Iodine-129 as a Tracer of Fluids Associated with Hydrocarbon Systems: Results from

the Fruitland Formation, San Juan Basin, CO, and from Other Depositional Settings

By

G.T. Snyder and Udo Fehn

University of Rochester, Rochester, NY

William C. Riese

BP America Production Co, Houston

Jean E. Moran

Lawrence Livermore National Laboratory, Livermore

William L. Pelzmann

BP America Production Co, Houston

Stephen Franks

RockFluid Systems Inc, Plano

Anthony Gorody

Universal Geoscience Consulting Inc, Houston

The long half-life of iodine-129 (15.7 Ma) is particularly appropriate for addressing a number of questions related to hydrocarbon formation, since it can be used to determine fluid ages up to 80 Ma. One of the largest studies of ¹²⁹I within a single sedimentary basin was carried out within the Fruitland Formation coals of the San Juan Basin, CO and NM. Iodine extracted from production well brines yielded adjusted iodine ages of 70 to 75 Ma, in good agreement with the depositional age of the coals. Relationships between ¹²⁹I, ³⁶Cl, and the stable isotopes indicate that the waters are predominantly connate, and have not undergone extensive migration since deposition.

We compare the Fruitland Formation iodine-isotopic signatures with those of other systems, including gas-hydrate deposits, gas seeps in fore-arc regions, and basin deposits. One general trend is that lower ¹²⁹I ratios are correlated with isotopically heavy dD values, indicating a derivation form either brackish or marine waters or from subsequent diagenetic reactions with organic matter. Iodine-ages can be used as indicator of fluid migration, since iodine-ages of the fluids may differ from those of the host formations. Recent meteoric input may be identified, not only by more negative dD values, but also by elevated iodine ratios; however, this is rarely widespread in systems where hydrocarbons are present. As greater attention is directed towards non-conventional reserves, ¹²⁹I will become increasingly important in determining the sources, and possible migration of fluids within these systems.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming