Abstract. Changes in oil composition in the course of the development depend on numerous factors including both technogenic and natural ones. Physicochemical compositional analysis of 2456 oil samples collected in the Romashkino field in 1982–2000 from two productive strata (Devonian and Carboniferous reservoirs) has indicated statistically significant variations in the i-butane to n-butane content ratio (i-b / n-b). This ratio is characterized not only by a trend of steady growth but also by a significant variation with a period of ∼4.5 years. The i-b / n-b ratio variations are well correlated (R2=0.61) between these two reservoirs (Devonian and Carboniferous) occurring at different depths. However, the average i-b / n-b ratios recorded in these reservoirs are essentially different (∼0.44 and ∼0.55, respectively). A possible relationship is discussed between the i-b / n-b ratio variations and those in global seismicity.

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