Matrix Retention of Hydrocarbons

During the Rock-Eval pyrolysis S2 stage, the heavier hydrocarbon molecules generated during to the thermal maturation of kerogen, are often retained by the shale-matrix. Argillaceous matrices of organic-rich shales generally results in a greater amount of petroleum retention. Under such matrix retention conditions, the best-fit line of a S2 versus total organic carbon (TOC) cross-plot does not pass through the origin but intercepts the TOC-axis. Furthermore, as the S2 yield is reduced, the hydrogen indices (HI) tend to be undercounted and thereby lead to inaccurate source rock assessment. However, such effects may also be caused due to the impact of inert organic-matter which does not contribute much to the S2 peak magnitude, but does contribute to the TOC content. Thus, correction of the effects from both inert organic-matter and shale-matrix retention are needed for accurate petroleum source rock estimation. The effects of matrix retention tend to be more pronounced for type III kerogen-bearing shales, than type I-II kerogen-bearing shales.