Seismic polarity reversals in higher impedance gas sands
- Neidell & Associates, Houston, TX (United States)
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 {open_quotes}bright-spot{close_quotes} world, or zone I condition. For such reflectivity type, impedances ascribed to a single-layer sand model would yield the classic {open_quotes}trough-over-peak{close_quotes} signature on zero-phase wavelet-processed data, with a composite waveform for the {open_quotes}thin bed.{close_quotes} 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 {open_quotes}dim spots{close_quotes} we might expect if they were gas bearing. 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 analyses, 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.
- OSTI ID:
- 96238
- Report Number(s):
- CONF-941065-; ISSN 0149-1423; TRN: 95:005100-0042
- Journal Information:
- AAPG Bulletin, Vol. 78, Issue 9; Conference: 44. annual convention of the Gulf Coast Association of Geological Societies and the 41st annual convention of the Gulf Coast Section of the Society of Economic Paleontologists and Mineralogists, Austin, TX (United States), 5-7 Oct 1994; Other Information: PBD: Sep 1994
- Country of Publication:
- United States
- Language:
- English
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