Changes

Seismic amplitude is defined by the difference of acoustic impedance between two layers, where higher difference in AI has “brighter” seismic amplitude at the interface between both layers such as the interface between claystone and high porosity gas sand that is indicated by sharp decrease in AI or the interface between claystone and tight carbonate that is indicated by sharp increase in AI. On well data, synthetic seismic (Figure 5) can be generated by convoluting reflectivity coefficient (product of the difference in AI between two layers) with a wavelet that is mathematically defined as follow:

Seismic amplitude is defined by the difference of acoustic impedance between two layers, where higher difference in AI has “brighter” seismic amplitude at the interface between both layers such as the interface between claystone and high porosity gas sand that is indicated by sharp decrease in AI or the interface between claystone and tight carbonate that is indicated by sharp increase in AI. On well data, synthetic seismic (Figure 5) can be generated by convoluting reflectivity coefficient (product of the difference in AI between two layers) with a wavelet that is mathematically defined as follow:

::<math>R =\frac{(AI_2 - AI_1)}{(AI_2 + AI_1)} = \frac{(V_p2 * Rho_2)-(V_p1 * Rho_1)}{(V_p2 * Rho_2) + (V_p1 * Rho_1)}</math>

::<math>R =\frac{(AI_2 - AI_1)}{(AI_2 + AI_1)} = \frac{(V_p2 * Rho_2)-(V_p1 * Rho_1)}{(V_p2 * Rho_2) + (V_p1 * Rho_1)}</math>

where R is the reflectivity coefficient, AI1, Vp1, and Rho1 are the acoustic impedance, P-wave velocity, and density of the upper layer, whereas AI2, Vp2, and Rho2 are the acoustic impedance, P-wave velocity, and density of the lower layer. On Figure 5, (1) denotes the “bright” amplitude due to the sharp change in AI, whereas (2) denotes the “dim” amplitude due to the smaller change in AI.

where R is the reflectivity coefficient, AI1, Vp1, and Rho1 are the acoustic impedance, P-wave velocity, and density of the upper layer, whereas AI2, Vp2, and Rho2 are the acoustic impedance, P-wave velocity, and density of the lower layer. On [[:File:GeoWikiWriteOff2021-Muamamr-Figure5.png|Figure 5]], (1) denotes the “bright” amplitude due to the sharp change in AI, whereas (2) denotes the “dim” amplitude due to the smaller change in AI.

[[File:GeoWikiWriteOff2021-Muamamr-Figure5.png|framed|center|{{Figure number|5}}Schematic diagram to create synthetic seismogram.]]   

[[File:GeoWikiWriteOff2021-Muamamr-Figure5.png|framed|center|{{Figure number|5}}Schematic diagram to create synthetic seismogram.]]   

To predict the seismic amplitude variation with offset (AVO) or angle (AVA), several approximations can be used.<ref name=6AkiRichards>Aki, K. and P. G. Richards, 1980, Quantitative seismology: Theory and methods, San Francisco, W. H. Freeman and Co., 557 p.</ref><ref name=7Shuey>Shuey, R. T., 1985, A simplification of the Zoeppritz equations: Geophysics, v. 50, no. 4, p. 609-614.</ref><ref name=8Fattietal>Fatti, J. L., G. C. Smith, P. J. Vail, P. J. Strauss, and P. R. Levitt, 1994, Detection of gas in sandstone reservoirs using AVO analysis: A 3-D seismic case history using the geostack technique: Geophysics, v. 59, no. 9, p. 1362-1376.</ref> To predict this AVO response however, Vs data is required. Figure 6 shows the expected seismic response between Sand A or Sand B (under gas saturated and wet condition) with their respective overlying and underlying claystones. It can be observed that under gas saturated condition, both sands have Class III AVO (increase in amplitude with increasing angle), whereas under wet condition, both sands have class II AVO (dim amplitude at Near and Far angle). It should be noted that different lithology (e.g. cemented sand and unconsolidated sand) may have different amplitude response. The occurrence of such sands can be interpreted by carrying out petrography analysis or rock physics diagnostic.  

To predict the seismic amplitude variation with offset (AVO) or angle (AVA), several approximations can be used.<ref name=6AkiRichards>Aki, K. and P. G. Richards, 1980, Quantitative seismology: Theory and methods, San Francisco, W. H. Freeman and Co., 557 p.</ref><ref name=7Shuey>Shuey, R. T., 1985, A simplification of the Zoeppritz equations: Geophysics, v. 50, no. 4, p. 609-614.</ref><ref name=8Fattietal>Fatti, J. L., G. C. Smith, P. J. Vail, P. J. Strauss, and P. R. Levitt, 1994, Detection of gas in sandstone reservoirs using AVO analysis: A 3-D seismic case history using the geostack technique: Geophysics, v. 59, no. 9, p. 1362-1376.</ref> To predict this AVO response however, Vs data is required. Figure 6 shows the expected seismic response between Sand A or Sand B (under gas saturated and wet condition) with their respective overlying and underlying claystones. It can be observed that under gas saturated condition, both sands have Class III AVO (increase in amplitude with increasing angle), whereas under wet condition, both sands have class II AVO (dim amplitude at Near and Far angle). It should be noted that different lithology (e.g. cemented sand and unconsolidated sand) may have different amplitude response. The occurrence of such sands can be interpreted by carrying out petrography analysis or rock physics diagnostic.  

[[File:GeoWikiWriteOff2021-Muamamr-Figure6.png|thumbnail|Figure 6. Expected AVO response of (A) Sand A under gas case, (B) Sand A under wet condition, (C) Sand B under gas case, and (D) Sand B under wet condition.]] 

GeoWikiWriteOff2021-Muamamr-Figure6.png|{{Figure number|6}}Expected AVO response of (A) Sand A under gas case, (B) Sand A under wet condition, (C) Sand B under gas case, and (D) Sand B under wet condition.

 

GeoWikiWriteOff2021-Muamamr-Figure7.png|{{Figure number|7}}(A) Synthetic seismic of Sand A and (B) schematic cartoon of the modeled structure.

[[File:GeoWikiWriteOff2021-Muamamr-Figure7.png|thumbnail|Figure 7. (A) Synthetic seismic of Sand A and (B) schematic cartoon of the modeled structure.]] 

   

   

Synthetic seismic view of Sand A on an asymmetrical anticline structure is shown on Figure 7. Gas-water contact was modeled which properties were derived from fluid substitution. It can be observed that below the modeled GWC, amplitude polarity reversal can be identified due to the change from acoustically softer gas saturated sand to acoustically harder water saturated sand. This method is a common workflow to guide the seismic interpretation.

Synthetic seismic view of Sand A on an asymmetrical anticline structure is shown on Figure 7. Gas-water contact was modeled which properties were derived from fluid substitution. It can be observed that below the modeled GWC, amplitude polarity reversal can be identified due to the change from acoustically softer gas saturated sand to acoustically harder water saturated sand. This method is a common workflow to guide the seismic interpretation.