Oils from the Santa Cruz basin, southeastern Bolivia, probably were derived from a common source. These oils, however, are in reservoirs of different ages (Tertiary

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to Devonian) and have a wide range in gravity (45 to 65° API). The highest gravity oils typically are in Tertiary units. Geochemical analyses show systematic compositional trends in the C⁵ to C¹⁰ molecular weight range of these oils. Isoparaffins and cycloparaffins tend to increase in relative abundance, whereas normal paraffins and aromatics tend to decrease with increasing gravity.

It is proposed that these compositional trends result from fractionation during migration by accommodation in water. This origin requires that normal paraffins essentially be excluded at the onset of the migration event while aromatics are "swept" through the reservoir site. The enhanced isoparaffin and cycloparaffin content of the most fractionated oils is attributed to their intermediate solubilities. Exsolution of these hydrocarbons is attributed to solubility reduction caused by temperature and pressure decreases and the probable presence of a gas cap. Processes such as thermal fractionation and biodegradation fail to account satisfactorily for observed compositional trends of these oils.

The wide range in reservoir ages and gravities of the oils in the Santa Cruz basin, coupled with the likelihood that the oils were derived from a single source, provide a natural laboratory in which the chemical effects of migration-fractionation can be studied.

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