CALIBRATION OF SEISMIC RESPONSE FOR 3D-AVO ANALYSIS
JAMES DISIENA, FRED HILTERMAN, CONNIE VANSCHUYVER, 1996. "CALIBRATION OF SEISMIC RESPONSE FOR 3D-AVO ANALYSIS", Stratigraphic Analysis Utilizing Advanced Geophysical, Wireline and Borehole Technology for Petroleum Exploration and Production, Jory A. Pacht, Robert E. Sheriff, Bob F. Perkins
Download citation file:
In this Gulf of Mexico case study, extended Amplitude Versus Offset (AVO) analysis enables the generation of sand and gas reservoir maps not obtainable from conventional 3D data. This interpretation results from calibrating the 3D-AVO response to lithology and pore fluid variations, in particular:
Analysis of well data provides lithology, velocity, and density control necessary to estimate Poisson’s ratio, as well as rock property variations from wet to gas filled conditions,
AVO modeling based on these variations yields the anticipated seismic response, thereby linking the log properties to the 3D seismic data, and
Processing to recover the AVO response from data with source-to-receiver offsets out to two times the target depth, enables lithologic mapping when supported by the above calibration.
The reservoir sands range from 6,000 to 13,000 ft depth, lying above and below geopressure. Conventional seismic analysis associates the amplitude of the reflections to the contrast in acoustic impedance, the product of velocity and density. On the log data, the target sands appear to have little contrast in acoustic impedance with their encasing shales. Because of this low contrast, the amplitudes on the seismic stack sections show little-to-no discrimination between sand-shale lithologic variations or pore fluid variations.
A two-term model for the seismic amplitude provides the basis for unraveling the complex AVO responses of lithologic variations and gas sands. The first term, the Normal Incidence reflectivity (NI), responds to changes in acoustic impedance. The second term, defined as the Poisson reflectivity (PR), relates to changes in Poisson’s ratio. Unlike NI, PR remains sensitive to lithologic variations within this geologic environment. The rock property contrasts, which generate the NI and PR response, become evident by crossplotting well-log values of acoustic impedance versus Poisson’s ratio. The crossplots show that even when the sands have the same acoustic impedance as the encasing shales, Poisson’s ratio discriminates between them. To obtain a robust estimate of PR, the AVO processing incorporates corrections for anisotropy, which extends the AVO analysis out to very-far offset traces. Finally, a view of the lithostratigraphic properties develops by displaying the NI and PR estimates from the seismic data with a novel color crossplotting method. Unlike the stack amplitude, distinct reservoir features appear in the seismic crossplot volume. After calibration, these 3D crossplot sections provide maps of reservoir quality sands and potential pay intervals.