Oil-oil mixing in deeper water areas of the Gulf of Mexico Basin is demonstrated using geochemical data obtained from more than 1300 oils. Oils that are compositionally intermediate between well characterized end-member families, and oils that have experienced bacterial alteration but contain “fresh” gasoline-range chemistries, are interpreted to have originated through oil-oil mixing. Some oils possess both compositional patterns.

Oils possessing compositions similar to those occurring naturally have been produced in the laboratory by mixing Cretaceous oils from shale-rich sources occurring in the basin with Jurassic oils from carbonate-rich sources. The map distribution of compositionally intermediate oils additionally supports a mixing interpretation.

“Fresh” oils derived from Gulf of Mexico Cretaceous sources tend to contain minor amounts of sulfur and moderate to high API gravities. Oils from Jurassic sources tend to have larger sulfur contents and lower average gravities. Mixing of these end members can be viewed as degrading the quality of the Cretaceous oils, or as improving the quality of the Jurassic oils.

Occurrence of mixed oils indicates overlap of petroleum systems, a phenomenon directly impacting exploration risk. Additionally, total oil volumes might be anticipated to be greater in basinal areas where the Cretaceous and Jurassic systems overlap.

Mixing resulting from multiple migration-accumulation episodes may mainly characterize basins where the principal vector of fluid movement is vertical rather than lateral.

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