25th International Meeting on Organic Geochemistry IMOG 2011

P-288
Re/Os fractionation during generation and evolution of
hydrocarbons
Fatima MahdaouI 1,2 , Laurie Reisberg 1 , Raymond Michels 2 , Yann Hautevelle 2 , Yannick
Poirier 3 , Jean-Pierre Houzay 3
1 Centre de Recherches Petrographiques et Geochimiques, Nancy Université, CNRS UPR2300,
Vandoeuvre-lès-Nancy, France, 2 UMR7566 G2R, CNRS, CREGU, Nancy Université, Vandoeuvre-lès-Nancy,
France, 3 TOTAL Centre Scientifique et Technique Jean-Féger, Pau, France (corresponding
author:fatima.mahdaoui@g2r.uhp-nancy.fr)
Rhenium (Re) and osmium (Os) are organophile
elements. For this reason, the Re-Os radiogenic
system is well adapted to the dating of organic-rich
rocks such as black shales. Recently, it has been
suggested that this system can also be used to date
the major events occurring during the evolution of
hydrocarbons (Selby and Creaser, 2005) [1]. In
particular, in their study of oil sand from western
Canada, these authors found a correlation between
187 Os/ 188 Os and 187 Re/ 188 Os, which they interpreted
as an isochron dating the time of oil migration.
In order to permit the effective use of the Re-Os
system to date hydrocarbon evolution, we need a
better understanding of the events that could
fractionate the Re/Os ratios of petroleum fluids. One
process that is particulary important in the evolution of
these fluids is the precipitation of asphaltenes. The
asphaltene fraction frequently concentrates most of
the metals contained in petroleum. For this reason,
we have undertaken an experimental study of the
effects of progressive asphaltene precipitation on Re
and Os contents and Re/Os ratio.
The experiments were performed on two oils. The first
is a conventional oil, containing about 6%
asphaltenes. Given the very chalcophile behavior of
rhenium and osmium, we also chose to work on a
sulfur rich oil. The asphaltenes used in the
precipitation experiments were pentane-insoluble
fractions of these oil samples.
The first phase of our study was aimed at developing
a method of sequential precipitation. For this, we used
techniques inspired by Nalwaya (1999) [2]. For the
fractionation procedure, we used a binary mixture of
dichloromethane (DCM) and n-pentane: the bulk
asphaltene fraction was dissolved in DCM. Increasing
quantities of pentane were then added, permitting the
precipitation of progressively less polar asphaltenes.
These various fractions revealed differences in
structure or composition when analyzed by gel
permeation chromatography and FTIR spectroscopy
The asphaltene fractions underwent a chemical
treatment to extract, separate and purify Os and Re.
Re and Os concentrations were determined by
isotope dilution. Os isotopic compositions were
determined by Negative Thermal Ionization Mass
Spectrometry (N-TIMS) using a Finnigan MAT262
instrument. Re isotopic compositions were
determined by Inductively Coupled Plasma Mass
Spectrometry (ICP-MS), using an Elan 6000.
We found similar Re and Os profiles as a function of
the asphaltene precipitation sequence for the two oils.
We also analyzed other metals such as vanadium and
nickel. Comparison with the Re-Os results suggested
that that these metals do not occupy the same sites
as rhenium and osmium. This observation provides
important constraints on the nature of the association
between Re, Os and organic matter.
Our results also suggest fractionation of the Re/Os
ratio after more than 50% of the asphaltenes are
precipitated. This mechanism could be responsible for
fractionation of Re and Os during the evolution of
hydrocarbon in petroleum systems. Consequences for
the use of the Re/Os geochronometer will be
discussed.
References
[1] Selby D. and Creaser R.A. (2005b). Direct
radiometric dating of hydrocarbon deposits using
rhenium-osmium isotopes. Sciences 308, 1293-1295.
[2] Nalwaya V., Tantayakom V., Puimsomboon P. and
Folger S. (1999). Studies on Asphaltenes through
Analysis of Polar Fractions. Ind. Eng. Chem. Res. 38,
964-972.
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