A number of useful shale volume methods have more restricted application than the more common methods described earlier. The four listed below have proved useful on particular projects that needed help. The reader should take a moment to define the parameters and work a hypothetical numerical example.
Shale Volume from Electromagnetic Propagation
1. Vshept = (ATTEN - ATTEN_CLN) / (ATTEN_SHL - ATTEN_CLN)
VSH_rESd - Shale Volume from Deep Resistivity
The clean and shale resistivity values can be picked from suitable cross plots or from depth plots.
For the resistivity log method, the use of the logarithm of the resistivity log values (and base line values) works better than linear values, as follows:
2. Vshrd = (log(RESD) - log(RESD_CLN)) / (log(RESD_SHL) - log(RESD_CLN))
Note that RESD_CLN is greater than RESD_SHL
Shale Volume from Shallow Resistivity
3. Vshrs = (log(RESS) - log(RESS_CLN)) / (log(RESS_SHL) - log(RESS_CLN)
4. Vshsig = (SIGMA – SIG_CLN) / (SIG_SHL – SIG_CLN)
All cased hole thermal decay time logs display a SIGMA curve as one of the primary measurements. Although there are hydrocarbon effects, the curve can sometimes be used to overcome problems with the gamma ray log, such as uranium precipitation on casing or tubing, or missing GR log.
5. Vshpe = (PE – PE_CLN) / (PE_SHL – PE_CLN)
PE of quartz is between 1.6 to 1.8 and illite shale is near 3.5. l
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