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
where:
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 sonic log crossplot. It uses these
equations:
Stt1*V1 + Stt2*V2 + Sttfluid*Porosity = SttLog
Nma1*V1 + Nma2*V2 + Nmafluid*Porosity = NtnLog
1*V1 + 1*V2 + 1*Porosity = 1
where:
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 shalecorrection based upon the gammaray curve,
using Larionov's equations.
For shale correction: I prefer using the gammaray for Vshale
because the neutron and density logs don't respond to the same
kinds of shale in the same way. The gammaray responds to high
cation exchange capacity (CEC) shales, which are the main ones
of interest.
The program assumes metric units are in use if the fluid travel
time is over 600. It 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 the Schlumberger Comp.
neutron porosity is 15% and the sonic travel time is 70
microsec/ft, you will get the following compositions:
Pass 1: 93.25% LS, 8.899% DOLO, 15.649% XPOR (gas/gyp, sec por)
Pass 2: 75.58% LS, 9.036% SS, 15.384% XPOR
Pass 3: 44.486% SS, 41% DOLO, 14.514% XPOR
Which run is accurate? They all are, pending more information.
You can try correcting the input values for shale effect, or
better yet, check other sources of information, like your
zone's sample descriptions on a mud log, to pin things down.
Also note that while the presence of gas tends to decrease the
neutron porosity, it doesn't generally affect the sonic unless
the zone is unconsolidated. This makes the sonicneutron
crossplot (and the sonicdensity crossplot, for that matter)
inferior to the neutrondensity crossplot for identifying gas
effects.
