Basic Petrophysics · Module B4
Porosity Calculation
Density, neutron and sonic porosity, the matrix and fluid assumptions behind each, and how to combine them: the density–neutron crossplot for lithology, the density–neutron average in gas, and total versus effective porosity in shaly rock. It ends with thickness-weighted averaging over net pay.
- Level
- Beginner
- Position
- Module 4 of 7
- Video
- About 18 minutes, narrated
- Format
- Narrated video lesson with chapters; with the certified option, a graded module quiz.
What the module covers
- Density porosity and the matrix density assumption
- Neutron porosity and its lithology scaling
- Sonic porosity by the time-average equation, and where it applies
- Reading lithology from the density–neutron crossplot
- Gas-corrected porosity from density and neutron together
- Total versus effective porosity, and averaging over net pay
Learning outcomes
After the module you should be able to:
- Compute porosity from density, neutron and sonic logs and state the assumptions
- Use the density–neutron crossplot to check lithology
- Report a thickness-weighted net pay porosity
A short preview
Density porosity is φD = (ρma − ρb) / (ρma − ρf). With a sandstone matrix of 2.65 g/cc, a filtrate density of 1.00 g/cc and a bulk density of 2.32 g/cc, φD = 0.33 / 1.65 = 0.200. If the rock is in fact a limy sand with a matrix density of 2.71 g/cc, the same log gives 0.39 / 1.71 = 0.228. A 0.06 g/cc matrix error is almost three porosity units, which is why matrix density is treated as a parameter to calibrate rather than a default to accept.
Who it is for
Geoscientists, reservoir and production engineers, and graduates who need to read a standard log suite and build a first-pass interpretation they can defend.
Prerequisites: None beyond basic geology and algebra.
Take the module in the Learning Hub
Basic Petrophysics: Seven foundational modules with narrated lessons and chapter-based navigation. The lesson opens in the Learning Hub once the track is purchased; graded quizzes and the certificate come with the certified option.