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The Leading Edge; April 2001; v. 20; no. 4; p. 425-428; DOI: 10.1190/1.1438966
© 2001 Society of Exploration Geophysicists
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AVO inversion, Part 2

Isolating rock property contrasts

Chuck Skidmore, Mike Kelly and Ray Cotton

Diamond Geoscience Research Corporation, Tulsa, Oklahoma, U.S.

Corresponding author: C. Skidmore, chucks@dgrc.com

The first 20% of the full text of this article appears below.

Part 1 of this article (TLE, March 2001) described a method for extracting rock property contrasts from seismic data. This has opened many doors previously thought to have been closed to the explorationist. This paper will address issues such as determining hydrocarbon saturation, production effects, nonbright pay, and S-wave seismic from P-wave acquisition and processing. These are pertinent problems in today's industry, and many require great expense to solve.


   Hydrocarbon saturation
 
Fizz gas, residual hydrocarbons, and low gas saturation are terms associated with less than economic amounts of gas in a reservoir. This problem, usually related to low-impedance sands, is a major cause for many amplitude-related dry holes.

Vp undergoes the greatest transformation in the presence of minor amounts of gas. Density ({rho}) and VS are insensitive to low gas saturation. The inability of acoustic impedance inversion or simple AVO to separate out the Vp term minimizes the hope of distinguishing saturation effects. However, rock property contrast volumes differentiate the individual elastic terms, making determination of noneconomic gas from economic gas a possibility. Figures 1 and 2 show amplitude maps and cross-sections over two known gas fields in the central Gulf of Mexico. Figure 3 indicates that both reservoirs have a strong positive AVO.


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  		Figure 1. (a) Amplitude map from Gulf of Mexico field. Contours are two-way time. (b) Amplitude cross-section from Gulf of Mexico field.

 

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  		Figure 2. (a) Amplitude map. (b) Amplitude cross-section. All amplitudes are currently producing.

 

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  		Figure 3. (a) AVO strength map. (b) Yellow and red indicate increasing amplitudes with offset.

 
AVO and amplitude analyses are standard approaches to exploration and development and appear to have performed adequately. The two dry holes shown in Figure 1 are exceptions to the rule that strong amplitudes plus positive AVO equals a hydrocarbon indicator. These wells penetrate the reservoir where both . . . [Full Text of this Article]






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