Abstract

Seismic amplitude anomalies are a compelling direct hydrocarbon indicator but unfortunately cannot discriminate between high and low levels of gas saturation. This is demonstrated dramatically in the King Kong/Lisa Anne minibasin in the deep-water Gulf of Mexico. Both the King Kong and Lisa Anne features, located on opposing sides of the minibasin at the same stratigraphic level, display very similar amplitude anomalies on migrated seismic volumes. However King Kong contains gas at commercial levels while Lisa Anne contains gas only at low saturation (5% - 25%). Wireline logs recorded at Lisa Anne show the presence of low saturation gas sands with low acoustic impedance, comparable to that of fully saturated gas sands. Thus the recorded seismic amplitude anomalies are valid, but indicate the presence of low saturation gas sands rather than sands with commercial gas levels.

Log data document a complex distribution of pore fluids in the Lisa Anne sands with multiple low-gas-saturation and brine saturated sands stacked one above another. The data also show that sand bodies of considerable thickness can be charged with low saturation gas, in this case thicknesses in excess of 80 ft. The absence of a gas cap argues that this is a dynamic system which is not in hydrodynamic equilibrium; fluids have moved through the system in the recent past and possibly continue to do so at the present day.