Eugenie F. Grace¹, David L. Young², Arthur H. Johnson¹
(1) Chevron USA Production Company, New Orleans, LA
(2) Chevron Overseas Petroleum Inc, Cabinda, Angola

Abstract: 3D Seismic Evidence for Gas Hydrates: Deepwater Offshore Angola, West Africa

Evaluation and interpretation of conventional 3D seismic data in offshore Angola, West Africa, indicates the presence of a gas hydrate layer. Evidence for gas hydrates includes (1) the presence of a Bottom Simulating Reflector (BSR), which cuts across structure and stratigraphy, is subparallel to the mudline, and occurs at a depth below the mudline consistent with the pressure and temperature stability regime for gas hydrates at that water depth, and (2) apparent "seismic blanking" of reflectors above the BSR. Other related characteristics of the survey area are the presence of numerous gas vents and locally raised BSR, areally confined stacked amplitudes at and below the BSR, areally confined stacked amplitudes above the BSR. These features are interpreted to reflect vent-related pressure and temperature changes affecting the BSR, accumulations of free gas trapped below the BSR due to the sealing nature of the overlying gas hydrate layer, and accumulations of free gas within the Hydrate Stability Zone, respectively.

MWD logs in an 8 1/2" pilot hole drilled through the expected gas hydrate interval showed no evidence of hydrates or of free gas at the base of hydrates. However, the pilot hole was drilled at a favorable location with respect to potential drilling hazards associated with the gas hydrates. In addition, a lithology change occurred at the expected depth of the BSR, which could have masked any potential hydrate-related resistivity change at the base of the Hydrate Stability Zone. Questions still remain whether shallow drilling hazards exist elsewhere through the survey area.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana