Litho Density Log

The Litho-Density Log is an improved version of the Formation Density Compensated (FDC) log. It measures both bulk density (ρb) and the photoelectric absorption index (Pe). While ρb primarily responds to porosity, Pe is mainly influenced by the rock matrix (lithology), making it valuable for identifying lithology.

The tool resembles the FDC log, featuring a pad containing a gamma-ray source and two detectors, pressed against the borehole wall by a backup arm. Gamma rays emitted at 662?keV scatter in the formation, losing energy until absorbed via the photoelectric effect.

At the far detector, the energy spectrum shows two regions (see Figure 1):

• Compton Scattering Region: Higher energy gamma rays inversely related to electron density.
• Photoelectric Effect Region: Lower energy gamma rays inversely related to both electron density and photoelectric absorption.

By comparing counts in these regions, the Pe index is determined.

The near detector corrects the density measurement for mudcake and borehole rugosity.

Factors Influencing the PEF Log

1. Mean Atomic Number of the Matrix: High Pe Values: Indicate higher atomic numbers; may suggest heavy minerals or igneous rocks. Low Pe Values: Indicate lower atomic numbers; may suggest coal seams.
2. Formation Fluids: Pe values are relatively unaffected by porosity and fluid saturations since fluids have low Pe compared to the matrix. Exception: Highly saturated brines may impact Pe.

Figure 2 shows minimal Pe changes with porosity variations up to 35% for quartz, limestone, and dolomite.

Photoelectric Absorption

The cross section measures the probability of a nuclear reaction. The photoelectric cross section index (Pe) relates to the atomic number (Z) of elements. For compounds, Pe is calculated based on atomic fractions.

Table 1 lists Pe values for common rocks and minerals, showing that Pe is relatively independent of porosity and fluids.

Volumetric Photoelectric Absorption Index

Defined as:

U = Pe ρe

Since Pe has the units of barns per electron and ρe the units of electrons per cubic

centimetre, U has the units of barns per cubic centimetre. This parameter permits

the cross sections of the various volumetric components of a formation to be added

in a simple weighted average manner. Thus,

where U, Uf, and Uma are the photoelectric absorption cross sections of the mixture,

pore fluid, and matrix, respectively; all are expressed in barns per cubic centimetre.

The Litho-Density tool offers higher counting rates and better resolution than the FDC tool, useful for detecting fractures and thin beds (see Figure 3).

Mudcake and borehole rugosity compensation use the "spine and rib" method. Borehole size affects measurements due to the tool's fixed curvature (see Figure 4).

Uses of the Litho-Density Log

• Lithology Determination: Pe helps identify rock types by comparing measured values to known Pe values.

• Detection of Heavy Minerals: High Pe values indicate heavy minerals, aiding in inter-well correlation.
• Fracture Identification: Enhanced resolution detects fractures within the formation.