Porosity - Lithology Case History
These case histories represent just a few of the possibilities for processing seismic inversion data to aobtain various attributes that help to represent the seismic data in terms of rock type, reservoir quality, and spmetimes fluid type.

Vp/Vs Ratio and Poisson's Ratioo
Thedisplays below demonstrate this technique. A standard compressional seismic section is shown in the first illustration. The color code represents compressional interval velocity, determined from detailed velocity analysis. It can also be found from seismic inversion as described earlier. The shear wave seismic section is first stretched so that all reflections are displayed at compressional arrival times, then it is color coded to display shear interval velocity. The discreet interval velocity data is transformed into Poisson's ratio by the above equation and presented as a cross section, color coded in steps of Poisson's ratio.

 


Compressional wave inverted velocity section


Shear wave inverted velocity section


Poisson's ratio seismic section

The light yellow color on the final plot probably indicates a gas sand which is not directly visible on either the seismic amplitude or velocity displays. Control of seismically derived Poisson's ratio data by comparison to well logs is complicated by the fact that the sonic and density log data is affected by mud filtrate invasion into gas zones. Thus the logs must be modeled for this effect before they can be used. This type of log modeling is called fluid replacement and is best accomplished using the log response equation.

A handy chart for determining lithology directly from Vc and Vs is shown below.


Lithology from shear and compressional velocity


Qualitative Porosity Indications
The following illustration shows a porosity study based on inversion and calibration of porosity to the inverted acoustic impedance curves. Seismic shear data was not required for this work, but it would be helpful in a similar study in carbonate reservoirs.


Sand - shale porosity analysis from inverted acoustic impedance calibrated to well log data

Determining which attribute or combination of attributes will correlate to reservoir properties may require some trial and error testing. There are more than 20 possible attributes and their permutations and combinations can be quite large. The usual choices are Vp/Vs, Poisson's ratio, instantaneous compressional amplitude, compressional/shear amplitude ratio, and other related combinations. In all cases, log modeling and quantitative log analysis will be required to control the inversions and attribute calibration.

There are many connecting links between the seismic and well logging domains. Both develop velocity, density, and lithologic relationships from their measured data. A synthetic view of seismic response can be made from well logs, as can the inverse process create a sonic log from seismic data. A clear understanding of the sources and definitions of acoustic velocity information, and the ability to communicate these differences, will go a long way toward integrating exploration and evaluation techniques.

Analysis and interpretation of this diverse suite of data leads to a petrophysical description of the reservoir. In many cases, rock and fluid properties can be inferred and mapped. When calibrated to "ground truth", meaningful exploration and development decisions can be made with less risk. However, if the calibration is not attempted or done poorly, the results are mere arm-waving that may increase the chance of failure.

Do the work. Integrate the geo-science disciplines. Check your work. You'll be a happy puppy after it's all over.
 

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