Well Logging Types for Drilling Professionals — a practical field guide

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Well logs are detailed records that capture various physical properties measured along the borehole. Each type of log focuses on one or more formation properties, such as radioactivity, electrical resistivity, acoustic velocity, and hydrogen content. In the field, it's important to combine complementary logs. This approach helps minimize uncertainty and provides a solid foundation for making informed drilling decisions, such as determining mud weight, establishing casing points, identifying hydrocarbon zones, and detecting overpressure. 

  1. High-level categories of logs 

  • Lithology / natural radioactivity logs — gamma ray (GR). 

  • Electrical logs — resistivity family and spontaneous potential (SP). 

  • Porosity logs — density, neutron, and sonic. 

  • Advanced fluid/flow logs — nuclear magnetic resonance (NMR), dielectric. 

  • Mechanical and geometric logs — caliper, temperature, pressure. 

  • Imaging logs — resistivity or ultrasonic 360° borehole images. 

  • Run Modes — Wireline (after drilling) vs Logging-While-Drilling (LWD) (real-time while drilling).  

    2. Common logs, what they measure, how they work, and field use 

  • Gamma Ray (GR) — Quick Lithology Indicator 

    • Function: 
      The gamma-ray log is primarily used to distinguish clay or shale zones (which emit higher natural gamma radiation) from cleaner formations such as sands or carbonates (which emit lower radiation). It serves as a quick-look lithology indicator and is often the first curve interpreted when reviewing a log. GR data helps identify formation tops, shale breaks, and clean reservoir intervals. 

    • Technology: 
      The GR tool measures the natural gamma radiation emitted by radioactive elements, mainly potassium (K), uranium (U), and thorium (Th) within the formation. The readings are reported in API units (American Petroleum Institute standardized scale). 

    • Field Interpretation: 
      High GR readings usually indicate shale, claystone, or other radioactive materials, while low readings represent cleaner, non-shaly formations. Sudden drops in GR values often mark cleaner sands or potential reservoir zones. In carbonate formations, elevated GR readings can sometimes be attributed to uranium enrichment rather than true shale content; therefore, it is always advisable to cross-check with other logs, such as density or neutron. 

    • Field Tip: 
      Always plot the GR log in the first track during data display or logging runs. This provides a quick visual reference for lithological changes, helping to plan which intervals require detailed analysis or sidewall coring. 

  • Spontaneous Potential (SP) 

    • Function: 
      The SP log detects natural electrical potentials generated between the borehole fluid and the formation, making it useful for identifying permeable, water-bearing, or flushed zones. It also helps differentiate between permeable sands and impermeable shales, assisting in stratigraphic correlation. 

    • Technology: 
      The tool measures the natural voltage difference between an electrode at the surface and one in the borehole. This voltage arises from the electrochemical interaction between the drilling mud filtrate and the formation water. 

    • Field Interpretation: 
      A negative SP deflection generally indicates a permeable sand or carbonate zone relative to the adjacent shales, which form the baseline. The magnitude of SP deflection depends on the contrast in salinity between the drilling mud and the formation water. When the salinity contrast is small, the SP response may be muted. SP is beneficial when resistivity tools cannot be run due to poor borehole conditions. 

  • Resistivity Logs (Shallow, Medium, Deep) 

    • Function: 
      Resistivity logs are key for identifying the presence and type of formation fluids, such as hydrocarbons or water, and for estimating water saturation. They also reveal the extent of mud filtrate invasion and help differentiate between flushed and uninvaded zones. 

    • Technology: 
      These tools use either electrodes or electromagnetic induction coils to measure the formation’s electrical resistance to current flow. Multiple measurements are taken at varying depths of investigation—shallow, medium, and deep to evaluate both invaded and uninvaded zones. 

    • Field Interpretation: 
      Hydrocarbon-bearing zones usually show high resistivity, while water-bearing zones show lower values. The difference between shallow and deep resistivity readings (curve separation) indicates the presence of filtrate invasion. Deep resistivity reflects the true formation fluid conditions. Always use multiple resistivity scales when evaluating thin beds or highly resistive formations. 

  • Density Log 

    • Function: 
      The density log measures the formation’s bulk density, which helps estimate porosity when the matrix density is known. It also aids in identifying lithology and gas-bearing formations. 

    • Technology: 
      A gamma-ray source emits high-energy photons into the formation, and detectors measure the intensity of backscattered gamma rays. The amount of scattering depends on the electron density of the formation, which correlates with bulk density. 

    • Field Interpretation: 
      Low density indicates high porosity or the presence of gas, since gas has a much lower density than liquids. High density suggests tighter, less porous formations such as limestone or dolomite. Combining density and neutron logs helps confirm the presence of gas. This is observed as a “crossover” where the two curves diverge. 

  • Neutron Porosity Log 

    • Function: 
      This log measures the hydrogen content of the formation, which is a good indicator of porosity and fluid type. This complements the density log for cross-validation. 

    • Technology: 
      The tool emits fast neutrons into the formation. These neutrons lose energy mainly through collisions with hydrogen atoms. The number of slowed neutrons captured by the detector correlates with hydrogen concentration and, therefore, porosity. 

    • Field Interpretation: 
      High neutron readings indicate a higher hydrogen content, typically due to water- or oil-filled pores or clay-bound water in shales. Gas-bearing formations exhibit low neutron readings, resulting in a density-neutron crossover on the combined plots. Always cross-reference neutron data with GR to differentiate between shale effects and actual porosity. 

  • Sonic / Acoustic Log 

    • Function: 
      The sonic log measures the travel time of a sound wave through the formation. It is used to estimate porosity, determine lithology, assess mechanical properties, and predict zones of overpressure. 

    • Technology: 
      The tool transmits an acoustic pulse and records the time it takes for the wave to travel between a pair of receivers spaced along the tool body. The output is given in microseconds per foot (µs/ft) or milliseconds per meter (ms/m). 

    • Field Interpretation: 
      Longer travel times (slower velocities) suggest porous or fractured zones, while shorter times indicate tighter, denser formations. Sudden increases in transit time without a corresponding lithology change may indicate the development of overpressure. Sonic data is also valuable for geomechanical analysis and seismic tie-ins. 

  • Nuclear Magnetic Resonance (NMR) Log 

    • Function: 
      NMR logging directly measures hydrogen nuclei in the formation fluids to determine pore-size distribution, porosity, permeability, and the proportion of movable versus bound fluids. This is particularly effective for evaluating complex or shaly formations where conventional logs are ambiguous. 

    • Technology: 
      The tool generates a magnetic field and applies radio-frequency (RF) pulses to align hydrogen protons. The time it takes for these protons to return to their normal state (relaxation times T1 and T2) provides information about the type of fluid and the pore structure. 

    • Field Interpretation: 
      NMR differentiates between free fluid (movable hydrocarbons or water) and bound fluid (clay or capillary-bound water). The resulting T2 distribution curve enables the estimation of permeability and the producible hydrocarbon volume. It provides superior fluid typing in low-resistivity pay zones. 

  • Dielectric Logs 

    • Function: 
      Dielectric logs measure the formation’s ability to transmit electromagnetic energy, which depends on water content. They are especially useful in evaluating fresh-water or variable-salinity formations where resistivity readings can be misleading. 

    • Technology: 
      Microwave or radio-frequency signals are transmitted into the formation, and the response is analyzed to determine its dielectric permittivity. Water, due to its high permittivity, produces strong signals compared to hydrocarbons. 

    • Field Interpretation: 
      High permittivity readings indicate water-bearing zones, while low readings suggest the presence of hydrocarbons or gas. Dielectric measurements serve as a valuable cross-check against water saturation derived from resistivity in complex reservoirs. 

  • Borehole Image Logs (Resistivity or Ultrasonic) 

    • Function: 
      Borehole imaging provides a high-resolution 360° visual representation of the borehole wall. It helps identify fractures, bedding planes, faults, vugs, and stress directions, which are essential for wellbore stability and completion design. 

    • Technology: 
      Resistivity image tools utilize pad-mounted electrodes to measure microresistivity variations, whereas ultrasonic image tools employ acoustic pulses reflected from the borehole wall to construct an image. Data are processed to create unwrapped “map” views of the borehole interior. 

    • Field Interpretation: 
      Sinusoidal features indicate bed dips and orientations; dark or linear patterns reveal open fractures or faults. Image logs guide horizontal well placement, fracture stimulation design, and structural interpretation. 

  • Caliper Log 

    • Function: 
      The caliper log measures the diameter and shape of the borehole. It detects washouts, cavings, or tight spots that may cause logging or casing problems. 

    • Technology: 
      Mechanical caliper tools utilize spring-loaded arms that extend to the borehole wall, while advanced versions employ ultrasonic pulses to measure the wall distance continuously. 

    • Field Interpretation: 
      Borehole enlargements suggest washouts due to unstable formations or excessive mud weight. The caliper log is essential for correcting density and sonic data (since poor wall contact can distort readings) and for selecting the proper casing and centralizer sizes before running tubulars. 

3. Standard Formation Evaluation Suite (Wireline or LWD) 

Typical log combination of tools is Gamma Ray (GR) + Spontaneous Potential (SP) + Resistivity (deep and shallow) + Density + Neutron + Sonic 

  • Purpose and value: 
    This standard set of logs provides the core dataset for formation evaluation. Together, they help determine lithology, identify hydrocarbon-bearing zones, estimate porosity, and verify the integrity of the formation. The combined interpretation also helps to recognize mud filtrate invasion profiles and distinguish between permeable and tight intervals. 

  • How it works in practice: 

    • Gamma Ray (GR) indicates shale content and differentiates clean sands or carbonates from shaly layers. 

    • Spontaneous Potential (SP) helps detect permeable sands and define formation boundaries. 

    • Resistivity logs (both deep and shallow) reveal the types of fluids: hydrocarbons (high resistivity) versus water (low resistivity), and indicate the depth of filtrate invasion. 

    • Density and Neutron logs measure porosity and provide a cross-check for gas zones through “crossover” behavior. 

    • Sonic log adds another independent porosity measurement, helping to assess rock mechanical properties, which are crucial for wellbore stability and pressure prediction. 

  • Field application: 
    This standard suite is typically acquired in all exploration and appraisal wells. In development wells, it provides key data for real-time decision-making on casing points, mud weights, and completion planning. When interpreted together, these logs offer a reliable foundation for petrophysical analysis and reservoir characterization. 

When Fluid Discrimination Is Difficult 

Run additional logs; Nuclear Magnetic Resonance (NMR) and Dielectric Logs 

  • Purpose: 
    In cases where conventional logs cannot clearly distinguish between hydrocarbons and water, such as in shaly sands, low-resistivity reservoirs, or variable-salinity zones, NMR and dielectric logs provide essential complementary data. 

  • How they help: 

    • NMR logs directly measure pore fluid behavior and relaxation times to separate movable hydrocarbons from bound water. They also estimate permeability and effective porosity more accurately than conventional tools. 

    • Dielectric logs measure rock permittivity to evaluate water saturation where resistivity data are inconclusive (for example, in freshwater or mixed-salinity formations). 

  • Field application: 
    Use these advanced logs selectively where standard interpretation is ambiguous. They are particularly valuable in complex reservoirs such as tight sands, shaly carbonates, or formations with conductive drilling fluids that mask resistivity responses. 

When Structural or Geological Detail Is Needed 

Run additional logs; Borehole Image Log (Microresistivity or Ultrasonic) 

  • Purpose: 
    Borehole image logs provide a 360° visual representation of the wellbore wall, enabling detailed structural, stratigraphic, and fracture analysis. 

  • How it helps: 
    These logs identify bedding planes, fractures, faults, and vugs, as well as formation dip and stress orientation. The detailed images improve structural interpretation, well placement, and geomechanical modeling. 

  • Field application: 
    Imaging logs are essential for horizontal wells, fracture studies, and geosteering operations. They help confirm wellbore stability and optimize completion strategies in formations with fractures or faults. 

How logs guide drilling decisions 

  • Mud weight and well control: Rapid detection of low-density, high-porosity units or signs of overpressure (sonic slowness, unexpected resistivity patterns) informs mud weight adjustment to avoid kicks or loss of circulation.  

  • Casing point selection: Use GR plus resistivity/porosity logs to choose casing shoe above weak or high-pressure zones and to ensure cementable intervals. 

  • Formation evaluation while drilling (LWD): Gives near-real-time input (porosity, gamma, resistivity) so the driller can alter trajectory or drilling parameters immediately. LWD reduces non-productive time versus waiting for wireline. 

  • Hydrocarbon identification and initial fluid contact: Resistivity + density + neutron crossover and NMR free fluid index combine to indicate movable hydrocarbons and estimate initial water contact. 

  • Detecting invasion and filtrate effects: Compare shallow versus deep resistivity and neutron/density shapes to determine how far the mud filtrate has invaded and whether resistivity reflects the proper formation fluid. 

4. Quality control, common errors, and field checks  

  • Tool Calibration and Temperature Effects 

    Before running any logging tool, always verify that it has been properly calibrated at the surface according to the manufacturer's specifications. Calibration ensures that tool responses are within acceptable limits and that the baseline measurements are reliable. Be aware that high borehole temperatures can significantly influence tool performance, for example, by reducing the output strength of nuclear sources or altering NMR relaxation times. Regular calibration checks and temperature compensation are essential for maintaining data accuracy during high-temperature logging operations. 

  • Borehole Conditions 

    Unstable borehole conditions can severely impact log quality. Large washouts, heavy mud cake buildup, or irregular hole geometry can distort both density and resistivity readings by increasing the distance between the tool sensors and the formation wall. To minimize such effects, always review caliper log data before interpretation. If significant washouts are present, apply appropriate borehole corrections or repeat logging runs after conditioning the hole. Reliable density and resistivity data depend on good wall contact and a stable borehole environment. 

  • Source–Detector Standoff and Tool Centralization 

    Proper tool centralization and pad contact are crucial, especially for image logs, micro-resistivity pads, and density tools. Even small standoffs between the pad and the formation wall can cause data distortion or image blurring. If contact appears poor or tool pads fail to seat firmly against the wall, consider reducing logging speed, adjusting tool weight, or re-running the log after circulating the borehole to remove debris or mud cake. Consistent pad contact ensures sharp, accurate images and representative formation resistivity readings. 

  • Data Management and Archiving 

    Always document, label, and securely archive both the raw field curves and final processed logs immediately after acquisition. Downstream petrophysicists and reservoir engineers rely on the original, depth-synchronized datasets for detailed formation evaluation, including saturation calculations, net pay estimation, and reservoir modeling. Maintaining complete and traceable data records also supports quality audits and future re-evaluation of the well. 

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