Oil formation and fluid convection in Railroad Valley, NV: a study using cosmogenic isotopes to determine the onset of hydrocarbon migration

The onset of hydrocarbon migration is a critical question for the understanding of the formation processes of ail reservoirs. We present here first results on the use of cosmogenic isotopes to address this question. We tested the application of I-129 and Cl-36 determinations on oil field brines from Railroad Valley, NV, where several oil fields exist in close association with geothermal activity. Residence times in the reservoir rocks calculated using the I-129 system in oil field brines coexisting with oils in Railroad Valley range from 6-24 Ma. This range is shifted to 7-28 Ma if the fissiogenic contribution from reservoir rocks is taken into account. The spread in residence times indicates that oils were separated from source regions at different times during the evolution of fluid convection. The longest residence time is interpreted as the onset of fluid convection in this region. This corresponds well with the dating of structural activity in Railroad Valley, a likely mechanism for geothermal activity in this region.