CPG CPG Formation Evaluation & Petrophysics 1 — Questions and Answers
Question 1: Which petrophysical equation is most commonly used to calculate water saturation from resistivity logs in clean sandstone formations?
- Archie's equation (Correct answer)
- Waxman-Smits equation
- Dual-water model
- Indonesia equation
Correct answer: Archie's equation
Archie's equation (Sw = (a × Rw) / (φ^m × Rt))^(1/n) is the foundational method for calculating water saturation in clean, non-shaly sandstones.
Question 2: What does the neutron-density crossplot primarily identify in formation evaluation?
- Lithology and gas-bearing zones (Correct answer)
- Permeability and flow rate
- Formation pressure gradients
- Fault orientations
Correct answer: Lithology and gas-bearing zones
The neutron-density crossplot identifies lithology by the crossplot position and detects gas-bearing zones via the characteristic 'gas effect' crossover.
Question 3: In petrophysics, the term 'cementation exponent' (m) in Archie's equation typically ranges from what values for consolidated sandstones?
- 1.8 to 2.0 (Correct answer)
- 0.5 to 1.0
- 3.0 to 4.0
- 0.1 to 0.5
Correct answer: 1.8 to 2.0
For consolidated sandstones, the cementation exponent m typically ranges from 1.8 to 2.0, reflecting the tortuous pathways through cemented grains.
Question 4: Which log measurement is the primary direct indicator of formation fluid pressure in permeable zones while drilling?
- Repeat Formation Tester (RFT) / MDT (Correct answer)
- Gamma ray log
- Sonic log
- Caliper log
Correct answer: Repeat Formation Tester (RFT) / MDT
The Repeat Formation Tester (RFT) or Modular Dynamic Tester (MDT) directly samples formation fluid and measures pore pressure at discrete depth intervals.
Question 5: What is the primary purpose of the photoelectric factor (Pe) log in formation evaluation?
- Lithology identification (Correct answer)
- Porosity calculation
- Permeability estimation
- Fluid contact detection
Correct answer: Lithology identification
The photoelectric factor log is highly sensitive to the atomic number of rock-forming minerals and is therefore used primarily for lithology identification.
Question 6: Which core analysis measurement distinguishes macroporosity from microporosity in carbonates?
- Mercury injection capillary pressure (MICP) (Correct answer)
- Routine helium porosity
- Klinkenberg-corrected permeability
- Dean-Stark water saturation
Correct answer: Mercury injection capillary pressure (MICP)
Mercury injection capillary pressure (MICP) reveals the full pore-throat size distribution, allowing geologists to distinguish macro- and microporosity systems in carbonates.
CPG CPG Formation Evaluation & Petrophysics 1
Which petrophysical equation is most commonly used to calculate water saturation from resistivity logs in clean sandstone formations?