Hydrocarbon seeps harbor copious chemosynthesis-dependent life, the traces of which are preserved in the fossil record within authigenic carbonates. These environments are mostly characterized by seepage of methane-rich fluids, yet numerous crude oil–dominated seeps have been discovered in recent years. Oil seepage has a profound influence on the local fauna, but recognizing such seeps in the rock record remains elusive. This study presents new geochemical data that will allow for a more confident identification of ancient oil-seep deposits. Geochemical data from modern and ancient seep limestones reveal that oil-dominated seep carbonates are enriched in rare earth elements and uranium compared to their methane-dominated counterparts. These trace element patterns have the potential to serve as a basis for an improved understanding of the adaptation of chemosynthetic life to oil seepage, and to better constrain the marine carbon cycle in the geologic past.