How TOC affects Rock-Eval pyrolysis and hydrocarbon generation kinetics: an example of Yanchang Shale (T3y) from Ordos Basin, China

Abstract

Rock-Eval pyrolysis and kinetics are widely used to evaluate hydrocarbon generations. Due to heterogeneity of shale, even a series of samples come from the same drill core rock will have a wide range of total organic carbon (TOC). It is important to select propriate TOC samples to acquire Rock-Eval and kinetic parameters. However, influence of different TOC levels to Rock-Eval and kinetics are still not well known. In this study, different samples with different TOC of 3.87%, 13.59%, 18.17%, 23.93%, 25.93% and 35.35% taken from one drill core were selected and analysed. And all samples are prepared to 4mm grain-size samples for Rock-Eval pyrolysis to reflect hydrocarbon generation and expulsion in realistic conditions. The results show that generation rate gradually decreases from 0.0064 to 0.0053 mg/g·s−1 when TOC increase from 13.59 to 35.35%. And the samples with 35.35% and 25.53% TOC show highest transformation ratio, while the samples with 3.87% show the lowest transformation ratio. In addition, the samples with 3.87% TOC only shows one main activation energy peak (56Kcal/mol). Yet the samples with 35.35% TOC shows three activation energy peaks (53, 54, 56Kcal/mol). With increasing of TOC levels of samples, percentage of main activation energy decrease from 69.43 to 25.96 and 29.62%. Therefore, generation rate of high TOC shale will decrease and transformation ratio will increase. And hydrocarbon generation and expulsion of high TOC samples need lower action energies.