Carbazole distributions in carbonate and clastic source rocks

Carbazole, the benzocarbazoles, and their alkyl derivatives have been studied from rock extracts to assess the contribution of facies and maturity in controlling their distribution. Carbonates from the Keg River Formation (Elk Point Group, Middle Devonian, Western Canada) consisting of two different facies reflecting deposition under trangressive and regressive events were used to assess facies influences. To study maturation, a uniform organo-facies of organic rich Posidonia Shale (Lower Toarcian, Jurassic, Northern Germany), from 0.48 to 1.45% R(r) was used. The organic rich Lower Keg Member, deposited under photic zone anoxia, contained predominantly C(4)-C(5) carbazoles. By contrast, the Upper Keg River Member deposited under regressive higher salinity environment contains predominantly carbazole itself, 1-methylcarbazole, and a high abundance of benzo[c]carbazole relative to benzo[a]carbazole. The distribution of the C(1) to C(3) carbazoles was seen to vary with facies. An examination of the carbazole derivatives within the Posidonia Shale reveals that maturation strongly influences the distribution of carbazole, methylcarbazoles, I,8-dimethylcarbazole, I-ethylcarbazole, benzo[a]carbazole, and benzo[c]carbazole. Quantitative determination of these compounds indicates a maximum yield at 0.88% R, for all derivatives, with the exception of carbazole which displays a maximum at 0.53% R(r). A systematic variation of compound ratios with maturity to 0.88% R(r) suggests that selective isomerisation/destruction/generation of carbazole derivatives may occur. In contrast to earlier publications which stress the importance of carbazoles as migration markers in migrated oils, the results presented here indicate that facies and maturation do play a significant role in controlling the distribution of carbazole derivatives in source rocks and other sediments. Copyright (C) 1997 Elsevier Science Ltd.