Lithology
from logs
Lithology
from the spontaneous potential log :
The SP log has 2 main
uses :
• Primarily used for determining gross lithology, i.ereservoir vsnon-reservoir (permeable vs
non-permeable).
• indication of shaliness of formation.
Lithology from caliper
log:
If the hole diameter is
on gauge lithology may be(massive s.s ,calcareousshales, igneous rocks or
metamorphic rocks).
If the hole diameter is
larger than bit size lithology may be (unconsolidated sand, gravels or brittle
shales).
If the hole diameter is
smaller than bit size lithology may be (swelling shales or porous and permeable
sandstones
Lithology
from Natural Gamma Ray (ɣ-ray) Logs:
Because radioactive
material is concentrated in shale, shale has high gamma ray log readings.
Clay-free sandstone and carbonate rocks have low gamma ray log readings.
Lithology from NGS Log:
Because some minerals (radioactive elements) have characteristic
energy levelsof thorium, uranium, and potassium (), the NGS log can be used to
identify minerals or mineral type.
Lithology from
the Litho Density log :
The litho-density log measures the photoelectric absorption(Pe)of
formation which is influenced by the mean atomic number of the matrix and formation
fluids co it can be directly related to lithology.
Lithology from cross plots :
Cross plots are a convenient way to demonstrate how various combination
of logs respond to lithology and porosity.They also provide visual insight
into the type of mixtures that the combinations is most useful in
unraveling.
Neutron-Density CrossPlots
• Neutron and density
porosities are crossplotted on linear scales
• Points corresponding to
particular water-saturated pure lithologies define curves (sandstone, limestone
,dolomite ,etc).
• When the matrix
lithology is a binary mixture (e.g.,sandstone-lime or limedolomite or
sandstone-dolomite) the point plotted from the log readings will fall between
the corresponding lithology lines .
Sonic-Density Crossplots
• Crossplots of sonic 𝓉 versus density ρb or ФD have poor porosity and reservoir rock (sandstone, limestone, dolomite)
resolution, but they are quite useful for determining some evaporate minerals.
Sonic-Neutron crossplots
• It is a plot of sonic tversus porosity from anSNP
log.
• As with the
density-neutron plots, resolution between sandstone, limestone and dolomite
lithologies is good, and errors in choosing the lithology pair will have only a
small effect on the porosity value found.However, resolution is lost if
evaporites are present.
M-N PLOT
• In more complex mineral
mixtures, lithology interpretation is facilitated by use of the M-N plot.
• These plots combine the
data of all three porosity logs to provide the lithology-dependent quantities M
and N.
• M and N are simply the slopes of the
individual lithology lines on the sonic-density and density-neutron crossplotcharts
,respectively
Neutron
density crossplot
