Biomarker: Assessment of Thermal Maturity
Thermal maturity consists of temperature/time-driven disproportionation reactions that convert sedimentary organic matter into light and heavy fractions of petroleum and finally into hydrocarbon gas and pyrobitumen or graphite. Different geochemical scales commonly used to describe the extent of thermal maturation include vitrinite reflectance (Ro), programmed pyrolysis (e.g., Rock-Eval) Tmax and production index, and biomarker maturity ratios (e.g., Peters et al. 2005).
Geochemists divide thermal maturation into three stages: diagenesis, catagenesis, and metagenesis. Temperature and vitrinite reflectance values at the transitions between these stages vary depending on the burial heating rate and type of organic matter and are thus only approximate. Diagenesis consists of chemical, physical, and biological changes in the organic matter during and after sediment deposition and lithification at temperatures below ~80 °C (Ro<0.6%). Diagenesis occurs prior to...
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