Assessment of generation temperatures of crude oils

Biological marker maturity parameters were used to estimate the minimum HC generation temperatures of crude oils from Eastern Hungary. More than 50 oils and oil shows were analysed. Molecular- and homologous-ratios of biological marker compounds (triterpanes, steranes, mono- and triaromatic steroid hydrocarbons) were used as maturation parameters. The oils have at least five maturity stages, i.e. they have been generated under different thermal conditions. The highest reservoir temperature in each group was chosen as the best estimate of the groups' temperature just below the generation temperature, i.e. reservoirs of the group might be expected to be at shallower depths (lower temperatures) than those of the generation zone due to vertical migration into pools. For each maturation level, a threshold temperature range for genesis was inferred from reservoir temperatures; they are from 130-135 degreesC for the least mature oils to 210-215 degreesC for the most mature oils. In the least mature oils cracking was not observed, hence carbon-carbon cracking reactions had not taken place during their genesis. The most mature oils an intensively cracked oils; they are almost condensates. Two major genetic groups (families) of oils were found in the area. Both are present in each maturation level. The effects of migration were checked, and no influence on maturation was found. A number of the oils are in overpressured reservoirs within, or just above, the zone of the present-day active oil generation, hence the present-day temperatures of the pools must have been maximum temperatures. Contrary to the traditionally accepted temperature range for petroleum generation-maturation reactions (50-150 degreesC), there is strong evidence from this study that the onset of oil generation requires temperatures higher than 130 degreesC and is still proceeding above 215 degreesC. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.