Heterocyclic polycyclic aromatic compounds in source rock extract and crude oil: method development and geochemical assessment

Petroleum is one of the most complex organic matrices, comprising hydrocarbons and heteroatomic compounds generated and transformed throughout diagenetic
and catagenetic processes. Among these compounds, nitrogen-, sulfur-, and oxygen-containing heterocyclic polycyclic aromatic (NSO-HETs) have gained
attention as molecular markers for geochemical characterization, providing information on depositional paleoenvironment and thermal maturity. However,
their determination is challenging due to their low concentration and the high complexity of petroleum matrices. In this context, gas chromatography coupled
with triple-quadrupole mass spectrometry (GC-MS/MS) offers high selectivity and sensitivity for the determination of NSO-HET markers. The aim of this study was
to apply GC-MS/MS to quantify NSO-HETs markers in Brazilian source rocks and crude oils for assess geochemical interpretation. Source rocks and crude oils
samples from the Amazonas, Solimões, Paraná, Recôncavo, Sergipe, and Santos basins were investigated. Source rock extracts and crude oils were
fractionated using SARA method, and fractions containing aromatic and NSOHET were analyzed in a GC-MS/MS. Specific methods for each nitrogen-, sulfur-
, and oxygen class were optimized using individual certified standards. The NSOHET markers revealed compositional differences and similarities among source
rocks and crude oils and provided information on the origin and input of organic matter, as well as depositional paleoenvironmental conditions and thermal
maturity. Overall, higher concentrations of nitrogen- and sulfur-containing compounds were obtained in source rocks extracts and crude oil associated with
marine origin, while samples of non-marine origin showed higher amounts of oxygen-containing compounds. Diagnostic ratios and relative proportions of
NSO-HET markers further refined interpretations, indicating predominantly algal organic matter with subordinate terrestrial input and deposition under different
redox conditions. Variations in NSO-HET concentrations reflected differences in environments and conditions associated with source rocks formations and the
crude oil evolution. These results demonstrate the potential of GC-MS/MS for quantification of NSO-HET markers profiling as a selective approach for the
characterization of organic matter input and conditions of the depositional paleoenvironment from organic extracts of source rocks and crude oils.