9.2 Types of well logs and their interpretation

Well logs are essential tools in borehole geophysics, providing crucial data about subsurface formations. They measure various properties like electrical resistivity, radioactivity, and acoustic velocity, helping geologists identify rock types, estimate porosity, and determine fluid content.

Interpreting well logs requires analyzing multiple datasets to build a comprehensive picture of the subsurface. By combining different log types and integrating them with other geological data, geoscientists can create detailed subsurface models, characterize reservoirs, and make informed decisions about resource exploration and extraction.

Well Log Types

Electrical Logs

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• Measure the electrical properties of subsurface formations, including resistivity and spontaneous potential (SP)
• Used to identify permeable zones and fluid content
• Resistivity logs help distinguish between water-bearing zones (lower resistivity) and hydrocarbon-bearing zones (higher resistivity)
• SP logs measure the natural electrical potential difference between the borehole and the formation, indicating permeable beds and lithology changes

Nuclear Logs

• Measure the natural radioactivity and induced radioactivity of subsurface formations
• Include gamma ray, neutron, and density logs
• Gamma ray logs measure the natural radioactivity of formations, helping to identify lithology (shales have higher gamma ray values)
• Neutron logs measure the hydrogen content of formations, providing information on porosity and fluid content
• Density logs measure the bulk density of formations, which is influenced by lithology, porosity, and fluid content

Acoustic Logs

• Measure the velocity and attenuation of sound waves in subsurface formations
• Used to determine porosity, lithology, and mechanical properties
• Acoustic velocity is influenced by rock matrix, porosity, and fluid content
• Attenuation of sound waves can indicate fractures or other inhomogeneities in the formation

Other Log Types

• Caliper logs measure borehole diameter, identifying washouts, cavings, and fractures
• Temperature logs measure borehole temperature, detecting geothermal gradients and fluid flow zones
• Dipmeter logs measure the orientation of bedding planes, providing information on structural dip and stratigraphic features
• Image logs (resistivity or acoustic) provide high-resolution images of the borehole wall, revealing sedimentary structures, fractures, and faults

Interpreting Well Log Data

Lithology Determination

• Analyze responses of gamma ray, density, and neutron logs to identify rock types
• Different rock types have characteristic log signatures based on mineral composition and texture
• Shales typically have high gamma ray values, low density, and high neutron porosity
• Sandstones and carbonates have lower gamma ray values, higher density, and lower neutron porosity
• Combinations of log responses help to distinguish between different lithologies (limestone vs. dolomite, quartz sandstone vs. arkosic sandstone)

Porosity Estimation

• Use density, neutron, and acoustic logs to estimate porosity
• Density logs respond to the presence of pore spaces filled with fluids or gases (lower density indicates higher porosity)
• Neutron logs measure the hydrogen content, which is related to the amount of pore space (higher neutron porosity indicates higher total porosity)
• Acoustic logs measure the velocity of sound waves, which is influenced by the rock matrix and the presence of pore spaces (lower velocity indicates higher porosity)
• Porosity can be calculated using appropriate equations and assumptions based on lithology and fluid content

Fluid Content Interpretation

• Infer fluid content from resistivity logs and the combination of other log responses
• Formation water is typically more conductive than hydrocarbons, resulting in lower resistivity values for water-bearing zones
• Hydrocarbons (oil and gas) have higher resistivity values compared to water-bearing zones
• The separation between neutron and density logs can help distinguish between gas, oil, and water-bearing zones (gas zones have larger separation)
• Resistivity logs, in combination with porosity logs, can be used to estimate water saturation and hydrocarbon saturation in reservoir rocks

Identifying Subsurface Features

Stratigraphic Features

• Identify bedding planes, unconformities, and lateral facies changes by examining variations in log responses across different depth intervals
• Gamma ray logs can be used to identify shale intervals, which often serve as marker beds for correlating stratigraphic units between wells
• Abrupt changes in log responses may indicate unconformities or sequence boundaries
• Gradual changes in log responses may represent lateral facies changes or gradational contacts between stratigraphic units

Structural Features

• Recognize faults and folds by abrupt changes in log responses, repeated sections, or missing sections in the well log data
• Faults may be indicated by sudden offsets in log responses, changes in dip angles, or the presence of fault gouge or breccia
• Folds may be identified by repeated sections or gradual changes in dip angles across the well log
• Dipmeter logs can provide direct measurements of the orientation of bedding planes, helping to identify and characterize structural features

Reservoir Characterization

• Identify permeable zones, such as sandstones or carbonates, which may act as reservoir rocks for hydrocarbons or groundwater
• Resistivity logs can help identify zones with high permeability and potential fluid flow
• Porosity logs (density, neutron, acoustic) can be used to estimate the storage capacity of reservoir rocks
• Combination of log responses and other data (core, seismic) can be used to assess reservoir quality and continuity

Integrating Well Log Datasets

Subsurface Modeling

• Correlate well logs from multiple wells across an area to develop 2D and 3D subsurface models
• Create cross-sections by correlating well logs along a profile, showing lateral and vertical variations in lithology, porosity, and fluid content
• Construct isopach maps to display the thickness of specific stratigraphic units or reservoir intervals
• Generate structural contour maps to represent the depth or elevation of key stratigraphic or structural surfaces

Multi-disciplinary Integration

• Integrate different types of well logs (electrical, nuclear, acoustic) to provide a more comprehensive understanding of subsurface properties
• Combine well log data with other geological and geophysical data (seismic, core, outcrop) to refine subsurface models and improve the understanding of regional geology
• Use well log data to calibrate seismic data and improve the interpretation of seismic reflectors and facies
• Incorporate well log data into reservoir models to characterize the spatial distribution of porosity, permeability, and fluid content

Applications

• Use subsurface models developed from well log data for various applications
• Reservoir characterization: Assess the quality, heterogeneity, and continuity of reservoir rocks for hydrocarbon or groundwater exploration and production
• Resource estimation: Calculate the volume and distribution of hydrocarbons or groundwater resources based on well log-derived properties
• Well placement: Optimize the location and trajectory of new wells based on the subsurface models and target zones
• Geohazard assessment: Identify potential drilling hazards, such as overpressured zones, lost circulation zones, or unstable formations, based on well log responses and subsurface models