Bright Spots Related to High GOR Oil Reservoir in Green Canyon

Hwang Long-Far, Peter J. Lellis

Reflection seismic amplitude anomalies (bright spots) commonly are thought to be related to gas reservoirs but not oil reservoirs. However, recent drilling in Green Canyon, offshore Louisiana, indicates there are bright spots related to oil reservoirs that contain no free gas. The oil does have a high gas-to-oil ratio (GOR). The physical basis for oil-related bright spots can be quantified using pressure-volume-temperature (PVT) measurements to predict acoustic properties. This was done for 468 worldwide crude oil samples from wells with low (~100) to high (~3000) GOR. The results show (1) the compressional-wave velocity (Vp) decreases as GOR increases, (2) Vp of crude oil with high GOR (e.g., 3,000) can be as low as 2,200 ft/sec. This crude oil velocity is much closer to the velocity of gas (1,400 ft/sec) than that of water 5,300 ft/sec). Therefore, bright spots can be caused by oil reservoirs with high GOR but no free gas.

PVT analyses were also performed on 11 crude oil samples from seven Green Canyon wells. GOR varied from 154 to 1,672. As with the worldwide data, Vp decreased with increased GOR. The wells are located in areas surveyed with three-dimensional seismic data. Bright spots associated with high GOR oil reservoirs are visible on the seismic sections. These bright spots are compared to seismic amplitudes associated with reservoirs containing water, free gas, and low GOR oils.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.