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.