| The trend and variance in Amplitude Variation with Offset (AVO) seismic observations caused by near-surface structure, attenuation, and scattering are numerically synthesized by pseudospectral viscoelastic 2-D modeling. Depending on the relative influence of intrinsic attenuation, apparent attenuation associated with scattering, and geometrical focusing, a normalized AVO response can increase or decrease with offset and so mimic the behavior predicted as a function of contrasts in elastic and petrophysical properties. Lateral variations in AVO parameters are the key to detecting hydrocarbons, so lateral changes in AVO produced by lateral changes in the overburden properties have a potential for being misinterpreted, especially if the recording aperture is small. Interpretation of AVO based on homogeneous layered elastic models must be used with caution as they are, in general, nonunique.; A positive and high amplitude gradient of the Amplitude Variation with Angle (AVA) response for an interface between overburden shale and a carbonate reef in the Parigi formation in the Northwest Java Basin is hypothesized to be caused by the presence of gas. The AVA is affected by a reduction of the bulk density of the reservoir, a decrease of the overburden Vp/Vs ratio, and by local overburden attenuation caused by gas seeping through the overlying sediments.; Quantitative interpretation for reservoirs that are thinner than the tuning thickness can not be based solely on the reflection amplitudes, since the tuning effects from the interference between reflection from the tops and bottoms of the bed will complicate the interpretation of results. A 2-D distribution of acoustic impedance derived from the reflection amplitudes, constrained and calibrated by the proven hydrocarbon reservoirs in a well, shows consistent detectable impedance anomalies associated with reservoir depths. Anomalies are also observed in the instantaneous frequency. The results can be used to guide the search for anomalies at nearby locations for evaluating potential for hydrocarbon accumulation. |
