Well Log Analysis

Open-hole logs are recorded before the well is cased; the tool is in direct contact with the formation and provides the most detailed and quantitatively accurate measurements. Cased-hole logs are recorded after pipe is installed; tool contact is indirect, attenuation is greater, and interpretation is more qualitative but still useful for detecting bypassed pay and monitoring production changes.

The Archie equation (Sw = [(Ro/Rt)^0.5]/m^n) calculates water saturation from resistivity. It assumes a clean sandstone with negligible clay conductivity and works well in simple sequences but fails in shales, argillaceous sandstones, and tight carbonates where clay minerals conduct electricity. Empirical and local modifications improve accuracy in non-ideal lithologies.

Log porosity (from density or neutron tools) measures connected pore space in a volume around the borehole; it is largely reliable in sandstones but sensitive to borehole effects and mud invasion. Core porosity is measured on small samples in the laboratory and may not represent in-situ conditions (pressure, fluid saturation). The two should be compared and reconciled; large discrepancies signal tool calibration issues or core alteration.

Hole deviation is the angle of the wellbore from vertical. Highly deviated and horizontal wells distort log response because sensors may not be parallel to formation layers or stress. Correction algorithms account for deviation, but their accuracy decreases as deviation angles increase. Very steep boreholes may require special logging tools or interpretation caution.

Yes. Neural networks and random forests trained on large sets of core-calibrated logs can automate lithofacies classification with 80–95% accuracy, faster than manual interpretation. Limitations include dependency on training data quality, difficulty extrapolating to new geological settings, and black-box opacity. Human expert review remains essential.