Influence of maturity on carbazole and benzocarbazole distributions in crude oils and source rocks from the Sonda de Campeche, Gulf of Mexico

Carbazole geochemistry has been studied for a petroleum system in which vertical migration is prominent and where reservoirs are locally sourced. An essentially uniform organofacies of organic-rich Tithonian source rocks from the Sonda de Campeche (Gulf of Mexico), covering a well defined maturity sequence (0.36-1.29% R-r) and associated crude oils (0.49-0.92% R-c) from Palaeocene reservoirs formed the sampling base. Comparison of the distribution of alkylcarbazoles and benzocarbazoles in rock bitumens and oils revealed that fractionation due to primary expulsion had no effect on the distribution of shielded/exposed carbazoles within crude oils. Perhaps more importantly, the benzocarbazole ratio in both rock extracts and crude-oils increased with maturity, indicating that this parameter cannot be directly used as a migration indicator in petroleum systems where vertical migration through faults and fissures represents the main avenues of oil migration. (C) 1998 Elsevier Science Ltd. All rights reserved.