Identification of phenyldibenzothiophenes in coals and the effects of thermal maturity on their distributions based on geochemical data and theoretical calculations

Highlights

• All four phenyldibenzothiophene isomers are unequivocally identified in coals.

• The thermodynamic stabilities and geometric optimizations of PhDBTs were calculated.

• Two maturity indicators of PhDBTs for highly mature geological samples are proposed.

• The distributions of PhDBTs and MDBTs were compared.

• The origins of the phenyldibenzothiophenes in coals are discussed.

Abstract

All four phenyldibenzothiophene (PhDBT) isomers were unequivocally identified in coals for the first time by the co-injection of authentic standards and comparison of the retention indices with those reported in previous studies. Based on density functional theory (DFT) calculations, the relative thermodynamic stability order of the phenyldibenzothiophene isomers (PhDBTs) was determined to be 3-PhDBT > 2-PhDBT > 4-PhDBT > 1-PhDBT. All PhDBT isomers were observed in a suite of coals from the Junggar and Ordos basins, and the effects of thermal maturity on their distributions were systematically investigated. Based on geochemical data and theoretical calculations, two phenyldibenzothiophene maturity indicators, i.e., PhDR1 (2-PhDBT/4-PhDBT) and PhDR2 ((2-PhDBT + 3-PhDBT)/4-PhDBT), are proposed. For high maturity coals (≥0.96 %Ro), PhDR1 and PhDR2 increase gradually with increasing thermal maturity. Two preliminary calibrations of these parameters against measured %Ro were made: %Rc = 0.60 × PhDR1 + 1.00 (≥1.00 %Ro) and %Rc = 0.30 × PhDR2 + 1.00 (≥1.00 %Ro). Compared to their corresponding methylated counterparts, phenyldibenzothiophenes are particularly useful as molecular markers of maturity at higher levels of thermal stress and provide a molecular assessment for sedimentary rocks containing types II and III kerogens where few biomarker parameters are available. PhDBTs in coals are likely to be generated during diagenesis/catagenesis as intermediate products in the formation of more condensed heterocyclic polycyclic aromatic compounds, such as triphenyleno[1,12-bcd]thiophene and benzobisbenzothiophenes, which were detected in the highly mature samples. This study broadens the current understanding of the occurrence, distribution, geochemical significance, and origin of phenyldibenzothiophenes in sedimentary organic matter.

Introduction

Methyldibenzothiophenes (MDBTs) and their corresponding phenyl substituted counterparts are important sulfur-heterocyclic aromatic compounds in crude oils and ancient sedimentary rocks (e.g., Radke et al., 1986, Radke, 1988, Marynowski et al., 2002, Li et al., 2008, Li et al., 2013, Li et al., 2014). However, compared with MDBTs, the identification, occurrence, distribution, geochemical application, and origin of phenyldibenzothiophene (PhDBT) have received less attention.

Three phenyldibenzothiophene isomers (PhDBTs) were first detected in volatiles formed during the pyrolytic carbonization of coal tar pitches for gas chromatography–mass spectrometry (GC–MS) analysis, but the specific positional isomers were not identified due to a lack of synthetic standards (Meyer zu Reckendorf, 1997). Later, all four phenyldibenzothiophene isomers were identified in coal tar pitches and ring furnace flue gases by injection of authentic standards (Meyer zu Reckendorf, 2000). Marynowski et al. (2002) identified the four phenyldibenzothiophene isomers in Paleozoic marine sedimentary rocks, hydrothermal oil, and bitumen by comparing their retention indices and mass spectra with those of synthetic standards and the results of previous studies. Subsequently, PhDBTs were detected in other ancient sedimentary rocks (Rospondek et al., 2007, Rospondek et al., 2008, Rospondek et al., 2009, Grafka et al., 2015). However, the identification, occurrence, and distribution of phenyldibenzothiophenes in coals have not yet been reported.

The origin of phenyldibenzothiophenes remains controversial. Meyer zu Reckendorf (2000) proposed that PhDBTs are intermediates in the transformation of dibenzothiophene (DBT) into benzobisbenzothiophenes and triphenyleno[1,12-bcd]thiophene during oxidative pyrolysis. Marynowski et al. (2002) speculated that 1-PhDBT might originate from sulfur incorporation during diagenesis into unsaturated aliphatic moieties (Sinninghe Damsté and de Leeuw, 1990) or carbohydrates (Sinninghe Damsté et al., 1998). In addition, Marynowski et al. (2002) suggested that hydrothermal oxidation is important for the formation of PhDBTs as these compounds are absent in associated samples that have not experienced diagenetic or catagenetic oxidation. Rospondek et al. (2007) reported that the free radical phenylation of dibenzothiophene can account for the occurrence and distributions of phenyldibenzothiophenes in natural rocks based on laboratory simulation experiments.

Several studies have been conducted to investigate the geochemical application and formation of PhDBTs. Marynowski et al. (2002) proposed the 2-PhDBT/(2- + 1-)PhDBTs ratio as a maturity parameter based on the relative abundances of phenyldibenzothiophene isomers in sedimentary rocks of various thermal maturities. Rospondek et al. (2007) determined that the PhDBTs/(∑2-PhN[b]T + 2-(2-naphthyl)B[b]T + PhDBTs) ratio can be used as a maturity indicator and exhibits a good correlation with the vitrinite reflectance (%Ro). Rospondek et al. (2008) evaluated the relative thermodynamic stabilities of phenyldibenzothiophene isomers and investigated equilibrium mixture compositions of PhDBTs using laboratory maturation experiments, molecular modeling, and geological data. In addition, Rospondek et al. (2008) proposed that 1-PhDBT can isomerize to 2-PhDBT through the 1,2-phenyl shift and cyclize to triphenyleno[1,12-bcd]thiophene. While previous studies have demonstrated the potential application of phenyldibenzothiophenes, the proposed PhDBTs maturity parameters have only been preliminarily established and have not been calibrated for practical application (e.g., for any specific relationship with vitrinite reflectance).

In this study, we unequivocally identified all four phenyldibenzothiophene isomers (m/z 260) in coals for the first time by co-injection of authentic standards in GC–MS analyses and through comparison of their retention indices (I) with those previously reported. We present quantum chemical calculations to determine the thermodynamic properties of the PhDBTs in order to evaluate their relative stabilities. The occurrence and distributions of PhDBTs in coals with various thermal maturities are systematically described. Based on geochemical data and theoretical calculations, we discuss the implications of using PhDBTs in the assessment of thermal maturity and the origin of PhDBTs through comparisons with MDBTs.