Determination of source ages and migration patterns of brines from the U.S. Gulf Coast basin using 129I

The long-lived cosmogenic and fissiogenic isotope 129I has been applied to the study of source ages and migration patterns of oil field brines from the U.S. Gulf Coast basin. 129II ratios were measured in sixty samples of saline formation water from depths of 660–4503 m in several oil fields in southeast Texas, south-central Louisiana, and offshore Louisiana. Comparisons between measured ratios and the decay curve for surface hydrospheric 129I result in minimum source ages much older than present host formation ages, indicating vertical migration of brine from older, deeper sources. Conservative corrections for a fissiogenic component due to spontaneous fission of 238U in the subsurface result in the following median source ages: Iberia field: 55 Ma, Port Barre field: 53 Ma, and Eugene Island Blocks 316 and 330: 55 Ma, pointing to Wilcox Group shales as source formations. Mesozoic sources cannot be ruled out, due to the uncertainties in estimating the fissiogenic component. Some of the ratios measured in Texas Gulf Coast brines show evidence of a greater fissiogenic component which indicates that the brines have resided in formations with locally high U concentrations.