Large Angle AVO in the Presence of Strong Shale Anisotropy: Understanding Angle Dependant Reflectivity to Help Locate Thick Turbidite Reservoir Sands in the Lower Miocene of Block 14, Offshore Angola

By

Ernesto Taia¹, Virgilio Pinto¹, Simao Jose¹, Guy Delorme²

(1) Sonangol P&P, Luanda, Angola (2) ChevronTexaco Overseas Petroleum, Luanda, Angola

 Turbidite channel reservoir sands in the Lower Miocene of Block 14 can be difficult to detect on near to mid-offset range data because many of the sands have weak to moderate positive acoustic impedance contrasts relative to the encasing shales. Excellent images of the reservoirs can however be obtained when AVO attributes derived from the full offset range of 400 to 4000 meters are used.

To retain the very long offset data an anisotropic moveout correction had to be used in the data processing since the tertiary shales are strongly anisotropic. This anisotropy causes the reflections on the CDP gathers to be severely over-corrected at the very long offsets when simple hyperbolic moveout corrections are used.

To understand the expected AVO behavior of the reservoir sands a simplified anisotropized Zoeppritz equation was used to calculate angle dependant reflectivity from which models were derived that tie the real seismic with remarkable accuracy. Model analysis led to the development of a seismic attribute aimed specifically at revealing thick sands based on their expected AVO behavior. By subtracting - 90 degree phase near angle data from - 90 degree phase, very large angle data, a seismic trace is obtained which closely mimics a Gamma Ray curve.

The technique has been calibrated against Block 14 well penetrations and has been used to locate drilling targets. Stunning images of subtle channels that were difficult to detect on the conventional seismic data (0 to 30 degrees) are now obvious from amplitude maps derived from this “Pseudo-Gamma Ray” data.