CPG Formation Evaluation & Petrophysics Flashcards

Which petrophysical equation is most commonly used to calculate water saturation from resistivity logs in clean sandstone formations?

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.

What does the neutron-density crossplot primarily identify in formation evaluation?

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.

In petrophysics, the term 'cementation exponent' (m) in Archie's equation typically ranges from what values for consolidated sandstones?

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.

Which log measurement is the primary direct indicator of formation fluid pressure in permeable zones while drilling?

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.

What is the primary purpose of the photoelectric factor (Pe) log in formation evaluation?

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.

Which core analysis measurement distinguishes macroporosity from microporosity in carbonates?

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.