FREQUENCY-DEPENDENT AMPLITUDE ANALYSIS FOR OIL DETECTION WITHIN THE MIDDLE JURASSIC SEDIMENTARY ROCKS IN THE SOUTHERN PART OF WESTERN SIBERIA
Privalova, Irina 1988-
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The Middle Jurassic (Ju3-4) sandstone reservoirs are characterized by the presence of non-structural types of traps, which are usually located on the slopes of anticlines and have stratigraphic, tectonic, and lithological barriers. Therefore, the structural element is not a determinative factor for identification of the hydrocarbon deposits within the Ju3-4 formation. In order to detect hydrocarbons, amplitude variations versus offset and frequency were analyzed. The amplitude of the reflected wave from the Ju3-4 formation is affected by an interference with the strong reflected wave from the Bazhenovskaya (B) formation. Frequency-dependent amplitude analysis shows that at low frequencies (10 Hz) the effect of the strong reflected wave of the B formation is almost suppressed. Moreover, at this frequency the amplitude anomalies of the target reflected wave (Ju3-4) were detected near the oil wells. This phenomenon allowed consideration of the low-frequency (LF) as a direct hydrocarbon indicator. However, the modelling shows that the variation of the reservoir thickness has a remarkable influence on the reflected wave from the reservoir at low frequencies. In order to consider the effect of the thickness variation, the additive model of the influence of thickness variations on the LF attribute was developed and applied. As a result of the thickness compensation, a better correlation between the LF attribute anomalies and pure oil wells was obtained. The LF attribute only allows for a qualitative separation of oil-saturated zones from water-saturated zones. The correlation between the LF attribute and formation resistivity was investigated in order to create quantitative criteria that could distinguish the oil-saturated zones from water-saturated zones. Finally, the resistivity map was constructed. The map shows sufficient correlation with the oil saturation obtained from the well drilling.