Seventeen oils from the Middle Magdalena Valley of Colombia were studied for the purpose of acquiring an improved view of the number and nature of hydrocarbon sources in the region. Oils from this basin possess a range of bulk compositions (API gravity, sulfur content, gasoline content, etc) and occur mainly in Tertiary reservoirs developed in nonmarine sediments. Several explanations have been informally offered regarding the origin of the oils. Most of these explanations have depended on a concept of the "immature" oil.

Examination of the chemistry of the oils and comparison of these data to oils from other basins lead to the conclusion that the Magdalena oils were derived from a single source or series of closely related sources. The sources for the oils are believed to be stratigraphic units deposited in marine environments. This conclusion is based mainly upon the isoprenoid, normal paraffin, and polycyclic alkane composition of the oils. A marine source virtually eliminates the possibility of the source occurring within the Tertiary section.

Study of lithologic descriptions of the Cretaceous sediments stratigraphically underlying the Tertiary allows the nomination of several units that may have been the source for the oils: the La Luna, Simiti, and Paja formations. Each of these formations was deposited under anoxic marine conditions, and some of them contain enough organic debris to be described as "carbonaceous."

Differences in the bulk compositions of the oils are interpreted to be related to secondary alteration processes. Altered oils from the Magdalena possess compositional attributes similar to oils that have been modified during in-vitro bacterial degradation experiments. There is little evidence that collectively the oils represent a range of maturities; consequently, the idea that the more dense fluids are examples of immature oils is rejected.

The Middle Magdalena oils are derived from a single source or a series of closely related sources that are probably Cretaceous in age. Some of the oils have experienced secondary alteration that is believed to account for the range of bulk compositions observed in the region.

The complicated source-reservoir relationships suggested by knowledge of the oil chemistry are undoubtedly contributory to protracted exploration history of the Magdalena. Improved knowledge of these relationships should be of assistance in the search for new reserves that may be obscured by the complex structural features characteristic of the Magdalena and adjacent terrain.

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