Rock Density From Porosity Calculator
Compute rock density porosity using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.
Formula
rho_bulk = (1 - phi) x rho_grain + phi x rho_fluid
Bulk density is a linear combination of grain density and fluid density weighted by porosity (phi). The solid fraction (1 - phi) contributes the mineral density, and the pore fraction (phi) contributes the fluid density. This can be rearranged to solve for porosity from measured bulk density.
Worked Examples
Example 1: Sandstone Bulk Density
Problem: A sandstone has 20% porosity, quartz grain density of 2,650 kg/m\u00B3, and is saturated with water (1,000 kg/m\u00B3).
Solution: rho_bulk = (1 - 0.20) x 2650 + 0.20 x 1000\nrho_bulk = 0.80 x 2650 + 200\nrho_bulk = 2120 + 200 = 2320 kg/m\u00B3
Result: Bulk density = 2,320 kg/m\u00B3 (2.32 g/cm\u00B3)
Example 2: Porosity from Well Log
Problem: A density log reads 2,450 kg/m\u00B3 in a limestone with calcite grain density 2,710 kg/m\u00B3 and brine at 1,050 kg/m\u00B3.
Solution: phi = (2710 - 2450) / (2710 - 1050)\nphi = 260 / 1660 = 0.1566 = 15.66%
Result: Porosity = 15.66%
Frequently Asked Questions
What is the relationship between rock density and porosity?
Rock bulk density is determined by a linear mixing model that combines the grain (mineral) density and the fluid density in proportion to the porosity. The formula is rho_bulk = (1 - phi) x rho_grain + phi x rho_fluid, where phi is the fractional porosity. As porosity increases, bulk density decreases because lighter pore fluid replaces denser mineral grains. This relationship is fundamental in well-log interpretation and reservoir characterization.
How does porosity affect seismic velocity in rocks?
Porosity strongly influences seismic wave velocities because pore fluids are more compressible and less rigid than mineral grains. Higher porosity generally results in lower P-wave and S-wave velocities. The Wyllie time-average equation approximates this relationship for P-waves: 1/V = phi/V_fluid + (1-phi)/V_matrix. This makes density-porosity relationships critical for interpreting seismic surveys and sonic well logs in exploration geophysics.
What is void ratio and how does it relate to porosity?
Void ratio (e) is the ratio of the volume of voids to the volume of solids in a rock or soil, while porosity (phi) is the ratio of void volume to total volume. They are related by e = phi / (1 - phi) and phi = e / (1 + e). Void ratio is commonly used in geotechnical engineering and soil mechanics because it relates linearly to volume change during compression, while porosity is preferred in petroleum engineering and hydrogeology.
What are the stages of the rock cycle?
The rock cycle describes transformations among three rock types. Igneous rocks form from cooled magma or lava. Sedimentary rocks form from compressed and cemented sediments. Metamorphic rocks form when existing rocks are changed by heat and pressure. Weathering, erosion, melting, and tectonic forces drive these transitions.
What formula does Rock Density From Porosity Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
Does Rock Density From Porosity Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.