Résumé en français

Les cas d’applications de la méthode Re-Os sur des huiles et bitumes naturels montrent que les données s’alignent sur des droites interprétées comme étant des droites isochrones. Cependant, les mécanismes permettant la réinitialisation du géochronomètre ne sont pas clairement établis. Les événements réellement datés par cette méthode restent alors ambigus. Certains âges sont attribués à la génération ou la migration de l’huile mais d’autres à des évènements plus récents. Ainsi, l’utilisation du système Re-Os nécessite une meilleure compréhension des événements permettant une homogénéisation isotopique des huiles à l’échelle d’un bassin pétrolier et du fractionnement de l’élément père (Re) par rapport à l’élément fils (Os). L’interaction entre les eaux de formation et l’huile et un phénomène omniprésent lors de la migration et du séjour en réservoir de l’huile. Ainsi, un processus important qui pourrait potentiellement affecté le système Re-Os des huiles est l’échange de rhénium et d’osmium entre l’huile et les eaux de formations durant sa migration et son séjour en réservoir. L’effet de ce processus sur le géochronomètre a été étudié par la mise en place d’expériences de contact entre des solutions aqueuses de rhénium et d’osmium.

Les résultats de cette étude montrent que le rhénium (ReO4^-) et l’osmium (OsCl6^2-) sont transferés de façon très efficace d’une phase aqueuse vers le pétrole. Ce transfert a été constaté sur une large gamme de concentration (1-100µg/g) et de températures (25-150°C), sur des temps de 1 à 15 jours et pour des huiles aux caractériques différentes (lourdes, légère, soufrée, non soufrée). Les fortes teneurs mesurées dans les huiles après les expériences par rapports aux teneurs observées dans la nature dénotent une capacité exceptionnelle des huiles à capturer le Re et Os. Ces résultats suggèrent alors que le rhénium et l’osmium des huiles peuvent être hérités de l’histoire des intérations eau-huile et non seulement du mécanisme de génération-expulsion de l’huile comme l’ont suggéré certains auteurs. Sur la base de ces constations, cette étude permet de proposer un mécanisme de réinitialisation du géochronomètre Re-Os dans les huiles. Ainsi, l’homogénéisation isotopique de l’osmium dans pétroles pourrait être réalisée par le transfert du rhénium et de l’osmium des eaux de formations vers les huiles des pièges pétroliers en contact avec ces eaux. Tous les pièges baignés par un même aquifère auraient des rapports isotopiques d’osmium identiques. La composition chimique hétérogène des eaux pourrait controler la variation des rapports Re/Os dans les aquifers et par conséquent la variation dans les pétroles. Ainsi, une fermeture

hydrodynamique du piège marquerait la fermeture isotopique du système permettant, dans un scénario simple la datation de la dernière fermeture hydrodynamique du piège.

3 Introduction

The understanding of petroleum occurrences in sedimentary basins depends strongly on our grasp of the factors controlling oil generation, migration and emplacement in traps. One particularly critical parameter in the construction of models of petroleum systems is the timing of events that affect oils (e.g. formation, trapping, dysmigration, alteration…). Most time constraints are currently obtained through numerical modeling (Al-Hajeri et al., 2009), with the results depending on the choice of the parameters and the assumptions of the model. Further advances require development of an absolute temporal framework, based on direct radiometric dating of the petroleum fluids.

Recent work by Selby and Creaser (2005) suggests that the rhenium-osmium (Re-Os) radioisotope system may provide the much needed tool for the direct dating of oils. In this system, 187Re decays to form 187Os with a decay constant of about 1.666 x 10-11 yr-1 (Smoliar et al., 1996), equivalent to a half-life of about 43 billion years. The particular advantage of the Re-Os system is that, unlike the parent and daughter elements of most other radioisotope systems, Re and Os are organophile elements (Miller et al., 2004; Selby et al., 2005; Selby and Creaser, 2005a, b). This characteristic has allowed the Re-Os system to be used quite successfully for the dating of organic-rich rocks such as black shales, which are highly enriched in Re and Os relative to most other crustal rocks (Ravizza and Turekian, 1989; Cohen et al., 1999; Selby and Creaser, 2003, Xu et al., 2009). Moreover, studies have shown that the process of petroleum generation does not disturb the Re-Os systematics of black shales (Creaser et al., 2002; Selby and Creaser, 2003).

The Re-Os ages obtained for black shales indicate the time of sedimentary deposition (Ravizza and Turekian, 1989; Cohen et al., 1999; Creaser et al., 2002). In contrast, dating of petroleum poses specific challenges. Throughout its history (generation, expulsion, migration, and trapping) oil undergoes continuous evolution, which could modify its metal content. Even if Re-Os data produce an apparent age, its interpretation may not be obvious. Indeed, as is true for all radiometric isotope couples, use of the Re-Os system for dating requires isotopic resetting and equilibration of Os isotopes at the time of the event of interest. Therefore, the use of the Re-Os system to date hydrocarbon evolution requires a better understanding of events which could lead to the isotopic homogenization of oils at the scale of oil fields and the

subsequent or concomitant fractionation of the parent-daughter ratio (i.e. Re/Os) (Selby and Creaser, 2005). This in turn requires over all a better understanding of the geochemical behavior of Re and Os, particularly with regards to their affinity for organic matter. In particular, it is important to understand how, and at what point, Re and Os are incorporated into oil.

Natural oils are known to contain significant concentrations of Re and Os (Barre et al., 1995; Tanner and Holland, 2001; Selby and Creaser, 2005a,b; Selby et al., 2005). Since the kerogen of potential source rocks is highly enriched in these elements, it seems reasonable to think that Re and Os in oil are derived from this kerogen during oil generation (Selby and Creaser, 2005; Selby et al., 2005, 2007). To verify this assumption, Rooney et al. (2012) and later Cumming (2012) conducted hydrous pyrolysis experiments to determine how Re and Os are partitioned between generated oil and its source rock. These authors found that after heating, the generated bitumen and the remaining kerogen have very similar 187Os/188Os ratios, but different Re/Os ratios. Thus bitumen generation is capable of both homogenizing the Os isotopes and fractionating the parent/daughter ratios, two conditions essential for the development of an isochron. However, surprisingly, the oils generated during these experiments contained very low concentrations of Re and Os, suggesting very little transfer of these elements from kerogen to oil. Rooney et al. (2012) suspect that the absence of transfer of Re-Os to the generated oil may imply that hydrous pyrolysis experiments do not completely mimic natural hydrocarbon generation.

Taking the experimental results of Rooney et al. (2012) into account, Lillis and Selby (2013) suggested that the Re-Os isochron they obtained from natural oils of the Phosphoria petroleum system represented the time of bitumen generation and concomitant oil expulsion. These authors therefore consider that bitumen generation is the key process responsible for resetting the geochronometer in oils. Re-Os isochrons measured on petroleum samples would thus reflect the age of generation from the source-rock. Nevertheless, in a subset of their samples, these authors found an isochron with a much younger age, which they relate to alteration by thermochemical sulfate reduction. Interaction with hydrothermal fluids could therefore provide another means of resetting the Re-Os isotopic system.

To understand the behavior of Re and Os in oil it is helpful to consider that of other metals. The most commonly investigated elements are vanadium and nickel, which have been

shown to bind at least partially to porphyrins, structures of biotic origin (Treibs, 1934; Parnell, 1988) known to be inherited from the source kerogen. Thus Ni/V ratios can be used for petroleum source-rock correlations (Ali et al., 1983; López et al., 1995). However, other metals may be incorporated into oils after generation (Parnell, 1988). This is for instance the case for uranium. Interaction with uranium bearing aqueous fluids can produce uranium concentrations in bitumen of up to several percent (Landais, 1993 and references therein) Recent LA-ICPMS analyses show that hydrocarbon inclusions in quartz and fluorite associated with ore deposits from throughout the world are strongly enriched in metals such as Fe, Zn, Cu and Pb compared to crude oil samples from various petroleum fields (Demange et al., 2011). Other authors (eg. Saxby, 1976; Parnell, 1988; Giordano, 1994) suggested that the organic matter may play a role in the metal complexation (for temperatures less than 200 °C) and contribute to the metal transport in interstitial waters by formation of metal-humate, metal-fulvate or metal-amino acid complexes. Giordano (1994) also suggested that organosulfur ligands could contribute to both metal and sulfur transport in moderate temperature. These lines of evidence suggest that certain metals can be transferred from aqueous fluids to petroleum. Throughout its evolution, petroleum is in contact with formation waters. Given these observations, one mechanism that could explain the high contents of Re and Os found in oils would be the transfer of metals from formation waters to petroleum during expulsion, secondary migration or trapping.

In order to investigate this potential process and its effects on the geochronometer, we present results from contact experiments between oil and aqueous solutions enriched in Re and Os at temperatures similar to those expected in petroleum reservoirs. Our goal is to determine whether under these conditions Re and Os are efficiently transferred from the aqueous phase into the oil.