Gas geochemistry: a new technology to evaluate petroleum systems

Abstract

In the last decade hydrocarbon gas geochemistry has significantly evolved especially regarding to the widespread use of GC-C-IRMS (Gas chromatography-combustion-ion ratio monitoring system) techniques that allowed accurate measurements of F13 C in the C1-C4 fraction. Also, due to the improvement of the sampling procedures, sample collection - relatively small amounts of gas samples at low pressure - has become easier, sample transportation has become safer – the risk of accidents has been reduced - and analysis has become faster. The “state-of-the-art” of the use of gas geochemistry to study processes within petroleum systems is discussed in this paper. The discussion is mainly focused on the identification of the hydrocarbon gas origin (biogenesis versus hermogenesis, mixing) and hydrocarbon gas generation (primary versus secondary cracking), including the inference of the relative thermal evolution and possible secondary alterations caused by biodegradation, secondary migration and eventually leakage from petroleum accumulations. Brief comments were also made about the new technologies involving noble gases and their potential use as tracers of processes in petroleum systems. Additionally, the origin of non-hydrocarbon gases in petroleum accumulations is succinctly discussed, as well as their possible economic impacts. The use of gas geochemistry techniques has brought a significant improvement in the understanding of petroleum systems under exploration in Brazil and in other parts of the world. Besides, on-going research projects suggest that there is a great potential for technological advances, e.g., adding the hydrogen isotopic composition of hydrocarbons to the interpretations, and also integrating the available results to the isotopic variations of noble gases.