1. Faculty of Chemical Engineering and Materials Science, Organic Geochemistry Unit, Delft University of Technology, De Vries van Heystplantsoen 2, 2628 RZ Delft, Netherlands
2. Current address: Institut Français du Pétrole, BP 311, F—92506, Rueil Malmaison, Cedex, France
3. Organic Geochemistry Unit, Delft University of Technology, De Vries van Heystplantsoen 2, NL—2628 RZ Delft, Netherlands