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Twenty-five crude oil samples from Tertiary reservoirs in the Middle Magdalena Valley, Colombia, were studied by geochemical methods in order to understand their genetic relationships. The oils were probably derived from both calcareous and siliciclastic facies of the Upper Cretaceous La Luna Formation. Some oils in the eastern margin contain relatively higher concentrations of land plant derived biomarkers than the remaining oil samples. The molecular compositions of the oils observed in this study appear to be consistent with the inferred depositional sequence of anoxic marine/pelagic carbonate facies in the north and western sectors of the Middle Magdalena Valley, while the eastern sector received a higher proportion of clastic input. Biomarker maturity parameters indicate that most of the oils were generated from the early phase of the oil window, except for the Colorado-38 oil that was generated during the peak of the oil window. Geochemical data support “local” generation and migration of the Middle Magdalena oils as opposed to longer distance migration from the region of the eastern Cordillera. The range of oil gravities measured in the Middle Magdalena Valley is attributed to organic facies variations, different levels of biodegradation, and late charge of condensate rather than to thermal maturity and migration-related effects. |
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