Rock Density From Porosity Calculator

Bulk density of a porous rock is a volume-weighted average of its two phases. Writing porosity as the fraction phi of total volume occupied by pore space, the solid framework occupies (1 - phi), and the mixing law rho_bulk = (1 - phi) x rho_grain + phi x rho_fluid follows from conservation of mass with no fitted parameter. The same expression inverts to give density porosity, phi = (rho_grain - rho_bulk) / (rho_grain - rho_fluid). The grain term is purely mineralogical: quartz 2650, calcite 2710, dolomite 2870, anhydrite 2960 and halite 2160 kg/m3. Pore fluids span fresh water at 1000, formation brine at 1050 to 1200, oil at 700 to 900, and gas at only 100 to 300 kg/m3. Realistic values follow from those endpoints. A water-filled quartz sandstone at 20 percent porosity has a bulk density near 2320 kg/m3; clean reservoir sandstones usually fall between 10 and 30 percent porosity, chalk can exceed 40 percent, and crystalline basement is typically under 1 percent. The commonest practical error is a wrong matrix assumption: computing porosity with a quartz grain density in a dolomite understates it by several porosity units, and gas in the flushed zone drives apparent porosity too high because the fluid density used is far too large. The model also treats all pore space as connected and fluid-filled, so clay-bound water and isolated vugs are lumped into phi.

Density has been obtained by weighing and displacement since antiquity, but the numbers used in subsurface work come from two much younger traditions. Laboratory practice settled on weighing core plugs dry and again saturated, with grain volume measured by helium expansion in a Boyle law porosimeter, a technique standardized by the petroleum industry through the 1930s and 1940s. That gave grain density and porosity directly, but only for the few centimetres of rock that coring recovers. The in-situ measurement arrived with gamma-gamma logging. A sealed chemical source, usually caesium-137, irradiates the formation, and the count rate of gamma rays reaching a detector after Compton scattering depends on electron density, which for most rock-forming minerals is very nearly proportional to bulk density. Early single-detector tools of the 1950s were badly biased by mudcake between the sonde and the borehole wall. The dual-spacing compensated density tool described by J. S. Wahl and colleagues at Schlumberger in 1964 solved this by adding a short-spacing detector whose reading corrects the long-spacing one, and that two-detector geometry remains the basis of wireline and logging-while-drilling density tools today. Turning the log reading into porosity requires exactly the mixing law above, which is why matrix and fluid densities are entered explicitly rather than assumed.