Identification, distribution and geochemical significance of phenyldibenzofurans in coals

Abstract

Four phenyldibenzofuran (PhDBF) isomers were unambiguously identified for the first time in coals through co-injection of commercially available standards. PhDBF isomers were detected in all coals from the East China Sea Shelf and the Ordos basins. The stability order of PhDBFs was confirmed to be 4-PhDBF > 2-PhDBF > 3-PhDBF > 1-PhDBF. The degree of molecular structural deformation of PhDBFs was also determined. The influence of thermal maturity on the distributions of PhDBFs in sedimentary organic matter containing Type II–III kerogen was systematically investigated, and the concentration of 4-PhDBF relative to 2-PhDBF and 3-PhDBF was found to be influenced by thermal maturation. The previously proposed maturity indices based on PhDBF ratios (PhFR-1 and PhFR-2) were found to be useful in coal facies with high thermal maturity (≥ 1.0 %Ro) on the basis of density functional theory (DFT) calculations and geochemical data. Two calibrations of PhFR-1 and PhFR-2 against the measured vitrinite reflectance (%Ro) were established, which are as follows: %Rc = 3.0 × PhFR-1/100 + 1.0 (≥ 1.0 %Ro) (R2 = 0.95) and %Rc = 7.0 × PhFR-2/100 + 1.0 (≥ 1.0 %Ro) (R2 = 0.96). PhFR-1 and PhFR-2 have good correlations with the other widely used thermal maturity indicators. The oxic sedimentary environment is likely more beneficial to the generation of phenyldibenzofurans and methyldibenzofurans. PhDBFs in coals may be formed during diagenesis or early catagenesis and intermediates in the producing of more condensed aromatic compounds from high thermal maturity. The results of this study provide a deeper insight into the occurrence, distribution and significance of PhDBFs in sedimentary deposits.