petrophysics · porosity
Density Porosity
φD = (ρma − ρb) / (ρma − ρfl)
click formula to derive ↑
Inputs
g/cc
g/cc
g/cc
Description
Calculates porosity from the bulk density log assuming a two-component (matrix + fluid) mass balance. The result is highly sensitive to the assumed matrix density, so lithology must be reasonably well known before relying on this value.
Variables
| Symbol | Unit | Description |
|---|---|---|
| φD | fraction | Density Porosity |
| ρma | g/cc | Grain/matrix density of the rock framework. Use 2.65 g/cc for sandstone, 2.71 for limestone, 2.87 for dolomite — this choice has the largest impact on the result. |
| ρb | g/cc | Formation bulk density read directly from the density log, including pore fluid and matrix contributions. |
| ρfl | g/cc | Density of the fluid occupying the pore space at the depth of interest — about 1.0 g/cc for fresh mud filtrate, up to 1.1 g/cc for saline mud filtrate. |
Assumptions
- Formation is composed of a single dominant matrix mineral with a known grain density
- Pore space is filled with a fluid of known, uniform density (no gas effect)
- Density log is properly calibrated and corrected for borehole effects
Limitations
- Highly sensitive to the matrix density assumption — a 0.05 g/cc error in ρma can shift φD by several porosity units
- Overestimates porosity in gas-bearing zones because gas reduces ρb without the formula accounting for it
- Not reliable in heterogeneous lithologies (e.g., interbedded sand/shale) without a shale or lithology correction
Use Cases
- → Carbonate and sandstone porosity: Primary porosity curve when neutron data is unavailable or unreliable, using a matrix density appropriate to the known lithology.
- → Neutron-density crossplot input: Feed φD alongside φN into the crossplot porosity calculation to identify gas effect and reduce single-log bias.
- → Core porosity QC: Compare φD against core porosity measurements over a cored interval to validate the matrix density choice before applying it field-wide.
Related Calculations
Region Notes
Permian Basin
Wolfcamp and Spraberry carbonate-rich intervals often require ρma between 2.68-2.71 g/cc rather than pure sandstone 2.65; calibrate against core where available.
Eagle Ford
Mixed carbonate-clay mudrock typically uses ρma ≈ 2.68-2.70 g/cc; using 2.65 (sandstone) overstates porosity by 1-2 pu.
Middle East Carbonates
Limestone (ρma=2.71) and dolomite (ρma=2.87) intervals can alternate within a single reservoir — use a lithology log or PEF to select ρma zone-by-zone.
Global
When gas effect is suspected (φD > φN), prefer the neutron-density crossplot porosity over φD alone.
References
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