PO.PP.Sw.Fertl

Formation resistivity factor from Archie's equation. It is the ratio of the resistivity of a rock fully saturated with water to the resistivity of that water, expressed through porosity. F = a / φ^m. Porosity is a fraction. a and m come from a log-log plot of F against porosity, or from a named pair such as Humble.

Water saturation in a clean formation from the general Archie equation. Porosity, water resistivity and true resistivity fix the bulk volume of water once the tortuosity factor, cementation exponent and saturation exponent are chosen. Sw = ((a / φ^m) * (Rw / Rt))^(1/n). Porosity is a fraction, resistivities are ohm·m, and a, m and n are dimensionless. With Vsh = 0 the shaly-sand equations in this batch reduce toward this one, at the a and m those equations bake in.

Sandstone permeability from the formation factor, the correlation Carothers published in 1968. It is a statistical fit, not a pore-scale derivation, and it applies to sandstones. The limestone fit is a different equation and is not this one. k = 7×10^8 / F^4.5, with k in millidarcies and F dimensionless.

Porosity from the density log in a clean formation. The bulk density sits between the matrix density and the fluid density, and the fraction of the way across is the porosity. φ = (ρma − ρb) / (ρma − ρf). All three densities share one unit, g/cm³ on a density log. Shale and gas corrections are not in this equation.

Equivalent formation-water resistivity from the static spontaneous potential, for water in which NaCl dominates. The SP equation is SSP = −K log10(Rmfe / Rwe), with K = 61 + 0.133 T and T in °F. Solving for the water resistivity gives Rwe = Rmfe · 10^(SSP / K). SSP is negative in the usual case of a fresher mud filtrate opposite a saltier formation water, and Rwe then comes out smaller than Rmfe. The result is the equivalent resistivity at formation temperature, which is what the NaCl form of the SP equation solves. The chart that turns Rwe into true Rw is not a closed form and is not in this lambda.

The linear gamma-ray shale index. It places a reading between the clean-sand and shale picks on a 0 to 1 scale. It is the input the Larionov shale-volume equations expect, and it is the shale volume itself only when the index is taken as linear. IGR = (GR − GRclean) / (GRshale − GRclean). Readings outside the two picks fall outside 0–1; the index is not clipped.