Use this geological calculation for a transparent first-pass interpretation of measured rock, soil, fluid, well, stratigraphic, isotopic, geophysical, or seismic data under an explicitly selected model.

GR

Gamma-ray log reading at the depth of interest in API units.

GR clean

Clean (zero-shale) baseline reading in API units.

GR shale

Shale-point reading in API units.

Linear gamma-ray index IGR = (GR - GR_clean) / (GR_shale - GR_clean) from log readings in API units; the standard first step toward Larionov-style Vshale estimates. The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

IGR

Gamma-ray index between the clean and shale end members.

• Log-derived porosity, shale volume, permeability, and saturation depend on tool type, borehole conditions, invasion, lithology, matrix and fluid properties, environmental corrections, calibration, and depth matching. Core and pressure data should constrain interpretation.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Compare independent measurements and models, retain units and analytical metadata, propagate uncertainty, and validate interpretations against logs, cores, maps, laboratory data, or field observations.

• Well Log Shale Volume Calculator

  Convert a gamma ray reading in API units to shale volume using the linear or Larionov transforms.

• Well Log Porosity Neutron Calculator

  Correct a neutron porosity log reading (limestone porosity units) for shale content using V_shale times the shale neutron reading.

• Well Log Porosity Density Calculator

  Density-log porosity phi_D = (rho_ma - rho_b) / (rho_ma - rho_f) from bulk, matrix and pore-fluid densities in kilograms per cubic meter; the standard formation-density porosity relation.