129I and 36Cl in dilute hydrocarbon waters: Marine-cosmogenic, in situ, and anthropogenic sources

The long-lived halogen radioisotopes 129I and 36Cl provide valuable information regarding the source of fluids in hydrocarbon systems and in localized areas where infiltration of younger meteoric water has occurred. Despite the utility of these two isotopes in providing time-signatures for fluid end-members, considerable uncertainty remains regarding the interpretation of “intermediate-age” waters in hydrologic systems. These waters are likely the result of the combination of two or more halogen sources at some time in the past, each with its own characteristic concentration and isotopic composition. In order to unravel the evolution of these “intermediate-age” waters, the effect that infiltration of meteoric water has on the isotopic composition of older formation waters is modeled. Also evaluated is the effect that the timing of dilution has on 129I and 36Cl signatures observed in the present, specifically, the hypothesis that halogen isotopic signatures imparted by the mixing of brine and meteoric waters early in the development of a sedimentary basin are quantitatively different from those imparted by the mixing of old brines with recent meteoric waters.