Attempts to determine the geologic ages of crude oils based on their stable carbon isotope compositions have been unsuccessful. This is a consequence of the large range of δ13C values for crude oils from any specific time interval. These ranges have generally been attributed to differences in the relative contributions of biomass inputs of varying isotopic compositions and, to a lesser extent, isotopic fractionations associated with crude oil formation. However, a potentially important parameter that appears to have been largely overlooked is the effect of paleolatitude on δ13C of the marine biomass. In this study, Upper Jurassic–sourced oils representing most major and minor petroleum basins were investigated to assess the effects of paleogeography on their δ13C values. From high to low paleolatitudes, the oils become increasingly enriched in 13C. Thus, for this specific geologic time interval (and likely others), the δ13C values of oils from the major marine basins reflect that of the primary marine biomass, which varied as a function of spatial paleoenvironmental parameters, in particular sea-surface paleotemperature.