Maximum Porosity MODEL
The PHIMAX method can be used in shaly sands as a porosity method,  when only a GR or SP log is available with no porosity indicating logs. It does not work in carbonates:
      1: IF PHIe > PHIMAX * (1.00 - Vsh)
      2: THEN PHIe = PHIMAX * (1.00 - Vsh)

Where:
  PHIe = porosity from any method (fractional)
  Vsh = shale content from any method (fractional)
  PHIMAX = maximum expected porosity in clean rock (fractional)
 
RECOMMENDED PARAMETERS:
Normal values for PHIMAX:
Very high porosity sandstone (and tar sands) PHIMAX = 0.36 to 0.42
High to medium porosity PHIMAX = 0.25 to 0.36
Low or shaly medium porosity PHIMAX = 0.18 to 0.25
Very low porosity, PHIMAX = 0.10 to 0.18

NUMERICAL EXAMPLE:
1. For example, if Vsh = 0.65 and PHIe = 0.30, this would mean the sand fraction has 85% porosity, which is unreasonable. The sand fraction porosity is derived as follows:
PHIsd = PHIe / (1 - Vsh) = 0.30 / (1 - 0.65) = 0.85

Using the PHIMAX equation with a PHIMAX of 0.30:PHIe = (1 - 0.65) * 0.30 = 0.105 which indicates the sand fraction has a porosity of 0.30 by edict of the analyst. This is more reasonable than the value of 0.85 derived earlier.