Novel arylated polyaromatic thiophenes: Phenylnaphtho[ b]thiophenes and naphthylbenzo[ b]thiophenes as markers of organic matter diagenesis buffered by oxidising solutions

Abstract

Three naphtho[ b]thiophenes (asymmetric dibenzothiophene isomers) are minor components relative to the prevailing dibenzothiophene constituents of aromatic fractions of diagenetically oxidised marine sedimentary rocks. Interestingly, the rocks containing naphtho[ b]thiophenes often also contain their arylated derivatives, phenylnaphtho[ b]thiophenes, as major components relative to phenyldibenzothiophenes. Both these series are accompanied by other structural isomers, i.e., naphthylbenzo[ b]thiophenes. To our knowledge, phenylnaphtho[ b]thiophenes and their naphthylbenzo[ b]thiophene isomers have not been reported in geological samples and are unambiguously identified here using synthetic standards. The distribution of arylated thiophenic polycyclic aromatic compounds strongly depends on maturity. At low maturity, where kinetically controlled distribution of the arylated thiophenes is expected, the greatest number of isomers is observed, suggesting that low regioselectivity reactions are involved in their formation. At the beginning of the oil window, small amounts of unstable 3-phenylnaphtho[ b]thiophenes, together with three 2-phenylnaphtho[ b]thiophenes, naphthylbenzo[ b]thiophenes, with major 2-(2-naphthyl)benzo[ b]thiophene, and all four possible phenyldibenzothiophenes, with abundant kinetically favoured 1- and 4-substituted isomers, are present. Experimental free radical phenylation of dibenzothiophene yielded a phenyldibenzothiophene isomeric distribution very similar to that observed in natural samples, but different from that resulting from ionic phenylation. Since these arylated polycyclic aromatic thiophenes occur exclusively in samples in which sedimentary pyrite was hydrothermally transformed to hematite/goethite as a result of rock interaction with oxidising brines, such solutions, usually rich in benzene, can be considered as a potential source of phenyl species. At more advanced stages of maturity, 3-phenylnaphtho[2,1- b]thiophenes are transformed to 2-phenylnaphtho[2,1- b]thiophenes, as revealed by artificial maturation experiments. In turn, severe heating of 2-phenylnaphtho[2,1- b]thiophene leads to its complete disappearance and the formation of dibenzothiophene and phenyldibenzothiophenes. Under the same conditions, only phenyldibenzothiophenes isomerise, presumably by a 1,2-phenyl shift, and 1-phenyldibenzothiophene also cyclises to triphenyleno[1,12- bcd]thiophene. The thermodynamically controlled distribution of phenyldibenzothiophenes, dominated by 3- and 2-substituted isomers, is reached at about 1.2% R r (vitrinite reflectance). The distribution is similar to that encountered in hydrothermal petroleum generated above 300 °C in the rift system of the Guayamas Basin. Such a distribution can be experimentally obtained via prolonged heating a mixture of all possible phenyldibenzothiophene isomers at 330 °C in the presence of an acidic clay catalyst. The ratio between the sum of all phenyldibenzothiophenes and 2-phenylnaphtho[ b]thiophene plus 2-(2-naphthyl)benzo[ b]thiophene changes regularly with advancing maturity, suggesting a potential application of these newly identified compounds for maturity assessment.