Lithology from logs

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(P_e)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 ρ_b or Ф_D 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