Abstract
Radioactive 129I method is very useful for petroleum exploration. 129I is naturally produced by cosmic-ray-induced spallation of 129Xe in the upper atmosphere, and by spontaneous fission of 238U in the Earth's crust.
129I dating method gives an isolation age of interstitial water from the sea to the sedimentary system. A half-life of 129I is 15.7m.y., so that its application age is restricted between 3 to 90Ma. This is the first and the only geochronologic method to determine the age of fluid in the Tertiary sedimentary basin, while conventional methods are available only for solid or younger fluid. An application to U.S. Gulf Coast basin (Moran et al., 1995) revealed vertical migration of brine from older, deeper sources to present young host formations.
Another application of fissiogenic 129I as a tracer in Paleozoic Anadarko basin, Oklahoma (Moran, 1996), to determine the source rock was also presented.
129I dating method will be expected to give geochemical evidences about fluid migration in the basin, and resolve the genetics of dissolved-in-water type iodine-natural gas deposits in Neogene to Quaternary sediments in Japan.
