Porosity - Lithology Case History
Ratio and Poisson's Ratioo
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.
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.
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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