Rock Density and Porosity Calculator
Calculate bulk density, grain (matrix) density, fluid density, and porosity using standard petrophysical relationships. Provide any three known values to solve for the fourth.
Measured density of the rock sample (solid + pore fluid)
Density of the solid mineral grains (quartz ≈ 2.65, calcite ≈ 2.71, dolomite ≈ 2.87)
Density of pore-filling fluid (water ≈ 1.00, oil ≈ 0.80, gas ≈ 0.001–0.3)
Fraction of rock volume occupied by pores (0–100%)
⚠️ Leave exactly one field blank — it will be calculated from the other three.
Formula
The fundamental petrophysical relationship linking bulk density, grain density, fluid density, and porosity:
ρb = φ · ρf + (1 − φ) · ρg
Rearranged to solve for each unknown:
- Grain density: ρg = (ρb − φ · ρf) / (1 − φ)
- Fluid density: ρf = (ρb − (1 − φ) · ρg) / φ
- Porosity: φ = (ρg − ρb) / (ρg − ρf)
- Void ratio: e = φ / (1 − φ)
Where: ρb = bulk density, ρg = grain (matrix) density, ρf = pore fluid density, φ = porosity (fraction).
Assumptions & References
- The rock is modeled as a two-component system: solid mineral grains and a single pore-filling fluid (no partial saturation mixing).
- Porosity is total porosity (connected + isolated pores); effective porosity may differ in tight or clay-rich rocks.
- Fluid is assumed homogeneous throughout the pore space (fully saturated with one fluid phase).
- Grain density is assumed uniform (single mineral or effective average for polymineralic rocks).
- Temperature and pressure effects on density are not accounted for; use in-situ corrected values for reservoir conditions.
- Typical bulk density range: 1.8–3.0 g/cm³ for sedimentary rocks; up to ~3.5 g/cm³ for mafic igneous rocks.
- References: Archie (1942), AAPG Bulletin; Schlumberger Log Interpretation Principles (1989); Tiab & Donaldson, Petrophysics (4th ed., 2015).