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C2-carbazole isomers have been investigated in crude oils from the Hui-Liu Structure Ridge (HLSR) in the Pearl River Mouth Basin (PRMB), South China Sea. The NH shielded isomer, as well as the NH partially shielded isomers, was detected in high abundance and the NH exposed isomers in lower abundance. A small-enrichment trend of 1,8-dimethylcarbazole (DMC) was observed in crude oils along the western part of HLSR (WPHLSR), which may indicate little effect of migration on the C2-carbazole distributions. Two strikingly different distribution patterns of NH partially shielded isomers were observed in the reservoirs along the WPHLSR: one with a preference of 1,3-and 1,6-DMCs and the other with a preference of 1,4-and 1,5-DMCs. All of the oils occurring in the Upper reservoirs have a preference of 1,3-and 1,6-DMCs, whereas those trapped in the Lower reservoirs show a preference of 1,4-and 1,5-DMCs, which may indicate there are two petroleum migration systems in the WPHLSR. [ABSTRACT FROM AUTHOR]

A remarkable invariance in the ratio of 1,3-dimethylcarbazole (DMC) to 1 ,6-DMC was discovered in crude oils from the Pearl River Mouth Basin, South China Sea. The remarkably invariant ratio is kept at a constant of about unity regardless of their concentrations, sources or maturities for the samples. In combination with the molecular structures of 1,3- and 1,6-DMCs, the invariance might indicate that the nitrogen compounds share a common precursor with a skeleton of 1-methylcarbazole and are formed through methylation at C3 and C6 with an essentially identical rate. [ABSTRACT FROM AUTHOR]