pr/n-C17 Ph/nC18, PhnC18 PhnC Ph/n C18, Ph C18 Ph/n-C18 IIIII II III input steranes R 20S S evolved MSMS 10 (1 nC17 nC Pr/nC17 PrnC1 Prn C1 Pr/n-C1 nC18 Ph/nC18 PhnC1 Phn Ph/n-C1 1 ( nC1 Pr/nC1 Pr/n-C Ph/nC1 Ph/n-C Pr/nC Ph/nC

The Barreirinha Formation-Upper Devonian, is the main petroleum source rock of the Amazon Basin, deposited during the great Devonian Transgression, contributing to significant accumulations of organic matter (OM) in anoxic conditions, which allowed its preservation. The present work had the objective of characterizing the molecular composition of biomarkers in outcrops samples of the Barreirinha Formation, aiming to evaluate the paleoenvironment, thermal evolution, and the preservation of OM total organic carbon (TOC) and Rock-Eval pyrolysis indicate considerable amounts of immature OM deposited in a low oxygenation environment. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-tandem mass spectrometry (GC-MS/MS) data corroborate that the OM was deposited in a suboxic to the oxic environment and low salinity (absence or low relative abundance of β-carotane and gammacerane). 24-N-Propyl-cholestane was detected and identified by synthetic pattern co-injection. High concentrations of tetracyclic polyprenoids (TPPs) in ascending order from base to top, high hopane/sterane ratios, to suggest that the samples had a high molecular weight n-alkanes, C29 steranes, low thermal evolution, and anoxic depositional paleoenvironment.