Effects of paleolatitude on the stable carbon isotope composition of crudeoils

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 delta(13)C values for crude oils from any specific ti me interval. These ranges have generally been attributed to differences in the relative contributions of biomass inputs of varying isotopic compositio ns and, to a lesser extent, isotopic fractionations associated with crude o il formation. However, a potentially important parameter that appears to ha ve been largely overlooked is the effect of paleolatitude on delta(13)C of the marine biomass. In this study, Upper Jurassic-sourced oils representing most major and minor petroleum basins were investigated to assess the effe cts of paleogeography on their delta(13)C values. From high to low paleolat itudes, the oils become increasingly enriched in C-13. Thus, for this speci fic geologic time interval (and likely others), the delta(13)C values of oi ls from the major marine basins reflect that of the primary marine biomass, which varied as a function of spatial paleoenvironmental parameters, in pa rticular sea-surface paleotemperature.