Hydrocarbon generation and retention potential of Chang 7 organic-rich shale in the Ordos Basin, China

Abstract

This study investigates the hydrocarbon generation and retention potential of Chang 7 organic-rich shale, with an emphasis on the producibility of retained hydrocarbons, using a sample set chosen to represent a maturity spectrum of 0.54–0.9% R o and organic matter of type II and mixed type II–III. Based on the present-day hydrogen index (HI pd ), the sample sets were divided into three sections: Upper, Middle and Lower. The three sections have a high hydrocarbon generation potential, with an average original total organic carbon (TOC o ) content of 12.27, 3.10 and 5.13 wt%, of which 49.39, 23.62 and 49.86 wt% represents generative organic carbon (GOC), and an original hydrogen index (HI o ) of 581.27, 278.05 and 586.82 HC g −1 rock in the Upper, Middle, and Lower sections, respectively. The bulk of the analysed samples exhibited moderate–high oil saturation, yet the oil crossover effect was only observed in two organic-rich samples, indicating organic-rich shale-oil resource systems. The sorption capacity of organic matter controls oil retention in the Chang 7 shale system, where the oil saturation index increases with increasing maturity in the oil window until a maximum retention capacity of about 82–83 mgHC g −1 TOC is reached at a vitrinite reflectance of 0.8% and thereafter decreases with further maturity. Supplementary material: A detailed spreadsheet of the back-calculated original geochemical parameters using the mass-balance method of Jarvie (2012 a ) is available at https://doi.org/10.6084/m9.figshare.c.6387577