Oxidation Tmax: A new thermal maturity indicator for hydrocarbon source rocks

We report a new thermal maturity indicator for hydrocarbon source rocks based on non-isothermal programmed temperature oxidation of organic matter. Open system pyrolysis (e.g., Rock-Eval) is a fast and inexpensive analysis method and thus it is widely used in geochemical screening of source rocks. Pyrolysis Tmax based maturity assessment is known to be adversely affected by the presence of oil-based mud and organic contaminants. Moreover, pyrolysis does not yield reliable Tmax for overmature or organic-lean samples. With these limitations in mind, we aimed at utilizing thermal oxidation behavior of organic matter that could be used as an indicator for its maturity. A suite of organic-rich shale samples was analyzed using routine pyrolysis, and pyrolysis Tmax values were obtained. Then the same suite of samples were subjected to thermal oxidation using a LECO analyzer, and the maximum temperature of oxidation was measured. A strong correlation (R² ≥ 0.88) was found between pyrolysis Tmax and oxidation Tmax values. Furthermore, oxidation analyses of pre-pyrolyzed (charred) samples also produced a strong relationship between oxidation Tmax of the pre-pyrolyzed (charred) samples and the pyrolysis Tmax of the original sample. A final trial was conducted using routine pyrolysis instrumentation, namely Rock-Eval 6 (RE6) to measure the maximum temperature of oxidation (S4 peak) and compare it to pyrolysis Tmax. A cross plot of pyrolysis Tmax and oxidation Tmax values both acquired from RE6 analyses led to the following relationship: Pyrolysis Tmax(°C)=(oxidation Tmax(S4)+272.82)/1.71(R²=0.89) We note that oxidation Tmax is a reliable new thermal maturity indicator, especially for contaminated and/or overmature samples.