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GC Frequency in Defining Trend of Productive Thin-bedded Sandstone*

By

Bob Hardage1

 

Search and Discovery Article #40284 (2008)

Posted March 31, 2008

 

*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, March, 2008, and entitled “ ‘Previous HitInstantaneousNext Hit’ an Ideal Indicator.” Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.

 

1 Bureau of Economic Geology, The University of Texas at Austin ([email protected])

 

General Statement

The numerous seismic Previous HitattributesNext Hit that can be calculated with various interpretation software packages are based on three fundamental wiggle-trace Previous HitattributesNext Hit: amplitude, phase, and frequency. Because these Previous HitattributesNext Hit are calculated at every time-sample of a seismic trace, they are referred to as “Previous HitinstantaneousNext Hit” Previous HitattributesNext Hit.

Constructing Previous HitattributesNext Hit as Previous HitinstantaneousNext Hit functions is important because interpreters then have more flexibility in how they use the Previous HitattributesNext Hit. For example, a time-based attribute can be analyzed along an interpreted horizon (only one data point thick); within a thin data window (three or four data points thick) that conforms to a reference surface; or averaged throughout an extensive data window (several tens of data points thick) that spans some portion of seismic image space.

Most seismic interpreters, including the author, tend to focus on amplitude-based Previous HitattributesNext Hit as they search a 3-D seismic volume for geologic information. However, phase-based and frequency-based Previous HitattributesNext Hit are valuable for depicting subtle targets in many instances. This article illustrates an application in which Previous HitinstantaneousNext Hit frequency was used to define a stratigraphic trend of a productive thin-bedded sandstone.

 

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Figure Captions

Figure 1. Net thickness of a Caddo sandstone unit defined by well logs from numerous wells across a small area of the Fort Worth Basin. Locations of only a few wells of particular interest in a research study are labeled.

Figure 2. Seismic section along profile AA’ labeled in Figure 1 that identifies the unique reflection response of the Caddo sandstone unit.

Figure 3. Map of average Previous HitinstantaneousNext Hit frequency in a 10-ms window positioned 10 to 20 ms below the Caddo horizon. The trend of 60- to 75-Hz average-frequency Previous HitattributesNext Hit on this map is a close approximation to the log-based map in Figure 1 and identifies both new drilling locations and locations to avoid.

 

Click to view in sequence a map of Caddo sandstone defined by well logs (Figure 1) and a map of Previous HitinstantaneousNext Hit frequency near the Caddo horizon (Figure 3).

 

Example

The net-sand-thickness map in Figure 1 illustrates the distribution of a productive Caddo sandstone in the Bend Conglomerate interval of the Fort Worth Basin. Considerable contour detail is shown on the map because reservoir thickness was defined from logs acquired in about 30 wells across the mapped area. Only a few of these well locations are labeled in Figure 1.

A vertical seismic section along profile AA’ is shown in Figure 2. The labeled feature shows the reflection character across the thin-bedded sandstone and illustrates that the sandstone target is stratigraphically trapped and is not a structural feature.

Several seismic Previous HitattributesNext Hit were calculated and analyzed in an attempt to follow the trend of this productive Caddo target through 3-D seismic image space. Of all the Previous HitattributesNext Hit that responded to the presence of this thin-bedded unit, the attribute that produced the optimal stratigraphic trap image was Previous HitinstantaneousNext Hit frequency. For example, the Previous HitinstantaneousNext Hit-frequency map in Figure 3 is a close match to the log-based map in Figure 1 because:

  • The frequency of the reflection signal reacts to the presence of this thin-bedded unit.
  • An appropriate color bar is used to display this frequency behavior.

Selection of the color bar used to display a seismic attribute is often the key to attribute interpretation. The correspondence between a seismic attribute value and a targeted geologic condition can be enhanced by the proper choice of color bar that displays the attribute – and, unfortunately, attribute-to-geology relationships can be obliterated by a poor choice of color bar. Some interpreters justifiably take as much time creating an appropriate color bar for attribute maps as they do creating the attribute that is being mapped.

 

Conclusion

The fundamental message from this example is that frequency-based Previous HitattributesTop at times can be ideal indicators of stratigraphic-trap conditions.

 

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