Questions
6–8 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip for ONGC/OIL exams
Overview
Well logging is the practice of making a detailed record of the geologic formations penetrated by a borehole. For PSU exams like ONGC and OIL, it is a high-yield topic focused on identifying lithology, porosity, and hydrocarbon saturation. Aspirants must master the characteristic responses of various logging tools to differentiate between reservoir and non-reservoir rocks.
Gamma Ray and SP Logs
Gamma Ray (GR) logs measure natural radioactivity to identify lithology, while Spontaneous Potential (SP) logs measure the electrical potential difference between a downhole electrode and a surface electrode. These are primarily used for depth correlation and identifying permeable zones.
- GR is high in shales due to Potassium, Uranium, and Thorium content
- SP deflection is negative in porous, permeable, water-bearing sands
- SP log baseline shifts in the presence of hydrocarbons
- Used for bed boundary identification and correlation
Resistivity Logs
Resistivity logs measure the ability of a formation to conduct electric current, which is inversely proportional to hydrocarbon saturation. Since hydrocarbons are insulators, high resistivity generally indicates potential oil or gas zones.
- Archie's Law: Sw = (a * Rw / (phi^m * Rt))^(1/n)
- Lateral and Laterolog devices for deep investigation
- Resistivity is high in massive crystalline rocks and hydrocarbons
- Deep investigation tools bypass the invaded zone (Rxo)
Porosity Logs (Density & Neutron)
Porosity logs determine the volume of voids in a rock formation; Density logs measure electron density using gamma-ray scattering, while Neutron logs measure hydrogen index.
- Density porosity formula: phi = (rho_matrix - rho_bulk) / (rho_matrix - rho_fluid)
- Neutron logs respond directly to hydrogen concentration
- Gas effect: Density porosity increases and Neutron porosity decreases (crossover)
- Compensated tools are used to account for borehole size variations
Sonic Log
The Sonic log records the interval transit time (Delta-t) of compressional sound waves through the formation. It is the primary tool for determining porosity in consolidated rocks where fluid effects are minimized.
- Wyllie Time-Average Equation: phi = (dt_log - dt_matrix) / (dt_fluid - dt_matrix)
- Used for seismic calibration and rock strength analysis
- Affected by secondary porosity (vugs and fractures)
- Delta-t increases as porosity increases
Formula Sheet
Archie's Equation: Sw = (a * Rw / (phi^m * Rt))^(1/n)
Density Porosity: phi = (rhoma - rhob) / (rhoma - rhof)
Wyllie Time-Average: phi = (dt_log - dt_matrix) / (dt_fluid - dt_matrix)
Exam Tip
Always memorize the 'Gas Effect' pattern (Density and Neutron curve crossover) as it is a favorite question in technical exams.
Common Mistakes
- Confusing the direction of SP deflection in hydrocarbon versus water-bearing zones.
- Ignoring the 'Gas Effect' where density and neutron curves cross over.
- Applying Archie's Law to formations with high shale content without correcting for clay conductivity.
More Revision Notes
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