liminfoWell Log Reference
Free reference guide: Well Log Reference
About Well Log Reference
The Well Log Reference is a comprehensive, searchable guide to wireline and LWD log interpretation for petroleum engineers, petrophysicists, geologists, and geoscience students. It covers natural gamma ray (GR) and spectral gamma ray (SGR) logs for lithology identification and shale volume calculation, spontaneous potential (SP) logs for permeable zone identification and formation water resistivity (Rw) estimation, and five shale volume calculation methods including Larionov, Clavier, and Steiber equations with the recommendation to use the minimum of all methods for Vsh_final.
The resistivity section provides detailed explanations of Laterolog (LLD/LLS) for saltwater mud environments, Induction logs (ILD/ILM) for freshwater/oil-base mud, Array resistivity tools (AIT/HRLA) with multiple depths of investigation for invasion profiling, microresistivity logs (MSFL/MLL) for flushed zone Rxo measurement, and LWD propagation resistivity for real-time while-drilling measurement. The porosity section covers sonic log (DT) with Wyllie time-average and Raymer-Hunt-Gardner equations, density log (RHOB) with photoelectric factor (Pe) for lithology, neutron log (NPHI) with matrix corrections, neutron-density crossplot interpretation, the Archie equation and shaly sand models (Simandoux, Indonesia, Waxman-Smits), BVW analysis for production potential, and permeability estimation formulas.
Special logging sections cover NMR logs with T2 distribution analysis and SDR permeability, image logs (FMI/FMS) for fracture and structural interpretation, dipmeter for formation dip measurement, caliper logs for borehole condition assessment, cement bond logs (CBL/VDL) for casing integrity, and temperature logs for geothermal gradient analysis. All content loads in your browser with no server communication, organized into five categories with instant search and dark mode support.
Key Features
- Gamma ray log interpretation: GR lithology values (15-300+ API), spectral GR mineral identification (K/U/Th), and five Vsh calculation methods
- SP log analysis: SSP/PSP concepts, Rw calculation from SP deflection, and temperature correction formulas
- Complete resistivity log guide: Laterolog, Induction, Array, Microresistivity, and LWD propagation tools with invasion profile interpretation
- Three porosity logs compared: sonic (DT with Wyllie and RHG), density (RHOB with Pe), and neutron (NPHI with matrix corrections)
- Archie equation and four shaly sand saturation models with application guidelines based on Vsh percentage
- BVW (Bulk Volume Water) analysis with Buckles plot interpretation and irreducible water saturation values by grain size
- NMR log interpretation: T2 distribution, cutoffs (33ms sand, 92ms carbonate), fluid typing, and SDR permeability
- Image log, dipmeter, caliper, CBL/VDL cement bond, and temperature log interpretation guides
Frequently Asked Questions
How do I calculate shale volume (Vsh) from a gamma ray log?
First calculate the gamma ray index: IGR = (GR_log - GR_min) / (GR_max - GR_min), where GR_min is the clean sand baseline and GR_max is the shale baseline. Then apply one or more non-linear corrections: Larionov Tertiary (Vsh = 0.083 * (2^(3.7*IGR) - 1)), Larionov Older (Vsh = 0.33 * (2^(2*IGR) - 1)), Clavier, or Steiber equations. The recommended practice is to take the minimum value from multiple methods: Vsh_final = min(Vsh_GR, Vsh_SP, Vsh_ND).
When should I use Laterolog vs Induction logs?
Use Laterolog (LLD/LLS) in saltwater (conductive) mud systems where formation resistivity is high. Use Induction logs (ILD/ILM) in freshwater or oil-base (non-conductive) mud systems where formation resistivity is low to moderate (<200 ohm-m). Laterolog focuses current beams into the formation, while Induction induces eddy currents. Array tools (AIT/HRLA) provide multiple depths of investigation for detailed invasion profiling regardless of mud type.
How does the Archie equation calculate water saturation?
The Archie equation is: Sw = (a * Rw / (phi^m * Rt))^(1/n), where a is the tortuosity factor, Rw is formation water resistivity, phi is porosity, m is the cementation exponent, Rt is true formation resistivity, and n is the saturation exponent. Common parameter sets include Humble (a=0.62, m=2.15, n=2.0) for sandstones and standard Archie (a=1.0, m=2.0, n=2.0) for carbonates. Sw < 0.5 generally suggests commercial hydrocarbon potential.
What are the shaly sand correction models?
For formations with Vsh > 15%, the standard Archie equation overestimates Sw because clay-bound water conducts electricity. Four correction models are covered: Simandoux (1963) and Modified Simandoux for moderate shale content, the Indonesia/Poupon-Leveaux (1971) model suited for SE Asian formations with dispersed shale, and Waxman-Smits (1968) using CEC-based Qv for high shale content (Vsh > 40%). Each model adds a shale conductivity term to account for clay-bound water.
How do I identify gas zones on logs?
Gas zones show a characteristic neutron-density crossover: the neutron log (NPHI) reads anomalously low (hydrogen deficiency in gas) while the density log (RHOB) also reads low (gas has low density). On a neutron-density crossplot, gas zones move to the northwest (upper-left) relative to the matrix line. Resistivity logs typically show elevated deep resistivity (LLD or ILD) in gas-bearing zones. For gas zone porosity, use the RMS formula: phi_ND = sqrt((phi_N^2 + phi_D^2) / 2) instead of the arithmetic average.
What does BVW (Bulk Volume Water) tell me about production?
BVW = porosity * water saturation. When BVW is approximately constant across a zone (independent of depth), the formation is at irreducible water saturation, meaning no free water exists and the zone will produce water-free hydrocarbons. When BVW varies, free water is present and water production is expected. Typical irreducible BVW values range from 0.02-0.04 for coarse sand to 0.10-0.14 for silty sand. A Buckles plot (phi vs Sw) with data clustering on a single BVW hyperbola confirms irreducible conditions.
How does NMR logging differ from conventional porosity logs?
NMR logs measure T2 relaxation time distribution of hydrogen nuclei, providing pore size distribution rather than a single porosity value. Using T2 cutoffs (33ms for sandstone, 92-100ms for carbonate), NMR separates total porosity into Bound Volume Irreducible (BVI), Free Fluid Index (FFI), and Clay Bound Water (CBW). This enables direct fluid typing (water: 100-1000ms, light oil: 300-3000ms, heavy oil: 3-100ms) and permeability estimation using the SDR equation: k = C * phi^4 * T2_LM^2.
Is this reference suitable for petrophysics coursework?
Yes. The reference covers the complete petrophysical analysis workflow from basic log responses through advanced interpretation. It starts with fundamental logs (GR, SP, resistivity, sonic, density, neutron), progresses through porosity calculation methods and saturation models, and covers special logs (NMR, image logs, dipmeter, CBL). Each entry includes representative values, formulas, and interpretation guidelines used in industry and academic petrophysics courses.