In many older fields, you only have resistivity logs to work with. What if you need a sonic log to generate seismic seismogram?
A quick solution is to generate your own pseudo sonic log.
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In 1951, Faust came up with a transform to estimate compressional sonic using resistivity. He found out that in wet clastic rocks, the resistivity log and the P-wave sonic track each other.
The method works best in a water bearing or in a fully invaded water based mud shallow clastic rocks.
You need to establish new parameters for each major geological layer. The transit times can vary greatly in shallow clastic sections, across formations with low-salinity water saturations, and in carbonate sections where formation resistivity are higher.
To determine the parameters, calibrate with offset well data, check shots or vertical seismic profile.
Vp = P-wave velocity (feet/second)
a = constants, typically from 1000 to 3400 depending on depth.
R = resistivity value (ohm-feet). Usually, Rt (Ro in water bearing zones) works best.
Z = depth (feet)
Do note that the method doesn't work in gas zones.
Gas has a very low density. If the invaded zone is not fully flushed with mud filtrate, the computed sonic log won't match the real sonic log from the same well or the offset wells. The good thing is, you can use it to detect a gas bearing interval.
Use the same technique to identify source rocks, and evaluate the organic richness of unconventional plays.
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3. Faust, L.Y., 1951, A Velocity Function Including Lithologic Variation: Geophysics, v. 18, p. 271-288.
4. Meyer, B.L. etal, 1984, Identification of Source Rocks on Wireline Logs by Density/Resistivity and Sonic Transit Time/Resistivity Crossplots, AAPG Bulletin, v. 68, p. 121-129.
5. Passey, Q.R., etal, 1990, A Practical Model for Organic Richness from Porosity and Resistivity Logs, AAPG Bulletin, v. 74, p. 1777-174.