Polycyclic aromatic sulfur heterocycles used as molecular markers in crude oils and source rocks

Abstract

Polycyclic aromatic sulfur heterocycles (PASHs), such as benzothiophenes (BTs), dibenzothiophenes (DBTs), and benzonaphthothiophenes (BNTs), are found in crude oil and source rocks and have been extensively studied in the last decades. PASHs play a significant role in organic geochemistry as molecular markers (S-markers) because they are common in a variety of both oils and source rocks and mainly due to their thermal stability. Furthermore, relative to other hydrocarbon groups such as alkanes and many aromatic compounds, S-markers are highly stable and degradation-resistant. Thus, S-markers have been studied to obtain important information about thermal evolution, depositional environment, maturity as well as about petroleum migration and biodegradation. DBT-related markers, such as 4-MDBT/1-MDBT, 4,6-DMDBT/1,4-DMDBT, and benzonaphtho[2,1-d]thiophene/benzonaphtho[1,2-d]thiophene ([2,1]BNT/[1,2]BNT) are good maturity indicators, particularly for high to over-mature oil. The application of these S-markers overcomes the limitation of hopane and sterane maturity indicators which are restricted to a relatively low maturation range. S-markers are also related to the type of source rock where oils derived from siliceous rocks are rich in BT, DBT, DMDBT, and TMDBT and their relative concentrations are lower than those in carbonate-sourced oils. Individual DBTs are used to indicate susceptibility to biodegradation in the following order: DBT > 4-MDBT > 2-+3-MDBT > 1-MDBT. In this study, PASHs used as S-markers individually or through diagnostic ratios in crude oils and source rocks were reviewed. Different analytical techniques (including classical and innovative approaches) used to quantify and characterize S-markers, are discussed. According to our knowledge, this article is the first overview of PASHs employed as geochemical markers and provides an overview of the different alternatives for sample preparation, instrumental analysis, identification, and quantification of these compounds.