Characteristics of carbazole compounds in ultra-deep marine oil from Fuman oilfield, Tarim Basin: Significance for thermal maturity assessment of crude oil

•Carbazoles in crude oil of Fuman Oilfield is affected by thermal maturity.•The concentrations of carbazole in marine oil decrease as maturity increases.•%Rc = −0.0335(1,8-DMCa/1-ECa) + 1.2902, %Rc calculated from MPI1.•%Rc1 = −0.0405 (1,8-DMCa/1-ECa) + 1.3418, %Rc1 calculated from MPR. Carbazoles in Ordovician ultra-deep marine oil from the FI17 fault zone in Fuman oilfield (Tarim Basin) were separated using a recently proposed silica gel column chromatographic method and the enriched fractions were analyzed by GC–MS. Biomarker and carbon isotope signatures revealed that all oil in the study area was produced from the same source rock and that compositional differences can be attributed to thermal maturation. The convergent ratios of N-H shielded isomers/N-H half shielded isomers and benzo[a]carbazole/(benzo[a]carbazole + benzo[c]carbazole) suggested that carbazoles in crude oil had not been affected by vertical migration. Therefore, thermal maturity was identified as the main controlling factor affecting chages of carbazole concentrations and ratios in crude oil. The concentrations of the total carbazole and its three isomers (N-H shielded isomers, N-H half shielded isomers and exposed isomers) in crude oil decreased sharply with increasing maturity. The 1,8-dimethylcarbazole/carbazole (1,8-MCa/Ca) and 1,8-dimethylcarbazole/2,4-dimethylcarbazole (1,8-MCa/2,4-MCa) ratios showed significant correlation with maturity expressed as %VRE (the vitrinite reflectance equivalent converted from MPI1 and MPR) when %VRE is