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Evidence for Low Free Gas Saturations in Water-Bearing Bright Spot Sands
Water-bearing sands occasionally have been observed to give seismic reflection amplitudes of sufficient magnitude to be considered Bright Spots. It has been shown theoretically that such “false” Bright Spots* might be produced by the presence of small amounts of free gas which can cause large changes in seismic velocity in clean unconsolidated sands. This paper describes work carried out in an effort to test the validity of this explanation for a number of false Bright Spots encountered by Shell Oil in the Offshore Gulf of Mexico Province. The particular examples of false Bright Spots studied were observed on acreage acquired in the December 1972 Federal Lease Sale. The investigation of these false Bright Spots involved laboratory measurements on cores, wireline formation tests and complete log suites.
Wireline formation test results were not conclusive. However, formation waters appeared to be saturated with methane gas at reservoir conditions. This methane gas was analyzed and found to be biogenic in origin. Core examinations in these anomalous sands show the presence of organic material. Therefore, generation of methane could be occurring within these sands at the present time. Additional core studies showed that no combination of sand porosity, grain size and/or sorting could produce the velocity changes that were needed to give the amplitude anomalies that were observed for some of these sands. Cycle skipping was observed on the sonic logs at several of the false Bright Spot zones which may indicate abnormally low velocities in these zones.
Considering all of the above lines of evidence, we concluded that the false Bright Spots which were studied were created by low saturations of free gas. All of these cases occurred in good quality Pleistocene sands.
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