Abstract: Seismic Polarity Reversals in Higher Impedance Gas Sands

W. R. Landwer, N. S. Neidell

Most interpreters view processed seismic displays with an understanding of sand/shale reflectivity as provided by available sonic and density logs. Simple synthetic trace seismograms are relatively straightforward for the well-known "bright-spot" world, or zone I (Neidell and Berry, 1989) condition (case 3 according to Rutherford and Williams, 1989). For such reflectivity type, impedances ascribed to a single-layer sand model would yield the classic "trough-over-peak" signature on zero-phase wavelet-processed data, with a composite waveform for the "thin bed."

This paper, however, considers anomalous reflections for the higher impedance sands in zone II (case 2) and zone III (case 1), which do not appear as the "dim spots" we might expect if they were gas bearing. Some zone-III type sand members (more consolidated and having greater acoustic impedance than their contemporary shale counterpart) frequently exhibit polarity reversed reflections when filled with gas in commercial quantities. This effect is also often exaggerated by tuning effects on amplitude. Using gas producing field case studies, seismic amplitude/tuning thickness models, subsurface well log information, and auxiliary analyses such as amplitude vs. offset and inversion, this phenomenon is examined for its interpretive significance. Also, by modeling and viewing additional an lyses, we demonstrate how such hydrocarbon-associated reflectivity changes occur for certain of the higher impedance members of the Miocene and Pliocene sections in the offshore Gulf of Mexico.

Many substantial pay zones found at depth are associated with high-magnitude (bright), trough-over-peak reflections, but derive this exaggerated signature from tuning. Logged impedance values in such sands typically show good contrast on the high side with the shales for the wet condition, and only modest impedance reductions below the shales for the gas pay. As this high-potential province remains yet largely unexploited, owing to poor understanding, improved insights here may result in many new and significant discoveries.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994