This article is based on
Pocket Pal" by E. R. (Ross) Crain, P.Eng.
2018 by the author. Copying or distribution in any form without
express written permission is prohibited.
Shale distribution can vary, as shown above. The shale volume calculations shown below are insensitive to the shale distribution. However, porosity and water saturation are strongly affected by laminated shale, so the methods shown in Sections 5, 6, 7, and 8 in this handbook do not apply to laminated shaly sands.
*** PLEASE NOTE ***
You must choose the appropriate methods for each zone, but the minimum rule works well in most cases, provided the usage rules have been honored first.
Shale volume estimation is the first calculation step in a log analysis. All other calculations depend on the shale volume being known from this step.
STEP 1: Convert density log (gm/cc or Kg/m3) to porosity units if a density porosity log is not available (skip this step if density data is already in porosity units):
1: PHIDSH = (DENSSH – KD2) / (KD1 – KD2) – do this once in an obvious shale zone
2: PHID = (DENS – KD2) / (KD1 – KD2) – do this for every data level
Where: KD1 = 1.00 for English units
KD1 = 1000 for Metric units
KD2 = 2.65 for English units Sandstone scale log
KD2 = 2650 for Metric units Sandstone scale log
KD2 = 2.71 for English units Limestone scale log
KD2 = 2710 for Metric units Limestone scale log
KD2 = 2.87 for English units Dolomite scale log
KD2 = 2870 for Metric units Dolomite scale log
NOTE: The choice for KD2 must match the neutron log units – if neutron is in Limestone units, KD2 must be 2.71 for gm/cc or 2710 for Kg/m3 log scale.
STEP 2: Calculate shale volume from the three common methods:
3: Vshg = (GR - GR0) / (GR100 - GR0)
4: Vshs = (SP - SP0) / (SP100 - SP0)
5: Vshx = (PHIN - PHID) / (PHINSH - PHIDSH)
In tar sands or heavy oil, add the resistivity method:
6: Vshr = (logRESS - logRMAX) / (logRSH - logRMAX)
In radioactive sands, replace the gamma ray method with Thorium method if gamma ray spectral data is available:
7: Vshth = (TH - TH0) / (TH100 - TH0)
NOTE: Trim Vsh values between 0.0 and 1.0. If too many values fall outside this range, check the clean and shale parameters. Do not calculate methods which fail to pass all usage rules listed below.
STEP 3: Adjust gamma ray method for young rocks, if needed:
8: Vshc = 1.7 - (3.38 - (Vshg + 0.7) ^ 2) ^ 0.5
STEP 4: Take minimum of available methods:
9: Vsh = Min (Vshg, Vshs, Vshr, Vshx, Vshc)
· Use uranium corrected gamma ray (CGR) in preference to uncorrected GR
· Do not use GR in radioactive sandstones or carbonates. Use Thorium curve from NGT for radioactive sandstone, and uranium corrected GR (CGR) curve for radioactive carbonates.
· Do not use SP in fresh water formations, salt mud systems, high resistivity zones, or in carbonates.
· Do not use density neutron crossplot when bad hole, gas, or heavy minerals are present.
· Do not use the nonlinear young rock model unless there is some evidence that it is needed.
If log analysis porosity is too low, calculated shale volume may be too high (or vice versa).
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