VARIATIONS IN CHEMICAL COMPOSITION OF HYDROCARBON BASES IN THE DEPLETED STRATA OF THE SEL-ROKHO (WESTERN FERGANA) AND ABUZ (KRASNODAR KRAY) OIL FIELDS

Abstract

Time-dependent variations in the chemical and even isotopic compositions of natural hydrocarbon gases released into the atmosphere from mineral springs and out-gassing wells have been reported before. This effect is especially large in seismically active zones, where it occurs even during seismic lulls. Given the above, the variations in chemical composition of hydrocarbon gases at points of their venting into the atmosphere, which we detected in the depleted oil fields of Sel'-Rokho (Western Fergana) and Abuz (Krasnodar Kray) are important, since they seem to be indicative of chemical instability of water-gas systems ascending from depth into the reservoir beds in which the gases should, in the ideal case, become chemically homogeneous. There seem to be two alternative explanations of the observed phenomenon. One of these postulates differentiation of hydrocarbon gases within the well on their path from the groundwater table (which is at a depth of 60 m in wells in the Sel'Rokho field and approximately 90 m in well No. 44 in the Abuz field) to the ground layer of the atmosphere. The other explanation assumes a periodicity in the gas supply (along fractures in the rock) into the flooded reservoir rock from two or more independent hydrocarbon sources (perhaps hydrocarbon reservoirs in the still-deeper sediments) with sharply differing chemical compositions. The second explanation seems more likely.