Norlex project workshop Oct 4th-5th 2007 (http://norges.uio.no) & Novaia Zemlya Project (www.uio.no/novazem)
THE 187Re-187Os GEOCHRONOMETER IN SEDIMENTARY SYSTEMS:
ABSOLUTE DATING OF SOURCE ROCKS AND CRUDE OIL GENERATION
D. Selby
University of Durham, Dept. Earth Sciences, Durham DH1 3LE, UK (david.Selby@durham.ac.uk)
Analytical advancements during the past 15 years has allowed wide application of the
(rhenium)-187Os (osmium) radioactive isotope system in Earth Science, both as a
geochronometer and process tracer. The geochemical character of Re and Os (chalcophile,
siderophile, and organophile) is quite different from other radioactive isotopic systems based on
lithophile elements (Rb-Sr, Sm-Nd, K-Ar, U-Pb). This allows access, with a geochronometer and
process tracer, to geochemical pathways otherwise not accessible. Applications relevant to
sedimentary systems are the determination of Re-Os depositional ages for clastic sedimentary
rocks that are common hydrocarbon source rocks, such as organic-rich shales and establishing
the timing of oil generation.
The 187Re-187Os system has been known to yield depositional ages for organic-bearing
clastic units, e.g., black shales for some time. The later results from Re and Os being
sequestered from seawater under anoxic / suboxic conditions and being incorporated in organic
matter. Improved analytical methodologies for digestion (organic-selective dissolution techniques)
and optimal sampling strategies high-precision (< ± 1% 2) Re-Os ages been attained. In
addition, it is now established that Re-Os geochronometer in black shale remains undisturbed
through hydrocarbon maturation and in some cases chlorite-grade metamorphism allowing
precise depositional ages to be determined from a wider range of shales than previously thought
possible (e.g., units with TOC contents ~ 0.5%).
Accurate Re-Os black shale dates are best illustrated by comparison to units or
boundaries for which precise U-Pb zircon age determinations exist. A black shale at the
Devonian-Carboniferous (DC) boundary in Western Canada, has yielded a Re-Os age of 361.3 
2.4 Ma (2, including  uncertainty), in accord with the most recent U-Pb zircon age
interpolations. This result, and other high-precision results for Jurassic units (OxfordianKimmeridgian Staffin shale), demonstrates that the Re-Os shale geochronometer has a role to
play in timescale calibration research and stratigraphic correlation, especially in sections with
limited potential for ashbed U-Pb zircon dating.
187Re
The known associated of Re and Os with organic matter in black shale, a common
source of hydrocarbons, has led to the identification of the utility of crude oil for Re-Os
geochronology. The Re-Os data for oil from six Alberta Oil Sand deposits yield a Re-Os date of
112 ± 5.3 Ma (Isoplot Model 3, MSWD = 2.2, 2) with an initial 187Os/188Os of 1.43 ± 0.11. This
date likely represents the period of in-situ radiogenic growth of 187Os following hydrocarbon
migration and reservoir filling, thus directly dating these processes, and this formation age is in
agreement with recent burial history models for parts of the Western Canada Sedimentary Basin.
Oil generation ages have also been established form the Shetland basin, North Sea that are in
good agreement with the burial history models and fluid flow age. These studies suggest that ReOs isotopes in crude oil may approach isotopic resetting, or homogenization, during generation,
but that Re/Os fractionation also exists that might be organic-based. Recent work has also shown
that the asphaltene fraction of crude oil can be used to approximate the Re-Os isotopic character
of the whole oil. The Re-Os data demonstrates the promising utility of organic-rich sediments and
crude oil has to yield valuable age information on the geochronology of sedimentary / petroleum
systems.