Changes in properties of crude oils throughout Paleozoic section of Volga-Ural Region,and method of their study

Discusses stratigraphic aspects of crude-oil gravities and various factors influencing changes in gravity of a given crude. Conclusions reached are that 1) Permian and Middle Carboniferous crudes are essentially similar in the Volga-Ural region, 2) that they are lighter than Lower Carboniferous crudes, and 3) that the Devonian crudes are the lightest of all the Paleozoic crudes in the region. The oils of the Saratov-Volgagrad district differ from those of other parts of the Volga- Ural region and appear to have become lighter with age. Differentiating factors in the Permian and Carboniferous crudes are oxidation processes, whereas those in the Devonian are thermodynamic and catalytic. Waters in the petroliferous strata reflect the basic processes affecting oil gravity. Except in the Saratov-Volgagrad area, high pH, high sulfates, hydrogen sulfide and bacteria characterize Permian and Carboniferous waters. These conditions are reversed in the Saratov-Volgagrad district. Methods for evaluating actual differences in gravity of crudes of different ages in the same fields are discussed. — B.N. Cooper.