An empirical model is presented in which the origin of most natural gases can be deduced from compositional variations in the gases.

The genetic characterization is performed using the C₂₊ composition of the gases, the ¹³C concentration in methane (^dgr¹³C₁) and ethane (^dgr¹³C₂), as well as the deuterium concentration in methane ^dgrD. Three diagrams are designed in which C₂₊, ^dgr¹³D, and ^dgr¹³C₂ are plotted versus ^dgr¹³C₁. In these three diagrams, compositional fields can be defined for primary gases, i.e., those gases for which compositional changes are due to processes occurring during their formation (biogenic and thermogenic associated and nonassociated gases, respectively). Secondary gases result from mixing processes after formation of gases. The variability of natural gases can be described by (and reduced to) predominantly various mixing processes of primary gases. Two types of migration of gases can be recognized: shallow migration where thermogenic gases are stripped of C₂₊ or become mixed with biogenic gases, and deep migration where deep dry gases become mixed with gases from more immature sources. Case histories will be presented to demonstrate the applicability of the model.

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