Three types of natural gases can be distinguished in the Po Basin: (1) Bacterial gases (delta 13 C CH4 : -60 ppt, delta D CH4 : -180 to -200 ppt, C (sub 2+) : 0.2%), which are found predominantly in Pliocene-Pleistocene reservoirs. (2) Mixed gases (delta 13 C CH4 : -60 to -50 ppt, delta D CH4 : -180 to -200 ppt, C (sub 2+) : 0.2 to 5%, which are predominantly in reservoirs of Messinian (upper Miocene) age, but also in a few places in older and younger reservoirs. (3) Thermogenic gases (delta 13 C: -50 ppt, delta D: -200 to -150 ppt, C (sub 2+) :5%), which commonly are associated with oil and are produced predominantly from Miocene and Mesozoic reservoirs. A quantitative assessment of gas in place for these three gas types is: (1) 80%, (2) 10%, and (3) 10%, respectively. The areal distribution and amount of the three gas types are controlled strictly by geologic factors.--Modified journal abstract.