Origins of Natural Gas in the Po Valley, N. Italy

Abstract

Summary A study on natural gases sampled from 48 gas fields in the Po Basin is reported. A total of 155 wells are involved in the present work, representing 90% of the total original gas in place. The chemical and isotopic analyses have been considered based on the regional geological setting. A correlation between chemical and isotopic composition is shown by the presence of pure and isotopically light methane (13C from −76 to −60 parts/10 18 ) in the Plio-Pleistocene fields and by gas less depleted in 13C (13C from −60 to −30 parts/10 18 ) and associated with heavier hydrocarbons in Miocene and Mesozoic gas fields. No relationship exists within the Tertiary sequences between depth and isotopic composition. The organic matter of the Tertiary sequences is mainly of terrestrial origin and is immature down to 5000 m. On the basis of these data it is possible to distinguish three types of methane in the Po Basin: (1) Biogenetic and/or diagenetic gas (80% of the original gas in place) from Tertiary reservoirs, originated in situ from terrestrial immature organic matter of the Tertiary sequence themselves. (2) Thermogenetic gas (10% of the original gas in place) which migrated from more than 5000 m depth, associated with heavier hydrocarbons. (3) Gas of mixed origin (10% of the original gas in place). The areal distribution and the amount of the three types of methane are strictly controlled by geological factors. As a matter of fact the synsedimentary tectonics and the turbiditic sedimentation have determined the best conditions for the trapping of the biogenetic gas in the southeastern part of the area, where 70% of the original Tertiary gas in place is present. In the pedealpine homocline sediments have not been affected by strong Plio-Pleistocene folding, and therefore only a small amount of biogenetic gas was accumulated in the stratigraphic and in the rare structural traps. On the other hand the thermogenic methane is found in the vicinity of the Apennines where strong tectonic movements have favoured its migration from deeper zones and into Mezozoic deposits of the pedealpine homocline. Furthermore for the first time the presence of pure isotopically light methane is here reported at 4500 m depth, that is from depths greater than so far reported in literature (3350 m). This fact is due to peculiar geological settings of the southeastern area comprising the low geothermal gradient, the synsedimentary tectonics and the strong Plio-Pleistocene subsidence (8000 m).