Effect of the progressive precipitation of petroleum asphaltenes on the Re-Os radioisotope system
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- 作者：Fatima Mahdaoui ; Laurie Reisberg ; Raymond Michels ; Yann Hautevelle ; Yannick Poirier ; Jean-Pierre Girard
- 关键词：Re&ndash ; Os ; Radiochronometer ; Petroleum ; Asphaltene precipitations
- 刊名：Chemical Geology
- 出版年：4 November, 2013
- 年：2013
- 卷：358
- 期：Complete
- 页码：90-100
- 全文大小：968 K
文摘
Recent studies (e.g., Selby and Creaser, 2005) have raised the intriguing possibility that the evolution of oil can be dated using the Re-Os radioisotope system. However, in order to understand which specific events are being dated, it is essential to determine the processes that could create the range in the parent-daughter ratio necessary for the use of the geochronometer. One particularly important process in the evolution of petroleum is the sequential and progressive precipitation of asphaltenes during oil migration, mixing and reservoir filling. As asphaltenes are known to contain the near totality of the Re and Os present in oil, this event could potentially lead to the required fractionation of Re/Os ratios. Sequential asphaltene precipitation was performed in the laboratory with the aim of assessing the effect of progressive precipitation on relative and absolute Re and Os contents. The experiments were performed on two well preserved oils with distinct properties: one was a medium density, sulfur-poor crude oil containing about 7% asphaltenes; the other a heavy, sulfur-rich oil containing about 20% asphaltenes. Bulk asphaltenes were extracted from each oil and dissolved in an organic solvent (dichloromethane). Starting with these bulk fractions, sequential precipitation into fractions of decreasing polarity was provoked by adding increasing quantities of precipitant (n-heptane or n-pentane). For both oils, the Re and Os concentrations of the successive asphaltene fractions decreased steadily during the progressive precipitation. However the Re/Os ratio remained constant, except at very high degrees of asphaltene precipitation. This suggests that natural asphaltene precipitation during the evolution of hydrocarbons in a petroleum system has very little effect on the Re/Os ratio. Thus asphaltene precipitation during the evolution of hydrocarbons in a petroleum system or during oil recovery is unlikely to perturb the Re-Os geochronometer. Conversely, asphaltene precipitation is probably not the process responsible for the Re/Os fractionation required to develop an isochron.
The similarity of the concentration profiles with progressive precipitation obtained for Re and Os suggests that these metals have similar binding properties in asphaltenes. In contrast, V and Ni concentration profiles obtained during these experiments display a different character. Unlike Re and Os, which are hosted by the most polar asphaltenes, V and Ni are preferentially bound to asphaltenes of intermediate polarity. This suggests that Re and Os are not hosted by the same organic entities that carry V and Ni (e.g., metallo-porphyrins).