Abstract: Thermocatalytically generated gaseous hydrocarbons from gas or oil prone source rocks in subsiding sedimentary basins and those penetrating the seal of gas or oil accumulations at the surface reach the surface in geologically short times and in low concentrations. These transported hydrocarbon gases transmit information to the surface concerning hydrocarbon potential, organic facies and maturity of the generating source rock and characteristics of the gas accumulation. In the present invention, these near surface gaseous traces are analyzed according to their molecular and isotopic composition, and from that information the status of gas and oil generation is derived by analyzing the compositional shifts of the gas due to migration. The shifts in isotopic composition of methane gas during migration are quantified. Relationships of the original gas parameters are reconstructed in this way to derive information as to the geological status and potential of their source i.e. the prospectivity of the basin.