The basic interpretation of well logs allows for identification of rock types based on their characteristic log responses:
- High GR, High Sonic: Indicates shale or clay-rich rocks
- Low GR, Low Sonic: Indicates clean sandstone or carbonate rocks
- Low GR, High Resistivity: Indicates tight carbonates or hydrocarbon-bearing reservoirs
- High GR, Low Resistivity: Indicates high clay content, often non-reservoir quality
Rock types are differentiated based on a combination of their log signatures. The tool uses a nearest-neighbor algorithm to identify the most likely rock type based on the input parameters.
Advanced interpretation considers multiple log interactions:
Reservoir Quality Assessment:
- Porosity can be estimated from Sonic logs where ΔT = 189 μs/ft indicates 100% porosity
- Reservoir quality correlates with low GR (<60 API) and high resistivity (>10 ohm-m)
- Log signatures must be considered collectively for accurate lithology identification
Environmental Corrections:
- Borehole effects can alter log measurements
- Temperature affects resistivity readings (increases with depth)
- Formation water salinity impacts baseline resistivity values
Geological context enhances log interpretation:
Depositional Environments:
- Clean Sandstone: Often indicates high-energy fluvial or beach environments
- Shaly Sandstone: Commonly found in deltaic or shallow marine settings
- Limestone: Typically develops in clear, warm marine environments
- Dolomite: Often a diagenetic alteration of limestone
- Shale: Represents low-energy, deep marine or lacustrine environments
Vertical succession of rock types can reveal transgressive-regressive cycles and basin evolution patterns.