1Department of Earth Sciences, Oxford University, South Parks Road, Oxford OX1 3AN, UK
2PTT Exploration and Production, 555 Vibhavadi Rangsit Road, Chatuchak, Bangkok 10900, Thailand
3Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
4NERC Isotope Geoscience Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK