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