Organic Sulfur Markers as Proxies of Depositional Paleoeenvironments Related to Recôncavo and Amazon Basins, Brazil

Abstract

This study employed organic sulfur markers (S-markers) associated with geochemistry parameters to evaluate the paleoenvironment of different depositional settings in 24 samples collected in vertical sections of outcrops of the Candeias and Barreirinha Formations in Recôncavo and Amazon basins, respectively. A total of twenty-one S-markers from benzothiophene (BT), dibenzothiophene (DBT), and benzonaphtothiophenes (BNT) classes were optimized and quantified by gas chromatography-triple quadrupole mass spectrometry (GC–MS/MS). S-markers efficiently evaluated and differentiated the depositional paleoenvironment in the source rocks based on the individual compound, in cross-validation with saturated biomarkers, and associated with parameters such as total organic carbon (TOC) and Rock-Eval pyrolysis. Samples from the lacustrine environment presented low concentrations of BT, DBT, and BNT, and samples from the marine environment showed high BT, DBT, and BNT concentrations. The variations in ∑DBT and TOC indicated that the quantity and/or the type of organic matter exert some control over the distribution of DBTs. Although the formations are from different paleoenvironments, the organic matter input was similar, as indicated by high proportions of 1,2-BNT and 2,1-BNT relative to 2,3-BNT, thus characterizing the algal input with a microbial contribution for both sites. The sum of the BNTs was directly related to the amounts of amorphous organic matter (AOM) in the vertical distribution of outcrops. These results are in accordance with the finding that BNTs may originate from the microbial activity. The DBT/Phen vs pristane/phytane (Pr/Ph) relationship attested to differences in the redox conditions of the depositional paleoenvironments of the formations under study. The 4,6-DMDBT/2,4,6-TMDBT and 2,4,6-TMDBT/(2,4,7 + 2,4,8)-TMDBT ratios indicated immaturity for hydrocarbon generation.