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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 44 (1994), Pages 765-765

Abstract: Seismic Polarity Reversals in Higher Impedance Gas Sandstones

W. R. Landwer, N. S. Neidell


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

In this study, however, we consider anomalous reflections of the higher impedance sandstones 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 sandstone 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 case studies from gas-producing fields, seismic amplitude/tuning thickness models, subsurface well log information, and auxiliary analyses such as amplitude versus offset and inversion, we examined this phenomenon for its interpretive significance. Also, by modeling and viewing additional analyses, we demonstrate how such hydrocarbon-associated reflectivity changes occur for certain higher impedance members of 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 they derive this exaggerated signature from tuning. Logged impedance values in such sandstones typically show good contrast on the high side with the shales for the water-wet condition and only modest impedance reductions below the shales for the gas pay. Because this high-potential province remains largely unexploited, owing to poor understanding, improved insights here may result in many new and significant discoveries.

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