CPG CPG Geochemistry & Source Rock Analysis 1 — Questions and Answers
Question 1: What does a Total Organic Carbon (TOC) value of 2.0 wt% or greater generally indicate about a source rock?
- Good to excellent source rock potential for oil or gas generation (Correct answer)
- Reservoir quality with high storage capacity
- Thermal maturity above the oil window
- High permeability for tight gas extraction
Correct answer: Good to excellent source rock potential for oil or gas generation
A TOC of 2.0 wt% or higher indicates good to excellent organic richness, meaning the rock contains sufficient organic matter to generate commercial quantities of hydrocarbons.
Question 2: Vitrinite reflectance (%Ro) is used as a thermal maturity indicator. What %Ro value marks the onset of the main oil generation window?
- ~0.6% Ro (Correct answer)
- ~0.2% Ro
- ~1.3% Ro
- ~2.5% Ro
Correct answer: ~0.6% Ro
The main oil generation window typically begins at approximately 0.6% Ro, where sufficient thermal energy has been applied to crack kerogen into liquid hydrocarbons.
Question 3: In Rock-Eval pyrolysis, what does the S2 peak represent?
- Remaining generative potential — hydrocarbons released by thermal cracking of kerogen (Correct answer)
- Free hydrocarbons already present in the rock
- Carbon dioxide released from inorganic carbonates
- Volatile hydrocarbons lost during sample preparation
Correct answer: Remaining generative potential — hydrocarbons released by thermal cracking of kerogen
The S2 peak measures the hydrocarbons generated by thermal cracking of kerogen during pyrolysis, representing the rock's remaining generative potential.
Question 4: Which kerogen type is primarily associated with marine algal organic matter and has the highest oil-generation potential?
- Type II kerogen (Correct answer)
- Type I kerogen
- Type III kerogen
- Type IV kerogen
Correct answer: Type II kerogen
Type II kerogen, derived largely from marine phytoplankton and algae mixed with bacterial material, has a high hydrogen index and excellent oil-generation potential.
Question 5: What is the 'Hydrogen Index' (HI) in Rock-Eval pyrolysis, and what does a declining HI with increasing maturity indicate?
- HI = (S2/TOC) × 100; declining HI shows kerogen is being consumed as it generates hydrocarbons (Correct answer)
- HI = (S1/S2) × 100; declining HI shows increasing permeability
- HI measures inorganic hydrogen in clays; decline indicates dehydration
- HI = TOC/S3; decline indicates carbonate dissolution
Correct answer: HI = (S2/TOC) × 100; declining HI shows kerogen is being consumed as it generates hydrocarbons
Hydrogen Index equals (S2/TOC) × 100 and decreases with maturity because hydrogen-rich kerogen is progressively converted into expelled hydrocarbons.
Question 6: What geochemical correlation technique is used to 'fingerprint' crude oil and match it to its source rock?
- Biomarker analysis (sterane and hopane ratios) by GC-MS (Correct answer)
- Chloroform bitumen extraction gravimetry
- Stable carbon isotope ratio of bulk crude
- Vitrinite reflectance of oil inclusions
Correct answer: Biomarker analysis (sterane and hopane ratios) by GC-MS
Biomarker analysis using gas chromatography–mass spectrometry identifies molecular fossils (steranes, hopanes, terpanes) whose ratios are diagnostic of source rock age, depositional environment, and maturity.
CPG CPG Geochemistry & Source Rock Analysis 1
What does a Total Organic Carbon (TOC) value of 2.0 wt% or greater generally indicate about a source rock?