Many young sedimentary basins produce a great variety of natural gases ranging from deep wet gases to shallow dry gases. It is of considerable interest to find evidence for the origin of these gases, especially with regard to deep exploration.

In the Tertiary subalpine Molasse basin of south Germany, dry gases occur in shallow Oligocene to Miocene reservoirs (^dgr¹³C₁ ~ -70 to -60 parts per thousand), low C₂₊ gases in upper Eocene reservoirs (^dgr¹³C₁ ~ -60 to -50 parts per thousand), and wet gases in other Eocene reservoirs (^dgr¹³C₁ ~ -50 to 62 parts per thousand; the wet gases being partly associated with crude oils). Two alternatives for the origin of the gases should be considered: (1) the dry gases may be migrated wet gases which have been stripped of their C₂₊ components and have been enriched in carbon 12 isotopes; and (2) the dry gases are of biogenic origin.

Carbon and hydrogen isotope analyses on gases in the Molasse basin have brought direct evidence for the dual origin of these gases. The shallow dry gases are of biogenic origin, as shown by the direct relation between the deuterium isotope ratios of the methane and their associated waters. The wet gases are of thermogenic origin. The scatter in the carbon isotopic composition and the C₂₊ concentration in this particular basin is due to downmixing of the bacterial gases to deeper strata owing to an underlying underpressured zone. C and H isotope analyses on gases from two other Tertiary subalpine basins (Austria and northern Italy) have shown the dual-origin concept to be generally applicable to these basins. In particular the D/H and ¹³C/¹²C patterns o the gases reflect mixing processes and thus give information on the general hydrodynamic situation of the basins.

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