Carbon, helium, neon and argon isotopes in a Po Basin (Northern Italy) Natural Gas Field.

Abstract The abundance and isotopic composition of He, Ne and Ar have been determined in 19 gas samples taken from the Dosso degli Angeli hydrocarbon gas field, between 3000- and 3750-m depth in the Po Basin. Carbon isotope ratios in these samples vary between δ 13 C(CH 4 ) = −67.5 and −73.4%, indicating a predominantly biogenic source for the methane. C 2+ [= Σ (C 2 H 6 + C 3 H 8 + …)] concentrations increase systematically from 0.04% to 0.17% by volume with depth, suggesting that there is a small additional thermogenic or diagenetic hydrocarbon contribution to these gases. CH 4 / 36 Ar ratios in the Dosso degli Angeli gases are in the range predicted for a groundwater which has derived its rare gas by equilibration with the atmosphere, and is CH 4 -saturated under reservoir conditions. There is also up to 10% excess of CH 4 which is not associated with the groundwater. These results are consistent with > 90% of the CH 4 in the Dosso degli Angeli reservoir originating from CH 4 -saturated groundwater. Measured 20 Ne/ 36 Ar ratios depart from the predicted ratio in air-equilibrated groundwater. This fractionation is consistent with water/gas phase equilibration under reservoir conditions, and is similar to that observed in gas reservoirs in the Vienna and Pannonian Basins, Austria and Hungary, respectively. The He in the Dosso degli Angeli gases is almost entirely radiogenic and derived from the crust. The concentration of 4 He in the reservoir gases requires that He is mostly sourced from outside the reservoir volume. Radiogenic 4 He/ 40 Ar ratios input into the reservoir gas must be much higher than averaged crustal production ratios, as no shift from the atmospheric 40 Ar/ 36 Ar ratio is observed. Preferential release of 4 He over 40 Ar from the sites of their production is the preferred explanation. This observation contrasts with previous studies from active extensional basins, in which radiogenic 4 He and 40 Ar appear to have been stored, released and transported at near production ratio on a regional scale. The most obvious difference between these studies is the comparatively low thermal gradient of the Po Basin.