Occurrence and Distribution of Carbazoles and Benzocarbazoles in Tertiary Niger Delta Source Rocks

The distributions of carbazoles and benzocarbazoles in source rocks from the Northern depobelt of the Tertiary Niger Delta, Nigeria, have been investigated by gas chromatography-mass spectrometry (GC-MS). Biomarker compositions of the source rocks indicate that the source rocks were formed from organic matter of mixed origin (terrestrial and marine) and are either immature or at early maturity stage. The carbazoles distributions in the source rocks are dominated by C-0-C-2-carbazoles and strong variation was observed in their distributions with increasing maturity. Among the C-1-carbazoles, 1-methylcarbazole shows a decrease trend at the immature stage and an increase trend at higher maturity levels. 2- and 3-methylcarbazoles display an increasing trend with increasing maturity while 4-methylcarbazole exhibits a decreasing pattern with increasing maturity. Within the C-2-carbazoles isomers, 1,8-and 2,7-dimethylcarbazoles, show an increasing trend with increasing maturity while the partially exposed isomers 1,3- and 1,6-dimethylcarbazoles decrease with increasing maturity. The ratio of benzo[a]carbazole/benzo[a]carbazole +benzo[c]carbazole varies significantly from 0.36 to 0.55 in the entire maturity range, indicating a strong maturity dependence. These results show that maturity will have great effect on the use of carbazoles and benzocarbazoles distributions as oil migration parameter in the Niger Delta.