compensated neutron log, Oil & Gas Glossary

What Is the Compensated Neutron Log?

The compensated neutron log (CNL) is a borehole nuclear logging tool that measures the concentration of hydrogen in the formation by bombarding it with fast neutrons from an Am-241/Be or Cf-252 source and counting the thermalized (slowed) neutrons at two receiver arrays at different distances from the source. Because hydrogen is the most effective element at slowing neutrons (due to its mass being nearly equal to that of a neutron), the neutron count rate at the far receiver decreases as hydrogen content increases — and since hydrogen in sedimentary rocks is predominantly in formation fluid (water, oil) and clay-bound water, the measurement closely approximates porosity in clean water-saturated or oil-saturated formations. The compensated (two-detector) design cancels the borehole effects present in single-detector tools — the ratio of far-to-near detector counts is converted to apparent limestone porosity (φ_N), which is the standard neutron log output regardless of actual lithology. Neutron porosity must be interpreted alongside bulk density from the density log (ρ_b) for lithology identification, gas detection, and accurate total porosity determination through the neutron-density crossplot.

Key Takeaways

Neutron log measures hydrogen index (HI) — the ratio of hydrogen atoms per unit volume relative to fresh water — which approximates porosity in liquid-filled formations but is significantly affected by gas (gas has lower HI than water, making gas-bearing zones appear lower porosity than they actually are).
Neutron log output is expressed in apparent limestone porosity units (%); for sandstones add ~4 pu (porosity units) and for dolomites subtract ~2–4 pu to convert to correct lithology porosity.
The neutron-density crossplot (N-D crossplot, "density-neutron") is the primary lithology and gas identification tool — limestone, dolomite, and sandstone points each fall on distinct matrix lines; gas-bearing zones "cross over" (neutron reads lower than density-derived porosity).
Clay minerals contain bound water in their crystal structure (clay-bound water, CBW) — neutron log reads this bound water as porosity, causing neutron-derived porosity to overestimate effective (producible) porosity in shaly sands.
The compensated design (two detectors at different spacings) removes borehole size and mudcake effects present in single-detector tools — but neutron porosity is still sensitive to borehole fluid salinity and formation water salinity.

Neutron Log Physics and Interpretation

The hydrogen index (HI) is the fundamental quantity measured by neutron tools — it is the ratio of hydrogen atoms per unit volume in the formation to the hydrogen atoms per unit volume in pure water at standard conditions. Fresh water has HI = 1.0; oil has HI ≈ 1.0–1.05 (slightly higher hydrogen density than water); natural gas has HI ≈ 0.1–0.5 depending on pressure (gas expands at lower pressure, reducing its hydrogen density). This is why the neutron log "reads porosity correctly" for water and oil-filled zones — their HI closely approximates the fraction of pore space — but underreads porosity in gas-bearing zones. The gas effect on the neutron log (neutron porosity lower than actual because gas has low HI) combined with the opposite gas effect on the density log (gas density ~0.1–0.3 g/cm³ vs fluid density 0.8–1.1 g/cm³ — low density makes density-derived porosity read too high) produces the characteristic "cross-over" pattern: neutron porosity < density porosity in gas zones. This cross-over is the most reliable gas indicator on wireline logs.

The neutron-density crossplot simultaneously plots neutron porosity on the x-axis and density porosity on the y-axis — or sometimes bulk density vs neutron porosity on separate axes. Limestone, sandstone, and dolomite matrix lines plot at distinct positions because each mineral has a different photoelectric factor, bulk density, and neutron response. A formation point that plots above the limestone matrix line (toward lower density, higher neutron) is dolomite; one that plots below (toward higher density, lower neutron) is anhydrite or calcite-cemented sandstone. Gas-bearing zones plot below and to the left of the water-saturated matrix line — neutron decreases (gas effect), density decreases (gas in pore space reduces bulk density). The separation between neutron and density curves on a log track (with both scaled in apparent porosity units) is the field log equivalent of the crossplot cross-over — immediately recognisable to any petrophysicist as a potential gas indicator requiring follow-up with resistivity logs and well test data for confirmation.

Fast Facts: Compensated Neutron Log

Source: Am-241/Be (americium-beryllium) most common; provides 4–5 MeV fast neutrons
Detectors: two helium-3 (³He) or BF₃ thermal neutron detectors at different spacings (near ~15 cm, far ~30 cm)
Output: apparent limestone porosity (NPHI or φ_N) in decimal or % porosity units
Lithology correction: sandstone = φ_N − 4 pu; dolomite = φ_N + 2–4 pu (varies by tool and formation)
Gas effect: neutron reads 5–15 pu too low in gas-bearing zones — combined with density cross-over for gas identification
Clay effect: neutron reads 5–25 pu too high in shaly sands due to clay-bound water (CBW)
Tool variants: CNL-T (thermal), CNL-E (epithermal — less affected by chlorine in saline muds), HNGS
LWD neutron: azimuthal neutron density (AND) provides real-time neutron porosity while drilling for geosteering

Petrophysics Tip:

Always interpret the neutron log in conjunction with the density log — never in isolation. The neutron log alone cannot distinguish between: high porosity water sand (neutron reads high, density reads high porosity — both agree); shaly sand (neutron reads high from clay-bound water, density reads moderate — separation indicates clay); and gas sand (neutron reads low from gas effect, density reads high from gas effect — the cross-over). These three cases require the density log to separate. Equally important: correct both logs to the same lithology before crossplotting. If your formation is predominantly sandstone, convert both neutron and density to sandstone porosity units before the crossplot — using limestone apparent porosity on a sandstone formation introduces a systematic 4 pu offset that shifts all the points off the correct matrix line, making clean sand look like dolomite. The correction from limestone apparent porosity to true lithology porosity is a standard petrophysical step that should never be skipped, particularly in frontier wells where lithology is not pre-determined.

The compensated neutron log is also referred to as:

CNL — the Schlumberger trade name (Compensated Neutron Log) that has become the generic industry acronym; equivalent tools from other vendors are also called CNL generically
NPHI — the standard log curve mnemonic for neutron porosity in limestone apparent units; universal in digital well log databases (LAS format)
Thermal neutron log — a specific variant measuring thermal (fully slowed) neutrons; more sensitive to chlorine in saline formation water and mud
Epithermal neutron log — measures slightly higher-energy (epithermal) neutrons; less sensitive to chlorine, better gas indicator in saline conditions

Related terms: Density Log, Porosity, Dual Water Model, NMR Log

Frequently Asked Questions About the Compensated Neutron Log

How is the neutron-density crossplot used to identify formation lithology?

The neutron-density crossplot plots neutron porosity (NPHI) against bulk density (RHOB) or density porosity (DPHI) for each depth point. The three clean matrix lines — sandstone, limestone, and dolomite — form a triangular region. A point on the limestone line is clean limestone; below (higher density, lower neutron) is dolomite or anhydrite; above (lower density, higher neutron) is sandstone. Binary mixtures of two lithologies plot between end-member matrix lines. Gas-bearing zones plot below and to the left of the matrix line — neutron decreases and density decreases simultaneously ("gas flag"). Shaly sands plot upper-right — neutron increases from clay-bound water while density increases only moderately. Crossplot clusters allow volumetric mineral analysis (MRIAN, ELAN) that solves simultaneously for quartz, calcite, dolomite, clay, and pore fluid proportions — the standard quantitative petrophysical workflow.

Why does the neutron log overestimate porosity in shaly sands?

Shale minerals — particularly smectite, illite, and mixed-layer clays — contain hydrogen-bearing water molecules in two forms: interlayer water (trapped between clay sheets in the crystal structure) and adsorbed water (coating the external surface of clay particles). Both forms contribute hydrogen index to the neutron measurement. The neutron log cannot distinguish between hydrogen in clay-bound water (which is not producible and should not count as effective porosity) and hydrogen in free pore water or oil (which represents genuine effective porosity and hydrocarbon storage capacity). In a sandstone with 20% effective porosity and 15% clay volume, the neutron log might read 30–35% apparent porosity — the 10–15 pu overestimate from the clay-bound water volume. This overestimate is called the "shale effect" or "clay correction" in petrophysical analysis. The correction uses the clay volume (V_sh, estimated from the gamma ray, SP, or other clay indicators) to subtract the clay-bound water contribution: φ_N_corrected = φ_N_measured − V_sh × φ_N_shale, where φ_N_shale is the neutron porosity of a 100% clay (read from a pure shale layer on the log). After this correction, the neutron-density crossplot porosity (using the average of corrected neutron and density) gives a better approximation of total porosity, from which the CBW fraction (from NMR) is subtracted to obtain effective porosity.

How does the neutron log behave differently in gas versus oil reservoirs?

The neutron log responds to the hydrogen content of the fluids in the invaded and uninvaded zones around the wellbore — the shallow invasion zone is dominated by mud filtrate, and the deep zone retains original formation fluid. In an oil reservoir, oil has hydrogen index close to 1.0 (similar to water), so the neutron log reads approximately the same whether the pore space contains oil or water — the neutron log cannot discriminate oil from water alone. The density log also shows minimal separation between oil and water (oil density ~0.75–0.85 g/cm³ vs water ~1.0 g/cm³ — small contrast). Gas is fundamentally different: gas at reservoir conditions (say 3,000 psia and 200°F) has hydrogen index of only 0.2–0.4, depending on pressure and composition. The neutron reads 10–20 pu lower in gas than water at the same porosity, while the density log reads 8–15 pu higher (because gas in the pores reduces bulk density, which the density-to-porosity conversion interprets as higher porosity). This opposing response — neutron goes down, density-derived porosity goes up — is uniquely diagnostic of gas. In a gas-condensate reservoir, the condensate in the deeper invaded zone has higher HI than gas and closer to oil HI, partially masking the gas effect and reducing the cross-over magnitude. LWD neutron and density logs run while drilling record the gas cross-over in real time, enabling immediate geosteering decisions to stay within gas-bearing reservoir intervals.

Why the Compensated Neutron Log Matters in Oil and Gas

The compensated neutron log is one of the three standard porosity logs (alongside bulk density and sonic) that form the petrophysical backbone of every well evaluation programme — it is run on virtually every exploration and development well drilled globally. Its hydrogen index measurement provides essential information for lithology discrimination (when combined with density), gas identification (the cross-over with density is the most reliable wireline gas indicator), and clay volume quantification (from the shale effect magnitude). The neutron-density combination underpins every porosity calculation used to compute hydrocarbon volumes for reserves assessment — the quality of this measurement directly affects the accuracy of proven plus probable reserves estimates reported to regulatory authorities. In shale gas plays where the neutron log's clay sensitivity allows continuous mineralogical profiling at reservoir scale, the CNL data drives completion optimisation — brittle (siliceous) intervals with low neutron clay response are preferentially perforated and hydraulically fractured because they fracture more easily and create more complex fracture networks than ductile clay-rich intervals.

Compensated Neutron Log: Definition, Porosity Measurement, and Fluid Effects