Hydrocarbon generation and retention potential of Chang 7 organic-rich shale in 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 % to 0.9 % Ro and organic matter of type II and mixed type II-III. Based on the present-day hydrogen index (HI pd ), the sample sets are divided into three sections, Upper, Middle, and Lower. The three sections have a high hydrocarbons generation potential, with an average original TOC (TOC o ) of 12.27, 3.10, and 5.13 wt.% of which 49.39, 23.62, and 49.86 wt.% represent generative organic carbon (GOC), an original hydrogen index (HI o ) of 581.27, 278.05 and 586.82 HC/g rock, in the Upper, Middle, and Lower Sections, respectively. The bulk of analyzed samples exhibit moderate-high oil saturation, yet the oil crossover effect is observed only 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 mg HC/g TOC is reached at a vitrinite reflectance of 0.8% and thereafter decreases with further maturity. Supplementary material: [Detailed spreadsheet of the back-calculated original geochemical parameters using the mass-balance method of Jarvie (2012a)], are available at https://doi.org/10.6084/m9.figshare.c.6387577 .