Gas Isotopes Tracing: an Important Tool for Hydrocarbons Exploration

Gas may be considered today both as a reliable tracer for the understanding of associated liquid hydrocarbons and as an economic target. Isotopic measurements of stables isotopes and noble gases give important clues to reconstruct the geological history of hydrocarbons from their generation to their accumulation. Recent analytical advances in carbon isotopes of natural gases (methane to butane and carbon dioxide) duemainly to the development of GC-C-IRMS (Gas Chromatograph-Combustion-Isotopic ratios mass spectrometer) have allowed to reconstruct some of the physico-chemical processes which affect naturalgas chemical signatures, instead of using these signatures as simple fingerprinting of origins as it was the case some time ago. These reconstructions provide important information on both the origins and the dynamic behavior of hydrocarbon fluids between the source rocks and the accumulations in reservoirs. Correlating this methodology with other natural tracers increases the understanding of hydrocarbon history in sedimentary basins. Among other new potential methodologies, noble gas coupled to stable isotopes are the new frontier tool, as their chemical inertia enables their use as precise tracers of sources and of associated physical processes (state of the phases, migration and leakage). Moreover, because some isotopes (4He, 40Ar for example) are produced by natural radioactivity, they may very well represent geological "clocks", giving potentially a quantification of the residence times of hydrocarbons in a reservoir. Several new applications of this association of stable isotopes and noble gases are presented in this paper: a new way for distinguishing bacterial and thermogenic gas origins, and the characterization of the parameters related to the genesis of thermogenic gas (primary versus secondary cracking, openness of the system, relations between gas isotope signatures and biodegradation). At last is presented a tentative quantification of the proportion of hydrocarbon gases leaked from a reservoir, and a quantitative relative residence times of hydrocarbons in heterogeneous reservoirs