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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
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Petroleum is a fragile fluid, a part of which is likely to be destroyed or lost to the atmosphere. The moment crude oil separates from bitumen in the source rock, it begins to undergo compositional changes that continue throughout migration and accumulation. The processes and factors influencing oil composition in an accumulation take effect before, during, and after entrapment. The primary factors influencing oil composition before entrapment are source rock characteristics, primary migration, and secondary migration. The main factors influencing oil composition during entrapment are pressure and temperature, as they establish the conditions under which secondary alteration processes take place. After entrapment, secondary alteration processes influencing oil composition are the effect of thermal maturation, physical and biological degradation, gravity segregation, dysmigration (separation-migration), and deasphalting. These alteration processes can increase the API gravity and gas-oil ratio of the oil, but generally decrease the API gravity.
Knowledge of the conditions and mechanisms of degradation are needed to understand the nature and distribution of oil shows and to carry out oil-oil and oil-source rock correlations. Considerable progress has been made in understanding secondary alteration processes, but more is expected as the more poorly understood macromolecular and polar fractions are investigated. Presently, molecular chemistry is a useful tool that can recognize incipient biodegradation, detect oil gravity segregation, and discriminate a pyrobitumen from a precipitated asphaltene, whereas bulk analyses are currently inaccurate.
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