Identification, distribution and geochemical significance of dinaphthofurans in coals

Abstract

All six dinaphthofuran (DNF) isomers have been unequivocally identified in sedimentary organic matter through the co-injection with commercially available standards for the first time. The elution order and retention indices (I) of dinaphthofurans (DNFs) were obtained by GC–MS analysis. Quantum chemical computations were carried out to determine the thermodynamic stability sequence of DNF isomers, which is as follows: dinaphtho[1,2-b;2′,1′-d]furan > dinaphtho[1,2-b;1′,2′-d]furan > dinaphtho[1,2-b;2′,3′-d]furan > dinaphtho[2,1-b;2′,3′-d]furan > dinaphtho[2,3-b;2′,3′-d]furan > dinaphtho[2,1-b;1′,2′-d]furan. The DNF isomers are ubiquitous in coal samples in this study. The effects of maturity on the distribution patterns of DNFs in the coals were investigated and the relative abundance of dinaphtho[1,2-b;2′,1′-d]furan (1221DNF) to dinaphtho[1,2-b;2′,3′-d]furan (1223DNF) is significantly controlled by maturation. Based on theoretical calculations and geochemical data, a dinaphthofuran maturity indicator, defined as DNFR (1221DNF/1223DNF), is proposed. A preliminary calibration of DNFR against the vitrinite reflectance (%Ro) was established with the following relationship: %Rc = 1.27 × DNFR – 1.12 (for %Ro values > 1.00). DNFR is a useful indicator in evaluating the maturity for sediments with Type II–III organic matter. DNFs in coals may be produced during diagenesis and catagenesis and oxic environment contributes to their generation. This study helps us better understand the occurrence and significance of complex oxygen-containing organic compounds in sedimentary organic matter.