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The AAPG/Datapages Combined Publications Database

Montana Geological Society

Abstract

MTGS-AAPG

Expanded Abstracts Volume: Rocky Mountain Section Meeting: AAPG
July 28-31, 1996

Pages 57 - 67

Seismic Stratigraphy — Lodgepole Play Examples

John A. Andrew, Consultant, StratSeis Geophysical Inc., Englewood, Colorado

ABSTRACT

The ability to "see" a stratigraphic feature, such as a Lodgepole Mound, in seismic data depends on it's subsurface depth, size and acoustic contrast relative to the wavelength of the seismic signal. Frequency is the only Previous HitvariableNext Hit affecting wavelength that can be controlled during a seismic survey. It is possible to preserve and enhance high frequencies by: (1) using a seismic source rich in high frequencies, (2) recording with modern high fidelity instruments and (3) data processing with modern techniques, equipment and software.

The vertical resolution (bed thickness) of seismic data is wavelength (frequency) dependent. Sheriff (1977) says that vertical resolution can be estimated by multiplying a 1/4 cycle of the effective frequency by the estimated rock Previous HitvelocityNext Hit. He also says that lateral resolution is a function of both wavelength and depth. Energy reflected from a Fresnel Zone (area imaged by a seismic wavefront) will arrive at the surface within a 1/2 wavelength and will interfere constructively, producing the signal wavelet. Being wavelength (frequency) dependent, a Fresnel Zone is smaller for high frequency data than low frequency seismic data from the same depth. Also, the Fresnel Zone size geometrically increases with depth as the wavefront expands, further limiting lateral resolution.

Identifying stratigraphic features in seismic data is fundamentally different from mapping structure. Stratigraphic features are often difficult to detect in routinely acquired and displayed seismic data. The ability to detect and study stratigraphy is strongly dependent on the frequency content of the seismic data. Often stratigraphic effects in seismic data are dismissed or overlooked as geophysical problems (processing, statics, multiples, etc.), thus inhibiting stratigraphic interpretations.

Some stratigraphic features can be visually identified by their geometric shape and/or internal acoustic contacts, especially, if the feature involves a distinctive shape encased in rocks with contrasting Previous HitvelocityNext Hit. Conversely, stratigraphic bodies are less visible if they involve rocks with small Previous HitvelocityTop contrast. For example, Lodgepole Mounds near Dickinson, North Dakota are relatively low porosity carbonate bodies and associated thin shales encased in shaly carbonate rocks. They are difficult to see in routinely displayed seismic data, but are more recognizable in vertically exaggerated displays.

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