Print this pageThe System U/Th/Pb in the Kerogen and Associated Oils: Is It Possible to Calculate from Age for from Oil Accumulation?
Ricard, E.1
Gourlan, A. T.1
Prinzhofer, 
Alain
1
Pecheyran, C.2
Donard, O. F.2
The age of expulsion of oils and its residence time in an accumulation is still one of the most difficult problem to quantify in Earth Sciences. Until now, except some attempt with the system Re/Os, the main possibility is to model the oil and gas generation through basin modelling, and deduce an age of generation without any external boundary condition. We suggest to use the geochronological system Uranium/thorium/lead in the organic matter to assess the possibility of using such a radioactive/radiogenic system to access to chronological information about hydrocarbons. This is now feasible thanks to new analytical techniques such as the coupling of femtosecond laser (allowing the measurements on a very small part of a source rock or an oil without any fractionation effect) with an ICP-MS with muticollectors detection system. The first results indicate that the parent elements (Uranium and Thorium) are more concentrated in the source rock than in the produced oil, when compared to the daugther element (Lead). This implies that the calculation of a model age has to be done through the coupled analyses of the kerogen and of the associated oil: the oil will present almost the same lead isotopic ratio than it was at the time of expulsion, whereas the kerogen will have continued to increase the proportion of radiogenic isotopes (206Pb, 207Pb and 208Pb). The shift between the two families of isotopic signatures, associated with the measurement of the U/Pb and Th/Pb ratios, should give the age of expulsion, if there is no opening of the hydrocarbon system from that time to the present. First results on different petroleum systems in the world, chosen for a simple geological history (single source rock, and age of expulsion already assessed by basin modelling) and relatively ancient timing of expulsion (several tenths of million years) give promising results.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil