Re-Os Geochemistry of Source Rocks and Hydrocarbons: Ages, Correlations, and Timing of Migration

Holly J. Stein and Judith L. Hannah
Geosciences, Colorado State University, Fort Collins, CO

Advances in Re-Os geochemistry now permit direct dating of black shales and tracing of hydrocarbons. These two redox-sensitive elements are concentrated in organic material, and remain fixed as long as conditions remain reducing. The Re-Os isotopic composition of organic material deposited in anoxic environments reflects that of seawater at the time of deposition. The ¹⁸⁷Os/¹⁸⁸Os ratio in seawater is controlled by input of Os from hydrothermal systems and cosmic dust (chondritic ¹⁸⁷Os/¹⁸⁸Os = 0.127 today), and from continental runoff (average ¹⁸⁷Os/¹⁸⁸Os ≈ 1 today). Over time, seawater ¹⁸⁷Os/¹⁸⁸Os is variable, changing with the quantity and composition of Os that is delivered to the oceans from the continents. Decay of ¹⁸⁷Re to ¹⁸⁷Os provides a radiometric clock for the depositional age of black shales, and generates a unique ¹⁸⁷Os/¹⁸⁸Os ratio that fingerprints the organic material in the shales. Thus, the Re-Os system serves as both a chronometer and a tracer. Thoughtful sampling strategy is crucial for accurate, high-precision Re-Os ages determined by the isochron method. Combining widely spaced samples or mixing Re and Os from hydrogenous and detrital sources will introduce scatter. By chemically extracting Re and Os from only the organic material in black shales, we can selectively analyze hydrogenous Re and Os. Both Re and Os are strongly fractionated into the heavier organic compounds (Selby et al. 2007), so that Re-Os systematics in source rocks are maintained through petroleum maturation; thus, Re and Os concentrations in migrated hydrocarbons are lower, but still measurable. As hydrocarbons may be derived from mixed sources and/or accumulate over time, isochrons for migrated oils may show considerable scatter. Nevertheless, the mean age and initial ¹⁸⁷Os/¹⁸⁸Os constrain possible models for source rocks and timing of migration.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas