Neutron/Density Crossplot

General comments on Porosity Crossplots

This crossplot solves a subset of equations listed below:

Density:   D1*V1 + D2*V2 + D3*V3 + D4*V4 + Dfluid*Por = DBulk
Neutron:   N1*V1 + N2*V2 + N3*V3 + N4*V4 + Nfluid*Por = NLog
Sonic:     S1*V1 + S2*V2 + S3*V3 + S4*V4 + Sfluid*Por = SLog
Pe: P1*D1*V1+ P2*D2*V2 +P3*D3*V3 +P4*D4*V4 +Pf*Df*Por = PLog*D
Volumes:    1*V1 +  1*V2 +  1*V3 +  1*V4 +      1*Por = 1


V1,V2,V3,V4 = end members	   Por=volume frac void space
D=bulk density			   Dbulk=log's density reading
N=neutron apparent matrix effect   NLog=log's neutron reading
S=sonic travel time		   SLog=log's sonic reading
P=Photoelectric index		   fluid=log's fluid response
The program assumes that the response of a log is the sum of the individual responses of the rock constituents times their respective volume fractions. (The photoelectric index curve, Pe, is multiplied by density to make it conform to volumetric mixing rules.) The acceptable porosity range=-1% to 41%, and the acceptable end member range =-1% to 101% (with +- 1% for statistical error). Each calculated composition is evaluated and flagged as [ok] or labeled with possible effects ([gas], [shale], etc.).

Comments specific to this crossplot:

This program solves the neutron (either sidewall or compensated) and compensated density log crossplot. It uses these equations:

     Rho1*V1 + Rho2*V2 + Rhofluid*Porosity = RhoBulk
     Nma1*V1 + Nma2*V2 + Nmafluid*Porosity = NtnLog
        1*V1 +    1*V2 +        1*Porosity = 1


V1=volume fraction of end member #1
V2=volume fraction of end member #2
The program evaluates inputs for the neutron and density logs, with optional shale-correction based upon the gamma-ray curve, using Larionov's equations.

Since it is usually the case that density porosity and neutron porosity appear on the same track, the program lets you enter a zone's density porosity directly, and skip the bulk density. You have the same option if you are correcting for shale: you can skip entering the shale's bulk density and enter the shale's density porosity intead.

As to shale correction: I prefer using the gamma-ray for Vshale because the neutron and density logs don't respond to the same kinds of shale in the same way. The gamma-ray responds to high cation exchange capacity (CEC) shales, which are the main ones of interest.

The program solves the simultaneous equations and returns three successive answer sets. Each set is labeled for feasibility, giving possible reasons if the results seem unlikely.

KEEP IN MIND: you can and will get multiple valid compositions for many data points! For example: if both the neutron and density porosities in a zone are 10%, and the logs are run on a limestone matrix, all compositions returned by this program will be valid. You either have 90% limestone and 10% crossplot porosity, or 57% sandstone, 32% dolomite, and 11% crossplot porosity. Check other sources of information, like your zone's sample descriptions on the mud log, to pin things down.

The presence of gas will tend to decrease the neutron porosity and increase the density porosity (or decrease the apparent density), which makes the neutron and density combination well-suited for identifying gas.


Do you want to save your calculations? The input box at the very bottom of the screen records all the inputs and outputs for each calculation run. To save this information, select all the text in the box and copy it, then open a spreadsheet and paste it in as comma-separated values. Each data type will land in its own column, and each calculation run, or depth, will occupy a row. Format the spreadsheet to separate rows into different geologic formations, and you're done. Isn't that easier than writing everything down?

Don't have a spreadsheet handy? If you are working on a phone or a tablet, you can still copy the text and paste it into a note or an email.

The Recording box will reset if you press the "Help" or "Reset" buttons, or if you navigate to a different page.