Abstract

The possibilities inherent in reflection amplitude anomalies or 'bright spots' on seismic cross-sections as indications of gaseous hydrocarbons have stimulated widespread interest. To help evaluate these possibilities, laboratory measurements were made on specimens of compacted Ottawa sand of approximately 30 percent porosity. The measurements were made at five different water saturations (0 percent and 100 percent, and approximately 9 percent, 45 percent, and 85 percent) and net confining pressures to 10,000 psi. The gas was air. Measurements were made with acoustic pulses with a peak frequency of about 700 khz.Results show compressional velocity relatively independent of water saturation in the range from 9 to 85 percent. Examples of velocity values are:TableThe Wyllie time-average equation, useful in estimating porosities of water-saturated sands at depth (the application intended by Wyllie), was found inappropriate for prediction of compressional velocities in partially gas-saturated, unconsolidated sands. It ignores sensitivity to net confining pressures and does not adequately distinguish between acoustic effects of gases and liquids. Reflection coefficients up to .30 for the interface between a gas-filled and a water-saturated sand can be expected at shallow depths; though decreasing with depth, this reflection coefficient should remain appreciable ( approximately .15-.20) to depths below 10,000 ft.Wherever large seismic reflection amplitude anomalies occur in geologically young clastic provinces, the presence of gaseous hydrocarbons must be considered in the interpretation if the presence of such hydrocarbons is consistent with other evidence. A caution to observe is that small gas saturations will produce an effect similar to large gas saturations.

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