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Geochemical studies provide important data relevant to the origin of the oils in the Santa Cruz basin, Bolivia. Production from this basin occurs from rocks of Devonian, Carboniferous, Cretaceous, and Tertiary ages. The productive structures are usually undisturbed by major faulting. The Devonian sediments are composed of sandstones and dark marine shales. The post-Devonian rocks are generally oxidized and probably nonmarine. The Tertiary and Cretaceous reservoirs usually contain the highest API° gravity oils.
Comparison of geochemical data (N5-N10 molecular weight range) shows that the oils are very similar; however, systematic compositional trends occur as a function of API° gravity. These trends are interpreted from gross structural group data. Isoparaffins and cycloparaffins increase in relative abundance, while normal paraffins and aromatics decrease with increasing API° gravity. A model is proposed that rationalizes these compositional trends by a mechanism of "accommodation" in water. The model requires enrichment of hydrocarbons of intermediate solubility, partial exclusion of hydrocarbons of low solubility, and retention in solution of the more soluble hydrocarbons. Processes such as thermal fractionation and biodegradation fail to account satisfacto ily for the observed compositional trends.
The compositional interrelationships of the oils coupled with the geologic framework suggest that these oils have a common source, most probably the Devonian. Differences between the oils are attributed to fractionation occurring during migration.
Exploration risk for areas such as the Santa Cruz basin can be substantially reduced by use of the knowledge derived from petroleum geochemistry.
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