Summary

Thermal maturation is often a key element in resource-play exploration, frequently relying on TMAX from pyrolysis experiments. This value is only a proxy for Transformation Ratio (TR) which measures fractional conversion of kerogen to hydrocarbons. Further, TMAX says nothing about the possible retention of these products in the source rock. This paper outlines an original analytical method for using pyrolysis data including Normalized Oil Content (NOC) from S1 and Hydrogen Index (HI) from S2 in conjunction with TMAX to generate estimates of TR and Retention Ratio (RR).

The key to these calculations is finding a reliable estimate of original Hydrogen Index (HIo) that characterizes the immature kerogen associated with each sample. Published methods for this critical initial step are currently too subjective for effective evaluation. This analysis predicts HIo from a cross plot with TMAX on the x-axis (linear scale) and HI on the y-axis (log10 scale) in which the traditional curved trajectories from initial to terminal transformation become straight lines. The interpretation then reverses the maturation trajectory and predicts HIo by projecting from the maturation terminus at the high-TMAX, low-HI "vanishing point" to an "immature horizon" at the left edge of the plot. This boundary varies as a function of source rock type defined by (HI+NOC) for the most immature samples in a data set. All (TMAX, HI) pairs with a similar slope from the vanishing point intersect the immature horizon at the same position and have a common HIo. But each (TMAX, HI) pair will have a unique TR that is calculated based on its progress across the plot, and RR can be derived by comparing the measured NOC with a calculated Expected NOC (NOCe).

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