Exploring the coupling relationship between hydrocarbon generation of continental shale and nanopore structure evolution—A case study of Shahejie formation in Bohai Bay Basin

Abstract

The nano-scale pore structure of shale is closely related to the self-generated and self-accumulated shale oil and gas. The Bohai Bay Basin is a crucial oil-bearing basin in eastern China, and the Paleogene Shahejie Formation is the most important source rock section in this area. In order to study the internal relationship between hydrocarbon generation evolution and pore structure characteristics of source rocks, we conducted hydrocarbon generation simulation tests with a closed gold tube system, and heated up original rocks from Shahejie Formation in Laizhou Bay Sag, southern Bohai Bay Basin, from 290 °C to 440°C at different heating rates. Besides, we carried out low-temperature N2 adsorption experiments on sample residues, and measured their pore structure characteristic parameters. The results show that with the increase of simulated temperature, the specific surface area and pore volume of nano-pores below 10 nm (which are mainly organic pores) decrease first and then increase, while those of nano-pores above 10 nm increase all the way. The evolution trend of total specific surface area and pore volume is mainly controlled by pores below 10 nm which are mainly organic pores, especially micropores below 2 nm. There are two main factors affecting the development of inorganic pores: (1) Dissolution of organic acids produced by pyrolysis of organic matter in hydrocarbon-generation evolution; (2) Deformation of crystal structure of mineral components under the combined action of temperature and pressure. The experimental results at different heating rates demonstrate that rapid settlement under geological conditions is not conducive to the development of nano-pores, especially micro-pores composed of organic pores.

Characterization of the evolution of thermal maturity and pore structure of continental organic-rich shales

“…Among them, thermal maturity has been most widely used (Liang et al, 2014;Sun et al, 2015;Gao et al, 2021;Xu et al, 2021), because pores induced by the generation of hydrocarbons are the main contributors to secondary porosity in shale. Hence, highly productive shale gas has been strongly associated with high-maturity shales (Chatellier et al, 2011;Kotarba and Lewan, 2013;Ji et al, 2017;Wu et al, 2019;Chen et al, 2020;Song et al, 2021;Zheng et al, 2021;Cao et al, 2022). Rapid and effective characterization of maturity is important for shale gas exploration.…”

“…Highly productive shale gas has been shown to be strongly associated with high-maturity shales (Wu et al, 2019;Chen et al, 2020;Song et al, 2021;Zheng et al, 2021;Cao et al, 2022), indicating that highly productive shale gas has inevitably experienced a high evolutionary stage and deep burial. In this long and complicated evolution process, the hydrocarbon generation of organic matter and the transformation of clay minerals in shale may have mutual influence.…”