Application of 129I / 127I to define the source of hydrocarbons of the Pol-Chuc, Abkatún and Taratunich–Batab oil reservoirs, Bay of Campeche, southern Mexico

Homogeneous 129I / 127I ratios from 6.51 ± 1.36 × 10− 14 to 12.6 ± 1.49 × 10− 14 were measured in formation brine at the Pol-Chuc, Abkatún, Taratunich–Batab off-shore oil reservoirs, Bay of Campeche in S-Mexico. Cosmogenic production could account for a homogeneous, Late Cretaceous/Paleocene time period (71.3 ± 5.3 to 56.3 ± 2.9 Ma) for the sedimentation and burial of organic material in the source formation. As the actual reservoir column is formed by Paleocene to Kimmeridgian sediments, the lower part of the lithological column must have received hydrocarbons that migrated downward from an initial source rock (Upper Cretaceous?) during a post-Paleocene event, probably during Miocene. Cosmogenic production from Tithonian shales can be excluded, as 129I would have been decayed. As an alternative or complementary process, the subsurface, radiogenic production of 129I / 127I by 238U-fission in Uranium-enriched sediments should also be considered to explain the present, low 129I / 127I ratios.