Gaseous hydrocarbons, which are thermocatalytically stripped from natural gas/petroleum parent rocks in subsiding sediment basins or which penetrate the seal of natural gas/petroleum deposits, reach the earth's surface in trace concentration within a geologically short time. The transported hydrocarbon gases transfer information about the hydrocarbon potential, the organic facies and the stage of maturity of the stripping parent rock or about the characteristics of a deposit to the surface. According to the invention, this circumstance is utilised for making statements regarding the prospectivity of the underlying rock by means of measurements at the surface. The molecular and isotopic composition of the trace gases is measured and, allowing for the changes caused by the transport, the abovementioned information about the natural gas-/petroleum-geological state in the underlying rock is derived therefrom. The isotopic changes in the methane during the migration to the surface are quantified. The relationship of the original gas parameters, reconstructed in this way, from parent rock and deposit up to their present geological state and potential - that is to say the prospectivity - of the basin is derived. This makes it possible to provide, before drilling, data about the hydrocarbon potential which is available for filling a certain structure or regionally in a sediment basin, and about lateral changes thereof. Under special geological conditions, the existence of a deposit in the underlying rock can be directly proved, and lateral changes in the effectiveness of stripping can be recognised. Application of the method is therefore suitable for a considerable reduction in the risk of development prospects for crude oil and natural gas.
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