PEF (Photoelectric Factor): Definition, Lithology Identification, and Formation Evaluation

What Is the PEF?

The photoelectric factor (PEF) is a wireline log measurement that quantifies the photoelectric absorption of gamma rays by formation electrons, expressed in barns per electron, and serves as a direct lithology indicator because PEF is strongly dependent on atomic number and therefore identifies mineral type independently of porosity, fluid, and formation water salinity.

How the Photoelectric Factor Is Measured

The PEF measurement is made by the litho-density tool (LDT), which uses a caesium-137 gamma ray source pressed against the borehole wall by a pad and measures gamma ray count rates at two detector spacings. The long-spacing detector provides the bulk density measurement through Compton scattering, which is sensitive to electron density (and therefore to bulk density). The short-spacing detector measures a count rate that is sensitive to photoelectric absorption in addition to Compton scattering. At low gamma ray energies (below approximately 100 keV), photoelectric absorption dominates over Compton scattering; the photoelectric cross-section per electron is proportional to approximately Z^3.6, where Z is the atomic number. The ratio of short-spacing to long-spacing count rates is processed to extract the photoelectric absorption parameter Pe (in barns per electron), which is reported on the log as the PEF or Pe curve.

Because photoelectric absorption depends on Z^3.6, PEF is exquisitely sensitive to elements with high atomic numbers. Calcium (Z=20) in calcite gives PEF of 5.08; silicon (Z=14) in quartz gives 1.81; magnesium (Z=12) in dolomite gives a lower contribution that, combined with the calcium, yields dolomite's PEF of 3.14. Iron (Z=26) in siderite, pyrite, and chlorite clay gives PEF values of 14-17. Barium (Z=56) in barite gives PEF of 267, meaning that even a thin layer of barite mudcake or barite contamination in the tool's measurement volume completely overwhelms the formation PEF and renders the measurement unreliable in barite-weighted muds unless barite contamination is detected and flagged.

PEF Applications Across International Jurisdictions

In Canada, PEF is a standard measurement in the wireline log suite for carbonate evaluation in the WCSB. The Nisku and Leduc Devonian carbonates of Alberta exhibit systematic PEF variations between tight dolomite (PEF near 3.14), porous limestone (PEF near 5.08), and anhydrite stringers (PEF near 5.05). AER pool establishment petrophysical reports use PEF-density-neutron crossplots to identify mineral fractions in complex carbonate-evaporite sequences where gamma ray alone cannot distinguish the lithology types that control reservoir quality and seal integrity. The Triassic Montney tight gas play in the WCSB Deep Basin uses PEF to identify clay-rich intervals with elevated iron content that indicate reduced reservoir quality.

In the United States, PEF is used in Permian Basin carbonate evaluation to distinguish Wolfcamp and Spraberry limestone from dolomite and to identify siliceous intervals in mixed carbonate-silica systems. BSEE well completion reports for Gulf of Mexico wells include litho-density log data as part of the reservoir characterisation submission; PEF is used to confirm lithology interpretations in deepwater turbidite systems where core coverage is limited. In Norway, Equinor's Ekofisk Field chalk evaluation uses PEF to identify tight chalk facies with high carbonate content versus slightly lower-PEF intervals with clay contamination. In the Middle East, Saudi Aramco's Arab Formation carbonate petrophysical evaluation uses PEF systematically to identify anhydrite zones (PEF near 5.05, almost identical to limestone PEF near 5.08, but distinguished by density) and to map dolomite distribution across the giant Ghawar structure.

PEF in Multi-Mineral Lithology Solutions

The most powerful application of PEF is in simultaneous multi-mineral lithology solutions using two or three logs. A three-mineral system (for example calcite-dolomite-quartz in a mixed carbonate-sandstone formation) requires three independent equations to solve for three unknown mineral fractions. The density equation, neutron equation, and PEF equation together provide three equations from three different physical measurements. The PEF's sensitivity to lithology without porosity dependence makes it a particularly valuable third equation that is nearly orthogonal to the porosity-sensitive density and neutron measurements. Software solvers that minimise the sum of squared residuals across all three equations simultaneously provide mineral fractions and porosity as a function of depth, giving petrophysicists a continuous quantitative lithology log that replaces manual crossplot interpretation.

PEF Synonyms and Related Terminology

PEF is also referenced as:

• Pe — the single-letter symbol used in log headers, petrophysical software, and technical papers; Pe and PEF are interchangeable in all contexts
• Photoelectric absorption factor — the descriptive full name used in some service company tool descriptions and SPE technical papers
• Litho-density Pe — used when specifying that the Pe measurement is from the litho-density tool rather than from any other potential source

Related terms: litho-density log, bulk density, lithology, neutron porosity, crossplot

Frequently Asked Questions

Why is PEF essentially independent of porosity and fluid content?

Photoelectric absorption depends on the atomic number of the electrons in the material. Common pore fluids — water, oil, and gas — are composed of oxygen, hydrogen, and carbon, all of which have low atomic numbers (8, 1, and 6 respectively) and very low PEF values. At typical reservoir conditions, replacing mineral-filled pore space with water or oil changes the bulk composition of the measured volume only slightly toward lower-Z elements, producing a small, predictable change in PEF that is easily corrected. Gas, with its very low density and predominantly hydrogen content (Z=1), can cause a minor reduction in apparent PEF in gas-bearing formations, but the effect is small compared to the lithology signal. This near-independence from pore fluid content is what makes PEF a fundamentally better lithology indicator than any of the porosity-sensitive logs.

How does PEF help identify clay minerals?

Different clay minerals have different iron content and therefore different PEF values. Illite and smectite, which are predominantly silica-alumina structures, have PEF values of 3.0-3.5. Chlorite, which contains significant iron in its structure, has PEF values of 6.0-6.8. Kaolinite has PEF near 1.8. An elevated PEF in a siliciclastic reservoir where dolomite and calcite are absent indicates iron-bearing minerals, most likely chlorite clay or siderite cement. This clay mineralogy information from PEF is useful for diagnosing permeability damage risk, since chlorite-coated pore surfaces are susceptible to fines mobilisation during production.

Why PEF Matters in Oil and Gas

Accurate lithology identification from logs is the foundation of petrophysical interpretation, because every calculation from matrix density to mineralogy-dependent neutron response correction depends on knowing what minerals are present. Before the litho-density tool introduced PEF in the early 1980s, lithology was inferred from the density-neutron crossplot alone, which is ambiguous for mixed carbonate-evaporite-siliciclastic sequences because multiple mineral combinations can produce the same density-neutron coordinate. PEF broke this ambiguity by providing a truly lithology-specific measurement that functions as an independent third axis in crossplot space. In the Arab Formation carbonates of the Middle East, the Devonian reefs of the WCSB, and the chalk fields of the North Sea, PEF-based multi-mineral solutions have replaced guesswork with quantitative mineral fractions that improve the accuracy of porosity and water saturation calculations across billions of barrels of carbonate reserves.