Well Log InterpretationDensity Log

Earth & Environmental Science

University of Texas at Arlington

Density Log

The Density log measures electron density in the rock and fluids, which is related to the bulk (combined) density of the rock and fluids together (b or RHOB)

Density LogElectrons scatter gamma rays and the electron density is estimated by measuring those scattered gamma rays.

There are two energy levels measured: high energy scattering which is related to electron density, and a low energy scattering due to the photoelectric effect.

Density Log

The Logging tool has a gamma ray source (usually radioactive Cobalt or Cesium) and a gamma ray spectrometer which measures the energy levels of the scattered gamma rays.

Density LogThe depth of

penetration is small, so the logging tool must be next to the side wall which has mud cake on it. As a result, the tool measurement must be corrected for bore hole effects.

Density LogThe borehole

correction is done automatically by the logging software, but the correction factor is reported on the log as DRHO (). If this correction factor is more than 0.2, the density reading is probably wrong.

Density LogThe density log is

used to Estimate porosity Estimate lithology

from the PE log Detect gas bearing

zones Identify high

density zones such as evaporites.

Density Log

Porosity can be calculated from bulk density because b is the sum of the weighted components of the bulk density: fluid density, fl, and the

matrix density, ma maflb 1

Density Log

maflb 1Rearranging

Produces the equation for porosity ():

flma

bmaD

Density Log

flma

bmaD

The required fluid densities (fl) and matrix densities (ma) for common lithologies are shown at right.

Density LogAssignmentMake a spreadsheet Similar to Fig. 4.5 in

the text (below) and to compute D (DPHI) assuming dolomite and limestone. Use the the log on the next slide and take readings of RHOB at depths shown on the table at left. Assume salt mud.

9305

9310

9332

9337

9351

9380

9384

Density LogAssignmentOn the spreadsheet, add readings from

the sonic log of the previous assignment at the same depths shown at left.

Is there much shale at any of these depths? Why?

9305

9310

9332

9337

9351

9380

9384