Origin of CO₂ in petroleum gases (from the isotopic composition of carbon)

To ascertain the feasibility of the use of the isotopic composition of carbon (i.c.c.) to establish the origin of CO/sub 2/, investigations were conducted using 19 gas samples from fields of the Volga-Ural region and the West Ciscaucasus. The characteristics of the i.c.c. of the CO/sub 2/ of petroleum gases of the Arlan field are outlined. The principal commerical oil-bearing horizon is the Coal-bearing horizon composed of terrigenous rocks: the Kashir horizon, represented by carbonates, contains a small oil pool. The i.c.c. of the CO/sub 2/ of these fields differs: The first one exhibits marked enrichment with /sup 12/C (from -26.3% to 19.6%) in comparison with the second (-4.6%). Judging from the i.c.c., the CO/sub 2/ in the pool of the coal-bearing horizon was formed during bacterial oxidation of the petroleums. The petroleums of this horizon display signs of oxidation, and their i.c.c. is characterized by an enhanced /sup 12/C content (-24.3%). The use of isotopic data appears to be helpful in investigations of the origin of CO/sub 2/ in hydrocarbon gases. 2 figures, 3 tables.