Petrophysical Attribute Evaluation and Anisotropic AVO Modeling in the Chad Basin, NE, Nigeria

Petrophysical analysis is an important aspect in AVO modeling. The relationship between petrophysical and seismic rock properties can be established for reservoir characterization since petrophysical rock properties are directly related to seismic rock properties. Both the qualitative and quantitative petrophysical analysis of thirteen well logs from Chad Basin was carried out. P-wave velocities were obtained from sonic logs and using an empirical relationship for shear wave prediction, the shear wave velocity was derived. AVO attributes were derived from the well logs. Velocities of the Primary and Secondary waves, and its bulk densities were convolved with a zero phase wavelet to generate synthetic gathers for the drilled column. With well-logs as control points, AVO synthetic data (isotropic) were generated using the exact Zoeppritz equations. Anisotropic AVO synthetic data were generated using the Aki and Richards equation to a maximum angle of 50 degrees. The isotropic and anisotropic PP, isotropic and anisotropic PS were analyzed within hydrocarbon intervals. A 2-D stratigraphic model which takes into account lateral variations in thickness of reservoirs, porosity, lithology, structure, fluid type and saturation was produced using the petrophysical properties within separating wells with the exact Zoeppritz equation, and the delineated reservoir sands within each well were identified on the stratigraphic model. Cross-plots of acoustic impedance against Poisson’s ratio was able to differentiate the hydrocarbon sands from the shale. The crossplots of Vp/Vs ratio was able to distinguish the wet saturated sands from hydrocarbon sands. LMR cross plots clearly delineated the gas sands from the oil saturated sands where they exist. The isotropic and anisotropic PP helped to identify AVO gas sand type while the isotropic and anisotropic PS showed greater information about the gas reservoirs. The lambda-rho and the difference between the lambda-rho and mu-rho attributes clearly show the gas zones. The isotropic and anisotropic PS also shows that the S wave information would greatly assist in the search of hydrocarbon in the Basin. Elastic properties indicative of hydrocarbon were seen to tie with high hydrocarbon saturation areas and were interpreted as probable prospects. Pseudo well logs were finally generated for the probable prospects.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California