Rock-Eval Pyrolysis : A Technique in Petroleum Geology

The Rock-Eval pyrolysis is a widely used geochemical technique in petroleum exploration and organic geochemistry. It provides valuable information on the quantity, type and thermal maturity of organic matter in sedimentary rocks, helping geologists asses hydrocarbon generation potential.

Principles of Rock-Eval Pyrolysis

Rock Eval pyrolysis involves heating a rock sample in an inert atmosphere and measuring the release hydrocarbons and CO2 at different temperature ranges. The process consists of two main phases :

1. Pyrolysis phase : ( 300 – 650^oC )
   • Organic matter breaks down under controlled heating, releasing hydrocarbons and CO2.
   • This phase measures the hydrocarbon generation potential of the rock.
2. Oxidation phase : ( > 650^oC )
   • The remaining organic carbon is oxidized to determine the total organic carbon (TOC).

• Formula for hydrocarbon potential (HP) : HP = S1 + S2
  • S1 : Free hydrocarbons already present in the rock (mg HC/g rock)
  • S2 : Hydrocarbons generated from kerogen breakdown (mg HC/g rock)
• S1 and S2 hydrocarbon potential
  • S1 : (mg HC/g rock) : represents pre-existing free hydrocarbons in the rock. High values indicate mature, oil -prone rocks.
  • S2 : (mg HC/g rock) : Indicates the hydrocarbon potential of kerogen. Higher values suggest good source rocks.

• Total Organic Carbon (TOC) – Source Rock Quality
  • TOC (%) measures the organic richness of the rock

• Hydrogen Index (HI) and Oxygen Index (OI) – Kerogen Type Classification
  • These indices help classify kerogen types based on their hydrogen and oxygen content :
    • HI = (S2/TOC) *100
    • OI = (S3/TOC) *100
  • HI (mg HC/g TOC) : high values indicate oil-prone kerogen.
  • OI (mg CO2/g TOC) : high values suggest gas-pron or oxidized organic matter.

• T_max – Thermal Maturity Indicator
  • T_max is the temperature at which S2 reaches its peak. It provides insights into the maturity of organic matter :

Tmax (oC)	Maturity Stage	Hydrocarbon Type
< 435 oC	Immature	No significant oil/gas
435 – 450 oC	Early maturity	Oil window
450 – 470 oC	Peak maturity	Oil + Gas
> 470 oC	Post mature	Dry Gas

Applications of Rock-Eval Pyrolysis

1. Petroleum Exploration
   • Identifies potential source rocks and evaluates their hydrocarbon potential.
   • Determines maturity levels to assess oil/gas generation.
2. Unconventional Resources (Shale oil & gas)
   • Evaluates kerogen type and TOC content in shale formations.
   • Helps estimate recoverable hydrocarbons in unconventional reservoir.
3. Environmental Geochemistry
   • Detects organic contamination in soils and sediments.
   • Helps assess pollution sources by analyzing hydrocarbon content.