Page 125 - The Geological Interpretation of Well Logs

9

THE DENSITY AND PHOTOELECTRIC

FACTOR LOGS

9.1 The density log, generalities the sum of 90% quartz grains (density 2.65g/em*) and
10% water (density 1.0g/cm*).
The log
The density log is a continuous record of a formation’s Principal uses
bulk density (Figure 9.1). This is the overall density of a Quantitatively, the density log is used to calculate poros-
rock including solid matrix and the fluid enclosed in the ity and indirectly, hydrocarbon density. It is also used to
pores. Geologically, bulk density is a function of the calculate acoustic impedance. Qualitatively, it is a useful
density of the minerals forming a rock (i.e. matrix) and lithology indicator, can be used to identify certain
the votume of free fluids which it encloses (i.e. porosity). minerals, can help to assess source rock organic matter
For example, a sandstone with no porosity will have a content (even quantitatively) and may help to identify
bulk density of 2.65g/cm?, the density of pure quartz. At overpressure and fracture porosity (Table 9.1).
10% porosity the bulk density is only 2.49g/cm*, being
DENSITY LOG
* Scale: gfem* bulk density
1.7 1.9 2.1 2.3 2.5 2.7 2.9

SHALE
QUARTZITE 60 2.65 g/ems

SANDSTONE &10% «(2.49 g/cm?)
oO 2.74 gicm®
LIMESTONE
610% *(2.54 g/cm)
go 2.87 g/om3
DOLOMITE
10% *{2.68 g/cm?)

SHALE
GAS \ (gas effect)

SANDSTONE OtL_
b& 20%
WATER (2.32 g/cm?)

poorly
density very variable
compacted
2-2.8 g/cm

SHALE
compact 1.2-1.5 giom?
SHALE
COAL
ORGANIC SHALE
2.03 g/cm?
SALT
SILLCIIGNEOQUS)

Figure 9,1 The density log: some typical responses. The density log shows bulk densiry. *Density and porosity with fresh
formation-water density 1.0g/em? (cf. Figure 10.1, which is on a compatible scale of neutron porosity).

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