Changes

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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 [[:file:GeoWikiWriteOff2021-Muamamr-Figure7.png|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.

==Rock Physics Diagnostic==

==Rock Physics Diagnostic==

This method was first introduced[9] to assess the change of velocity-porosity relations under different geological conditions, including pore shape, mineralogy, and post-depositional processes such as compaction and cementation (e.g. cemented sands are harder than uncemented rocks and compacted rocks at deeper depths are harder than the less compacted rocks at shallower depths due to the difference in effective stress)[10]. Each of these geological conditions follows different velocity-porosity relations (rock physics model) and therefore, in absence of geological information (e.g. petrography), we can predict the cause of such velocity-porosity relations. Brief discussion these rock physics models are summarized below[10]:  

This method was first introduced<ref name=9DvorkNur>Dvorkin, J. and A. Nur, 1996, Elasticity of high-porosity sandstones: Theory for two North Sea data sets: Geophysics, v. 61, no. 5, p 1363-1370.</ref> to assess the change of velocity-porosity relations under different geological conditions, including pore shape, mineralogy, and post-depositional processes such as compaction and cementation (e.g. cemented sands are harder than uncemented rocks and compacted rocks at deeper depths are harder than the less compacted rocks at shallower depths due to the difference in effective stress)[10]. Each of these geological conditions follows different velocity-porosity relations (rock physics model) and therefore, in absence of geological information (e.g. petrography), we can predict the cause of such velocity-porosity relations. Brief discussion these rock physics models are summarized below[10]:  

# Friable Sand[9]= this rock physics model (unconsolidated line) shows the change in velocity-porosity relations as the sorting deteriorates. Well sorted sands are interpreted to have higher porosity and therefore, lower velocity. Grain deterioration decreases the porosity but only slightly stiffening the rock.  

# Friable Sand<ref name=9DvorkNur /> = this rock physics model (unconsolidated line) shows the change in velocity-porosity relations as the sorting deteriorates. Well sorted sands are interpreted to have higher porosity and therefore, lower velocity. Grain deterioration decreases the porosity but only slightly stiffening the rock.  

# Friable Shale[9]= similar rock physics model as above but the grain is modeled to be shale instead of sand.  

# Friable Shale<ref name=9DvorkNur /> = similar rock physics model as above but the grain is modeled to be shale instead of sand.  

# Contact Cement[11]= this rock physics model describes the change in velocity-porosity relations due to the cementation at grain contacts, effectively “gluing” the grains together. The cement at grain contacts significantly reinforcing the rock (large increase in velocity) but with small decrease of porosity.  

# Contact Cement[11]= this rock physics model describes the change in velocity-porosity relations due to the cementation at grain contacts, effectively “gluing” the grains together. The cement at grain contacts significantly reinforcing the rock (large increase in velocity) but with small decrease of porosity.  

# Constant Cement[12]= explains a sand body with varying sorting and porosity but with similar amount of cement in which the decrease of porosity is attributed to sorting deterioration. This model is an expansion of Friable Sand model that takes into account the impact of cement to the velocity-porosity relations[10].  

# Constant Cement[12]= explains a sand body with varying sorting and porosity but with similar amount of cement in which the decrease of porosity is attributed to sorting deterioration. This model is an expansion of Friable Sand model that takes into account the impact of cement to the velocity-porosity relations[10].  

To conduct rock physics diagnostic, it is essential to eliminate as much variations as possible, such as saturation[9] because velocity depends on saturation. It is suggested to utilize the velocity log under wet condition to eliminate such variation. Figure 8 shows the total porosity-Vp crossplot of Sand A and Sand B where it can be observed that both sands follow the Friable Sand rock physics model indicating that the change in velocity and porosity of the sands are attributed to the difference in sorting[9]. Sand B has lower porosity but slightly higher velocity compared to Sand A due sorting deterioration caused by the clays.  

To conduct rock physics diagnostic, it is essential to eliminate as much variations as possible, such as saturation<ref name=9DvorkNur /> because velocity depends on saturation. It is suggested to utilize the velocity log under wet condition to eliminate such variation. Figure 8 shows the total porosity-Vp crossplot of Sand A and Sand B where it can be observed that both sands follow the Friable Sand rock physics model indicating that the change in velocity and porosity of the sands are attributed to the difference in sorting.<ref name=9DvorkNur /> Sand B has lower porosity but slightly higher velocity compared to Sand A due sorting deterioration caused by the clays.  

As this approach may help interpreting the cause of such change in velocity-porosity relations as a function of subsurface geology in an area, this workflow can be utilized to predict the elastic properties of the rocks away from well control (e.g. to expect what seismic amplitude that corresponds to sand reservoir). Other examples of the application of this method have been reported by several authors[13], [14], [15].  

As this approach may help interpreting the cause of such change in velocity-porosity relations as a function of subsurface geology in an area, this workflow can be utilized to predict the elastic properties of the rocks away from well control (e.g. to expect what seismic amplitude that corresponds to sand reservoir). Other examples of the application of this method have been reported by several authors[13], [14], [15].  

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9 Dvorkin, J. and Nur, A., 1996, Elasticity of High-Porosity Sandstones: Theory for Two North Sea Data Sets, Geophysics, 61(5), pp. 1363-1370.

9  

10 Avseth, P., Mukerji, T, Mavko, G. and Dvorkin, J., 2010, Rock-Physics Diagnostics of Depositional Texture, Diagenetic Alterations, and Reservoir Heterogeneity in High-Porosity Siliciclastic Sediments and Rocks – A Review of Selected Models and Suggested Work Flows, Geophysics, 75(5), pp. 75A31-75A47.  

10 Avseth, P., Mukerji, T, Mavko, G. and Dvorkin, J., 2010, Rock-Physics Diagnostics of Depositional Texture, Diagenetic Alterations, and Reservoir Heterogeneity in High-Porosity Siliciclastic Sediments and Rocks – A Review of Selected Models and Suggested Work Flows, Geophysics, 75(5), pp. 75A31-75A47.