Page 137 - The Geological Interpretation of Well Logs

~ THE DENSITY AND PHOTOELECTRIC FACTOR LOGS -

effects will be automatically accounted for, the principal THE PHOTOELECTRIC FACTOR LOG
unknown being the density of the organic matter (kero- (or Litho-Density)
gen). The method is presented below.
The following assumptions are made (for details see 9.7 Generalities
Myers and Jenkyns, 1992):
The log
1) The source rock is composed of mudrock (matrix The Photoelectric Factor (or PEF) log (the Litho-Density
density = 2.70g/cm), water filled porosity (density = log of Schlumberger) is a continuous record of the effec-
1.05g/cm*) and kerogen (density = 1.1 or 1.2g/em*). tive photoelectric absorption cross section index or Pe of
2) The non-source interval has the same mudrock matrix a formation. The photoelectric absorption index is strong-
density, water density and water filled porosity as the ly dependent on the average atomic number, Z, (i.e.
source interval (Figure 9.24). atomic complexity) of the constituents of the formation,
which implies the composition and by inference, the
lithology: the effects of porosity are kept to a minimum
= Pos ~ Pa
(Figure 9.25). For instance, the photoelectric absorption
te Pa Pina ())
index of a sandstone (quartz matrix) at 0% porosity is Pe
1.81 while at 35% fresh water filled porosity it is Pe 1.54.
— Ps— Pras For limestone (calcium carbonate matrix} the figures are, at
Dyer Pher — Dana (2)
0% porosity (i.e. matrix) Pe 5.08 and at 35% porosity Pe
4.23 (Figure 9.26) (Gardner and Dumanoir, 1980). That is,
the difference between the Pe values of the two matrix
TOC% = 0.85. Pre X Peer
types (i.e. 3.27 units) is more significant than the variations
Prer Ce) + Pins (1- $4 — Dre) (3) within each individual matrix type caused by porosity
changes (i.e. 0.27 and 0.85 respectively) (Figure 9.26).
Where:
Principal uses
p,, = density non-source interval (average from log)
The photoelectric absorption index is used principally in a
p, = density source interval (from log)
quantitative manner as a matrix indicator, either alone or,
Pras = 2-70g/em?, assumed mudrock density
especially when cross-multiplied with the corresponding
, = water filled porosity, derived with equation (1)
density log vatue, to produce the value U (Table 9.10).
,_, = kerogen filled porosity, derived with equation (2)
Qualitatively, in the correct borehole environment, it can be
P,., = 1.1 or 1.2g/cm’, kerogen density
used to indicate lithology and certain, mainly diagenetic
P, = 1.0Sg/cm’, density water
minerals.
TOC% = 0.85 X wt.% kerogen
The use of the litho-density log is severely restricted by
the fact that it is ineffective in holes with barite weighted
The water filled porosity of the contiguous non-source
mud, since the photoelectric absorption index for barite is
interval is given by equation (1). The volume (porosity)
nearly 150 times that of most of the common minerals’
filled with kerogen of a given density is given by equation
and when present will dominate the log response. If the
(2). The conversion of kerogen volume to weight and then
log is to be used effectively, the drilling mud must not
TOC% using a weight % kerogen equivalence to weight
contain barite.
% carbon of 0.85, is given by equation (3).
The litho-density log is sometimes, misleadingly,
called the lithology tog. It is not: it is a log of the
effective photoelectric absorption index. Its response is
an

Re KEROGEN ~RSe aN no closer to the lithology than that of any other log.
NEN ee or'1:2 gfemi § \

NSN NBN Table 9.10 Principal uses of the photoelectric factor
MATRIX REN N2Q
= N= (PEF) log.
p,, = 2.7g/em? Rs \ matrix Kn \
Dicipline
Used for
Knowing
NaN Pa, = 2.7giem? | &N
NN
NN
NEN NEA Quantitative Petrophysics Matrix Pinan, 2
REN
NEA
identification
NON-SOURCE SOURCE ROCK
(immature) Ursa Vans, U,

Figure 9.24 Schematic basis for using the density log Qualitative Geology Matrix type mineral
quantitatively to derive total organic carbon, TOC. The matrix values
and water content characteristics are assumed identical in Diagenetic elemental
both the source and non-source. Kerogen is an additional part minerals values
of the total volume in a source rock.

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