CARBON, HELIUM, NEON AND ARGON ISOTOPES IN A PO BASIN (NORTHERN ITALY) NATURAL-GAS FIELD

The abundance and isotopic composition of He, Ne and Ar have been dete rmined in 19 ps samples taken from the Dosso degli Angeli hydrocarbon ps field, between 3000- and 3750-m depth in the Po Basin. Carbon isoto pe ratios in these samples vary between deltaC-13(CH4) = -67.5 and -73 .4 parts per thousand, indicating a predominantly biogenic source for the methane. C2+[= SIGMA(C2H6 + C3H8 + ...)] concentrations increase s ystematically from 0.04% to 0.17% by volume with depth, suggesting tha t there is a small additional thermogenic or diagenetic hydrocarbon co ntribution to these gases. CH4/Ar-36 ratios in the Dosso degli Angeli gases are in the range predicted for a groundwater which has derived i ts rare ps by equilibration with the atmosphere, and is CH4-saturated under reservoir conditions. There is also up to 10% excess of CH4 whic h is not associated with the groundwater. These results are consistent with > 90% of the CH4 in the Dosso degli Angeli reservoir originating from CH4-saturated groundwater. Measured Ne-20/Ar-36 ratios depart fr om the predicted ratio in air-equilibrated groundwater. This fractiona tion is consistent with water/ps phase equilibration under reservoir c onditions, and is similar to that observed in ps reservoirs in the Vie nna and Pannonian Basins, Austria and Hungary, respectively. The He in the Dosso degli Angeli gases is almost entirely radiogenic and derive d from the crust. The concentration of He-4 in the reservoir gases req uires that He is mostly sourced from outside the reservoir volume. Rad iogenic He-4/Ar-40 ratios input into the reservoir ps must be much hig her than average crustal production ratios, as no shift from the atmos pheric Ar-40/Ar-36 ratio is observed. Preferential release of He-4 ove r Ar-40 from the sites of their production is the preferred explanatio n. This observation contrasts with previous studies from active extens ional basins, in which radiogenic He-4 and Ar-40 appear to have been s tored, released and transported at near production ratio on a regional scale. The most obvious difference between these studies is the compa ratively low thermal gradient of the Po Basin.