ABSTRACT: Carbon Isotopic Analysis of Individual Hydrocarbons: An Emergent Technique for Oiusource Correlation and Maturation Measurement in Wet Gas

SUMMONS, R. E., C. J. BOREHAM, and Z. ROKSANDIC, Onshore Sedimentary and Petroleum Geology, Bureau of Mineral Resources, Canberra, Australian Capital Territory, Australia

Carbon isotope measurements on whole oils or on the saturate and aromatic hydrocarbon fractions of crudes is a very rough means of correlating them with their putative source rocks or assigning marine versus terrestrial origins to oils. The new technique of specific compound isotope analysis (CSIA) enhances the scope and value of isotopic data because comparisons can be made on numerous individual components. Isotopic data can also be used to more effectively determine the origins of individual hydrocarbons in oils and source rocks and hence improve our understanding of the processes involved in petroleum formation.

Reliable source and maturity information on natural gases and condensates can also be established through isotopic means. This is because cracking of kerogen and larger liquid hydrocarbons into gaseous (C1-C5) hydrocarbons is accompanied by isotopic fractionations. The isotopic signatures of the large nalkanes may be relatively close to that of the composite carbon of the source organic matter while the gas range n-alkanes are shifted systematically, through kinetic isotope effects, as a function of maturity. Biogenic and thermogenic gases may also be distinguished while methane from different sources and/or reservoirs may be characterized readily.

This presentation will discuss the emergent SCIA technique with reference to Australian samples. The emphasis will be on analytical methods and pitfalls and the origins of specific carbon isotopic signatures in fossil organic matter.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)