Marcadores nitrogenados associados a parâmetros geoquímicos para interpretações paleodeposicionais das formações Candeias (Bacia do Recôncavo) e Barreirinha (Bacia do Amazonas).

Abstract

Nitrogen markers (N-markers), such as carbazoles, benzocarbazoles and their alkylated isomers, are specific to a small fraction of the chemical composition of petroleum and/or source rock extract (<4.0%), but can provide information on the origin, migration, depositional environment and maturity of petroleum, being evaluated in isolation and/or through diagnostic ratios. Presented in petroleum samples and source rock extracts mainly in aromatic and heterocyclic structures, the identification of N-markers involves a certain complexity, making, to date, the application of these compositions in geochemical evaluations applied to Brazilian sedimentary basins little explored. In view of this, the GC-MS/MS technique has been used in literature for the determination of organic compounds in geochemical studies to reduce sample preparation steps (such as the elimination of a fractionation step using acidic silica, in the case of nitrogen compounds) and to present high selectivity and sensitivity. The aim of this research was to use, in an innovative way, N-markers in conjunction with occasional geochemical interruptions (such as saturated biomarkers, total organic carbon and Rock-Eval pyrolysis data, for example) and also S-markers as new tools in the characterization of vertical successions of outcrops of oil source rocks in sedimentary basins with different origins, regarding depositional paleoenvironment, type of organic matter and depositional conditions, using samples from the Candeias Formations (Recôncavo Basin) and Barreirinha Formations (Amazon Basin). For this, methods for determination and quantification of the markers under study via GC-MS/MS were optimized. Samples were collected from three different outcrops: one in the Recôncavo Basin, with 10 samples collected vertically, two in the Amazon Basin, with 7 samples collected vertically in each outcrop. The organic matter contained in the samples was extracted via accelerated solvent protection (ASE) with subsequent fractionation via open column liquid chromatography and analysis of the combined aromatic and NSO fractions via GC-MS/MS. Together with conventional geochemical parameters, nitrogen and sulfur markers proved to be efficient in evaluating distinctions and similarities between the source rocks under study. Samples of the source rock from the Recôncavo Basin, of lacustrine origin, present, in general, lower concentrations of nitrogen compounds (carbazoles (CA), methylcarbazoles (MCA) and dimethylcarbazoles (DMCA)) and sulfur compounds (benzothiophenes (BT), dibenzothiophenes (DBT) and benzonaphthothiophenes (BNT)), while samples from the Amazon Basin, of marine origin, present high concentrations of these compounds. The input of algal organic matter was evidenced by ternary diagrams using isomers of N-markers (1-MCA, 2-MCA and 4-MCA) and S-markers (BNT-1,2, BNT-2,1 and BNT-2,3), highlighting isomeric differences inherent to the depositional environments (lacustrine and marine), which help to understand the behavior of these compounds in different environments. The values of the pristane/phytane ratio correspond to the concentrations of benzocarbazoles and the DBT/phenanthrene ratio, indicating that these compounds can be affected by the redox conditions of the depositional environments.