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

Abstract The long-lived halogen radioisotopes 129 I and 36 Cl 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 129 I and 36 Cl 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. The modeled results are compared to previously published isotopic data from production wells in the Fruitland Formation coalbed methane system. Portions of the basin preserve enough of the original brine source to retain 129 I/I ratios roughly recording the age of deposition of organic matter (73–76 Ma) even where some degree of dilution by meteoric water has occurred. However, the 129 I signatures of waters that presently contain less than 10% of the original formation water component indicate that mixing has occurred within the past 10 Ma, consistent with recently published 4 He data. Both 36 Cl concentrations and 36 Cl/Cl ratios show that waters containing more than 10% formation water are all in secular equilibrium with the host formation coals. In contrast, waters with less than 1% of the original formation water’s Cl − content all have 36 Cl/Cl ratios which are greater than secular equilibrium values, indicating the migration of meteoric water into the coal formation more recently than 2 Ma. Although 36 Cl signatures in the Fruitland Formation are not significantly affected by anthropogenic input, 129 I/I ratios in a subset of samples that retain less than 1% of the initial formation water component suggest input of anthropogenic 129 I during the past 50 a.