A mathematical representation of Rock-Eval hydrogen index vs Tmax profiles

Abstract

Different types of kerogens have different maturation paths on Rock-Eval Hydrogen Index (HI) vs T max plots. An equation: HI=1/(a* exp (b*(T max −435))+c), where a, b, and c are constants, can be used to define HI– T max profiles of particular source rock units. The constants a, b, and c, characterise the particular kerogen and also allow expression of its transformation ratio (TR) as a function of the T max by an equation: TR=(HI 0 *a* exp (b*(T max −435)))/( HI 0 *a* exp (b*(T max −435))+1), where HI 0=1/ c. The HI– T max equation constants were determined by nonlinear curve fitting for three standard Type I, II, and III kerogens and for two geological sections: Second White Speckled (2WS) Formation, Alberta Basin and a source rock unit of Cambay Basin, India. High correlation coefficients for the fitted equations for four HI– T max data sets of four groups of the 2WS samples, segregated using Oxygen Index (OI) vs T max cross plot, and for one data set of the Cambay Basin source rock unit, show the validity of the proposed HI– T max equation. Comparison of the calculated TR and measured production index (PI) values for the 2WS samples allow identification of samples that have expelled generated hydrocarbons and samples that contain unexpelled generated hydrocarbons. For the Cambay Basin example, where Rock-Eval data of silty and sandy samples were screened out, a reasonable match exists between the calculated TRs and measured production index values up to a T max value of 445°C, indicating onset of oil expulsion. The proposed HI– T max equation can thus characterise organic matter in a source rock unit with respect to its original generation potential and hydrocarbon generation threshold and rate. It may also help to identify the effective source sections that have expelled generated hydrocarbons, from several potential source rocks, if present in a basin.