CPG CPG Formation Evaluation & Petrophysics

Question 1

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

Accepted Answer

Archie's equation

Answer

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?

Accepted Answer

Lithology and gas-bearing zones

Answer

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?

Accepted Answer

1.8 to 2.0

Answer

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?

Accepted Answer

Repeat Formation Tester (RFT) / MDT

Answer

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?

Accepted Answer

Lithology identification

Answer

The photoelectric factor log is highly sensitive to the atomic number of rock-forming minerals and is therefore used primarily for lithology identification.

(CPG) Certified Petroleum Geologist Practice Test

(CPG) Certified Petroleum Geologist Practice Test

CPG CPG Formation Evaluation & Petrophysics