Abstract: Evidence for Low Free-Gas Saturations in Water-Bearing Bright-Spot Sands

H. J. Ritch, J. T. Smith

Water-bearing sands occasionally give seismic-reflection amplitudes of sufficient magnitude to be considered "bright spots." It has been shown theoretically that such "false" bright spots may be produced by the presence of small amounts of free gas which can cause large changes in seismic velocity in clean unconsolidated sands. False bright spots studied were observed on acreage acquired in the Gulf of Mexico in the December 1972 Federal Lease Sale. The investigation of the false bright spots involved laboratory measurements on cores, wireline formation tests, and complete log suites on good quality Pleistocene sands.

Wireline formation-test results were not conclusive. However, formation waters appeared to be saturated with methane gas at reservoir conditions. Analysis showed the methane to be biogenic in origin. Core examinations in these anomalous sands showed the presence of organic material. Therefore, generation of methane could be occurring within these sands now. Additional core studies showed that no combination of sand porosity, grain size, and/or sorting could produce the velocity changes needed to give the amplitude anomalies present in some of the sands. Cycle skipping on the sonic logs at several of the false bright-spot zones may indicate abnormally low velocities in these zones.

We concluded that the false bright spots studied were created by low saturations of free gas.

AAPG Search and Discovery Article #90976©1976 AAPG-SEPM-SEG Pacific Section Meeting, San Francisco, California