Pete J. Chimney1
(1) Chevron Overseas Petroleum, San Ramon, CA
Abstract: The 70-C prospect, offshore Cabinda, Angola. The amplitude anomaly that wasn't, an interpretation pitfall
The 70-C prospect is located in Area A, Cabinda, offshore Angola. The 70-C prospect was mapped in 1997 using the Kali 3D dataset as a follow-up to the 57-6X well drilled in 1983.
The 57-6X well was drilled using early 1980s vintage 2D time migrated seismic to test Cretaceous Vermelha (Cenomanian) and Pinda (Albian) reservoirs in a tilted fault block. The 57-6X well encountered over 500 feet of sand in the Pinda. The upper 65 feet were tested at 640 BOPD of 24° API oil.
A high amplitude anomaly exhibiting time dip was noted in the modern 3D seismic. The anomaly tied the base of the oil zone in the 57-6X well. Modeling of the expected seismic response indicated an oil-water or gas-water contact could produce the observed anomaly.
The 70-3X well was drilled directionally to test the updip attic in the Pinda above the position of the 57-6X well. Pinda reservoir sands encountered in the 57-6X well were not present in the 70-3X well. The amplitude anomaly ties the 70-3X well at a point 575 deeper than the same tie in the 57-6X well, precluding the amplitude anomaly as a DHI of the OWC.
The current interpretation suggests that the amplitude event is a low angle fault visible from the main bounding listric fault to the top of the Pinda reflector. Slower, less dense Vermelha resting on faster, denser Pinda yields a strong positive peak. Beyond the Pinda closure Vermelha rests on a similar Vermelha section yielding a low acoustic impedance.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana