Carbazole distributions in rocks from non-marine depositional environments

Abstract

Pyrrolic nitrogen compounds have been studied from extracts of the rocks collected from the Tertiary saline lacustrine strata in the Qaidam basin and the Middle-Lower Jurassic coal-bearing strata in the Turpan basin, NW China. Biomarker signatures revealed the presence of three sample groups, related to saline lacustrine conditions in the Qaidam Basin, and freshwater lake and swamp depositional settings in the Turpan Basin. Significant differences in carbazole distributions were observed among the sample groups from different depositional environments. Generally, a strong predominance of 1,4- + 1,5-dimethylcarbazoles over 1,3- + 1,6-dimethylcarbazoles was found for the Qaidam rocks from the saline lacustrine settings, with a ratio of 1,3- + 1,6-/1,4- + 1,5-dimethylcarbazole ranging from 0.35 to 0.66, while enhanced amounts of 1,3- + 1,6-dimethylcarbazoles relative to 1,4- + 1,5-dimethylcarbazoles were detected for the Turpan samples from the swamp environments, with the 1,3- + 1,6-/1,4- + 1,5-dimethylcarbazole ratio >0.8. Samples from the freshwater lake environments in the Turpan Basin showed intermediate values for the 1,3- + 1,6-/1,4- + 1,5-dimethylcarbazole ratio. In addition, a predominance of 2,4-dimethylcarbazole over 2,6-dimethylcarbazole was observed for the Qaidam samples, whereas most Turpan samples showed elevated amounts of 2,6-dimethylcarbazole relative to 2,4-dimethylcarbazole. Moreover, 3-methylcarbazole occurred as the least abundant C 1-carbazole isomer for most Qaidam rocks, while the Turpan samples from swamp settings often showed an enhanced abundance of 3-methylcarbazole. The 1,3- + 1,6-/1,4- + 1,5-dimethylcarbazole ratio was observed to have a positive correlation with 3-/4-methylcarbazole, 2,6-/2,4-dimethylcarbazole and Pr/Ph, respectively. These observations may indicate that C 1- and C 2-carbazoles with a methyl group at C-3 (C-6) prevailed for swamp settings, whereas isomers with a methyl at C-4 (C-5) were dominant for the freshwater or saline lacustrine environments. Although variations in benzocarbazoles were detected in the bitumens, an obvious correlation was not observed for the different depositional environments.