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Three types of natural gases can be distinguished in the Po basin. (1) Bacterial gases (^dgr13CCH4: -60 ppt, ^dgrDCH4: -180 to -200 ppt, C2+: 0.2%), which are found predominantly in Pliocene-Pleistocene reservoirs. (2) Mixed gases (^dgr13CCH4: -60 to -50 ppt, ^dgrDCH4: -180 to -200 ppt, C2+: 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 (^dgr13C: -50 ppt, ^dgrD: -200 to -150 ppt, C2+: 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. Synsedimentary tectonics and turbidite sedimentation are the best conditions for the trapping of biogenic gas in the southeastern part of the area, where 70% of the bacterial gas in place is produced. Here, bacterially formed methane is produced from depths of up to 4,500 m (15,000 ft), and is the deepest bacterial gas so far reported in the literature.
In the pedealpine homocline, lower sedimentation rates and a weak deformation of Pliocene-Pleistocene sediments prevented appreciable accumulations; only 15% of the bacterial gas in the basin is produced in this area.
Thermogenic gases are found in the vicinity of the Apennines, where strong tectonic movements have favored its migration from deeper Mesozoic sources into Tertiary reservoirs or into deep Mesozoic carbonate reservoirs of the pedealpine homocline.
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