A numerical method for calculating total oil yield using a single routine Rock-Eval program: A case study of the Eocene Shahejie Formation in Dongying Depression, Bohai Bay Basin, China

Obtaining reliable total oil yield in a source rock from routine programed pyrolysis data represents a technical challenge for the characterization of source rocks in early mature and oil window due to strong interactions between bitumen and kerogen/rock matrix. A common laboratory solution for a robust estimate of total oil yield requires two pyrolysis experiments using a whole rock sample and a post-solvent extracted replicate. In this paper, we present a numerical solution that provides a reliable estimate of total oil yield directly from a single routine Rock-Eval experiment. In temperature domain, the thermally vaporized products from heavy oil and bitumen adsorbed to the organic matter and the pyrolyzed products from kerogen show a certain degree of overlap in the default temperature range of S2 peak. By transforming the routine Rock-Eval pyrogram (FID curve) into activation energy variable space, it allows for separation of the reactants into subgroups by their responses to ramping temperature. The thermally vaporized products can be then discriminated from the thermally pyrolyzed products based on the differences in their thermal stability and decomposition behaviour. The proposed method was applied to the source rock samples in the Eocene-Oligocene Shahejie Formation of Dongying Depression, Bohai Bay Basin, where programed pyrolysis results from whole rock samples and post solvent extracted replicates were obtained. The case study suggests that (a) the two-step pyrolysis experiment approach may still under-estimate the total oil yield in case that solvent extraction treatment fails to remove all hydrocarbons in the sample; (b) the post-solvent extracted equivalent S2x values derived numerically from routine Rock-Eval analysis and those derived from post-extracted analysis are comparable with a correlation coefficient of 0.9766, suggesting numerical approach is as effective as additional laboratory experiment; and (c) the proposed approach can remove all petroleum in the sample numerically, thus providing an un-biased and robust estimate of total oil yield, a cost- and time-effective alternative to the traditional two experiment approach.